Past Colloquia

September 9, 2008
Eugene Shakhnovich
Harvard University
"Bridging scales in biological evolution: from atoms to organisms"
Host: Michael Hagan

One of the key unsolved problems in Biology today is understanding impact of classical evolution, on organismal and population level on molecular evolution of genes and proteins. In this lecture I will present a microscopic physical model of early, biological evolution, where phenotype - organism life expectancy - is directly related to genotype – the stability of its proteins and interactions between them which can be determined exactly in the model. Simulating the model on a computer, we consistently observe the ‘’Big Bang’’ scenario whereby exponential population growth ensues as favorable sequence-structure combinations (precursors of stable proteins) are discovered. After that, random diversity of the structural space abruptly collapses into a small set of preferred proteins. The key feature of evolved proteins is their remarkable robustness with respect to point mutations. Further, we noted that evolution of populations of organisms each carrying M genes is isomorphic to the problem of M-dimensional random walkers in space of protein sequences with two adsorbing boundary conditions: at high energies of protein native conformations where proteins become unviable and organisms carrying their genes die and at lower energies where proteins are depleted of possible stable sequences. This problem can be solved exactly and we obtained the relation between mutation rates, duplication rate and stability range of the proteins at which populations remain viable. This formula predicts the universal speed limit on molecular evolution at 6 mutations per genome per replication. It provides important insights into genomic organization and evolution of viruses and simple organisms. Further analysis highlights relationship between selection and mutations – an exact solution of the selection-mutation equation provides insights into evolution of bacteria viruses and immune response.

September 16, 2008
Albion Lawrence
Brandeis University
"(De)construcing spacetime in string theory"
Host: Michael Hagan

The famous difficulty of combining Einstein's theory of gravity with quantum mechanics teaches us that the degrees of freedom of nature should change drastically at short distances and high energies. String theory provides such a change of perspective by positing that the fundamental constituents of nature are extended objects. In this scenario, there is no operational notion of a "point" in space or in time. To probe this new description of spacetime, we will describe explicit examples of "stringy" spacetimes which have features that are very different from those that we are used to (these generalizations also have applications to particle physics and cosmology). We will then discuss how two familiar concepts--time, and the dimension of space--arise in string theory. We will close by discussing current work on recovering the physics of black holes.

Tuesday, September 23, 2008
James Bensinger
Brandeis University
"A precision muon detector for the ATLAS experiment at the LHC"

The muon detector, the outermost part of the ATLAS experiment at the LHC, was built to an unprecedented size and with unprecedented precision. The muon system occupies a volum 40 meters long and 25 meters in diameter. It contains aproximately 300,000 wires and the position of each one has to be known to 40 mm. This sytem is expected to measure the momentum of muons up to 1 TeV/c with 10% accuracy. As muons and electrons are expected to be significant signatures of new phenomenon, this is a critical part of the ATLAS experiment. I will describe the experiment and in particular the muon system and discuss the technological developments and advances that enabled us to build this system with emphasis on those developments achieved at Brandeis.

Tuesday, October 7, 2008
Paul Miller
Brandeis University
"Learning associations while retaining specificity: competing demands on network plasticity rules"

Many cognitive tasks require behavioral responses to specific pairings or combinations of stimuli (A with B or X with Y) that differ from responses to the same individual stimuli, but in alternative combinations (A with Y or X with B). Such pair-specific responses can require logic equivalent to XOR, and can not be solved with a single layer neural network. Generation of a neurons with responses to specific pairs of stimuli becomes essential. We assess how various plasticity rules, which describe how connections between neurons alter as a function of neural activity, can assist or detract from the necessary generation of neurons with responses to specific associations. We find that standard rules for spike-timing dependent plasticity tends to produce over-association (so that any cell responding to the pair "A and B" would also respond to "A and Y" or to "X and B". However, the more recently discovered long-term potentiation of inhibitory connections counters such over-association by generating cross-inhibition, which suppresses the unwanted responses. We show how such association-specific neurons can arise in an initially random network. We also address which types of network structure can generate stimulus-specific memory activity, which is needed when pairs of stimuli are separated in time. Some of these memory networks can retain information about the order of stimuli as well as the specific pairing.

Tuesday, October 28, 2008
Rudi Podgornik
Josef Stefan Institute and NIH
"The Nature and Characterization of Order in High Density DNA Mesophases"
Host: Jane' Kondev

I will describe the nature of known mesophases of DNA at high densities (cholesteric, line hexatic, hexagonal colunar, hexagonal and orthorhombic) and invoke some recent advances in their understanding. Concerted rotations and translations of long helical molecules around their long axes in bulk samples lead to a new mesophase with a screw-like order. This screw-like order actually expels the cholesteric twist from a line hexatic phase and allows it to show a typical sixfold lhexatic scattering intensity. Next I will describe a partially constrained system like DNA fibers where the molecules are allowed to rotate but are not allowed to fluctuate translationally. In this case, general considerations based on angular dependence of the DNA-DNA interaction potential, lead to new phases with non-hexagonal symmetry as already seen in classical DNA X-ray scattering studies in the '50s. I will also show how the azimutal interactions lead to crystallization of the fiber with an orthorhombic unit cell which can be observed in DNA at high density.

Tuesday, November 4, 2008
Martin Zwierlein
"Ultracold Fermi gases of atoms: Strongly interacting model matter"
Host: Albion Lawrence

Ultracold atomic gases allow us to study fermions, the building blocks of matter, in a highly controllable environment. The interactions between atoms can be varied at will over an enormous range via Feshbach scattering resonances. In an equal mixture of spin up and spin down fermions with attractive interactions, the gas forms a superfluid of fermion pairs. By varying the interaction strength, we can study the crossover from a Bose-Einstein condensate of tightly bound molecules to a Bardeen-Cooper-Schrieffer superfluid of long-range Cooper pairs. Superfluidity in this crossover regime is demonstrated by setting the gas under rotation and observing ordered lattices of quantized vortices. Thanks to its strong interactions, the gas is a high-temperature superfluid: Scaled to the density of electrons in a metal, superfluidity would occur already far above room temperature. A new regime is entered when the number of spin up versus spin down atoms is imbalanced. In this case, not every spin up atom can find a spin down partner. The ground state of such a system has been under debate for over 40 years. We observe the breakdown of the superfluid at a critical imbalance, giving way to an intriguing, strongly interacting Fermi gas with unequal spin populations.


Tuesday, November 11, 2008
Cristina Marchetti
Syracuse University
"Soft Active Matter"
Host: Bulbul Chakraborty

Assemblies of interacting self-driven units form a new type of active soft matter with intriguing collective behavior and mechanical properties. Active systems exhibit novel nonequilibrium phase transitions, can generate forces and adapt their structure and mechanical properties in response to stimuli. Examples include extracts of cytoskeletal filaments and motor proteins, bacterial colonies, migrating cells, and vibrated layers of granular rods. The most remarkable realization is perhaps the cell cytoskeleton, a viscoelastic polymer gel that controls cell shape, motility and division. In this talk I will review our work on using nonequilibrium statistical physics to derive a continuum description of active matter from specific models of single particle dynamics. Using a simple hydrodynamic model, I will demonstrate the surprising properties of active matter in specific geometries that may have relevance to some of the mechanical properties of living cells.

Tuesday, November 18, 2008
Alexander Grosberg
"To Knot or Not to Knot"
Host: Jane' Kondev

The mathematics and physics of knots has a long and fascinating history, starting from a model of an atom suggested by W.Thompson (Lord Kelvin) and enthusiastically supported by Maxwell. Knots in DNA are abundant and important. Recently, we surveyed the protein data bank and found that evolution for some as yet unknown reason strongly preferred unknotted proteins. In theoretical aspect, the field was long dominated by either highly abstract mathematics or computer simulations. Recently, some progress was made in the direction of physical understanding of knots. One fruit of it is the prediction that knots under certain circumstances behave like a material with negative Poisson ratio. In the talk, all these various aspects will be reviewed in some mixture.

Tuesday, November 25, 2008
Dr. Stefi Baum
Director, Center for Imaging Science, RIT
"The Universe Unveiled"
Host: John Wardle

This talk will review the remarkable history of imaging and its impact on discovery and modern society. Through technology, many things our eyes could never see--from images of earth from space to atoms and molecules at the smallest scale—are revealed with amazing resolution and detail. The images we form today use not only the visible light our eyes can see, but the full range of the electromagnetic spectrum, from gamma rays through infrared and on down to the lowest radio frequencies; while modern ultrasound and electron micoroscope imaging techniques transcend the realm of electromagnetic waves. To increase the physical information and diagnostic power of images, we employ a range of imaging techniques, such as spectroscopy, radar and polarimetry. We utilize in situ sensors to provide calibration for remotely sensed images. We manipulate particle beams, as well as electromagnetic radiation, to probe on nano scales at the highest energies. We create databases of the images so obtained, computer algorithms that fuse information from multiple imaging modalities, and visualization products that allow humans, aided by computers, to obtain the answers to fundamental questions critical to human knowledge, health, and security.

Tuesday, December 2, 2008
Priya Natarajan
Yale University and Harvard University
"Mapping dark energy and dark matter using cluster lensing"
Host: Albion Lawrence

Gravitational lensing has emerged as a powerful technique to map dark matter in the Universe. I will present the status of our current understanding of some key dark matter properties inferred from cluster lensing studies. I will also present new results on the feasibility of using cluster strong lensing to probe dark energy.

Tuesday, December 9, 2008
co-sponsored by MRSEC
Erik Luijten
University of Illinois, Urbana-Champaign
“Self-assembly of rod-like polyelectrolytes: from materials to cystic fibrosis”
Host: Michael Hagan

Electrostatic interactions play an important role in many biological problems and can lead to counterintuitive phenomena. I will highlight a number of problems in this area that we have addressed by means of computational methods. Specifically, we have used Monte Carlo and molecular dynamics simulations to better understand the self-assembly of stiff polyelectrolytes (charged polymers). Such molecules, e.g. filamentous actin, form close-packed bundles in the presence of multivalent ions or proteins. We elucidate the mechanism of this self-assembly process and are able to make direct comparison to experimental results obtained via small-angle x-ray scattering. I will also demonstrate how these findings pertain to fighting bacterial infections in cystic fibrosis patients.


Thursday, February 5, 2009
4:00 p.m.
Abelson 131
Gregg Lois, Yale University
"Frustrated phenomena in physics and biology: From supercooled liquids and glasses to protein folding dynamics."
Host: Seth Fraden

Frustration occurs when competing interactions between many degrees of freedom give rise to a large number of metastable states. Although some frustrated systems are able to equilibrate on experimental time-scales, others never equilibrate and their dynamics remain far from equilibrium on all accessible timescales. Frustrated systems far from equilibrium are typically termed glasses, and most materials will become a glass if quenched quickly from high to low temperature. Near the glass transition structural relaxation becomes sluggish, with relaxation times growing by many orders of magnitude over a relatively small temperature range. This slowdown is not accompanied by an obvious change in structure, and thus its origin has been the subject of intense study and debate. In the first half of the talk, I will discuss my recent work on the correspondence between the glass transition and mean-field percolation, as well as introduce a new simulation model for glass-forming materials that is inspired by this correspondence and yields an orders-of-magnitude reduction in computing time. In the second half of the talk, I will focus on my theoretical and computational studies of proteins. Proteins fold to a unique native state at room temperature despite being highly frustrated. How this occurs on timescales accessible to experiment, and relevant to biological function, is an open question that has intrigued scientists for decades. I will outline the Free Energy Reaction Path theory, a framework I developed for understanding the near-equilibrium dynamics of frustrated proteins, and discuss my recent protein folding simulations that validate its predictions. Results from both theory and simulation suggest a new non-equilibrium mechanism for reliable folding that could also be used to target metastable states in "structure-seeking" materials.

Tuesday, February 10, 2009
Abelson 131
Ali Khademhosseini, HST
"Microengineered hydrogels for stem cell bioengineering and tissue regeneration"
Host: Azadeh Samadani

Micro-and nanoscale technologies are emerging as powerful tools for controlling the interaction between cells and their surroundings for biological studies, tissue engineering, and cell-based screening. In addition, hydrogel biomaterials have been increasingly used in various tissue engineering applications since they provide cells with a hydrated 3D microenvironment that mimics the native extracellular matrix. In this talk, I will outline our lab's work in controlling the cell-microenvironment interactions by using patterned hydrogels to direct the differentiation of stem cells. In addition, I will describe the fabrication and the use of microscale hydrogels for tissue engineering by using a ‘bottom-up’ and a ‘top-down’ approach. Top-down approaches for fabricating complex engineered tissues involve the use of miniaturization techniques to control cell-cell interactions or to recreate biomimetic microvascular networks within mesoscale hydrogels. Our group has also pioneered bottom-up approaches to generate tissues by the assembly of shape-controlled cell-laden microgels (i.e. tissue building blocks), that resemble functional tissue units. In this approach, microgels were fabricated and seeded with different cell types and induced to self assemble to generate 3D tissue structures with controlled microarchitecture and cell-cell interactions.

Thursday, February 12, 2009
4:00 p.m.
Henry Fu, Brown University
"Swimming in viscoelastic fluids and gels"
Host: Seth Fraden

The swimming of microorganisms in Newtonian fluids has been, and still remains, an active area of research. However, in many cases the natural environments which microorganisms navigate are non- Newtonian fluids or even gels. For example, mammalian sperm swim through mucus in the female reproductive tract. In this talk I focus on swimming through polymeric viscoelastic fluids and gels. First, the forces exerted by a viscoelastic medium are different from those exerted by a Newtonian fluid. I address how this affects swimming shapes and speeds of flexible swimmers such as sperm. Second, the kinematic reversibility of the zero-Reynolds-number Stokes flow constrains what types of swimming motions are effective in Newtonian fluids (Purcell's "Scallop theorem"). I discuss how this is altered in nonlinearly viscoelastic fluids, and whether any new swimming strategies become available to swimmers in viscoelastic media. Finally, I describe issues that arise for swimmers moving through viscoelastic gels, which are solids rather than fluids.

Tuesday, February 24, 2009
4:00 p.m.
Abelson 131
Aparna Baskaran, Syracuse University
"Bacteria as a fluid: Applying the materials physics paradigm to biology"
Host: Bulbul Chakraborty

The advent of advanced imaging and other experimental techniques as applied to in vivo and in vitro biology experiments have brought many new systems under the purview of soft materials theorists. In this talk, I will give a flavor for the contributions that can be made by materials physicists to systems of biological relevance by describing recent work in the context of self-propelled particles. Self propelled particles are active units that live in a medium and exert forces on it and actively move through it. I will consider two simple theoretical models for this class of systems :
1) self propelled hard rods on a substrate
2) a stroke averaged swimmer in a viscous fluid.
I will use these to illustrate the interplay between self propulsion and physical interactions such as excluded volume interactions and hydrodynamic interactions on the large scale collective behavior of these systems

Thursday, February 26, 2009
Abelson 131
Vincenzo Vitelli, University of Pennsylvania
"Vibrational dynamics and heat conduction in amorphous solids"
Host: Seth Fraden

The development of a theoretical framework that encompasses the dynamical properties of amorphous solids stands as one of the lasting challenges of material science and mayunveil further technological potential for a vast class of soft materials such as emulsions, gels, colloidal suspensions as well as granular media and covalent glasses.

In recent years, an intriguing route to studying amorphous materials has emerged from investigating the random geometry of packings comprised of soft spheres interacting with finite-ranged repulsions. The central tenet of this approach is the existence of a zerotemperature jamming transition that occurs at a critical packing fraction, point J, at which the average number of contacts between particles barely satisfies the constraints of mechanical equilibrium.

At point J, the effect of disorder becomes overwhelming and the physics remarkably simple: we find that both the density of states and the thermal diffusivity of the vibrational modes exhibit a plateau extending to zero frequency. This talk focuses on the thermal conductivity of amorphous materials under pressure, which unlike crystals, increases monotonically with temperature. I will show that this distinct property emerges from the vibrational modes at Point J which controls energy transport in a manner akin to a critical point.

Tuesday, March 3, 2009
4:00 pm
Abelson 131
Andy Lau, Florida Atlantic University
"Response and Fluctuations in Active Systems"
Host: Zvomimir Dogic

Abstract: Active complex fluid systems like living cells, assemblies of motors and filaments, flocks of birds, and vibrated granular material are nonequilibrium systems that consume and dissipate energy. These active systems exhibit phenomena that can be quite distinct from those of conventional equilibrium soft materials. Nevertheless, their long-wavelength properties can at times be described by hydrodynamic-like stochastic equations similar to those of equilibrium systems but with additional terms that break Onsager reciprocity and noise sources that violate
the fluctuation dissipation theorem. In this talk, we focus on a model active system, namely, a bacterial bath, which consists of a population of rod-like motile or self-propelled bacteria suspended in a fluid environment. We discuss results of recent microrheological experiments in terms of a dynamical model for nematic liquid crystals in the isotropic state,appropriately modified to reflect the activity of bacteria. We show, in particular,that the non-equilibrium contributions to the stress arising from the swimming of the bacteria lead to a Ω scaling in the power spectrum of the active stress fluctuations, as observed experimentally.

Tuesday, March 10, 2009
4:00 pm
Abelson 131
Vinothan Manoharan, Harvard University
"The Physics of Self-Assembly in Simple Systems"
Host: Azadeh Samadani

Abstract: Self-assembly refers to any thermodynamic process in which a bunch of particles (molecules, biomolecules, polymers, colloids) come together in solution to form an ordered structure. In living things it is a widely used and robust manufacturing tool: DNA, RNA and proteins spontaneously form three dimensional structures, and supramolecular structures emerge from protein aggregates with staggering degrees of ordering and specificity. On the other hand, most synthetic systems in soft condensed matter do not, in general, assemble robustly. We use experimental model systems consisting of small numbers (N <= 10) of confined colloidal particles to try to understand what physical parameters (interactions) determine how a system will assemble. Using optical techniques such as microscopy and holographic microscopy, which allow to probe the dynamics and structure of these systems with high temporal and spatial precision, we measure the number of configurations observed in ensembles of particles interacting through a tunable short-range attraction. The results from these small model systems can be compared directly to simulation and theory, which yields some general insights into the design principles for robust self-assembly.

Tuesday, March 24, 2009
4:00 pm
Abelson 131
Aravi Samuel, Harvard University
"How worms and maggots navigate temperature gradients"
Host: Azadeh Samadani

Animals are intrinsically computational. We acquire sensory information about our environments, transform this information into neural representations and memories, and calculate and execute decisions based on recent and past experiences. Our own brains are staggeringly complex, with billions of neurons networked by trillions of synapses. But the basic "stuff" of our brains -- molecular and cellular structures and interactions -- is shared with our simplest animal relatives. Thus simple and well-chosen model organisms can be accessible vantage points with perspective over general biological principles. We study brain and behavior in the roundworm C. elegans and in the larval fruit fly. We focus specifically on navigational behaviors responding to physical sensory inputs. These inputs (e.g., temperature) can be delivered both reliably and quantifiably. Navigation of worms and maggots itself can be reduced to a quantified pattern as an alternating sequence of forward movements, turns, and reversals. From the systematic analysis of outward motile behavior we can infer the inner workings of neural algorithms. Applying recent advances in microscopy and optics, we are also able to manipulate and monitor the workings o-f these neural circuits in the intact animal. In this way, we strive to link brain and behavior in these simple but fascinating creatures.

Tuesday, March 31, 2009
4:00 p.m.
Abelson 131
"Expectations for the Large Hadron Collider"
Matthew Schwartz
Harvard University
Host: Matthew Headrick

Abstract: The Large Hadron Collider is the largest scientific instrument ever built. It had its first beam of protons last fall and plans to have the first collisions this summer. But what is it looking for? What are we hoping to find? There are many possibilities, including supersymmetry and the elusive Higgs boson. These concepts will be discussed through an exploration of the the theoretical motivation and historical context of this fantastic machine.

Tuesday, April 21
Abelson 131
Lecture I: "Making a Splash, Breaking a Neck: The Development of Complexity in Fluids"
Question: Have fluids been touched by Intelligent Design?
Eisenbud Lecture Series in Mathematics and Physics
Leo P. Kadanoff, University of Chicago
Host: Albion Lawrence

Wednesday, April 22
Abelson 131
Lecture II: "The Good the Bad and the Awful-- Scientific Simulation and Prediction"
Question: How far can you trust a computer simulation?
Eisenbud Lecture Series in Mathematics and Physics
Leo P. Kadanoff, University of Chicago
Host: Albion Lawrence

Thursday, April 23
Abelson 131
Lecture III:"Eigenvalues and Eigenfunctions of Toeplitz Matrices"
Question: Can you give an example of very, very, very bad convergence?
Eisenbud Lecture Series in Mathematics and Physics
Leo P. Kadanoff, University of Chicago
Host: Albion Lawrence


Tuesday, April 28, 2009
4:00 pm
Abelson 131
speaker: John Wardle, Brandeis University

Understanding Black Hole X-ray Binary Systems: Clues from Radio Polarization Imaging of the Microquasar GRS 1915+105


Black hole x-ray binary systems are superb laboratories for studying general relativity, special relativity, relativistic hydrodynamics and all the processes of high energy astrophysics. They consist of a several solar mass black hole and a normal star in orbit about their center of mass. Matter streams from the atmosphere of the normal star (often a giant filling its Roche lobe) and spirals down towards the black hole, forming a hot accretion disk around it. The inner disk is a source of intense thermal x-rays. Rapid fluctuations in the x-ray intensity reveal physical processes near the “innermost stable orbit” close to the event horizon. The 6.4 keV iron K alpha line shows directly the Doppler velocities in the disk and the depth of the gravitational potential well.

