Department Colloquia
Martin Weiner Lecture Series
Department of Physics Colloquium
4:00pm, Abelson 131
Refreshments at 3:30pm outside Abelson 131
Fall 2011 Colloquia
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.
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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!
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.
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.
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.
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.
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.
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.
Eisenbud Lecture Series in Mathematics and Physics,
Lecture I, 4pm, Abelson 131. Reception to follow.
Jennifer Chayes, Managing Director of Microsoft Research New England
Eisenbud Lecture Series in Mathematics and Physics,
Lecture II (4:30pm, Abelson 131)
Jennifer Chayes, Managing Director of Microsoft Research New England
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.
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
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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
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
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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)
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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.