In some systems, two jets of plasma are ejected at close to the speed of light along the spin axis of the black hole and accretion disk. The production of these jets seems to be coupled to the collapse of the inner accretion disk. The jets radiate synchrotron radiation at radio and infrared wavelengths which we can image with the Very Large Array radio telescope. The polarization of the radio waves reveals the structure of the magnetic field in the jet, which is sensitive to shocks and shear in the jet fluid. This talk will focus particular attention on the beautiful polarization images of the superluminal microquasar GRS 1915+105 made by Emily Dare (Brandeis ’08) as part of her honors research. These data have enabled us to model one particular outburst in some detail, to determine physical properties of the jet and to understand how the superluminal knots are made.


Fall 2009 Colloquia

Tuesday, September 8, 2009

The make-believe world of real-world physics
Eric Mazur, Harvard University


Tuesday, September 22, 2009

Intrinsic Alignments of Galaxies: Effects of Large-Scale Structure on Galaxy Formation
Tereasa Brainerd, Boston University

Abstract: It is not uncommon for one astronomer's noise to be another astronomer's signal, and this is the case for intrinsic alignments of galaxies. Popular theories of galaxy formation predict that, at a very small level, galaxies should be aligned with each other over length scales as large as a few tens of Mpc. Detection of these weak alignments (the "signal") then constitutes an important test of the theory. On the flip side, there is a great deal of effort being put into measuring cosmic shear, the weak gravitational lensing of distant galaxies by the overall mass of the universe. Cosmic shear tends to make galaxy images "point" in the same direction; that is, gravitational lensing makes images that were originally uncorrelated become aligned with each other. The basic cosmic shear theory was formulated under the assumption that galaxies are not intrinsically aligned with each other, hence intrinsic alignments of galaxies are "noise" for the lensers. Here I will show the first detection of intrinsic alignments of ordinary galaxies, similar in size and scale to the Milky Way. Accounting for mild observational errors and some small degree of misalignment of luminous galaxies with their dark matter halos, the detected signal agrees quite well with the predictions of the standard Cold Dark Matter model. I will also discuss the implications for future cosmic shear measurements, focusing on detection strategies that should eliminate the effects of intrinsic alignments of galaxies.


Tuesday, October 6, 2009

How the Hippies Saved Physics
David Kaiser, MIT

Abstract: In recent years, the field of quantum information science—an amalgam of topics ranging from quantum encryption, to quantum computing, quantum teleportation, and more—has catapulted to the cutting edge of physics, sporting a multi-billion-dollar research program, tens of thousands of published research articles, and a variety of device prototypes. This tremendous excitement marks the tail end of a long-simmering Cinderella story. Long before the big budgets and dedicated teams, the field moldered on the scientific sidelines. In fact, the field’s recent breakthroughs derive, in part, from the hazy, bong-filled excesses of the 1970s New Age movement. Many of the ideas that now occupy the core of quantum information science once found their home amid an anything-goes counterculture frenzy, a mishmash of spoon-bending psychics, Eastern mysticism, LSD trips, CIA spooks chasing mind-reading dreams, and comparable “Age of Aquarius” enthusiasms. For the better part of two decades, the concepts that would, in time, blossom into developments like quantum encryption were bandied about in late-night bull sessions and hawked by proponents of a burgeoning self-help movement—more snake oil than stock option. This talk describes the field’s bumpy transition from New Age to cutting edge.


Tuesday, October 13, 2009

Watching Worlds Collide
Matthew Kleban, NYU

Tuesday, October 20, 2009

No colloquium.

Tuesday, October 27, 2009

The Quantum and Fluid Mechanics of Global Warming
Brad Marston, Brown University

Quantum mechanics plays a crucial, albeit often overlooked, role in our understanding of the Earth's climate. In this talk three well-known aspects of quantum mechanics are invoked to present a simple physical picture of what may happen as the concentrations of greenhouse gases such as carbon dioxide continue to increase. Historical and paleoclimatic records are interpreted with some basic astronomy, fluid mechanics, and the use of fundamental laws of physics such as the conservation of angular momentum. I conclude by discussing some possible ways that theoretical physics might be able to contribute to a deeper understanding of climate change.

Tuesday, November 3, 2009

Precision Cosmology with 21 cm Tomography
Max Tegmark, MIT

With a cosmic flight simulator, we'll take a scenic journey through space and time. After exploring our local Galactic neighborhood, we'll travel back 13.7 billion years to explore the Big Bang itself and how state-of-the-art measurements and new theoretical insights are transforming our understanding of our cosmic origin and ultimate fate. We then discuss how mapping neutral hydrogen at high redshifts has the potential to dramatically improve the precision of our cosmology.

Tuesday, November 10, 2009

Models and manipulations: Min oscillations inside an E.coli bacterium
Andrew Rutenberg, Dalhousie University

Subcellular oscillations of Min proteins within individual cells of E. coli serve to localize division to midcell. While significant progress has been made to understand the Min oscillation both experimentally and in modeling, I will present three outstanding Min mysteries. I will then present a generic submodel of the Min oscillation, as well as systematic experimental manipulations of the Min oscillation. In particular, we find that the period of the Min oscillation responds dramatically to extracellular multivalent cations.

Tuesday, November 17, 2009

Ionic Membranes and Gels
Monica Olvera de la Cruz, Northwestern University

Ionic macromolecular liquids have fascinating properties. Charged molecules, such as nucleic acids, for example, co-assemble with oppositely charged multivalent ions and/or cationic proteins, to form complex structures, including ionic membranes and gels that respond rapidly to external stimuli including temperature, ionic concentrations, pH values and electric fields. The physical properties of ionic gels resemble those of chromosomes, and the structures of ionic membranes resemble those of viruses. Indeed, electrostatic interactions are essential to understand the structure and function of biological entities since most biomolecules are charged. Moreover, various co-assembled biomolecular units have some inherent asymmetry that renders them functionality. An important component of understanding function of assemblies is through the study of their symmetry or conversely the symmetries that they "break". Symmetric electrostatic interactions alone have shown to break symmetries, such as the buckling of cationic-anionic shells into icosahedra without rotational symmetry, and to lead to phase transition in ionic gels. We discuss symmetries and emergent properties of ionic membranes and gels.

Tuesday, November 24, 2009

The Shape Dependence of Fluctuation-Induced Forces
Mehran Kardar, MIT

The Casimir force is an attraction between parallel conducting plates due to quantum fluctuations of the electromagnetic (EM) field. Thermal fluctuations of correlated fluids (such as critical mixtures or superfluids) are also modified by boundaries, resulting in similar interactions. A nice demonstration is provided by the thinning of a wetting film of helium at and below the superfluid transition. Quantitative understanding of the latter requires inclusion of surface undulations. The EM Casimir force is also modified for corrugated surfaces in non-trivial fashion. I shall also discuss other geometries, such as spheres and cylinders.

Tuesday, December 1, 2009

The Remarkable Power of General Relativity
Gary Horowitz, UC Santa Barbara

Einstein's general theory of relativity describes gravity in terms of the curvature of space and time. This theory has been very successful in describing a wide range of gravitational phenomena. Surprisingly, it has recently been shown that general relativity can describe other areas of physics as well, including aspects of particle physics and condensed matter physics. This is a result of the gauge/gravity duality that has emerged from string theory. I will explain this remarkable development and show how general relativity reproduces properties of QCD and superconductivity.

Spring 2010 Colloquia

Tuesday, January 19, 2010

Friction: A Surprisingly Slippery Subject
Mark O. Robbins, Johns Hopkins University

Friction affects many aspects of everyday life and has played a central role in technology dating from the creation of fire by rubbing sticks together to current efforts to make nanodevices with moving parts. The friction "laws" we teach today date from empirical relationships observed by da Vinci and Amontons centuries ago. However, understanding the microscopic origins of these laws remains a challenge. Friction is a complex multiscale phenomenon that depends on both atomic interactions in contacts, and the macroscopic elastic and plastic deformation that determine the morphology and stress distribution within these contacts. Recent experiments and simulations have provided new information about friction at the atomic scale. The results are often counterintuitive, with solids sliding more easily than fluids and fluids resisting shear like solids. The talk will discuss the origins of friction, starting with how roughness affects the area where two surfaces actually touch, and then considering how surfaces can lock together in regions of contact to produce the macroscopic friction laws we teach.

Tuesday, January 26, 2010

Grabbing water with thin elastic sheets: The elasto-pipette
Pedro Miguel Reis, MIT

The study of the elasticity of thin objects (sheets or rods) is a rapidly burgeoning field. In this talk I will focus on the particular coupling of the elasticity of thin sheet and interfacial fluid forces. Inspired by real aquatic flowers, I shall present the results of a combined experimental and theoretical study of a system in which a thin petal-shaped plate is withdrawn from the flat interface of a liquid bath. As the flexible plate is drawn upwards, it deforms due to interfacial and hydrostatic forces, up to a point where it completely detaches from the interface as pinch-off occurs. If the bending stiffness of the plate is sufficiently low, upon detachment, a regime can be attained where the petal-shaped plate can fully enclose and therefore grab a drop from the liquid bath. We propose this mechanism as a robust means by which to grab, manipulate and transport small fluid droplets. This novel pipetting mechanism is passive and relies purely on the coupling of the elastic of thin plates and hydrodynamic forces at the interface.

Tuesday, February 2, 2010

Nanoparticle templated assembly of virus protein cages: principles and applications
Bogdan Dragnea, Indiana University

Self-assembling icosahedral protein cages have physical and chemical characteristics rendering them appealing in applications ranging from therapeutic or diagnostic vectors to building blocks for hierarchical biomaterials. For functional control of protein cage assemblies, a deeper understanding of the interaction between the protein cage and its foreign payload is required. Protein-cage encapsulated nanoparticles, with their well-defined surface chemistry, allow for systematic control over key parameters of encapsulation such as the surface charge, hydrophobicity, and size. Independent control over these variables allows experimental testing of assembly mechanism models. Previous studies done with Brome mosaic virus capsids and negatively-charged gold nanoparticles indicated that the result of the self-assembly process depends on the diameter of the particle. However, in these experiments, the surface-ligand density was maintained at saturation levels, while the total charge and the radius of curvature remained coupled variables, making the interpretation of the observed dependence on the core size difficult. The presentation provides an analysis of experiments aimed at decoupling the surface charge and the core size and a discussion of several potential applications.

Tuesday, February 9, 2010

Novel Colloidal and Interfacial Phenomena in Liquid Crystalline Systems
Nicholas Abbott, University of Wisconsin-Madison

Processes leading to the self-organization of molecules and colloids within and at the interfaces of isotropic liquids have been widely studied in the past. This talk will focus beyond those past studies by addressing interfacial and colloidal phenomena in systems in which the isotropic solvent is replaced by a nematic liquid crystal (LC). Observations derived from two experimental systems will be described. The first system involves LC-in-water emulsion droplets, and the influence of droplet size and interfacial chemistry on the structure of the droplets. Recent experimental observations in our laboratory have unmasked size-dependent ordering of the LC droplets that is not predicted by classical theories of LCs. Ordering transitions that are exquisitely sensitive to certain classes of biological lipids (e.g., endotoxin) have also been discovered. The second experimental system to be discussed involves the interfacial organization of solid microparticles at aqueous-LC interfaces. Our observations have revealed that the nematic order of a LC can give rise to new classes of inter-particle interactions at these interfaces. Significantly, the symmetries of the interactions differ from those encountered in isotropic solvent systems, thus giving rise to interfacial organizations of particles not previously reported. This presentation will highlight fundamental and unresolved issues related to the behaviors of these LC-colloidal systems..

Tuesday, February 23, 2010

Stochastic conformal geometry: applications in physics
Ilya Gruzberg, University of Chicago

Tuesday, March 2, 2010

Physical Aspects of Viral Infectivity
William Gelbart, UCLA

In this talk I introduce the two most prevalent kinds of viruses - those with single-stranded (ss) RNA genomes and those with double-stranded (ds) DNA genomes. I discuss their different "life cycles" in terms of the physical differences between ssRNA and dsDNA and of their co-evolution with bacterial, plant, and animal hosts. Three basic physical phenomena are argued to lie at the heart of the infectivity of these viruses, and are investigated by a combination of theory and experiment:

(1) high pressure in their capsids due to strong confinement of dsDNA genomes, illustrated by particular bacterial (e.g., lambda) and animal (Herpes) viruses; (2) self-assembly of ssRNA genomes and capsid proteins into infectious virions in the case of plant (bromo) viruses; and (3) "budding" of nucleocapsids, i.e., their spontaneous wrapping by lipid bilayer membrane as part of the exit of infectious virions from host cells, illustrated by a particular mammalian (Sindbis) virus.

Tuesday, March 9, 2010

Structure of trajectory space, broken symmetry and a glass transition

David Chandler, University of California Berkeley

Super-cooling a liquid often produces glass -- a solid with no apparent structural order. Unlike crystallization, a glass transition is not accompanied by a thermodynamic singularity. Nevertheless, a phase transition can underlie the formation of glass. Unlike equilibrium order-disorder phenomena, this transition appears as a singularity in a partition function of dynamical histories. I describe this transition -- its order parameters and phase diagrams.

Tuesday, March 23, 2010

Black Hole Collisions
Frans Pretorius
, Princeton University

Tuesday, April 13, 2010

Eisenbud Lecture Series in Mathematics and Physics
Daniel Freed
, University of Texas at Austin

Tuesday, April 20, 2010

Smectic Topology
Randall Kamien, University of Pennsylvania

The homotopy theory of topological defects in ordered media fails to completely characterize systems with broken translational symmetry. We argue that the problem can be understood in terms of the lack of rotational Goldstone modes in such systems and provide an alternate approach that correctly accounts for the interaction between translations and rotations. Dislocations are associated, as usual, with branch points in a phase field, whereas disclinations arise as critical points and singularities in the phase field. We introduce a three-dimensional model for two-dimensional smectics that clarifies the topology of disclinations and geometrically captures known results without the need to add compatibility conditions. Our work suggests natural generalizations of the two-dimensional smectic theory to higher dimensions and to crystals.

Tuesday, April 27, 2010

Black Holes, String Theory and Quantum Gravity
Alex Maloney, McGill University

Black Holes are among the most mysterious objects in physics, and provide a testing ground for many of the most important questions involving quantum mechanics and gravity. Recent advances -- based on the description of black holes in string theory -- have led to new and surprising insights into the microscopic structure of black holes. These have led to quantitative and qualitative insights into the physics of quantum black holes, and have the potential to shed light on basic conceptual questions involving the quantum structure of space-time.

Tuesday, May 4, 2010

Quantum Money from Knots
Eddie Farhi, MIT

An unknown quantum state can not be copied. This feature makes possible quantum money that can not be copied. I will discuss a quantum money scheme that also allows a merchant to verify that a tendered bill is legitimate. This scheme makes use of ideas from the theory of knots.

Tuesday, May 11, 2010, 3pm (note special time)

Dark Matter is from Cygnus: in search of a wind of Dark Matter in the Milky Way
Gabriella Sciolla, MIT

Astronomical and cosmological observations indicate that Dark Matter is responsible for 23% of the energy budget of the Universe and 83% of its mass. Yet, little is known about the identity of Dark Matter and its interactions, because Dark Matter particles have never been directly observed in the laboratory. In this talk I will discuss the challenges and rewards of direct detection of Dark Matter. In particular, I will discuss how a direction-sensitive detector can lead to the unambiguous observation of Dark Matter particles and provide unique information about the distribution of Dark Matter in the Milky Way.

Tuesday, September 7, 2010

Scandal in Physics: The Affair of Jan Hendrik Schön at Bell Labs
Eugenie Samuel Reich, science reporter

Abstract: In 2002, physicists were shocked to learn that a staff scientist at Bell Labs in New Jersey, Jan Hendrik Schön, had faked as many as 17 experiments reported on in Nature, Science and other leading journals of the physics community. In this talk I give an analysis of the case arising from the interviews and documents I obtained for my 2009 book on the case, "Plastic Fantastic: How the Biggest Fraud in Physics Shook the Scientific World". In the talk I will describe Schön's first few manipulations of data as a graduate student at the University of Konstanz, and the escalation of his fraudulent claims in the environment of Bell Laboratories. Based on reviews of his fraudulent papers and correspondence between him and journal editors, I also describe how his papers were handled by editors at the journals Nature and Science.


Tuesday, September 14, 2010

Chiral Self-Assembly
Zvonimir Dogic, Brandeis University

Abstract: Once initiated, most self-assembly pathways proceed without bound, resulting in the formation of three dimensional assemblages whose size is only limited by the number of the available building blocks. Designing an assembly process in which the final assemblage is limited to two or one dimensions presents a significantly more difficult challenge. In this talk I will described a robust and widely applicable mechanism by which homogeneous chiral rod-like molecules assemble into a myriad of self-limited structures. The exact geometry of the final assemblage is determined by the anisotropy of the constituent rodlike molecules as well as their molecular chirality. Because of the simplicity and universality of hard core repulsive interactions which dominate the phase behavior, these results are directly applicable to many anisotropic particles.

Tuesday, September 21, 2010

Is evolution reversible?
Jeff Gore, MIT

Abstract: The degree to which evolutionary adaptations are reversible has long intrigued biologists. A famous example of reversibility in evolution is the emergence and subsequent loss of legs in the lineage that led to whales. However, studies of enzymatic evolution suggest that it is possible for evolution to become "stuck", even if the original form of the enzyme is more fit in the new environment. In this talk I will describe our experiments using the evolution of antibiotic resistance in bacteria as a model system to probe the reversibility of evolution. I will also show how ideas from game theory can be used to determine when bacteria will cooperate to breakdown antibiotics.

Tuesday, September 7, 2010

Scandal in Physics: The Affair of Jan Hendrik Schön at Bell Labs
Eugenie Samuel Reich, science reporter

Abstract: In 2002, physicists were shocked to learn that a staff scientist at Bell Labs in New Jersey, Jan Hendrik Schön, had faked as many as 17 experiments reported on in Nature, Science and other leading journals of the physics community. In this talk I give an analysis of the case arising from the interviews and documents I obtained for my 2009 book on the case, "Plastic Fantastic: How the Biggest Fraud in Physics Shook the Scientific World". In the talk I will describe Schön's first few manipulations of data as a graduate student at the University of Konstanz, and the escalation of his fraudulent claims in the environment of Bell Laboratories. Based on reviews of his fraudulent papers and correspondence between him and journal editors, I also describe how his papers were handled by editors at the journals Nature and Science.

Tuesday, September 14, 2010

Chiral Self-Assembly
Zvonimir Dogic, Brandeis University

Abstract: Once initiated, most self-assembly pathways proceed without bound, resulting in the formation of three dimensional assemblages whose size is only limited by the number of the available building blocks. Designing an assembly process in which the final assemblage is limited to two or one dimensions presents a significantly more difficult challenge. In this talk I will described a robust and widely applicable mechanism by which homogeneous chiral rod-like molecules assemble into a myriad of self-limited structures. The exact geometry of the final assemblage is determined by the anisotropy of the constituent rodlike molecules as well as their molecular chirality. Because of the simplicity and universality of hard core repulsive interactions which dominate the phase behavior, these results are directly applicable to many anisotropic particles.

Tuesday, September 21, 2010

Is evolution reversible?
Jeff Gore, MIT

Abstract: The degree to which evolutionary adaptations are reversible has long intrigued biologists. A famous example of reversibility in evolution is the emergence and subsequent loss of legs in the lineage that led to whales. However, studies of enzymatic evolution suggest that it is possible for evolution to become "stuck", even if the original form of the enzyme is more fit in the new environment. In this talk I will describe our experiments using the evolution of antibiotic resistance in bacteria as a model system to probe the reversibility of evolution. I will also show how ideas from game theory can be used to determine when bacteria will cooperate to breakdown antibiotics.

Tuesday, September 28, 2010

No colloquium. Brandeis Thursday.

Tuesday, October 5, 2010

Gravity, entropy, and entanglement
Matthew Headrick, Brandeis University

It has been known since the 1970s that black holes, which are solutions to the field equations of general relativity, carry an intrinsic entropy. This suggests that general relativity is a coarse-grained theory, like fluid dynamics. Only recently, however, has it been understood (in certain cases) what the underlying microscopic theory is, and the answer turns out to be very surprising and extremely deep. In this colloquium, I will describe the so-called "holographic dualities" that provide the answer and explain a few of their implications and applications. In particular I will focus on a fascinating interplay between quantum information and the geometry of spacetime.

Tuesday, October 12, 2010

Searching for Exoplanets with Kepler
Andrea Dupree, Harvard-Smithsonian Center for Astrophysics

Abstract: NASA's Kepler satellite, launched in March 2009 is aimed at a 100 square degree area of the sky in Cygnus, and carries out photometry with exquisite precision on a star field of over 100,000 stars. Kepler will stare at this field for several years in order to detect transits of terrestrial planets in the 'habitable zone' where liquid water and possibly life might exist. Verifying the planetary candidates with ground-based spectroscopy, imaging, and modeling also comprises an integral part of the Kepler program. In addition to detecting planetary transits, Kepler obtains a wealth of information on stellar variability including studies of asteroseismology and gyrochronology. I will give an overview of the Kepler mission highlighting these new results.

Tuesday, October 19, 2010

No colloquium. DOE site visit.

Tuesday, October 26, 2010

Measuring the Spins of Stellar-Mass Black Holes
Jeffrey E. McClintock, Harvard-Smithsonian CfA

Starting in 1972 with Cygnus x-1, we now have a good sample of 23 stellar-mass black holes, which are located in x-ray binary systems. During the past five years, our team has measured the spins of eight of these black holes, and I will discuss the implications of our spin data for models of relativistic jets and black hole formation. We measure spin by fitting the thermal continuum X-ray spectrum of the black hole to the relativistic accretion-disk model of Novikov and Thorne, thereby determining the radius of the inner edge of the disk. We identify this disk radius with the black hole's innermost stable circular orbit (ISCO). It is then trivial to deduce the spin, which for a black hole of known mass depends solely on the radius of the ISCO via a standard GR formula. Strong theoretical evidence that the thin accretion disks we study are sharply truncated at the ISCO is provided by our GR MHD simulations. Likewise, strong empirical support for identifying the disk radius we measure with the radius of the ISCO is provided by our recent study of LMC X-3. We find for this persistent source, based on an analysis of hundreds of spectra collected over a span of 26 years, that the inner radius of the accretion disk is stable to a few percent. Thus, our measurements of spin are supported by both observational and theoretical evidence.

Tuesday, November 2, 2010

Joint Quantitative Biology/Physics Department Colloquium
Statistical Mechanics and Virus Assembly
Robijn Bruinsma, UCLA

Abstract: The assembly of a virus can be replicated in a controlled environment under conditions that are close to thermodynamic equilibrium. This suggests that the methods and concepts of statistical mechanics and thermodynamics might be applicable and informative for the analysis of the spontaneous self-assembly of viruses. In the seminar, some of the successes and failures of applying concepts borrowed from equilibrium and non-equilibrium statistical mechanics to viral assembly, such as the Law of Mass Action and the Gibbs-Thomas effect will be discussed, as well as outstanding riddles.

Tuesday, November 9, 2010

The Physics of Titan
Peter Ford, MIT

Abstract: Since the arrival in 2004 of the Cassini spacecraft in orbit about Saturn, its largest moon, Titan, has been studied in great detail, both by the Huygens lander and by instrument aboard the orbiter. This talk will focus on what can be deduced about the likely origin and composition of Titan and similar bodies in our solar system from studies of their orbital dynamics and global topography. Theoretical topics will include planetary accretion and internal differentiation, orbital dynamics and tidal resonances. On the experimental side, I'll describe the uses of Doppler tracking, radar imaging and altimetry, and speckle displacement interferometry in making the measurements.

Tuesday, November 16, 2010

Cell Shapes: From Waves to Motion
Wolfgang Losert, University of Maryland

Abstract: Migration of cells on surfaces and through tissue is an important part of life, from the amazingly coordinated migration of cells during development to the uncontrollable migration of a metastatic cancer. We investigate the physics of cell migration to understand how cells move, how they avoid obstacles, how the motion is directed, and how groups of cells coordinate their motion. Through detailed quantitative analysis of cell shapes, we demonstrate the existence of wave-like dynamic shape changes during the migration of Dictyostelium discoideum, a model system for the study of chemotaxis. Cell shapes have regions of high boundary curvature that propagate from the leading edge toward the back along roughly alternating sides of the cell. These curvature waves couple to the surface and facilitate forward motion of cells in a characteristic zig-zag manner with the ability to maintain overall direction for several minutes. Looking at large ensembles of thousands of cells we analyze how external chemical signals, signal relay between cells, or cell-cell contacts affect this cell behavior.

Tuesday, November 23, 2010

No colloquium.

Tuesday, November 30, 2010

Observing the High Redshift (z > 5) Universe with the Spitzer Space Telescope
Giovanni Fazio, Harvard University

One of the most important observations made by NASA's Spitzer Space Telescope has been the detection of luminous galaxies back to the era of reionization (z ~ 8) when the Universe was less that 700 million years old. The key advance made by Spitzer imaging is the ability, for the first time, to sample the redshifted rest-frame visible light of these galaxies. When combined with broadband multi-wavelength data, Spitzer observations can be fit to stellar population synthesis models to determine the spectral energy distribution of these galaxies and to constrain their stellar masses and ages and their star formation histories. As a result, there is evidence that most of the stellar mass of these galaxies formed at even higher redshifts (z > 9 - 10), and that a significant number of galaxies should exist in this region. Searches for galaxies at z ~ 9 - 10 continue. Spitzer observations of massive lensing clusters have also played a pivotal role in this study, and the first Spitzer/Infrared Array Camera (IRAC) detection of a z>6 galaxy came from such observations. Since most of these results were obtained with Spitzer/IRAC 3.6/4.5 micron bands, the Warm Spitzer Mission, when combined with Hubble Space Telescope observations, will provide a unique opportunity to obtain the first complete census of the assembly of stellar mass as a function of cosmic time back to the era of reionization, yielding unique information on galaxy formation in the early Universe.

Spring 2011 Colloquia

Tuesday, January 25, 2011

Nanostructures for Single-Molecule Biophysics
MRSEC Physics Department Colloquium
Derek Stein, Brown University

“Lab-on-a-chip” fluidic technology is improving the way we perform chemical and biological analyses. It was inspired by electronic integrated circuits, from which the name and a “smaller, cheaper, faster” approach are borrowed. As we shrink fluidic devices down to the nanoscale, however, new physical phenomena emerge, along with new opportunities for studying biophysics at the level of single molecules. I will describe how fundamental science and exciting technological possibilities converge at this scale. In particular, this talk will focus studies of single DNA molecules in nanochannels and nanopores, in which we control the Coulomb interactions and entropic forces. Our results shed light on the microscopic interactions that govern the behavior of long biopolymers, and reveal deep parallels between the physics of nanofluidic devices and integrated circuits.

Tuesday, February 1, 2011 (canceled due to inclement weather--rescheduled to March 15)

Collective molecular motor using chiral liquid crystalline thin films
Hiroshi Yokoyama, Kent State University

Tuesday, February 8, 2011

Self-Assembly of Photonic Nanostructures: Beyond Crystalline Sphere Packings
Eric Dufresne, Yale University

Visible light is scattered strongly by dielectric materials with structure on length scales around a few hundred nanometers. With careful design of geometry and selection of dielectric constants, these photonic materials can direct the flow of light with stunning results, including the vivid structural colors on the blue feathers of a jay and the green eye-spots on a butterfly’s wings. In this talk, I will describe our recent investigations into alternative strategies for the design and assembly of photonic materials. We find some of our inspiration from biology, where we have examined the mechanisms of color production and self-assembly across hundreds of species of birds and insects. Crystalline and amorphous photonic nanostructures appear to be formed by the arrested phase separation of proteins in birds and the bending of lipid membranes in insects. On another front, we are developing strategies based on recent advances in the synthesis of uniform nanoparticles with unusual geometries. In particular, I will describe the development of field-switchable photonic crystals based on dumbbell shaped polymer nanoparticles. Together, these approaches provide rich alternatives to the canonical path of self-assembly of photonic crystals from spherical particles.

Tuesday, February 15, 2011

Towards physical applications of holographic duality
John McGreevy, MIT

It turns out that string theory is useful: via the AdS/CFT correspondence, it provides a tractable description of certain quantum many-body systems, in terms of a gravitational theory in extra dimensions. This exotic-seeming description is most effective in regimes of strong interaction, where standard theoretical techniques flounder. This description organizes the solution by length scale, just as we are instructed to do by the renormalization group. I will motivate this duality and describe attempts to apply these ideas to real physical systems which have the common feature that interactions are important for determining their collective behavior. These include the quark-gluon plasma, cold atoms at unitarity, and the normal phase of high-temperature superconductors.

Tuesday, February 22, 2011

No colloquium. Midterm recess.

Tuesday, March 1, 2011

Exploring the String Axiverse with Astrophysical Black Holes
Sergei Dubovsky, NYU

Combining the QCD axion as a solution to the strong CP problem with the properties of axions in string theory suggests the simultaneous presence of many ultralight axions with masses homogeneously distributed over the log scale---the "Axiverse". These axions give rise to a number of distinctive observational signatures, including the rotation of the CMB polarizations at the level within the reach of the Planck satellite, and steps in the dark matter power spectrum. A surprising evidence for the axions with masses in the range 10^(-22) to 10^(-10) eV may come from observations of astrophysical black holes through the Penrose superradiance process. When the axion Compton wavelength is of order of the black hole size, the axions develop "superradiant" atomic bound states around the black hole "nucleus". Their occupation number grows exponentially by extracting rotational energy from the ergosphere, culminating in a rotating Bose-Einstein axion condensate emitting gravitational waves. This mechanism creates mass gaps in the spectrum of rapidly rotating black holes and gives rise to a distinctive gravity wave signal. In particular, the QCD axion with the decay constant of order the GUT scale affects the dynamics of stellar mass black holes. This opens a possibility for a discovery of the QCD axions through ongoing measurements of black hole spins. The corresponding gravity wave signal may be within the reach of the Advanced LIGO.

Tuesday, March 8, 2011

Rotator phases and twist solitons in polyethylene and n-alkanes
Scott Milner, Penn State University

Polyethylene (PE) and normal alkanes display multiple ordered phases --- a conventional crystal, and two "rotator'' phases RI and RII, in which chains are rotationally disordered about their axes. Experimental evidence suggests that the RII phase serves as a metastable intermediate in the nucleation of PE. We have computed the nucleation barrier for RII and direct crystal nuclei, combining theory and data for the bulk and surface free energies of the two structures, to see which barrier is lower. To deepen our understanding of the structure and dynamics of these multiple ordered phases, we performed atomistic MD simulations on C23 n-alkanes. This led us to focus on how such molecules rotate, by diffusive transport of twist solitons along the molecule. These motions occur even in crystalline PE, and play a key role in plastic deformation. Combining simulations and analytical theory, we determined the formation energy and mobility of twist solitons in the crystal and rotator phases, with results in good agreement with NMR experiments and MD observations.

Tuesday, March 15, 2011

Collective molecular motor using chiral liquid crystalline thin films
Hiroshi Yokoyama, Kent State University

Chirality often plays a decisive role in determining structures and properties of liquid crystalline phases. In certain circumstances, the chirality, macroscopic and molecular, can also manifest itself in dynamical behaviors. 100 years ago, Lehmann was the first to observe an unusual nonequilibrium dynamics of a chiral nematic liquid crystal subjected to a temperature gradient, now referred to as the Lehmann effect, in which the liquid crystal director makes a continuous precession at an angular velocity proportional to the magnitude and sign of the temperature gradient. In recent years, there is a resurgence of interest in the Lehmann effect, partly inspired by the authors’ discovery of mass flow-induced continuous precession of molecules in monomolecular thick film (Langmuir monolayer) of chiral liquid crystals [1], which may serve as a collective molecular motor. Given the absence of macroscopic twist in Langmuir monolayers, a naive interpretation of this phenomenon is that the chiral molecule behaves like a propeller placed in mass flow (typically water molecules), and the angular momentum at the molecular level is transferred to the macroscopic director rotation by a mechanism which we do not understand yet [2]. I shall review the present state of our understanding of this phenomenon drawing on our recent experimental and simulation studies.

[1] Y. Tabe and H. Yokoyama: "Coherent collective precession of molecular rotors with chiral propellers" Nature Materials 2, 806(2003).
[2]M. Yoneya, Y. Tabe and H. Yokoyama: "Molecular Dynamics Simulation of
Condensed-Phase Chiral Molecular Propellers" J. Phys. Chem. B 114,

Tuesday, March 22, 2011

No colloquium. APS March Meeting.

Tuesday, March 29, 2011

The two-body problem in general relativity

Scott Hughes, MIT

A simple problem in Newtonian gravity, the motion of two bodies about one another is far more challenging in general relativity (GR). Motivated largely by the anticipated importance of compact binaries as gravitational-wave sources, many years of effort have produced a suite of tools for modeling binaries with GR. In this talk, I will present an overview of how we model these sources in GR and what we have learned from the relativistic two-body problem. I will focus in particular on how unique aspects of relativistic gravity flavor the gravitational waves which binaries generate, and how these flavorings can be exploited to learn about compact bodies, especially black holes. I will emphasize analogs between the GR analysis and electromagnetic theory, hopefully demonstrating that the rich features of these models are in fact surprisingly intuitive.

Tuesday, April 5, 2011

Human Pulmonary Physiology with Hyperpolarized 129Xe Magnetic Resonance
Sam Patz, Associate Professor, Harvard Medical School; Scientific Director, Center for Pulmonary Functional Imaging, Brigham & Women's Hospital

Compared to other tissues in the body, traditional 1H Magnetic Resonance Imaging (MRI) of the lung is severely hampered because of its low proton density. In addition, because of the tremendous health burden of diseases such as Chronic Obstructive Pulmonary Disease (COPD), there is a great need for improved diagnostic techniques that provide noninvasive regional information about pulmonary function. To address this need, we have been developing MRI to measure pulmonary gas exchange using highly polarized (hyperpolarized) 129Xe .Using laser polarization techniques, it is possible to produce polarizations as high as 50%. Compared to the typical magnetization polarization produced by a high field MRI magnet of 1.5 Tesla, this represents an improvement by a factor of ~105 . Inhaled 129Xe follows a similar pathway as oxygen, first ventilating the alveolar gas spaces and then diffusing into septal tissue and blood. The 129Xe magnetization in the tissue (or dissolved) phase has a NMR frequency that is chemically shifted from that of the gas phase by ~200ppm. Thus by a simple chemical shift saturation recovery (CSSR) experiment where the tissue phase signal is initially destroyed by a selective RF pulse, one can observe the replenishment kinetics of 129Xe magnetization from alveolar gas spaces to septal tissue. The septal uptake curve of 129Xe as a function of time is characterized by three distinct regions: an early diffusive time behavior, a medium time regime where the septa begin to saturate, and a long time regime governed by blood flow. By fitting experimental data to an analytical diffusion model, we demonstrate one can obtain estimates for the alveolar surface area per unit gas volume, septal thickness, and capillary transit time through the gas exchange region. These are important quantities as alveolar surface area is a direct measure of emphysema, septal thickness is expected to increase with interstitial fibrosis and capillary transit time is a measure of pulmonary vascular health. We demonstrate these measurements in normal subjects, COPD subjects and subjects with interstitial disease.

Tuesday, April 12, 2011

Nearby, Thermally Emitting Neutron Stars
David Kaplan, University of Wisconsin-Milwaukee

Neutron stars are among the densest objects in the universe. The conditions in their centers are largely unconstrained by current theoretical physics or terrestrial laboratories, leaving a wide variety of compositions and structures possible. Observations of thermal emission from neutron stars -- specifically measurements of their sizes and cooling rates -- may therefore be the best way to constrain the behavior of matter in these extreme conditions. I will discuss a sample of nearby, cooling neutron stars that we are using for this purpose. We are attempting to pin down the basic parameters of these neutron stars with a variety of ground- and space-based observations, coupled with theoretical modeling. Along the way, we have encountered a number of interesting astrophysical puzzles that I will describe.

Tuesday, April 19, 2011

No colloquium. Passover and spring recess.

Tuesday, April 26, 2011

Hydration Phenomena at the Interface of Physics and Biology: A New Fluctuations-based Perspective
Shekhar Garde, Rensselar Polytechnic Institute

Water-mediated interactions (e.g., hydrophobic interactions) govern a host of biological and colloidal self-assembly phenomena from protein folding, micelle and membrane formation, to molecular recognition. Macroscopically, hydrophobicity is often characterized by measuring a droplet contact angle on a surface. At the nanoscale, such measurements are not feasible, e.g., for surfaces of proteins or nanoparticles. Using theory and molecular simulations, we present a new perspective that connects the behavior of water near nanoscale interfaces to their hydrophobicity/philicity. Specifically, we show that water density fluctuations (and not the average local density) provide a quantitative characterization of interfacial hydrophobicity. Density fluctuations are enhanced near hydrophobic interfaces and suppressed near hydrophilic ones. This new perspective provides a computational tool for characterizing the hydrophobicity patterns on protein surfaces, which are relevant for binding, recognition, and aggregation. Simulations also show how the properties of water at interfaces influence binding, folding, and dynamics of flexible molecules in interfacial environments. Our current understanding of the hydration of ions, osmolytes, and solution additives, when combined with this new perspective, provides additional insights into the role of water in multicomponent biological interactions.

Tuesday, May 3, 2011

The physical basis of morphogenesis
L. Mahadevan, Visiting Professor, Harvard University

The growth and form of a soft solid pose a range of problems that combine aspects of mathematics, physics and biology. I will discuss some examples of growth and form in the plant and animal world motivated by qualitative and quantitative biological observations. The problems include the shape and dynamics of a freely growing pollen tube, the undulating fringes on a leaf, the blooming of a flower and the looping of the vertebrate gut. In each case, we will see how a combination of physical experiments, mathematical models and simple computations allow us to begin unraveling the basis for the diversity of biological form, while suggesting a rich new lode of problems in geometry and physics.

Tuesday, September 6, 2011

Measuring a Black Hole Event Horizon: Very Long Baseline Interferometry of the Galactic Center
Shep Doeleman, Haystack Observatory
Watch the video

Abstract: It is now almost certain that at the center of our Milky Way Galaxy lies a super massive black hole - 4 million times more massive than our Sun. Because of its proximity to Earth, this object, known as Sagittarius A*, presents astronomers with the best opportunity in the Universe to spatially resolve and image a black hole Event Horizon. To do this requires using Very Long Baseline Interferometry (VLBI), the technique whereby radio telescopes around the world are linked together in a Global phased array. Very short wavelength VLBI observations have now confirmed structure on ~4 Schwarzschild radius scales within SgrA*, and have revealed time variability in this source on the same spatial scales. I will describe the instrumentation efforts that enable these observations, discuss what current and future VLBI observations of SgrA* tell us about this closest super-massive black hole, and describe plans for assembling a submm-VLBI Event Horizon Telescope.


Tuesday, September 13, 2011

Physics Applications In Upstream Oil & Gas
Martin Poitzsch, Schlumberger-Doll Research

The talk will focus on some specific examples of innovative and fit-for-purpose physics applied to solve real-world oil and gas exploration and production problems. The oil and gas industry is one of the largest and most geographically and organizationally diverse areas of business activity on earth; and as a “mature industry,” it is also characterized by a bewildering mix of technologies dating from the 19th century to the 21st. Oil well construction represents one of the largest volume markets for steel tubulars, Portland cement, and high-quality sand. On the other hand, 3D seismic data processing, shaped-charge perforating, and nuclear well logging have consistently driven forward the state of the art in their respective areas of applied science, as much or more so than defense or other industries. Moreover, a surprising number of physicists have made their careers in the oil industry. To be successful at introducing new technology requires understanding which problems most need to be solved. The most exotic or improbable technologies can take off in this industry if they honestly offer the best solution to a real problem that is costing millions of dollars in risk or inefficiency. On the other hand, any cheaper or simpler solution that performs as well would prevail, no matter how inelegant!

Tuesday, September 20, 2011

Non-equilibrium Dynamics in two-dimensional liquid crystals
Yuka Tabe, Waseda University, Tokyo

Liquid Crystal (LC) films composed of chiral compounds are known to exhibit a unidirectional molecular rotation under a transmembrane thermal, ionic or mass current. We have investigated the dynamics in ultrathin chiral LC films induced by transmembrane gas flow and found that there are two origins causing the unidirectional molecular rotation; one is the macroscopic helix and the other is the microscopic molecular propeller. When the two forces compete with each other, the rotational direction is switched at the intersection. The transmembrane gas flow induces not only the molecular precession but also the unidirectional hydrodynamic flow in the films. Depending on the elastic properties and the boundary conditions, the films showed both the director rotation and the molecular flow.

Tuesday, September 27, 2011

Street-fighting mathematics and science: The art of opportunistic problem solving
Sanjoy Mahajan, Olin College of Engineering

With traditional science and mathematics teaching, students struggle with fundamental concepts. For example, they cannot reason with graphs and have little feel for physical magnitudes. With such handicaps in intuition and reasoning, students can learn only by rote. I'll describe these difficulties using mathematical, physical, and engineering examples. Then I'll discuss how street-fighting mathematics and science, the art of approximation, can improve our teaching and thinking, the better to handle the complexity of the world.

Tuesday, October 4, 2011

The search for the Standard Model Higgs boson
Rozmin Daya, Southern Methodist University/Brandeis University

The proposed Higgs boson, a particle that is thought to give all matter its mass, is on the verge of being found--if it exists. Two of the Large Hadron Collider (LHC) experiments, ATLAS and CMS, are leading the search by analyzing the remnants of proton-proton collisions occurring at 7 TeV center of mass energy. Recent results from these experiments have improved significantly on the work of those from CERN's LEP accelerator and Fermilab's Tevatron, narrowing the possible mass range for the Standard Model Higgs boson. This talk summarizes the status of the search for this mysterious particle, and highlights the most promising channels for discovery using the ATLAS detector at the LHC.

Tuesday, October 11, 2011

No colloquium. Brandeis Thursday.

Tuesday, October 18, 2011

Specific and non-specific interactions of proteins: from networks to function
Gerhard Hummer, Laboratory of Chemical Physics at NIH

Many important biological functions are carried out by multi-protein assemblies that form only transiently and are held together by relatively weak pairwise interactions and by disordered linkers. In the first part of my talk I will present a computational method that allows us to study the structures and motions of large protein assemblies. Motivated in part by the results of these structural studies, I will then move beyond specific assemblies and instead employ a systems-level perspective to examine how competition between specific and non-specific binding limits the number of proteins in a cell and shapes their interaction network.

To study the dynamic structures of multiprotein assemblies, we developed a simulation model on the basis of residue-level coarse-graining and a transferable energy function. With this model, we were able to study protein binding and dissociation at equilibrium, including the formation of both the specific and non-specific complex structures, incorporating data from nuclear magnetic resonance (NMR), small-angle X-ray scattering (SAXS), spin-label (DEER/EPR), and single-molecule fluorescence (FRET) experiments for refinement and validation. Results for the ESCRT membrane-protein trafficking system and for protein kinase C highlight the dynamic character of these complexes, the importance of cooperative binding for proper assembly, and the significant contributions from non-specific interactions to the overall stability of the assemblies.

However, non-specific interactions can also result in disease-causing aggregation, and may provide a simple physical reason why multicellular organisms, from C. elegans to humans, have roughly the same number of protein encoding genes. To examine this question, we have developed a statistical-mechanical model. By collective evolution of the amino-acid sequences of protein binding interfaces we estimate the degree of mis-binding as a function of the number of distinct proteins. We show that the achievable energy gap favoring specific over nonspecific binding decreases with protein number in a power-law fashion. With this model, we determine a limit on the number of distinct proteins in a cell, and show that evolutionary pressure against nonspecific binding also shapes the topology of the protein interaction network.

Tuesday, October 25th, 2011

Computational Studies of Biomolecular Systems: Simulation
Methods, Force Fields, and Applications
Benoit Roux, University of Chicago
co-sponsored by Quantitative Biology Program

Classical simulations based on atomic models play an increasingly important role in a wide range of applications in physics, biology and chemistry. They are particularly valuable for the study of soft matter systems involving liquids, polymers, membranes, microemulsions and surfactants, as well as complex biomolecules like proteins and nucleic acids. Subjects of great interest in biophysics include, for example, the development and applications of effective methods for the computation of binding free energy, the determination of experimental structure using low-resolution information, and the simulation of large conformational transitions. Another subject of great interest is the development of advanced force fields taking induced electronic polarization into account explicitly. Recent applications to tyrosine kinases, the sodium-potassium pump, the voltage-gated potassium channel and the glutamate receptor will be discussed.

Tuesday, November 1, 2011

Neutrino Oscillations, Present and Future
Gary Feldman
, Harvard University

Abstract: Neutrinos are the least understood of fundamental building blocks of matter. The extremely small masses may offer a window on the physics at the grand unified scale. They may also be responsible for the existence of matter in the universe. Neutrino oscillations, the transformations from one form to another, provide a sensitive laboratory for studying neutrino properties. I will review our present knowledge of neutrino oscillations and the prospects for future studies.

Tuesday, November 8, 2011

The self organized beating of cilia and their control
Kenneth Foster, Syracuse University

Abstract: Cilia (eukaryotic flagella) are slender cylindrical active beams whose oscillation enables biological cells to swim. They are medically important as they push sperm, transport nourishment in the brain, clear particles from the lungs, organize sight and hearing, etc. A long standing question is how cilia beat. David Woolley has called this a problem with “an intractable level of difficulty”. Nevertheless an attempt will be made to explain in molecular model terms how cilia beat. The additional information needed to confirm models will be discussed. In order for cilia to fulfill multiple functions, Nature has superimposed multiple controls on this self-organized beating. Some of these controls will be explained based on experiments with the unicellular alga Chlamydomonas reinhardtii. Examples are the addition of ATP at a precise time in the ciliary beating cycle, the stabilization of beating frequency, rhodopsin control of cell steering, photosynthetic control of ciliary beating, the control of the orientation of the cilia, the ballistic/diffusive swimming ratio, the competition between obstacle avoidance and phototaxis, the control of phototactic direction, and multi-input few-output signal processing .

Tuesday, November 15, 2011

Irregularly Shaped Granular Materials: Geometric cohesion and other strange phenomena
Scott Franklin, RIT

Abstract: A number of strange phenomena can occur when granular particles have radically irregular shapes. Geometrically cohesive granular materials (GCGM) are collections of particles whose individual shape results in entanglements that resist extension forces. Examples include long, thin (anisometric) rods, arcs of varying length, and U-shaped "staples.'' At RIT and Georgia Tech we have conducted a number of experimental and computational investigations into the rigidity of piles of GCGM. These include canonical stress-strain tests to measure basic rheological properties and vibration-induced "melting'' experiments that have revealed an maximally rigid particle size. This optimal size results from a "competition" between the number of neighbors with which a particle can entangle, which goes down as the staple arms become long, and the entanglement region, which increases with arm length. Time permitting, I will also discuss new 2D simulations of self-propelled triangular particles. Triangles can tile the plane, and so pack efficiently, but the self-propulsion mechanism is chosen so as to inhibit collective motion. Simulations therefore test a tension between the geometric propensity to pack and the propulsive desire to separate.

Tuesday, November 22, 2011

Regulating Microtubules Through Severing
Jennifer Ross, UMass Amherst

Abstract:Microtubules are cytoskeletal filaments that organize intracellular space structurally and through active transport along their lengths. They need to be organized and remodeled quickly during development of differentiated cells or in mitosis. Much work has focused on remodeling from the ends because these long polymers can stochastically disassemble through dynamic instability or be actively disassembled. Microtubule-severing enzymes are a novel class of microtubule regulators that create new ends by cutting the filament. Thus, these proteins add a new dimension to microtubule regulation by their ability to create new microtubule ends. Interestingly, despite their destructive capabilities, severing has the ability to create new microtubule networks in cells. We are interested in the inherent biophysical activities of these proteins and their ability to remodel cellular microtubule networks. Interestingly, despite their destructive capabilities, severing has the ability to create new microtubule networks in cells. We use two-color single molecule total internal reflection fluorescence imaging to visualize purified severing enzymes and microtubules in vitro. We have examined two families of severing enzymes to find that their biophysical activities are distinct giving them different network-regulating abilities.

Tuesday, November 29, 2011

Eisenbud Lecture Series in Mathematics and Physics,
Lecture I, 4pm, Abelson 131. Reception to follow.

Jennifer Chayes, Managing Director of Microsoft Research New England

During the past decade, dynamic random networks have become increasingly important in communication and information technology. Vast, self-engineered networks, like the Internet, the World Wide Web, and online social networks, have facilitated the flow of information, and served as media for social and economic interaction. I will discuss both the mathematical challenges and opportunities that exist in describing these networks: How do we model these networks – taking into account both observed features and incentives? What processes occur on these networks, again motivated by strategic interactions and incentives, and how can we influence or control these processes? What algorithms can we construct on these networks to make them more valuable to the participants? In this talk, I will review the general classes of mathematical problems which arise on these networks, and present a few results which take into account mathematical, computer science and economic considerations. I will also present a general theory of limits of sequences of networks, and discuss what this theory may tell us about dynamically growing networks.
LECTURE 1: Models and Behavior of the Internet, the World Wide Web and Online Social Networks
Although the Internet, the World Wide Web and online social networks have many distinct features, all have a self-organized structure, rather than the engineered architecture of previous networks, such as phone or transportation systems. As a consequence of this self-organization, these networks have a host of properties which differ from those encountered in engineered structures: a broad "power-law" distribution of connections (so-called "scale-invariance"), short paths between two given points (so-called "small world phenomena" like "six degrees of separation"), strong clustering (leading to so-called "communities and subcultures"), robustness to random errors, but vulnerability to malicious attack, etc. During this lecture, I will first review some of the distinguishing observed features of these networks, and then discuss some of the models which have been devised to explain these features. I will also discuss processes and algorithms on these networks, focusing on a few particular examples.

Thursday, December 1, 2011

Eisenbud Lecture Series in Mathematics and Physics,
Lecture II (4:30pm, Abelson 131)
Jennifer Chayes, Managing Director of Microsoft Research New England

During the past decade, dynamic random networks have become increasingly important in communication and information technology. Vast, self-engineered networks, like the Internet, the World Wide Web, and online social networks, have facilitated the flow of information, and served as media for social and economic interaction. I will discuss both the mathematical challenges and opportunities that exist in describing these networks: How do we model these networks – taking into account both observed features and incentives? What processes occur on these networks, again motivated by strategic interactions and incentives, and how can we influence or control these processes? What algorithms can we construct on these networks to make them more valuable to the participants? In this talk, I will review the general classes of mathematical problems which arise on these networks, and present a few results which take into account mathematical, computer science and economic considerations. I will also present a general theory of limits of sequences of networks, and discuss what this theory may tell us about dynamically growing networks.

LECTURE 2: Convergent Sequences of Networks

In the second lecture of this series, I will abstract some of the lessons of the first lecture. Inspired by dynamically growing networks, I will ask how we can characterize general sequences of graphs in which the number of nodes grows without bound. In particular, I will define various natural notions of convergence for a sequence of graphs, and show that, in the case of dense graphs and even some sparse graphs, many of these notions are equivalent. I will also give a construction for a function representing the limit of a sequence of graphs. I’ll review examples of some simple growing network models, and illustrate the corresponding limit functions. I will also discuss the relationship between these convergent sequences and some notions from mathematical statistical physics.

Tuesday, December 6, 2011

Granular drag at the limit of pushing through boulders
Stephan Koehler, WPI

It is generally accepted that sheared granular media can be treated as a fluid-like continuum, although to date no commonly accepted theory has been developed. In particular understanding slow flows where frictional effects are dominant has proven very difficult.To probe the transition from discrete granules to the continuum we perform drag experiments involving intruders which range in size from being smaller than one bead diameter to being many times larger than a bead diameter. We find that the scale effect due to large beads can be well-accounted for by the concept of augmenting the intruder's dimensions. We also observe that there are transitions in the scaling behavior of the drag forces depending on the immersion depth relative to the intruder's diameter. This suggests that the forces on an intruder's surface can simply be modeled using the lithostatic pressure hypothesis; however we believe that there are edge and corner effects which become increasingly important with increasing system granularity.

Spring 2012 Colloquia

Tuesday, January 17, 2012

Harmony of scattering amplitudes: From colliders to supergravity
Zvi Bern, UCLA

Abstract: Feynman diagrams have long been the basic tool in quantum field theory giving a systematic description of the collisions of elementary particles. However, they can greatly obscure the beauty and simplicity, introducing complicated unphysical contributions that cancel only at the end of computations. We describe modern ideas that allow us to sneak past Heisenberg's uncertainty
principle to reveal a remarkable simplicity. We also describe relations between the way gravitons and gluons scatter, crying out for a unification of the sort seen in string theory. These ideas allow us to perform calculations addressing fundamental questions in quantum field theory that would have been extremely difficult, if not impossible, a few years ago. We describe state-of-the-art applications to physics at the Large Hadron Collider and to studies of the compatibility of General Relativity with quantum mechanics.

Tuesday, January 24, 2012

Knotted Fields

William Irvine, University of Chicago

Abstract: To tie a shoelace into a knot is a relatively simple affair. Tying a knot in a field is a different story, because the whole of space must be filled in a way that matches the knot being tied at the core. The possibility of such localized knottedness in a space-filling field has fascinated physicists and mathematicians ever since Kelvin’s 'vortex atom' hypothesis, in which the atoms of the periodic table were hypothesized to correspond to closed vortex loops of different knot types. I will discuss some remarkably intricate and stable topological structures that can exist in light fields whose evolution is governed entirely by the geometric structure of the field. A special solution based on a structure known as a Robinson Congruence that was re-discovered in different contexts will serve as a basis for the discussion. I will then discuss topologically non-trivial vortex configurations in fluids.

Tuesday, January 31, 2012

No colloquium

Tuesday, February 7, 2012

Black Holes- the Harmonic Oscillators of the 21st Century
Andrew Strominger, Harvard University

Abstract: In the twentieth century, many problems across all of physics were solved by perturbative methods which reduced them to harmonic oscillators. Black holes are poised to play a similar role for the problems of twenty-first century physics. They are at once the simplest and most complex objects in the physical universe. They are maximally complex in that the number of possible microstates, or entropy, of a black hole is believed to saturate a universal bound. They are maximally simple in that, according to Einstein's theory, they are featureless holes in space characterized only by their mass, charge and angular momentum. This dual relation between simplicity and complexity, as expressed in black holes, has recently been successfully applied to problems in a disparate variety of physical systems. I will give an introduction to the subject intended for a general audience.

Tuesday, February 14, 2012

Dripping, jetting, drops and wetting: the magic of microfluidics

David Weitz, Harvard University

Abstract: This talk will discuss the use of microfluidic devices to precisely control the flow and mixing of fluids to make drops, and will explore a variety of uses of these drops. These drops can be used to create new materials that are difficult to synthesize with any other method. These materials have great potential for encapsulation and release and for drug delivery. I will also show how the exquisite control afforded by the microfluidic devices provides enabling technology to use droplets as microreactors to perform biological reactions at remarkably high rates using very small quantities of fluids. I will demonstrate how this can be used for new fundamental and technological applications.

Tuesday, February 21, 2012

No colloquium. Midterm recess.

Tuesday, February 28, 2012

No colloquium. APS meeting.

Tuesday, March 6, 2012

A bacterium’s perspective on chemosensing and exploration
Thierry Emonet, Yale University

Abstract: Behavior is dynamics. Recent advances in high-throughput laboratory investigations provide unprecedented information about the identity, connectivity and spatiotemporal localization of molecular components that mediate cellular mechanisms. A major challenge now is to transform this data into a dynamical understanding of biological systems, connecting molecules to behavior. Ideas and concepts from physics are turning to be very helpful in that respect.
The detection, amplification, and processing of external chemical signals is affected by random fluctuations that arise within signaling pathways. In the case of the bacterial chemotaxis system we now have enough experimental data to go beyond ensemble averages. I will talk about our recent experimental and theoretical efforts to examine how the network design and spatial arrangement of this model signaling pathway shape the information processing and chemotactic capabilities of the single cell. An interesting result that emerges from this individual cell perspective is a molecular and dynamical systems understanding of how cells can resolve the compromise between the essential but likely competing behavioral modes of sensing and exploring. An introduction for general audience will be provided.

Tuesday, March 13, 2012

Black holes as mirrors
Patrick Hayden, McGill University
Watch video

Abstract: Are black holes the universe's perfect trash bins or does information about their contents leak out in Hawking radiation? I'll discuss information retrieval from evaporating black holes, assuming that the internal dynamics of a black hole is unitary and rapidly mixing, and assuming that the retriever has unlimited control over the emitted Hawking radiation. If the evaporation of the black hole has already proceeded past the "half-way" point, where half of the initial entropy has been radiated away, then additional quantum information deposited in the black hole is revealed in the Hawking radiation very rapidly. Information deposited prior to the half-way point remains concealed until the half-way point, and then emerges quickly. These conclusions hold because typical local quantum circuits are efficient encoders for quantum error-correcting codes that nearly achieve the capacity of the quantum erasure channel. The resulting estimate of a black hole's information retention time, based on speculative dynamical assumptions, is just barely compatible with the black hole complementarity hypothesis.
(Joint work with John Preskill.)

Tuesday, March 20, 2012

The role of quantum coherence in excitation energy transfer: New theoretical, computational and spectroscopy approaches.
Alan Aspuru-Guzik, Harvard University
Watch video

Abstract: Observed long-lived coherences in various photosynthetic complexes a at cryogenic and room temperature have generated vigorous efforts both in theory and experiment to understand their origins and explore their potential role to biological function. The ultrafast signals resulting from the experiments that show evidence for these coherences result from many contributions to the monitored polarization. These experiments raise the following specific questions: What is the role of quantum coherence, if any, in the energy transfer process of these systems?, and second: Why is the coherence preserved for these long times? In this talk, I will describe our recent efforts to address these two questions using tools from physical chemistry and quantum information theory. We employ and develop several techniques ranging from quantum master equations to explicit atomistic simulations and introduce measures of efficiency, partitioning of contributions to quantum transport, and non-Markovianity in these systems. We propose a new set of ultrafast experments (quantum process tomography, QPT) to extract the model-independent dynamical information, at the level of the electronic density matrix, about the energy transfer process from combinations of several ultrafast experiments designed to invert this quantum process matrix. This allows us to answer the crucial question of “How much information is in two-dimensional spectra?” and to make the case that QPT is a relevant reformulation of the problem with the goal to maximize the extracted information about the system as a function of the number of experiments carried out. I will describe QPT experiments currently underway. I will finish talking about our efforts to engineer novel materials for light harvesting and charge transport in organic photovoltaics, and our recent prediction and synthesis of the second largest hole-mobility organic material known using some of the tools mentioned above.

Tuesday, March 27, 2012

Light and Sound: An optical approach for illuminating the elusive mechanics of hearing in the mammalian cochlea
Jonathan Fisher, The Rockefeller University
Watch the video

Human hearing is exquisitely sensitive over a vast range of sounds. We can hear faint sounds down to the level of thermal fluctuations in the ear, and our ability to discern subtle differences in tone allows us to distinguish human voices of nearly identical timbre. We additionally perceive sounds of vastly differing intensities on a similar scale, enabling us to clearly hear the distant strumming of nylon strings from a classical guitar playing in concert with a full orchestra. These remarkable capabilities are largely lost in individuals with sensorineural hearing loss, which stems primarily from the failure of mechanosensory hair cells, and is associated with changes in the traveling waves that transmit acoustic signals along the cochlea. However, the connection between cochlear mechanics and the amplificatory function of hair cells remains unclear. Prestin, a unique "motor" protein, causes electrically evoked length changes in the cochlea's sensory hair cells. This effect is thought to be essential and even sufficient for cochlear amplification. Using a novel optical technique that permits the targeted inactivation of prestin in specific segments of the cochlea, we demonstrate that somatic motility is required to locally amplify the traveling wave, thus confirming directly that a force-producing mechanism originally characterized in isolated cells shapes the active traveling wave in vivo. By perturbing amplification in narrow segments of the basilar membrane, we further show that a cochlear traveling wave accumulates gain as it approaches its peak. Analysis of our results indicates that cochlear amplification produces negative damping that counters the viscous drag impeding traveling waves; targeted photoinactivation locally interrupts this compensation. These results reveal the locus of amplification in cochlear traveling waves and connect the characteristics of normal hearing to molecular forces.

Tuesday, April 3, 2012

Biological Flows and Mechanics
Michael Shelley NYU
Watch the video

Abstract: The mechanics of fluids and structures can sometimes be extremely useful in explaining biological phenomena. While bird flight and fish swimming are two well-known examples, and I will discuss a number of other instances where the role of mechanics seems not so obvious but turns out to be central and even surprising. These include the collective dynamics of swimming bacteria; observations and modeling of a simple undulating organism -- the nematode C. elegans -- negotiating a fluid-filled space full of obstacles; and the dynamics of the pronuclear complex in C. elegans embryo as it achieves proper position and orientation within the cell so that early development can successfully proceed.

Tuesday, April 10, 2012

No colloquium. Spring and Passover Recess.

Tuesday, April 17, 2012

Searching for Answers at The Large Hadron Collider
Matthew Strassler, Rutgers University

Abstract: The Large Hadron Collider (LHC), the world's most powerful particle accelerator, performed admirably in 2011, collecting more than 100 times as much data as in the previous year. The 2011 data was sufficient to allow particle physicists to begin addressing some long-standing and profound questions about nature, and 2012 may see a few of those questions settled. In this talk I will introduce the LHC as a machine and as an enterprise, and explain briefly how science is done there. After outlining the questions that the LHC was meant to explore, I will provide a progress report on what has been learned over the past year, as well as describing the long road that lies ahead.

This video is under copyright and may not be reproduced, in part or in total, without written permission of the speaker and of the Physics Department.

Tuesday, April 24, 2012

Time-reversal invariant topological insulators and superconductors
Taylor Hughes (UIUC)
Watch the video

Abstract: New states of matter have recently been discovered which are reminiscent of the quantum Hall effect found in two-dimensional electron systems, but which do not require the extreme environment to exist (e.g. high magnetic fields, low temperature). These states are the so-called time-reversal invariant topological insulators which were theoretically predicted and subsequently experimentally confirmed to exist in 2D and 3D. These states are nominally insulating in the bulk but harbor low-energy metallic states on their edges/surfaces which are robust to disorder and other imperfections. I will discuss the basic physics of topological insulators which can be modeled with simple, free-fermion Dirac Hamiltonians. My focus will be on the criteria that led the search for realistic materials, and on the novel response of these insulators to electromagnetic fields. In addition to this discussion, I will mention related topological superconducting states of matter as well as some interesting topological insulator/ferromagnet/superconductor hetero-structures which have recently received attention due to possible quantum computation applications.

Tuesday, May 1, 2012

No colloquium. Brandeis Friday.


Fall 2012 Colloquia

Tuesday, September 11, 2012

Craig Blocker, Brandeis University
Discovery of the Higgs Boson at the LHC

Abstract: In summer 2012, the two large detectors at the Large Hadron Collider, ATLAS and CMS, presented evidence for the discovery of a new particle consistent with the long sought Higgs boson. This talk will cover that evidence and discuss further measurements to determine the exact nature of this new particle.

Watch the video

Tuesday, September 18, 2012

No colloquium (Rosh Hashanah)

Tuesday, September 25, 2012

Heinrich Jaeger, University of Chicago

Dynamically Jammed Liquids: A New Perspective on Shear Thickening of Dense Suspensions

Dense suspensions of particles in a liquid exhibit a number of counter-intutive, non-Newtonian flow behaviors. Most remarkably, the application of stress can dramatically harden the material, transforming it from a liquid state at rest into a solid-like state when driven strongly. Shear-thickening-based models developed over the last 25 years cannot explain the observed large normal stresses (large enough to support a grown person's weight when running across a pool filled with a suspension such as cornstarch in water). This talk surveys some of the key issues, discusses the stress scales associated with shear thickening in dense suspensions, and outlines a new scenario for impact response. In particular, using high-speed video and x-ray imaging during sudden impact, we are able to link the nonlinear suspension dynamics in a new way to the jamming phase transition.

Watch the video

Tuesday, October 2, 2012

Christine Thomas, Brandeis University
Fundamental Aspects of Catalyst Design for Renewable Energy Applications

Abstract: In the search for alternatives to fossil fuels, a fundamental challenge is finding energy sources that are both renewable and naturally abundant. While solar power fits both of these criteria, variations in the sun's intensity necessitate storage of this energy. One strategy that has emerged is the storage of solar energy in the form of chemical bonds - that is, using solar energy make energy intensive molecules that can be stored for later use as energy sources. A typical example of this is the photocatalyzed splitting of water (H2O) into hydrogen (H2) and oxygen (O2). Other examples of such small molecule activation will be discussed, along with the challenges of designing efficient and economically viable catalysts for these purposes. Fundamental contributions from the Thomas laboratory will be presented with a particular emphasis on the tools used by synthetic inorganic chemists to probe their molecules and their properties.

Watch the video

Tuesday, October 9, 2012

No colloquium. Brandeis Monday.

Tuesday - Thursday October 16, 17, and 18, 2012

Eisenbud Lectures in Mathematics and Physics
Craig A. Tracy, UC Davis
Integrable Systems, Operator Determinants and Probabilistic Models

Abstract: This project involves the study of current fluctuations in the asymmetric simple exclusion process for a variety of initial configurations. This is a model of interacting particles on a one-dimensional lattice. The model has attracted wide attention from both mathematicians and physicists since it is one of the simplest models to incorporate far from equilibrium behavior with nonclassical fluctuations. These fluctuations are expected to have a new universal behavior similar in their applicability to the famous bell-shaped curve (the Gaussian distribution) of classical probability. A long-term goal of research in this area is the establishment of new limit laws similar in nature to the classical central limit theorem. Already these new universal distributions are being applied to various problems in growth processes, population genetics, and finance. This project will extend our knowledge of fluctuations to a much wider class of growth models.

The third lecture will take place at 4:30pm instead of 4pm (all lectures in Abelson 131)

Lecture I: Watch the video

Lecture III: Watch the video

Tuesday, October 23, 2012

Royce Zia, Virginia Tech
Non-equilibrium Statistical Mechanics: a growing frontier of “pure and applied” theoretical physics

Abstract: Founded over a century ago, statistical mechanics (SM) for systems in thermal equilibrium has been so successful that, nowadays, it forms part of our physics core curriculum. On the other hand, most of "real life" phenomena occur under non-equilibrium conditions. Unfortunately, statistical mechanics for such systems is far from being well established. The goal of understanding complex collective behavior from simple microscopic rules (of evolution, say) remains elusive. As an example of the difficulties we face, consider predicting the existence of a tree from an appropriate collection of
H,C,O,N,... atoms! Over the last two decades, an increasing number of condensed matter theorists are devoting their efforts to this frontier. After a brief summary of the crucial differences between text-book equilibrium SM and non-equilibrium SM, I will give a bird's-eye view of some key issues, ranging from the "fundamental" to (a small set of) the "applied." The methods used also span a wide spectrum, from "easy" computer simulations to sophisticated field theoretic techniques. These will be illustrated in the context of an overview of our work, as well as a simple model for transport.

Watch the video.

Tuesday, October 30, 2012

Douglas Weibel, University of Wisconsin, Madison
Co-sponsored with the Quantitative Biology Program
Protein regulation at curved bacterial membranes

Abstract: Using a combination of cell biological, biochemical, and biophysical techniques, we demonstrate that the Escherichia coli recombination repair enzyme, RecA is temporally localized and regulated by its interaction with anionic phospholipids at regions of large, negative membrane curvature. Early studies on RecA function in vivo demonstrated large amounts of protein associating with membrane fractions during DNA damage (Garvey et al. 1985, J. Biol. Chem. 285, 18984). A fundamental unanswered question is why does RecA bind the membrane and is this interaction conserved across organisms? To test whether this mechanism is evolutionarily conserved between bacteria and mitochondria, we are extending our studies to Rad51, which is a mitochondrial homolog of RecA. Recent studies have implicated Rad51 in mtDNA repair during damage due to lipid peroxidation. In this presentation I discuss several possible hypotheses that support our data.

Watch the video

Tuesday, November 6, 2012

No colloquium.

Tuesday, November 13, 2012

John Crocker, University of Pennsylvania
Interactions, directed assembly and transformations of colloids using DNA handshaking

Abstract: DNA is a versatile tool for directing the controlled self-assembly of nanoscopic and microscopic objects. The interactions between microspheres due to the hybridization of DNA strands grafted to their surface have been measured and can be modeled in detail, using well-known polymer physics and DNA thermodynamics. Knowledge of the potential, in turn, enables the exploration of the complex phase diagram and self-assembly kinetics in simulation. In experiment, at high densities of long grafted DNA strands, and temperatures where the binding is reversible, these system readily form colloidal crystals having several different structures. For interactions that favor alloying between two same-sized colloidal species, our experimental observations compare favorably to a simulation framework that predicts the equilibrium phase behavior, crystal growth kinetics and solid-solid transitions. We will discuss the crystallography of the novel alloy structures formed, and the interesting diffusionless transformations they undergo that resemble shape memory alloys.

Watch the video.

Tuesday, November 20, 2012

Pankaj Mehta, Boston University
Thermodynamics of cellular computation

Abstract: Cells often perform computations in response to environmental cues. On general theoretical grounds (Landauer's Principle), it is expected that such computations require cells to consume energy. We discuss the thermodynamics of cellular computations using two simple examples. First, we consider the classic problem, first considered by Berg and Purcell, of determining the concentration of a chemical ligand in the surrounding media. We show that learning about external concentrations necessitates the breaking of detailed balance and consumption of energy, with greater learning requiring more energy. Our calculations suggest that the energetic costs of cellular computation may be an important constraint on networks designed to function in resource poor environments such as the spore germination networks of bacteria. We will then discuss how Landauer's principle can be used to inform the design of synthetic memory modules in cells, focusing on protein-based switches and DNA-based memory.

Watch the video.

Tuesday, November 27, 2012

David Nelson, Harvard University
Life at High Reynolds Number

Abstract: Microorganisms living in the ocean can be subject to strong turbulence with cell division times in the middle of a Kolmogorov-like cascade of eddy turnover times. We explore the dynamics of a Fisher equation describing cell proliferation in one and two dimensions, as well as turbulent advection and diffusion. Because of inertial effects and cell buoyancy, we argue that the effective advecting velocity field is compressible. For strong enough compressible turbulence, bacteria, for example, can track, in a quasilocalized fashion (with remarkably long persistence times), sinks in the turbulent field. An important consequence is a large reduction in the carrying capacity of the fluid medium.

Watch the video.

Tuesday, December 4, 2012

Angelo Cacciuto, Columbia University

Crystallization of "Ugly" and "Soft" Particles

Abstract: Understanding how nanocomponents spontaneously organize into complex macroscopic structures is one of the great challenges in the field of complex fluids today. The process of self-assembly is in fact relevant to several biological processes, such as protein aggregation and intracellular trafficking, and has numerous applications in materials science. The ability to predict and control the phase behavior of a solution given a set of nanocomponents may open the way to the development of materials with novel optical, mechanical, and electronic properties. The main objective of our research is to establish rigorous links between the microscopic properties of nanocomponents and their ability to form ordered aggregates via the process of self-assembly. In this talk we will discuss two specific cases: (1) The phase behavior of hard irregularly shaped particles and their crystallizability limits, and (2) the exotic behavior (fanta-packing) of ultra-soft nanoparticles, such as dendrimers and star-polymers, at large densities.

Watch the video.

Tuesday, December 11, 2012

Alan Marscher, Boston University
Black Holes, Jets, and Gamma Rays in Active Galactic Nuclei

Abstract: Although black holes consume most of the matter that falls toward them, a small fraction gets heated and shot out along the poles in the form of ultra-high energy plasma jets having flow velocities very close to the speed of light. The most powerful jets are found in active galactic nuclei, where the mass of the central black hole is millions or even billions of solar masses. I will describe recent observations at gamma-ray, microwave, infrared, optical, and X-ray frequencies that probe the jets closer to the black hole than has been possible previously. The data so far are consistent with prevailing theoretical models, according to which the jets are propelled by magnetic forces. But the gamma rays originate mostly from regions parsecs from the black hole, contrary to most previous models.

Watch the video.

Spring 2013 Colloquia

Tuesday, January 15, 2013
Physics Department Colloquium
4pm in Abelson 131
Jane Luu , MIT
The New Solar System

Abstract: By the early 1990s, astronomers thought they knew all the major types of objects in the solar system: planets, satellites, comets, asteroid, etc. They also thought they had a good idea how the solar system obtained its current configuration. In 1992, the discovery of a new population of objects beyond Neptune, called the Kuiper Belt, turned this view upside down. Not only were astronomers far from knowing what the solar system contained, the original configuration is now up for debate. This talk describes the discovery of the Kuiper Belt and how it changed our view of the solar system.

Watch the video.

Tuesday, January 22, 2013

Doug Jerolmack, University of Pennsylvania
Surprising connections among aerodynamics, vegetation and groundwater in a desert dune field

Abstract: Desert dunes often exhibit remarkable pattern changes over short distances. For example, sediment-rich dunes can break up into smaller, isolated features, and then become stabilized by plants, over distances of kilometers. Such pattern transitions often coincide with spatial variations in sediment supply, transport rate, hydrology and vegetation, but have not been linked mechanistically. Here we hypothesize that the abrupt increase in roughness at the upwind margins of dune fields trips the development of an internal boundary layer, which thickens downwind and causes a spatial decrease in the surface wind stress. We demonstrate that this mechanism forces a downwind decline in sand flux at White Sands, New Mexico, using a combination of physical theory and field observations. Declining sand flux triggers an abrupt increase in vegetation density, which in turn drives changes in groundwater depth and salinity – showing that aerodynamics, sediment transport and ecohydrology are tightly interconnected in this landscape. Despite the complex climatic and geologic history of White Sands, internal boundary layer theory explains many of the observed first-order patterns of the dune field.

Watch the video.

Tuesday, January 29, 2013

Joseph Lehar, Novartis Institutes for BioMedical Research and Boston University
An astrophysicist turns to drugs

Novartis is undertaking a large-scale effort to comprehensively describe cancer through the lens of cell cultures and tissue samples. In collaboration with academic and industrial partners, we have generated mutation status, gene copy number, and gene expression data for a library of 1,000 cancer cell lines, representing most cancer lineages and common genetic backgrounds. Most of these cell lines have been tested for chemosensitivity against ~1,200 cancer-relevant compounds, and we are systematically exploring drug combinations for synergy against ~100 prioritized CCLE lines. We expect this large-scale campaign to enable efficient patient selection for clinical trials on existing cancer drugs, reveal many therapeutically promising drug synergies or anti-resistance combinations, and provide unprecedented detail on functional interactions between cancer signaling pathways. I will discuss early highlights of this work and describe our plans to make use of this resource.

Biography: Dr. Lehár heads the bioinformatics group for Novartis’s oncology translational research department, which is focused on identifying optimal treatment strategies for candidate drugs in the Novartis pipeline. Previously, he has played a central role in developing technology and analysis platforms for CombinatoRx (now Zalicus Inc.), a biotech company discovering combination therapies, in addition to researching the systems biology of drug combinations through his affiliation with BU. Prior to CombinatoRx, Dr. Lehár was a postdoctoral scientist at Whitehead Institute’s Center for Genome Research (now the Broad Institute), at the Harvard College Observatory, and at Cambridge University’s Institute of Astronomy. Dr. Lehár holds a Ph.D. in physics from MIT and a B.A. in physics from Brandeis.

Watch the video.

Tuesday, February 5, 2013

Jane' Kondev, Brandeis University
The physical nature of cellular decision making

Abstract: Cells make decisions all the time about what to eat, where to go, and what to become. At the heart of cellular decision making is gene regulation, the process by which cells selectively turn their genes on and off in response to environmental ques. Experiments have recently begun to probe gene regulation inside cells at the single molecule level, revealing the physical nature of this key biological process in quantitative detail. In this talk I will review recent experimental advances in single-cell gene expression measurements and the theoretical models that are being put forth to greet them. I will emphasize the interplay of theory and experiments and how it has revealed surprises about some of the best studied models of gene regulation in bacteria such as the lac operon.

Watch the video.

Tuesday, February 12, 2013

Silvan S. Schweber, Brandeis University and Harvard University
Hans Bethe and Physics in/of the 20th Century

Abstract: I will present some facets of Hans Bethe’s life to illustrate how I use biography to narrate aspects of the history of twentieth century physics. I will focus on post World War II quantum field theory, on the relation between solid state/condensed matter physics and high energy physics and make some comments regarding "revolutions" in science.

Watch the video.

Tuesday, February 19, 2013

No colloquium. Midterm recess.

Tuesday, February 26, 2013

Duncan J. Irschick, UMass Amherst
The making of GeckskinTM: A story of accidental science

Fortune Magazine Article about Geckskin

Gecko Feet Video

Watch the video.

Tuesday, March 5, 2013

Pablo Jarillo-Herrero, MIT
The Versatility of Dirac Electrons in Graphene

Abstract: Over the past few years, the physics of low dimensional electronic systems has been revolutionized by the discovery of materials with very unusual electronic structures. Among these, graphene has taken center stage due to its relativistic-like electron dynamics and potential applications in nanotechnology. Moreover, the recent discovery that hexagonal boron nitride (hBN) is a nearly-ideal substrate for high mobility graphene devices has enabled a new generation of quantum transport and optoelectronic experiments in graphene-based materials. In this talk I will review our recent experiments on graphene on hBN devices , where we explore different aspects of the "Dirac-ness" of charge carriers in graphene: from novel optoelectronic phenomena to a new type of quantum spin Hall effect.

Watch the video.

Tuesday, March 12, 2013

Chris White, Glasgow
From gluons to gravitons: a panoramic view of long distance behaviour

Abstract: Scattering amplitudes are important quantities in quantum field theory, which govern the probabilities for particle interactions. It has been known for decades that amplitudes contain "infrared divergences" associated with the emission of force-carrying particles (e.g. photons, gluons and gravitons) at large distances. Understanding these divergences is interesting in its own right, but also allows us to increase the precision of calculations at hadron colliders such as the Tevatron and the LHC. This talk will introduce scattering amplitudes and their associated infrared divergences, before summarising recent developments in the theory of quarks and gluons (Quantum Chromodynamics). We will also look at intriguing relationships between QCD and quantum gravity, which the infrared limit helps to clarify.

Watch the video.

Tuesday, March 19, 2013

No colloquium. APS meeting.

Tuesday, March 26, 2013

No colloquium. Passover and spring recess.

Tuesday, April 2, 2013

No colloquium. Passover and spring recess.

Tuesday, April 9, 2013

Robert Jaffe, MIT
Energy Critical Elements: More Precious than Gold

Elements that were once laboratory curiosities, like neodymium, tellurium, and terbium, now figure centrally when novel energy systems are discussed. Many of these elements are not at present mined, refined, or traded in large quantities. New technologies can only impact our energy needs, however, if they can be scaled from laboratory, to demonstration, to massive implementation. As a result, some previously unfamiliar elements will be needed in great quantities. Although every element has its unique story, these Energy Critical Elements have many features in common. I will describe the shared characteristics of these elements, their roles in emerging technologies, potential constraints on their availability, and government actions that can help avoid disruptive shortages. As an example, I will focus especially on elements that are required for photovoltaic technologies.

Watch the video.

Tuesday, April 16, 2013

Adam Cohen, Harvard University
Optical tools to monitor electrical activity in neurons

Our mental state is encoded in a set of electrical spikes that propagate through our neurons. These spikes are about 100 mV tall, last about 1 ms, and travel at a few meters per second. One can record these spikes with an electrode, but an electrode only reports voltage at a single point, while a human brain has ~10^11 neurons. For decades neuroscientists have sought an optical tool to convert neuronal spikes into light, with the goal of visualizing activity in large networks of neurons. We identified a protein from a Dead Sea microorganism that achieves this goal. In the wild the protein serves as a light-driven proton pump: it absorbs sunlight and generates a transmembrane voltage which its host uses as a source of metabolic energy. We found a way to run this protein "in reverse", to convert changes in membrane voltage into light. Upon expressing this gene in neurons, we acquired movies showing electrical impulses propagating through neurons.

D. Maclaurin*, V. Venkatachalam*, H. Lee, A. E. Cohen (*Co-first authors), “Mechanism of voltage-sensitive fluorescence in a microbial rhodopsin,” PNAS 10.1073/pnas.1215595110 (2013)

J. Kralj*, A. D. Douglass*, D. R. Hochbaum*, D. Maclaurin, A. E. Cohen (*Co-first authors), “Optical recording of action potentials in mammalian neurons using a microbial rhodopsin,” Nature Methods, 9,
90-95 (2012)

J. Kralj, D. R. Hochbaum, A. D. Douglass, A. E. Cohen, “Electrical spiking in Escherichia coli probed with a fluorescent voltage-indicating protein,” Science, 333, 345-348 (2011)

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Tuesday, April 23, 2013

Michael Brenner, Harvard University
Linear Algebra and the Shape of Bird Beaks

Abstract: Evolution by natural selection has resulted in a remarkable diversity of organism morphologies. But is it possible for developmental processes to create “any possible shape”? Or are there intrinsic constraints? I will discuss our recent exploration into the shapes of bird beaks. Initially, inspired by the discovery of genes controlling the shapes of beaks of Darwin's finches, we showed that the morphological diversity in the beaks of Darwin’s Finches is quantitatively accounted for by the mathematical group of affine transformations. We have extended this to show that the space of shapes of bird beaks is not large, and that a large phylogeny (including finches, cardinals, sparrows, etc) are accurately spanned by only three independent parameters-- the shapes of these bird beaks are all pieces of conic sections. After summarizing the evidence for these conclusions, I will delve into our efforts to create mathematical models that connect these patterns to the developmental mechanism leading to a beak. It turns out that there are simple (but precise) constraints on any mathematical model that leads to the observed phenomenology, leading to explicit predictions for the time dynamics of beak development in song birds. Experiments testing these predictions for the development of zebra finch beaks will be presented.
(joint work with J. Fritz and O. Campas; as well as Prof. Arkhat Abzhanov and members of his research group ( R. Mallarino, J. Brancale and M. Tokita)

Watch the video.

Tuesday, April 30, 2013

Hong Liu, MIT
From black holes to strange metals: many-body physics through a gravitational lens

Abstract: Ever since the end of the Stone Age, metals have fascinated humankind and have been vital in the development of civilization. During the last two decades, physicists have been mystified by a new class of “strange” metals, observed in high-temperature superconductors and other strongly interacting electron systems, and whose exotic properties challenge fundamental pillars of condensed-matter physics. I will describe how a string-theory concept called holographic duality has been applied to shed light on some of the mysteries of that novel metallic state.

Watch the video.

Fall 2013 Colloquia

Tuesday, September 3, 2013

Arup Chakraborty, MIT
How to hit HIV where it hurts

HIV continues to wreak havoc around the world, especially in poor countries. A vaccine is urgently needed to overcome this major global health challenge. I will describe key challenges that must be confronted to achieve this goal. I will then focus on some work that aims to address a part of these challenges by bringing together theory and computation (rooted in statistical physics), consideration of structures of multi-protein assemblies, basic immunology, and human clinical data. The results of these studies suggest the design of immunogens that could be components of vaccines that might elicit immune responses which might be able to hit HIV where it hurts upon natural infection. I shall also briefly touch upon some potentially generic features of viral evolution, which are superficially reminiscent of critical phenomena.

Watch the video.

Tuesday, September 10, 2013

Boris Shraiman, KITP
Statistical Genetics and the Dynamics of Natural Selection

Evolution works through natural selection that acts on genetic variation. A mounting body of evidence suggests that large populations harbor a great deal of such “selectable” variation. This implies that in order to understand how genetic variants (a.k.a. polymorphisms) spread through populations, theoretical models must account for interactions between polymorphisms at different genetic loci and in different individuals. The problem is further encumbered by the effect of sex and recombination that reassort polymorphisms between individual genomes. Yet, this “many-body problem” of evolutionary dynamics lends itself to a “Statistical Genetics” approach with many parallels to Statistical Physics. This lecture will present a statistical physicist’s view of natural selection acting in populations with high levels of genetic diversity and describe some of the new insights into the effects of different genetic interactions .

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Tuesday, September 17 (Brandeis Thursday), 2013

Robert Holyst, Polish Academy of Sciences
Biologistics:Mobility in cytoplasm of the eukaryotic and prokaryotic cells

Biologistics and biochemistry in a crowded environment are two emerging interdisciplinary fields of science. They provide quantitative analysis of transport of proteins and their interactions involved in gene expression and regulation. These processes inside living cells strongly depend on the physics of liquids at the nanoscale. As I will try to convince you during my talk, the length-scale dependent nanoviscosity [1-4] characterizing motion of proteins at the nanoscale is a key to quantitative analysis of biochemical reactions in living cells. Genes are activated and repressed by proteins referred to as transcription factors (TF). The binding of TFs to the operator region on DNA is diffusion limited. TFs search for operators by performing a combination of three-dimensional (3D) diffusion in a defined volume and one-dimensional (1D) diffusion along DNA molecule. The diffusion coefficients for 3D diffusion, D, and 1D diffusion, D1, are inversely proportional to the viscosity. For the model Gram-negative bacterium Escherichia coli, the nanoviscosity of the cytoplasm depends on the size of diffusing objects[2]. This scale dependent nanoviscosity changes by a factor of >104 between 0.001 Pas for water molecules (size 0.14 nm) and 18 Pas for large plasmids (size 300 nm). Accordingly D for biomolecules in E. coli varies by a factor of ~108. An understanding of how D, D1 and the reaction rates for gene expression depend on the length-scale dependent nanoviscosity and non-specific interactions between DNA and proteins is an essential step for understanding metabolic and proteomic networks. The final outcome of the work of my group is a database (6600 records) of diffusion coefficients for all proteins and their complexes from the proteome of E.coli. This is the first such database for any organism. Only 10-20 measurements of diffusion coefficients are needed to construct the databases for any cell or its organelles (nucleus, mitochondria).

Watch the video.


Tuesday, September 24, 2013

David Huse, Princeton University
Thermalization and localization in quantum statistical mechanics

Progress in atomic physics and quantum information science has motivated much recent study of the behavior of many-body quantum systems fully isolated from their environment, and thus undergoing coherent quantum time evolution. What does it mean for such a system to go to thermal equilibrium? I will explain the Eigenstate Thermalization Hypothesis (ETH), which says that each individual exact eigenstate of the system's Hamiltonian is at thermal equilibrium, and which appears to be true for most quantum many-body systems. But there are systems that do not obey this hypothesis, namely systems that are Anderson localized. These "many-body localized" systems can retain local memory of their initial state for infinite time and thus do not thermally equilibrate. A key issue here is whether or not the system itself constitutes a "thermal reservoir" that can equilibrate its parts.

Watch the video.

Tuesday, October 1, 2013

Peter J. Lu, Harvard University
Modern math in medieval islamic architecture

The conventional view holds that girih (geometric star-and-polygon) patterns in medieval Islamic architecture were conceived by their designers as a network of zigzagging lines, and drafted directly with a straightedge and a compass. I will describe recent findings that, by 1200 C.E., a conceptual breakthrough occurred in which girih patterns were reconceived as tessellations of a special set of equilateral polygons (girih tiles) decorated with lines. These girih tiles enabled the creation of increasingly complex periodic girih patterns, and by the 15th century, the tessellation approach was combined with self-similar transformations to construct nearly-perfect quasicrystalline patterns. Quasicrystal patterns have remarkable properties: they do not repeat periodically, and have special symmetry---and were not understood in the West until the 1970s. I will discuss some of the properties of Islamic quasicrystalline tilings, and their relation to the Penrose tiling, perhaps the best known quasicrystal pattern..

No video available.

Tuesday, October 8, 2013

Greg Bearman, Snapshot Spectra, USC Keck School of Medicine and the Israeli Antiquities Authority
Imaging Methods Applied to Conservation of Cultural Heritage

Spectral and computational imaging are two relatively new technologies to be applied to cultural heritage. There are two applications of imaging, content and conservation and imaging can work for both. Content, reading text, inscriptions and seals, for example, is typically the province of those interested in what the objects say or mean. Conservation, insuring the continued state of the object, is the field of conservation or site management in the case of outside sites. Dr. Bearman will discuss the application of spectral imaging as a conservation monitoring tool, using examples drawn from the Leon Levy Dead Sea Scrolls Digital Library, for which he is the technology and imaging consultant. Other imaging methods, such as reflectance transform imaging (RTI), will also be used to illustrate how digital imaging can significantly help conservators.

The key to developing a monitoring tool is a calibrated, repeatable, stable quantitated imaging system. Without such a system, one cannot compare images datasets over time; the user does not know if measured changes are due to changes in the object itself, system drift or just a poor imaging system with larger uncertainties in measured parameters, such as absolute reflectance. Spectral and RTI imaging systems will be used to illustrate the important parameters for monitoring.


Tuesday, October 15, 2013

Marin Soljacic, MIT
Exploring nanophotonics to tailor the laws of physics

By nano-structuring materials at length scales smaller than the wavelength of light, one can create effective materials, exhibiting optical properties unparalleled in any naturally occurring materials. The power of this approach is illustrated with two particularly important examples. Firstly, it is shown that the control over the density of photonic states via such effective materials provides a control over black body emission, which can now be tailored almost at-will. And since over 90% of all primary energy sources are converted into electrical and mechanical energy via thermal processes, exciting energy-related applications could be enabled. Secondly, a new way of confining light is presented. The ability to confine light is important both scientifically and technologically. Many light confinement methods exist, but they all achieve confinement with materials or systems that forbid outgoing waves. It is predicted and shown experimentally that light can be perfectly confined in a patterned dielectric slab, even though outgoing waves are allowed in the surrounding medium. Technically, this is an observation of an ‘embedded eigenvalue’—namely, a bound state in a continuum of radiation modes—that is not due to symmetry incompatibility.

No video available.

Tuesday, October 22, 2013

Patrick Charbonneau, Duke University
High-dimensional surprises near the glass and the jamming transitions

The glass problem is notoriously hard and controversial. Even at the mean-field level, there is little agreement about how a fluid turns sluggish while exhibiting but unremarkable structural changes. It is clear, however, that the process involves self-caging, which provides an order parameter for the transition. It is also broadly assumed that this cage should have a Gaussian shape in the mean-field limit. Here we show that this ansatz does not generally hold when increasing the dimension of space, and explore some its materials consequences. We notably examine the complex relationship between non-Gaussian caging, dynamical fluctuations, and dimensionality in the breakdown of the Stokes-Einstein relation near the glass transition. Non-Gaussian caging also persists in the jamming limit of infinitely compressed hard spheres, which affects the mechanical stability of these packings. The dimensional perspective thus establishes clear mileposts for the emergence of a complete mean-field description of the glass and the jamming transitions.

Watch the video.


Tuesday, October 29, 2013

Gabriella Sciolla, Brandeis University
The Higgs Boson: One Year Later

Particle physicists have been hunting for the elusive Higgs boson for over 40 years. On July 4th, 2012, a new particle was discovered at the Large Hadron Collider at CERN. The new particle had the potential of being the Higgs boson predicted by the Standard Model of Particle Physics. Unfortunately, the first measurement revealed only the mass of the new particle, the one parameter that the theory did not predict. In this colloquium I will discuss the measurements that the two LHC experiments have performed in the past year in order to shed light on the identity of the new particle.

Watch the video.

Tuesday, November 5, 2013

Efi Efrati, University of Chicago
Orientation dependent handedness and chiral design
(co-sponsored by MRSEC)

Handed phenomena are of central importance in fields ranging from biological self-assembly to the design of optical meta-materials. The definition of chirality (Greek for handedness), as given by lord Kelvin, associates it with the lack of mirror symmetry: the inability to superpose an object on its mirror image. While this definition has guided the classification of chiral objects for over a century, the quantification of handed phenomena based on this definition has proven elusive, if not impossible as manifest in the paradox of chiral connectedness. In this talk I will put forward a quantification scheme in which the handedness of an object depends on the direction in which it is viewed and thus best quantified by a pseudo-tensor. While consistent with familiar chiral notions, such as the right hand rule, this framework allows objects to be simultaneously right and left handed. The trace of the suggested handedness tensors recover Lord Kelvin's definition, yet their full structure is richer, and proven to be in quantitative agreement with the direction-dependent handed behavior of phenomena ranging from fluid flow to optical activity. I will review specific examples of handedness tensors, and discuss how the tensorial approach resolves the existing paradoxes and naturally enables the design of handed meta materials from symmetry principles.

No video available.

Tuesday, November 12, 2013

Anthony Dinsmore, UMass Amherst
Liquid Interfaces and Solid particles: geometry, physics and new materials

The interface between two liquids – such as the surface of a water droplet in oil – provides a versatile platform for assembly of small particles to make functional membranes or other materials. Owing to the large interfacial tension, nanometer-to-micron-sized colloidal particles readily adsorb at the interface and become confined there. While this adsorption phenomenon has been used in industry for decades, it continues to raise fundamentally fascinating and practically relevant questions. I will describe recent work developing materials for applications ranging from encapsulation & delivery to electronics. I will also describe recent work on the adsorption of spheres at anisotropically curved liquid interfaces, where a frustration arises from the contact condition on the particle and causes extended interfacial deformations. These deformation give rise to interactions among particles bound to curved fluid interfaces or fluid membranes. The results show how interfacial geometry might be used to control long-range forces.

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Tuesday, November 19, 2013

Mark Reid, Harvard University
Measuring the Cosmos

Over 2000 years ago, Hipparcus measured the distance to the Moon by triangulation from two locations across the Mediterranean Sea. However, determining distances to stars proved much more difficult. Many of the best scientists of the 16th through 18th centuries attempted to measure stellar parallax, not only to determine the scale of the cosmos but also to test Heliocentric cosmologies. While these efforts failed, along the way they lead to many discoveries, including atmospheric refraction, precession, and aberration of light. It was not until the 19th century that Bessel measured the first stellar parallax. Distance measurement in astronomy remained a difficult problem even into the early 20th century, when the nature of galaxies ("spiral nebulae") was still debated. While we now know the distances of galaxies at the edge of the Universe, we have only just begun to measure distances accurately throughout the Milky Way. Using the Very Long Baseline Array, we now can achieve positional accuracy approaching 10 micro-arcseconds! I will present new results on parallaxes and motions of star forming regions. These measurements address the nature of the spiral structure, size, rotation speed, and mass of the Milky Way.

Watch the video.

Tuesday, December 3, 2013

Richard Gaitskell, Brown University
LUX: NOBLE TRAVAILS: First Dark Matter Search Results from the Large Underground Xenon Detector

Particle dark matter is thought to be the overwhelming majority of the matter in the Universe. Its gravitational contribution overwhelms that from the ordinary matter that we, the earth and the stars, are composed of. However, direct evidence for the existence of particle dark matter remains controversial. I will discuss the LUX Experiment which has just reported world leading results in the search for WIMPs (weakly interacting massive particles). LUX is a 350 kg liquid Xe time projection chamber, and is operating underground at the Sanford Lab, Homestake, SD. I will also review a future noble liquid experiments, including, the 7 tonne liquid Xe LZ (LUX-ZEPLIN) which is proposed to be constructed at Sanford Lab in 2016.

Watch the video.

Spring 2014

Tuesday, January 14, 2014

Eric Brown, Yale University
Universal models for coherent flow structures in turbulence

Abstract: Turbulence is of tremendous importance in a wide range of astrophysical, geophysical, and engineering flow problems. Unfortunately, the largest-scale coherent structures such as convection rolls have different structures and dynamics in different flow geometries, so universal descriptions have been elusive. On the other hand, the robustness of these structures holds some promise for universal descriptions. I will present a new approach to universal models based on using robust empirical flow structure shapes as approximate solutions to the Navier-Stokes equations, which leads to low dimensional dynamical systems models for the flow. As an example, I will present results of Rayleigh-Benard convection experiments, in which a container is filled with water and heated from below. Buoyancy drives a flow which organizes into a roll-shaped circulation which spontaneously breaks the symmetry of the system. As a consequence, this roll exhibits a wide range of dynamics including erratic meandering, spontaneous flow reversals, and several oscillation modes. A simple model consisting of stochastic ordinary differential equations quantitatively reproduce these observed flow dynamics. The effects of boundary geometry and different forcings are each represented by different model terms. These results may lead to more general and relatively easy to solve models for turbulent flows with potential applications to climate, weather, and even the turbulent dynamo that is responsible for Earth's magnetic field.

Watch the video.


Tuesday, January 21, 2014

Daniel Eisenstein,The Harvard-Smithsonian Center for Astrophysics
Dark Energy and Cosmic Sound

Abstract: I will discuss how sound waves racing through the cosmos during the first million years of the Universe provide a robust method for measuring the low-redshift cosmological distance scale and thereby the properties of dark energy. The distance that the sound can travel can be computed to high precision and creates a signature in the late-time clustering of matter that serves as a standard ruler. Galaxy clustering results from the Sloan Digital Sky Survey and SDSS-III reveal this feature and allow us to measure distances to high accuracy, including a new 1% measurement to z=0.57.

Watch the video.

Tuesday, January 28, 2014

Leif Ristroph, Courant Institute at New York University
Learning aerodynamics from insects and dreaming up new ways to fly

Abstract: When viewed as miniature flying machines, insects are marvels of engineering that can teach us about the challenges and opportunities presented by flapping-wing aerodynamics. In the first part of this talk, I’ll show how a zoomed-in and slowed-down view of insect flight reveals the intricate strategies involved in orchestrating aerial maneuvers and in keeping up-right and on-course in the face of unexpected disturbances. In the second half, I’ll discuss where we are in terms of learning from insects in order to design and build small-scale flapping-wing robots. I’ll present a new aerodynamic apparatus – a kind of wind tunnel for flapping-wing flight – that serves to help inspire and test new flapping-wing concept vehicles. Specifically, I’ll show how this has led us to investigate unusual flying machines that look not like insects or birds but perhaps more like umbrellas, pyramids, UFOs, and jellyfish.

Watch the video.


Tuesday, February 4, 2014

Veronika Hubeny,University of Durham
From Black Holes to Fluid Holograms

This talk gives an overview of the Fluid/Gravity correspondence, which was developed 6 years ago in the context of the gauge/gravity duality. Mathematically, it posits that Einstien's equations of general relativity (with negative cosmological constant) in d+1 dimensions capture the (generalised) Navier-Stokes' equations of fluid dynamics in d dimen- sions. In particular, given an arbitrary fluid dynamical solution, we can systematically construct a corresponding black hole spacetime whose properties mimi that of the fluid flow. After reviewing the basic indredients, motivated partly by special properties of black holes, I will talk about the construction and implications of this remarkable correspondence.

Watch the video.


Tuesday, February 11, 2014

Herman Marshall, MIT Kavli Institute
A Soft X-ray Spectropolarimeter Telescope

We are developing instrumentation for a telescope capable of measuring linear X-ray polarization over a broad-band using conventional spectroscopic optics. Multilayer-coated mirrors are key to this approach, being used as Bragg reflectors at the Brewster angle. By laterally grading the multilayer mirrors and matching to the dispersion of a spectrometer, one may take advantage of high multilayer reflectivities and achieve modulation factors near 100% over the entire 0.2-0.8 keV band. We have a laboratory demonstration of the polarization of a pair of multilayer mirrors and will present progress on work to demonstrate the capabilities of laterally graded multilayer coated mirrors. We also present plans for a suborbital rocket experiment designed to detect a polarization level of <20% for an active galactic nucleus.

Watch the video.


Tuesday, February 18, 2014

No colloquium. Midterm Recess.


Tuesday, February 25, 2014

Christopher Rogan, Harvard University
Weakly Interacting Particles at the LHC: Searches for new forces, symmetries and dark matter

The Large Hadron Collider (LHC) is the world’s most powerful probe of the experimental high-energy frontier, where protons are accelerated and collided at energies previously inaccessible in a laboratory. These particle collisions are recorded and reconstructed by the CMS and ATLAS experiments, whose goals include trying to answer an array of open questions related to the nature of the dark matter that pervades our universe and whether there are new, un-discovered phenomena beyond the existing Standard Model (SM) of particle physics. Often, weakly interacting particles are a central part of these inquiries.

In this talk we will briefly review the CMS and ATLAS detectors, focusing on the elements of design that allow them to detect and study events with weakly interacting particles. The part that these ghostly particles play in models of physics beyond the SM, such as supersymmetry (SUSY), will be described along the strategies employed by the CMS and ATLAS experiments to discover them, illustrated through several examples of searches these new phenomena. Finally, the current experimental constraints on physics with new weakly interacting particles from Run I of the LHC will be summarized, along with some perspectives on the approaching Run II.

Watch the video.


Tuesday, March 4

No colloquium. APS Meeting.


Tuesday, March 11, 2014

Eisenbud Lecture Series in Mathematics and Physics--A Celebration of Leonard Eisenbud's 100th Birthday
Cumrun Vafa, Harvard University
Lecture 1 (4pm in Abelson 131): Strings and the Magic of Extra Dimensions

No video available.

Wednesday, March 12, 2014

Eisenbud Lecture Series in Mathematics and Physics--A Celebration of Leonard Eisenbud's 100th Birthday
Cumrun Vafa, Harvard University
Lecture 2 (11am in Abelson 333): Recent Progress in Toplogical Strings I
Lecture 3 (4pm in Abelson 229): Recent Progress in Topological Strings II

No video available.

Tuesday, March 18, 2014

No colloquium.


Tuesday, March 25, 2014

Bulbul Chakraborty, Brandeis University
The Physics of Sand: Emergent Behavior in the Macroworld

Abstract: Diversity in the natural world emerges from the collective behavior of large numbers of interacting objects. The origin of collectively organized structures over the vast range of length scales from the subatomic to colloidal is the competition between energy and entropy. Thermal motion provides the mechanism for organization by allowing particles to explore the space of configurations. This well-established paradigm of emergent behavior breaks down for collections of macroscopic objects ranging from grains of sand to asteroids. In this macro-world of particulate systems, thermal motion is absent, and mechanical forces are all important. We lack understanding of the basic, unifying principles that underlie the emergence of order in this world. In this talk, I will explore the origin of rigidity of granular solids, and present a new paradigm for emergence of order in these athermal systems.

Watch the video.


Tuesday, April 1, 2014

Katia Bertoldi, Harvard University
Performance through Deformation and Instability

Materials capable of undergoing large deformations like elastomers and gels are ubiquitous in daily life and nature. An exciting field of engineering is emerging that uses these compliant materials to design a active devices, such as actuators, adaptive optical systems and self-regulating fluidics. Compliant structures may significantly change their architecture in response to diverse stimuli. When excessive deformation is applied, they may eventually become unstable. Traditionally, mechanical instabilities have been viewed as an inconvenience, with research focusing on how to avoid them. Here, I will demonstrate that these instabilities can be exploited to design materials with novel, switchable functionalities. The abrupt changes introduced into the architecture of soft materials by instabilities will be used to change their shape in a sudden, but controlled manner. Possible and exciting applications include materials with unusual properties such negative Poisson’s ratio, phononic crystals with tunable low-frequency acoustic band gaps and reversible encapsulation systems.

Watch the video.


Tuesday, April 8, 2014

Daniel I. Goldman, Georgia Institute of Technology
Swimming in Sand

Abstract: Resistive force theory (RFT) is often used to analyze the movement of microscopic organisms that swim in true fluids. In RFT, a body is partitioned into infinitesimal segments, each which generates thrust and experiences drag as it moves through the medium with a given orientation and direction. Linear superposition of forces from elements over the body allows prediction of swimming kinematics and kinetics. We find that RFT works surprisingly well in dry granular media using empirically determined force-orientation relationships; within a given plane (horizontal or vertical) these relationships are functionally independent of the granular medium. It a variety of situations, RFT quantitatively models the below and above-surface locomotion of animals and robots that operate in the '' frictional fluid " regime (in which frictional forces dominate material inertial forces). In this talk I will discuss examples of granular RFT applied to subsurface swimming: these include prediction of muscle activation wave patterns in the sandfish lizard, elucidation of the benefits of a slender and slick body in desert dwelling reptiles, and, in combination with a geometric approach due to Shapere and Wilczek [PRL, 1987], the discovery of body undulation patterns which generate complex maneuvers, like turning in place.

Watch the video.


Tuesday, April 15, 2014

No colloquium. Spring Recess.


Tuesday, April 22

No colloquium. Spring Recess.


Tuesday, April 29, 2014
Michael J Naughton, Boston College
Nanoscale Coaxial Probes with Optical, Solar and Sensing Utility

We discuss a nanoscale coaxial architecture with potential utility in nanophotonics, photovoltaics, visual prosthetics, and biological, chemical and neuro sensing. As subwavelength optical waveguides, these nanostructures can be used in a range of nanoscale manipulations of light, including optical nanoscopy and lithography, high efficiency solar cells, high electrode-density retinal implants and discrete optical metamedia. A modification of the basic structure enables the fabrication of highly sensitive molecular sensors and high resolution optoelectronic neurostimulators/sensors (optrodes). We will report on aspects of these applications, including radial p-n junction "nanocoax" solar cells, and bio, electrochemical and neuro sensing.

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Fall 2014 Colloquia

Tuesday, September 2, 2014

Enectali Figueroa-Feliciano, MIT
What's the Matter with the Universe?

Abstract: Dark matter makes up 85% of the mass of the Universe, yet we know very little about what it is. We hunt for dark matter in an old iron mine half a mile underground using detectors operating only thousandths of a degree above absolute zero. I will present results from two experiments by the Cryogenic Dark Matter Search (CDMS) collaboration that are focused on light mass (< 10 GeV) dark matter, and discuss the future reach of this technology. Although the composition of the other 15% of the mass in the Universe is understood, plenty of questions about its origin and evolution remain. I will also introduce the Micro-X sounding rocket, a high-spectral resolution x-ray telescope which will study supernova remnants, the fascinating relics of stars whose explosive deaths gave birth to most of the atoms that form our planet.

Watch this video.

Tuesday, September 9, 2014

Matthew Headrick, Brandeis University
Quantum entanglement and the geometry of spacetime

Abstract: Recent developments have led to the discovery of a beautiful and surprising connection between the geometry of spacetime in quantum gravity and entanglement in quantum field theories. This discovery offers a new perspective on old puzzles concerning black holes, and may lead to a profoundly new way of thinking the emergence of spacetime from fundamental quantum-mechanical building blocks. I will describe these developments, explaining along the way the necessary background in general relativity, quantum field theory, and quantum information theory.

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Tuesday, September 16, 2014

Jeffrey J. Fredberg, Harvard School of Public Health
Collective migration and cell jamming

Abstract: Our traditional physical picture holds with the intuitive notion that each individual cell comprising the cellular collective senses signals or gradients and then mobilizes physical forces in response. Those forces, in turn, drive local cellular motions from which collective cellular migrations emerge. Although it does not account for spontaneous noisy fluctuations that can be quite large, the tacit assumption has been one of linear causality in which systematic local motions, on average, are the shadow of local forces, and these local forces are the shadow of the local signals. New lines of evidence now suggest a rather different physical picture in which dominant mechanical events may not be local, the cascade of mechanical causality may be not so linear, and, surprisingly, the fluctuations may not be noise as much as they are an essential feature of mechanism. Here we argue for a novel synthesis in which fluctuations and non-local cooperative events that typify the cellular collective might be illuminated by the unifying concept of cell jamming. Jamming has the potential to pull together diverse factors that are already known to contribute but previously had been considered for the most part as acting separately and independently. These include cellular crowding, intercellular force transmission, cadherin-dependent cell-cell adhesion, integrin-dependent cell-substrate adhesion, myosin-dependent motile force and contractility, actin-dependent deformability, proliferation, compression and stretch.

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Tuesday, September 23, 2014

No colloquium. Brandeis Thursday.

Tuesday, September 30, 2014

Mehran Kardar, MIT
Levitation by Casimir forces in and out of equilibrium

Abstract: A generalization of Earnshaw's theorem constrains the possibility of levitation by Casimir forces in equilibrium. The scattering formalism, which forms the basis of this proof, can be used to study fluctuation-induced forces for different materials, diverse geometries, both in and out of equilibrium. In the off-equilibrium context, I shall discuss non-classical heat transfer, and some manifestations of the dynamical Casimir effect.

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Tuesday, October 7, 2014

Christopher Rycroft, Harvard University
Modeling the toughness of metallic glasses

Abstract: Metallic glasses are a new type of alloy whose atoms form an amorphous structure in contrast to most metals. They have many favorable properties such as excellent wear resistance and high tensile strength, but are prone to breakage in some circumstances, depending on their method of preparation. The talk will describe the development of a quasi-static projection method within an Eulerian finite-difference framework, for simulating a new physical model of a metallic glass. The simulations are capable of resolving the multiple timescales that are involved, and provide an explanation of the experimentally observed differences in breakage strength.

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Tuesday, October 14, 2014

Dam Thanh Son, University of Chicago
Hydrodynamics and quantum anomalies

Abstract: Hydrodynamics is the theory describing collective behaviors of fluids and gases. It has a very long history and is usually considered to belong to the realm of classical physics. In recent years, it has been found that, in many cases, hydrodynamics can manifest a purely quantum effect --- anomalies. We will see how this new appreciation of the interplay between quantum and classical physics has emerged, unexpectedly, through the idea of gauge/gravity duality, which originates in modern string theory. I will briefly mention the possible relevance of the new findings to the physics of the quark gluon plasma.

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Tuesday, October 21, 2014

Matthew Reece, Harvard University
After the Higgs: What's Next for Particle Physics?

Abstract: The Large Hadron Collider's discovery of the Higgs boson in 2012 placed the capstone on the Standard Model of particle physics. In many ways, the theory is complete. Still, we have important unanswered questions. What is dark matter? Why is there more matter than antimatter? Do we live in a fine-tuned universe? I will discuss how the upcoming LHC run at higher energy, terrestrial and astrophysical experiments probing the nature of dark matter, and possible future high precision or high energy collider experiments could help us find answers to these questions.

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Tuesday, October 28, 2014

Raymond Brock, Michigan State
That Spin 0 Boson Changes Everything--The Future of the Energy Frontier in Particle Physics

Abstract: The "Higgs Boson" discovery requires us to think differently about planning for the future of Particle Physics. While the decades-long confirmation of the Standard Model itself an historic episode, as a dynamical model of nature it is unhelpful as a clear guide to the future. I’ll review the features of the Standard Model that make it superb, I’ll point out why it’s frustrating, and I’ll describe the hints that motivate us in the coming decades.

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Tuesday, November 4, 2014

Parthasarathi Majumdar, Ramakrishna Mission Vivekanandan University
The Quantum and the Continuum: Einstein's dichotomous legacies

Abstract: This talk begins with a summary of some of Einstein’s seminal contributions in the quantum domain, like Brownian motion and the Light Quantum Hypothesis, as well as on the spacetime continuum enshrined in the theories of special and general relativity. We then attempt to point to a possible dichotomy in his thinking about these two apparently disparate aspects of physics, which must have been noticed by him, but was not much discussed by him in the public domain. One may speculate that this may have had something to do with his well-known distaste for the probability interpretation of quantum mechanics as a fundamental interpretation. We argue that theorems ensuing from Einstein’s general relativity theory itself contain the seeds of a dramatic modification of our ideas of the Einsteinian spacetime continuum, thus underlining the dichotomy even more strongly. We then survey one modern attempt to resolve the dichotomy, at least partly, by bringing into the spacetime continuum, aspects of quantum mechanics with its underlying statistical interpretation, an approach which Einstein may not have thoroughly enjoyed, but which seems to work so far, with good prospects for the future.

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Tuesday, November 11, 2014

Seth Fraden, Brandeis University
Testing Turing's Theory of Morphogenesis

Abstract: A single fertilized egg first divides into two identical cells and then repeats that process producing hundreds more identical cells. Yet cells in mature animals are not identical; we have a head and tail, eyes and ears. In 1952 Alan Turing offered the first theory explaining cellular differentiation in his seminal paper, The Chemical Basis of Morphogenesis. Turing’s genius was the creation of a minimal model that raised very specific questions and has guided development biology even to this day. It turns out that biology does development somewhat differently than Turing envisioned. This raises the question as to whether or not any system exists in which Turing’s vision is realized. I’ll describe Turing’s model and an experimental system developed at Brandeis that is ideally suited for testing Turing's ideas in synthetic “cells” consisting of microfluidically produced emulsions. The Turing model is regarded as a metaphor for morphogenesis in biology; useful for a conceptual framework and to guide modeling, but not for prediction. In this chemical system, we quantitatively assess the extent to which the Turing model explains both pattern formation and temporal synchronization of chemical oscillators. I’ll describe my lab’s recent demonstration that chemical morphogenesis drives physical differentiation in synthetic cells and speculate on what technologies the future holds.

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Tuesday, November 18, 2014

Ian Hutchinson, MIT
Physics of Fusion Energy; What we know and what we don't know

Abstract: This talk will present an overview of the plasma physics that;determines whether or not we can make fusion, the energy sourse of the stars, a practical reality on the terrestrial scale. The main focus is on magnetic plasma confinement, in which we now know a very great deal;in a range of physics fields including MagnetoHydrodynamics, plasma collisionless heating and sustainment, cross-field transport, and the plasma boundary and materials interactions. There are several grand physics challenges that remain to be solved, and even more engineering challenges.

Watch this video.

Tuesday, November 25, 2014

No colloquium. Thanksgiving week.

Tuesday, December 2, 2014

Eisenbud Lectures in Mathematics and Physics
Peter Sarnak, Inste. for Advanced Study and Princeton University
Randomness in number theory and geometry

Abstract: The behavior of many arithmetic and geometric objects, from the zeros of zeta functions to the the topologies of random real algebraic varieties are apparently dictated by models from statistical physics. We will review some of these and highlight the basic conjectures and some of what is known towards them .

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Wednesday, December 3, 2014

Eisenbud Lectures in Mathematics and Physics, Lecture II
Peter Sarnak, Inste. for Advanced Study and Princeton University
Nodal domains for Maass (modular) forms

Abstract: The eigenstates of the quantization of a classically chaotic hamiltonian are expected to behave like random monochromatic waves .We discuss this in the context of the eigenfunctions on the modular surface -- i.e "Maass Forms ", and especially what can be proved about their nodal domains.

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Thursday, December 4, 2014

Eisenbud Lectures in Mathematics and Physics, Lecture III
Peter Sarnak, Inste. for Advanced Study and Princeton University
Families of zeta functions, their symmetries and applications

Abstract: The local statistical laws for the distribution of the zeros of the Riemann Zeta function and more generally of families of zeta functions ,follow one of 4 of the 10 universal random matrix ensembles. We review some this phenomenon ,especially in connection with applications.

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Spring 2015 Colloquia

Thursday, February 5, 2015
Special Colloquium at 1pm in Abelson 131

Karen Kasza , Sloan-Kettering Institute
S patiotemporal control of the active forces that shape living tissues

Abstract: The ability of multicellular tissues to physically change shape, move, and grow is a key feature of life. These behaviors are often accomplished by local movements or rearrangements of cells within the tissues. Many cell movements are actively driven by contractile forces generated in cells by the motor protein myosin II. During embryonic development, these forces are patterned to orient cell movements, resulting in changes in tissue shape and structure that build functional tissues and organs. To uncover how force-generation by myosin drives cell movement and determines the physical behavior of tissues, I use the fruit fly embryo as a model system, where polarized patterns of myosin activity orient cell movements and rapidly elongate the embryo. I will discuss how studying
embryos generated with engineered myosin variants allows us to dissect mechanisms underlying tissue behavior. In particular, I will describe how myosin variants with enhanced activity accelerate cell movement but, surprisingly, also alter the spatial pattern of forces and result in reduced tissue elongation. These experiments reveal that the levels and patterns of forces are controlled by the same biological cue and suggest a trade-off between the speed and orientation of cell movements within tissues. These studies of how forces shape the fruit fly embryo shed light not only on physical principles at work in active, living materials but also on how defects in cell movements contribute to human birth defects and tumor metastasis.

Watch the video .

Tuesday, February 10, 2015

Scott Forth, Rockefeller University
Examining the Mechanics of Dynamic Microtubule Networks

Abstract: Cells utilize dynamic biopolymer networks to carry out mechanical tasks during diverse processes such as cell division, migration, and development. For example, the microtubule-based mitotic spindle performs the physical work of segregating duplicated chromosomes into two daughter cells. The components that make up these networks are not typically at equilibrium, but instead operate under load. The mitotic spindle achieves a ‘steady state’ size and shape despite the components that make up the dense network undergoing constant motion, experiencing large forces, and turning over on rapid timescales. It has been challenging to understand how the forces that move microtubules and chromatin are regulated within these micron-sized dynamic networks by groups of diverse nanometer-sized proteins. Dr. Forth will present his postdoctoral work that directly addresses these problems by employing single molecule biophysical methods, including optical trapping and TIRF microscopy. His work reveals that proteins involved in cell division can generate frictional resistance as they move along microtubules, and that this friction can be harnessed within spindles to help ensure successful DNA segregation. Additionally, he has shown that motor proteins that push microtubules can also generate braking forces that scale with microtubule sliding velocity. Together these results can help us link the biophysical properties of essential proteins with their function in dividing cells.

Watch the video.

Thursday, February 12, 2015
Special colloquium, 1pm in Gzang 122

Wylie Ahmed, Institute Curie
Active mechanics in living oocytes reveals molecular - scale kinetics

Abstract: Living cells actively generate internal forces at the molecular scale to coordinate intracellular organization and transport. This requires force generation by molecular motors (myosin, kinesin,dynein) to drive motion and organization in the crowded intracellular environment. During meiosis,unfertilized eggs (i.e. oocytes) must actively position their nuclear structures to facilitate division for healthy development. This requires precise spatial and temporal coordination of molecular motors and the cytoskeleton. However, precise quantification of molecular motor force kinetics in their native environment is lacking. Here we show that force kinetics of myosin-V in living oocytes is revealed by measurement and modeling of active fluctuations and intracellular mechanics. We find that molecular force generation by myosin-V drives the oocyte cytoplasmic-skeleton out-ofequilibrium and actively softens the environment. Our results reveal that in-vivo myosin-V in oocytes generates 0.4 pN of force, with a power-stroke duration of 300 ms, and drives vesicle motion at 320 nm/s. Our results demonstrate that active forces generated in a living cell can tune intracellular mechanical properties and that in-vivo force kinetics at the molecular level can be extracted using our experimental and theoretical framework.

No video available.

Tuesday, February 17, 2015
Note time: 12noon in Abelson 131

Al Sanchez, Harvard University
Non linear dynamics and phase space behavior of interacting
biological components: From genes to ecosystems

Abstract: Biology operates across a very wide range of scales and levels of organization: molecules organize into cells, cells organize into organisms, and organisms organize into ecosystems. One of the main goals of biological physics is to find general laws and organizing principles that govern the behavior of biological systems across these scales. In this talk I will overview my recent work characterizing the dynamics and phase space behavior of two seemingly very different types of biological systems: gene networks and microbial ecosystems. In spite of their very different levels of organization, we found that many ecological and genetic networks are described by the same class of bifurcations and thus share similar dynamic behavior. First I will discuss our recent work on the behavior of gene networks near a critical bifurcation. By experimentally mapping their phase space dynamics, we found many surprising features of these systems. For instance, when they operate close to their critical point, cellular metabolic switches could be turned on and off not only by their specific biochemical signals, as the common textbook models suggest, but also by a large number of universal stressors such as high temperature. In the second part of this talk, I will discuss how we experimentally mapped the phase space dynamics of an ecological "cooperator-freeloader" ecosystem. Visual inspection of this phase space reveals the fate of microbial cooperator populations when they are invaded by freeloaders, and it also tells us how this invasion alters ecosystem level properties such as its productivity and resilience. Finally, we found that this ecosystem is also described by a catastrophic bifurcation, and that critical slowing down can forecast its proximity to the tipping point.

No video available.

Tuesday, February 24, 2015

Xuefeng Wang , University of Illinois, Urbana-Champaign
Cellular Forces Measured and Controlled by DNA-based Molecular Force Sensor & Modulator

Abstract: Mammalian cells are remarkable force processors. Cells adhere through membrane protein integrins and generate forces on integrins to probe the local environment. These forces regulate many fundamental cellular functions such as cell adhesion, proliferation, migration, and ultimately stem cell differentiation and cancer progression. Because of its critical importance, integrin force has long been a central topic in the field of cell mechanics. To study integrin forces at the molecular level, I developed a DNA-based force sensor and modulator termed tension gauge tether (TGT) which quantitatively reports and controls cellular forces on integrin molecules. Using TGT, I systematically studied the ranges and physiological roles of integrin forces and discovered two distinct force regimes: cell membrane generates ~40 pN molecular force on integrins to regulate cell adhesion and spreading, while actomyosin generates >54 pN molecular force on integrins to regulate cell polarization and migration. This work demonstrated the versatility of integrins and shed light on the mechanism how cells vary integrin forces to regulate different cellular functions. TGT was also applied to study other mechano-sensitive proteins such as cadherins and Notch receptors. Overall, TGT provides a novel avenue for the study of cell mechanics at the molecular level. In the future, I will apply TGT to study a series of mechanical–involved cellular processes such as kinase activation, durotaxis, stem cell differentation and endocytosis.

Watch the video .

Thursday, February 26, 2015
Special Colloquium, 1pm in Abelson 131
Benjamin Rogers , Harvard University
Sculpting phase diagrams: freezing by heating, switchable crystals, and more

Abstract: Physicists and chemists have long searched for ways to turn disordered states of matter (fluids) into ordered states (crystals). We know from everyday experience that such transitions can be triggered through changes in temperature: water freezes to become ice, and crystals like rock candy form when a hot solution cools. Colloidal suspensions--solutions of particles with diameters ranging from tens of nanometers to micrometers--also exhibit strikingly similar phase transitions under the right circumstances. In this talk, I will present experiments showing that these colloidal phase transitions can be controlled in unfamiliar ways. Using grafted DNA strands to induce specific attractions between the particles, and free DNA strands that compete to bind with the grafted ones, I will show that it is possible to create suspensions with exotic phase behavior, such as arbitrarily wide gas-solid coexistence, re-entrant melting, and even reversible transitions between different crystal phases. This work could prove especially useful in nanotechnology, where a central goal is to manufacture materials and devices by growing them directly from solution.

Watch the video .

Tuesday, March 3, 2015

No colloquium. APS Meeting.

Monday, March 9, 2015

Dustin Kleckner , University of Chicago
Vortex Knots in Fluids and Superfluids
Note time and place: 1pm in Gzang 122

Abstract: Linked and knotted field lines appear as important features in many physical fields, e.g., tangled vortex lines in turbulent flows or braided loops of plasma on the surface of the sun. In idealized examples, the topology (knottedness) of these field lines may never change, and is associated with a conserved quantity. Real dissipative fields, however, go through ‘reconnections’ which may untie knots and raise difficult questions about the role of topologically conserved quantities. I will describe novel techniques we have developed to generate vortex knots in experimental fluids and simulated superfluids. Our first glimpses into the behavior of these elemental knots show that their dynamics can be understood in terms of system-independent geometric principles.

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Tuesday, March 10, 2015

Markus Deserno, Carnegie Mellon Physics
Measuring Elastic Membrane Properties in Computer Simulations
Host: Prof. Michael Hagan

Abstract: One of the fascinating aspects of fluid lipid membranes is that on length scales not much bigger than their thickness they can be described exceedingly well by a completely geometric Hamiltonian. Such a continuum-level theory features two main input parameters, namely the moduli describing curvature elasticity. These are called "(mean) bending modulus" and "Gaussian curvature modulus", and their values are not predicted by the effective macroscopic theory. Instead, one either measures them in experiment, or one conducts computer simulations of realistic or coarse-grained membrane models which aim at extracting these values. In this talk I will present novel ways for doing the latter. I show that actively buckling a membrane leads to a clean and accurate signal for the mean bending modulus. The very beautiful and ancient theory of Euler Elastica helps to analyze the results, but we have to go beyond it when the membrane enters a gel phase. For the Gaussian modulus one needs to find a way to beat the insidious Gauss-Bonnet theorem, and a protocol that monitors the closure of open vesicle patches permits exactly that. I will also show how our results sometimes fit into existing knowledge and guesses, and how they sometimes trigger perplexing questions for which we do not know the answer yet.

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Tuesday, March 17, 2015

Robert J. Wood, Harvard University
Manufacturing, actuation, sensing, and control for robotic insects
Host: Prof. Aparna Baskaran

Abstract: As the characteristic size of a flying robot decreases, the challenges for successful flight revert to basic questions of fabrication, actuation, fluid mechanics, stabilization, and power - whereas such questions have in general been answered for larger aircraft. When developing a robot on the scale of a housefly, all hardware must be developed from scratch as there is nothing "off-the-shelf" which can be used for mechanisms, sensors, or computation that would satisfy the extreme mass and power limitations. With these challenges in mind, this talk will present progress in the essential technologies for insect-scale robots and the first flight experiments with robotic insects.

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Tuesday, March 24, 2015

Sanjoy Mahajan, MIT
Flipping the classroom to improve STEM learning
Host: Prof. Jane' Kondev

Abstract: A distressing amount of evidence indicates that our students learn little of what we teach them, having great difficulty transferring their knowledge to new problems and situations. I will give examples. A remedy is to flip the classroom---to ask students to start learning out of class and to use the classroom to foster higher-level learning. This valuable approach, enabled first by Gutenberg in the 15th century with the development of cheap books, can be enhanced with 21st-century digital technology (including for MOOCs). I will discuss a framework based on cognitive psychology, the ICAP framework, which is valuable for planning flipped classrooms that foster deep learning. I will illustrate the framework with examples from teaching mathematics, electrical engineering, and physics. In keeping with the first "O" (open) in MOOC, I will illustrate the framework and its digital implementation with freely licensed, nonproprietary tools, available for all to use, improve, and share.

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Tuesday, March 31, 2015

Daniel Prober, Yale University
Graphene Nanobolometers for Ultrasensitive Far-Infrared Detection
Host: Prof. Bulbul Chakraborty

Abstract: Graphene has recently been proposed as an ultrasensitive THz photon detector for space-based astronomy observations. We have studied the thermal properties of monolayer graphene for this application, and done extensive modeling of the detection processes. One would employ superconducting contacts to achieve energy confinement in the graphene. Recently we have studied experimentally the energy loss processes in graphene down to T = 0.1 K. The space-based observatories that could employ such detectors will be discussed, as well as the science that can be done with these observatories.

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Tuesday, April 7, 2015

No colloquium. Passover/Spring Recess.

Tuesday, April 14, 2015

Paul Townsend, Cambridge University
Host: Albion Lawrence
What happened to Einstein's Brane?

Abstract: In 1905 Einstein abolished the "luminiferous aether’’ of 19th century electromagnetic theory; in his new theory (which came to be known as Special Relativity) it was an unnecessary hypothesis. However, that doesn’t mean that it doesn’t exist! To be consistent with Special Relativity, an "aethereal’’ medium must have a tension (negative pressure) that is equal to its energy density. In 1917, two years after presenting his theory of General Relativity in its final form, Einstein proposed (although it took Lemaitre to see this) that the vacuum is precisely such a medium; it’s tension is Einstein’s "cosmological constant’’ and its energy, now called ``dark energy’’, appears to constitute (according to current consensus) about 70% of the energy of the observable universe. But what does this have to do with electrodynamics? General Relativity is contained in string theory, and the various possible string theories are unified by M-theory. The constituents of M-theory, as currently understood, are extended objects of various dimensions known as "branes’'. Their characteristic feature is a tension equal to their energy density, and photons are disturbances of some (effective) 3-brane, which is therefore the ``luminiferous aether’’. Dark matter `lives’ on other branes, and the cosmological constant has an interpretation as 3-brane tension. We may be living on Einstein's brane! His brain remains in a jar of formaldehyde. This talk will survey some of the basic physics of strings and branes, with an attempt to put them into an Einsteinian context.

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Tuesday, April 21, 2015

Klaus Schmidt-Rohr, Brandeis University
Dynamics and Nanostructure of Polymer Materials from NMR and Scattering Analysis

Abstract: The properties of polymer materials, including ultradrawn fibers and fuel-cell membranes, are strongly affected by their nanometer scale structure and molecular motions. Since these systems exhibit a large degree of disorder, diffraction methods are of limited use, and advanced solid-state nuclear magnetic resonance (NMR) techniques, complemented by small-angle scattering analysis, can provide unique insights. Surprising large-amplitude motions of chains in crystallites of many polymers have been identified as thermally activated helical jumps, using two-dimensional exchange NMR. The concomitant displacements of the chains along their axes result in chain diffusion between crystalline and amorphous regions. This affects macroscopic properties such as creep and ultradrawability. -- In hydrated Nafion, a fluoropolymer that is the benchmark material for proton exchange membranes in H2/O2 fuel cells, we have investigated the long contentious nanometer-scale structure. 13C and 19F NMR results have highlighted the significant stiffness of the perfluorinated polymer backbone. Using an algorithm based on 3D numerical Fourier transformation that yields the scattered intensity I(q) for any model implemented on a cubic lattice, small-angle scattering data of hydrated Nafion from the literature have been quantitatively simulated. The characteristic ”ionomer peak” is attributed to long, locally parallel but otherwise randomly packed water channels of ~2.4-nm diameter surrounded by the partially hydrophilic sidebranches, forming inverted-micelle cylinders. Simulations for a dozen other models of Nafion do not match the scattering data. The water-channel model is the first without constrictions of ~1.2 nm diameter and can explain the fast diffusion of water and protons through Nafion.

Fall 2015

Fall 2015 Colloquia

Tuesday , September 8, 2015

Jesse Thaler (MIT)
The Rise of Jet Substructure: Boosting the Search for New Physics at the LHC
Host: Albion Lawrence

Watch the video.

Tuesday, September 15, 2015

No colloquium (Rosh Hashanah).

Tuesday, September 22, 2015

Michael Rubinstein (UNC)
Airway Surface Brush Sweeps Lungs Clean: Polymer Physics Helps Us Breathe Easier
Host: Zvonimir Dogic

Abstract: The classical view of the airway surface liquid (ASL) is that it consists of two layers – mucus and periciliary layer (PCL). Mucus layer is propelled by cilia and rides on the top of PCL, which is assumed to be a low viscosity dilute liquid. This model of ASL does not explain what stabilizes the mucus layer and prevents it from penetrating the PCL. I propose a different model of ASL in which PCL consists of a dense brush of mucins attached to cilia. This brush stabilizes mucus layer and prevents it penetration into PCL, while providing lubrication and elastic coupling between beating cilia. Both physical and biological implications of the new model will be discussed.

Watch the video .

Tuesday, September 29, 2015

No colloquium (Brandeis Monday).

Tuesday, October 6, 2015

Markus Greiner (Harvard University)
Measuring Entanglement Entropy in Synthetic Quantum Matter

Abstract: With quantum gas microscopy we are now able to take the control of ultra cold quantum gases in an optical lattice to the next and ultimate level of high fidelity: addressing, manipulation and readout of single particles. In my talk I will first give an introduction to this field of research and present an overview of recent experiments. I will then focus on presenting experiments in which we are for the first time able to directly measure entanglement entropy in a quantum many-body system.

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Host: Matthew Headrick

Tuesday, October 13, 2015

Karsten Heeger (Yale)
Understanding the Nature of Neutrinos: Recent Discoveries and Future Prospects

Abstract: The discovery of neutrino mass and oscillations have opened an intense field of study in the properties of neutrinos and their role in the Universe but many open questions remain. Recently the Daya Bay reactor experiment observed the oscillation of antineutrinos over kilometer-scale baselines and opened the window to the study of CP violation in the lepton sector. The search for neutrinoless double beta decay with CUORE will probe if neutrinos are their own antiparticles and probe the effective mass of neutrinos. New experiments are getting underway to study neutrino oscillation at short and long baselines and search for signs of new physics in the neutrino sector. I will review recent results and future prospects for neutrino studies with reactor neutrinos and in double beta decay.

Watch the video .

Host: Gabriella Sciolla

Tuesday, October 20, 2015

No Colloquium due to overlap with Gabbay Award talk

Eisenbud Lectures in Mathematics and Physics
October 27 - October 29

Tuesday, October 27, 2015

Abelson 131
Jeffrey Harvey
, University of Chicago
Eisenbud Lectures in Mathematics and Physics
A physicist under the spell of Ramanujan and moonshine

Watch the video .

Wednesday, October 28, 2015

Abelson 131
Jeffrey Harvey , University of Chicago
Eisenbud Lectures in Mathematics and Physics
Mock modular forms in mathematics and physics

Watch the video .

Thursday, October 29, 2015

Abelson 333
Jeffrey Harvey , University of Chicago
Eisenbud Lectures in Mathematics and Physics
Umbral Moonshine

 Watch the video.

Tuesday, November 3, 2015

Cora Dvorkin (Harvard University)
Fingerprints of the Early Universe
Host: Gabriella Sciolla

Abstract: Cosmological observations have provided us with answers to age-old questions, involving the age, geometry, and composition of the universe. However, there are profound questions that still remain unanswered. The origin of the small anisotropies that later grew into the stars and galaxies that we see today is still unknown. In this talk, I will explain how we can use measurements of the Cosmic Microwave Background, which was last scattered when the universe was 380,000 years old, to reconstruct the detailed physics of much earlier epochs, when the universe was only a tiny fraction of a second old. I will also discuss the potential of current and upcoming measurements of the large-scale structure of the universe to further constrain the physics underlying inflation.

Watch the video.

Tuesday, November 10, 2015

Steve Whitelam (Lawrence Berkeley National Lab)
Joint Quantitative Biology/Martin Weiner Lecture Series
Physics Department Colloquium
Geometry and symmetry in nanomaterial self-assembly
Host: Michael Hagan

Watch the video.

Tuesday, November 17, 2015

Tom Powers (Brown University)
Hydrodynamics of Swimming Microorganisms in Complex Fluids
Host: Zvonimir Dogic

Abstract: Since fluid mechanics at the scale of the cell is dominated by viscosity, swimming microorganisms use drag for propulsion. While there has been much work on the mechanics of swimming in water at micron scale, many microorganisms usually encounter complex fluids such as mucus, which are full of polymers. I will address the emerging area of swimming in complex fluids. We use theory and simple scale-model experiments to study how viscoelasticity affects the swimming speed of swimmers with simple illustrative stroke patterns, such as small-amplitude traveling waves and rigid-body rotation of helices. We also study swimming mechanics in anisotropic media such as liquid crystals. We find that the nature of anchoring conditions for the liquid-crystalline degrees of freedom plays a critical role in determining the swimming speed. Furthermore, we study the fluid transport induced by the swimmers motion by calculating the flux of fluid in the laboratory frame.

Watch the video.

Tuesday, November 24, 2015

No colloquium (Thanksgiving week).

Tuesday, December 1, 2015

Joern Dunkel (MIT)
Pattern formation in soft and biological matter
Host: Zvonimir Dogic

Watch the video.

Tuesday, December 8, 2015

Ariel Amir (Harvard University)
Getting in Shape: how do microorganisms control their geometry?
Host: Zvonimir Dogic

Abstract: Microorganisms such as bacteria and budding yeast are remarkably successful in accurately self-replicating themselves within several tens of minutes. How do cells decide when to divide? How do they control their morphology? I will show how ideas from statistical mechanics and materials science can help answer these questions. In particular, I will show how a stochastic model of cell size control, combined with single cell data, can be used to infer a particular strategy for cell size control in bacteria and budding yeast, and how the theory of elasticity can be utilized to understand the coupling of mechanical stresses and cell wall growth in bacteria.

Watch the video.

Spring 2016

Spring 2016 Colloquia

Tuesday, January 19, 2016

Leonid Mirny (MIT)
Genome in 3D: some physical considerations
Host: Zvonimir Dogic


DNA of the human genome is 2m long and is folded into a structure that fits in a cell nucleus. One of the central physical questions here is the question of cross-scale communication: How can molecules a few nanometers in size control chromosome geometry and topology at micron length scales? Recently developed Chromosome Conformation Capture technique (Hi-C) provides comprehensive information about frequencies of spatial interactions between genomic loci. Inferring principles of 3D organization of chromosomes from these data is a challenging biophysical problem. We develop a top-down approach to biophysical modeling of chromosomes. Starting with a minimal set of biologically motivated interactions we build polymer models of chromosome organization that can reproduce major features observed in Hi-C experiments. I will present our work on modeling organization of human metaphase and interphase chromosomes. Our works suggests that active processes of loop extrusion can be a universal mechanism responsible for formation of domains in interphase and chromosome compaction in metaphase.

Tuesday, January 26, 2016

Ravi Radhakrishnan (University of Pennsylvania)
Can Soft Signals Be Oncogenic
Joint Quantitative Biology/Martin Weiner Lecture Series
Physics Department Colloquium
Host: Michael Hagan

Abstract: There are emerging links between the stiffness of the tissue microenvironment and the tumorogenicity in several tumors of soft tissues, thereby bringing to light the importance of how cells transduce mechanical signals to alter signals and cell fate. This talk will focus on molecular and subcellular mechanisms of curvature induction and sensing in cell membranes by a novel class of membrane remodeling proteins. I will discuss how thermally induced membrane undulations can couple to the induced curvature field by such proteins thereby providing a mechanism for curvature focusing. The curvature-undulation coupling also leads to a mechanism for long-range curvature sensing whereby such proteins can migrate toward preferred curvature locations at distances much larger than their size. Consistent with in vitro biophysical as well as cellular experiments, the curvature sensing/generating proteins can also be shown to be exquisite sensors of membrane tension thereby representing an important class of transducers of mechanical signals. I will describe a theory guided set of experiments, which demonstrate how such proteins can initiate and sustain survival signaling pathways that are initiated solely by physical stimulus and without any biochemical cues. We hypothesize that such survival mechanisms can be significant in enhanced survival of cells under conditions of altered mechanics (such as stiffness) of the tumor microenvironment.

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Tuesday, February 2, 2016

Zvonimir Dogic (Brandeis)
Spontaneous flow in active fluids

The laws of equilibrium statistical mechanics impose severe constraints on the properties of conventional materials assembled from inanimate building blocks. Consequently, such materials cannot exhibit spontaneous motion or perform macroscopic work. Inspired by biological phenomena such Drosophila cytoplasmic streaming, our goal is to develop a new category of soft active materials assembled from the bottom-up using animate, energy-consuming building blocks such as kinesin molecular motors and microtubule filaments. Released from the constraints of the equilibrium, these internally driven gels, liquid crystals and emulsions are able to change-shape, crawl, flow, swim, and exert forces on their boundaries to produce macroscopic work. In particular we describe properties of an active fluid that upon confinement transitions from a quiescent to a spontaneously flowing state. We characterize the properties of the emergent flows as well as how the transition to a flowing state depends on the properties of the confining geometry. Our results illustrate how active matter can serve as a platform for testing theoretical models of non-equilibrium statistical mechanics, developing new microfluidic applications and potentially even shedding light on self-organization processes occurring in living cells.

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Tuesday, February 9, 2016

Dan Pomeroy (MIT)
From Higgs Discovery to the Halls of Congress: How Scientists Can Engage In Public Policy
Host: Craig Blocker


National policy decisions increasingly involve complex issues with strong technological and scientific components. Scientists can play an important role in informing public policy both from within academia and by pursuing a career in public policy. This talk will provide examples of how a scientist may engage in public policy as well as a detailed discussion of the role of a science policy advisor to a United States Senator.

Tuesday, February 16, 2016

No colloquium (Midterm Recess).
 Tuesday, February 23, 2016

Aram Harrow (MIT)
What should we do with a small quantum computer?
Host: Matthew Headrick

A large-scale quantum computer would be able to solve problems that existing classical computers would take much longer than the age of the universe to solve. This would have dramatic implications for cryptography, chemistry, material science, nuclear physics and probably other areas that are still unknown. But what about quantum computers that will be available in the next few years? Experimentalists working with ion traps and superconducting qubits have plans to build quantum computers with 50-100 qubits capable of performing some thousands of quantum gates. The company D-Wave is already selling devices with over 1000 qubits, although they can only run a single algorithm (the adiabatic algorithm) and they suffer high rates of noise.In this talk, I will analyze two algorithms that can be run on current and near-term quantum computers. First I will look at the adiabatic algorithm, which has shown promise in its ability to use quantum tunneling to solve optimization problems more efficiently than classical local search. Here I will show that a different classical algorithm can simulate the adiabatic algorithm in these cases, suggesting that this is not a promising approach to quantum speedup. Second, I will look at a recently proposed Quantum Approximate Optimization Algorithm (QAOA), which is a method of performing combinatorial optimization using very few gates. I will show that conjectures from complexity theory imply that this algorithm cannot in general be simulated by classical computers.

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Tuesday, March 1, 2016

Marcelle Soares-Santos (Fermilab)
The Dark Energy Survey and Gravitational Waves
Host: Gabriella Sciolla

Abstract: DES is an ongoing imaging sky survey, the largest such survey to date. Its main science goal is to shed light onto dark energy by making precision measurements of the expansion history and growth of structure in the universe. In this talk I present our latest results and introduce a new DES initiative: searches for optical counterpart of gravitational wave events.

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Tuesday, March 8, 2016

Aditi Mitra (NYU)
Quantum Quenches
Host: Albion Lawrence

Abstract: The non-equilibrium dynamics of isolated quantum systems following a "quench" of some parameter of the Hamiltonian raises many fundamental questions, that can now even be probed in experiments. I will first give an overview of the topic and the status of experiments. I will then show how quantum quenches in the vicinity of a critical point can lead to universal out of equilibrium dynamics with new critical exponents that are not related to thermodynamic critical exponents. I will also highlight what quantum information measures such as entanglement entropy and entanglement spectrum reveal about a system which is out of equilibrium following a quantum quench, applying them both to systems near critical points, as well as to systems with topological order.

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Tuesday, March 15, 2016

No colloquium (APS).
Tuesday, March 22, 2016

Paul Mackenzie (Fermilab)
What's going on inside of a proton? The story of lattice QCD
Host: Albion Lawrence

Abstract: The forces between quarks inside a proton are much stronger than the other forces known in fundamental physics, so strong that the perturbative approximation methods that work well for the other forces don’t work for the strong interactions. They can be solved to high accuracy using large-scale computer simulations. The history of lattice QCD has been closely intertwined with the history of modern supercomputing, and I’ll also tell part of this story. Lattice calculations play a critical role in teasing out the fundamental properties of the quarks from the observed properties of the particles containing them. They are also critical in understanding the observed properties of nucleii in terms of fundamental physics.

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Tuesday, March 29, 2016

Vladan Vuletic (MIT)
What does the Golden Ratio have to do with friction? An answer atom by atom
Host: Matthew Headrick

Abstract: Friction is the basic, ubiquitous mechanical interaction between two surfaces that results in resistance to motion and energy dissipation. In spite of its technological and economic significance, our ability to control friction remains modest, and our understanding of the microscopic processes incomplete. To test long-standing atomistic models of friction processes at the nanoscale, we implemented a synthetic nanofriction interface using laser cooled ions subject to the periodic potential of an optical standing wave. We show that stick-slip friction can be tuned from maximal to nearly frictionless via arrangement of the atoms relative to the periodic potential, and that friction at the nanoscale can substantially differ from the simple phenomenological laws observed at the macroscale.

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Tuesday, April 5, 2016

Matthew Evans (MIT)
Gravitational Wave Detection with Advanced LIGO
Host: Matthew Headrick

Abstract: The Laser Interferometer Gravitational-wave Observatory (LIGO) recently made the first direct detection of gravitational waves; minute distortions in space-time caused by cataclysmic events far away in the universe. I will talk about the source of the signal we detected, the physics behind the detectors, and prospects for the future of this emerging field.

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Tuesday, April 12, 2016

Jeremy England (MIT)
Emergent Fine-tuning to Environmental Drives in a Random Chemical Mixture
Host: Michael Hagan

Abstract: The steady-state behavior of an undriven mixture of reaction chemical species is the equilibrium point where the concentrations obey a simple exponential relationship to free energy. Once external environmental drives are introduced, however, steady-state concentrations may deviate from these equilibrium values via processes that require sustained absorption and dissipation of work. From a physical standpoint, the living cell is a particularly intriguing example of such a nonequilibrium system because the environmental work sources that power it are relatively difficult to access – only the proper orchestration of many distinct catalytic actors leads to a collective behavior that is competent to harvest and exploit available metabolites. Here, we study the dynamics of an in silico chemical network with random connectivity in a driving environment that only makes strong chemical forcing available to rare combinations of concentrations of different molecular species. We find that the long-time dynamics of such systems are typified by the spontaneous extremization of forcing, so that the molecular composition converges on states that exhibit exquisite fine-tuning to available work sources.

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Tuesday, April 19, 2016

Gregory Grason (UMass Amherst)
Fiber-forming filaments figure out frustration: how impossible packing shapes the assembly of chiral filaments
Host: Zvonimir Dogic

Abstract: Filament assemblies are basic structural motifs of diverse materials, from macroscopic materials (textiles, cables) to the nanostructure assemblies that compose living matter (filamentous proteins). Yet, the highly non-trivial rules that govern packing and higher-order assembly of one-dimensional elements are largely unknown, arguably lagging decades, if not centuries, behind our knowledge of sphere-packing models of matter. In this talk, I will discuss the “metric” constraints imposed on dense bundles by the complex geometries (i.e. twist, bend) realized in self-organized assemblies. I will describe a surprising geometric connection between filament packing and the packing problem on non-Euclidean surfaces (e.g. the Thompson problem) that has critical implications for the structure and thermodynamics of “self-spinning” rope-like assemblies chiral filaments, a basic model of filamentous protein assembly, from extracellular bundles to amyloid fibers.

Tuesday, April 26, 2016

No colloquium (Passover and spring recess).