Past Colloquia

Spring 2018 Colloquia

"Soft, Structured, Living Materials"

January 23, 2018

Jesse L. Silverberg, Harvard University
Host: Seth Fraden

Abstract: The central narrative of contemporary biology is that DNA encodes all relevant information for an organism's function and form. While this genotype-to-phenotype framing is appealing for its reductionish simplicity, it has a substantial problem. Between nanometer-scale DNA and organismal-scale phenotype sits a gap of 5 to 9 orders of magnitude in length. This gap covers everything from active protein diffusion, and macromolecular self-assembly, to biopolymer networks and pattern forming mechanical instabilities. In other words, the story of how organisms get their function and form starts with genes, but rapidly transitions to the language of soft matter physics as we examine larger and longer length scales.

In this talk, I'll address technological challenges and solutions for studying multiscale biophysics in an experimental setting. Along the way, I will discuss how cm-scale cartilage tissue achieves its remarkable mechanical properties through biopolymer self-organization, and how advances in big data and cloud computing can be leveraged for visualizing this nm-scale structure. I will continue to develop the theme of multiscale biophysics in the context of cell-cell fusion, a remarkably common yet mysterious processes in which individual cells fuse together to increase their size ~1,000-fold while decreasing metabolic costs by ~75%. I will also briefly touch on current work studying embryonic morphogenesis where gradients in cell growth lead to the geometric nonlinearities driving epidermal pattern formation. The physics of phase transitions, instabilities, and networks will become reoccurring themes that appear in surprising and unexpected ways as we work to close the genotype-to-phenotype gap.

"Surface tension and surface elasticity of soft solids"

January 25, 2018

Qin Xu, ETH Zurich
Host: Seth Fraden

Abstract: Surface tension, also known as surface stress, is a fundamental physical property of any interface. However, surface tensions of solids are normally overlooked as they are too weak to significantly deform bulk solids. Measurements of solid surface tension in traditional engineering materials, such as metals and oxides, have proven to be very challenging. Consequently, our understanding of solid capillarity relies heavily on untested theories. Here, we take the advantage of high compliance and large deformability of a soft polymeric gels to directly measure solid surface tension under different deformation conditions. Under biaxial stretch, we find the surface tension depends on the strain via a surface modulus, which remarkably, is many times larger than the zero-strain surface tension. Under uniaxial stretch, solid surface tension becomes anisotropic and orientation dependent. In these experiments, we decompose the surface modulus into surface shear and surface bulk components. Further, we try to understand the origin of surface elasticity of soft gels by studying compression induced surface instability. These results suggest that solid surface tension, as a strain-dependent tensor, can play a dominant role in solid mechanics at much larger length scales than previously anticipated.

"Black Holes, Holography, and Quantum Error Correction"

January 30, 2018

Daniel Harlow, Massachusetts Institute of Technology
Host: Matthew Headrick

Abstract: Constructing a theory of quantum gravity is one of the grand challenges of theoretical physics.  In recent years considerable progress has been made on this problem in the special (and unfortunately unphysical) context of a negative cosmological constant, using the powerful tools of the anti de Sitter/ conformal field theory (AdS/CFT) correspondence.  In this talk I will explain how this correspondence can be naturally reformulated using the theory of quantum error correction, which was originally invented to protect quantum computers from decoherence.  I will also explain how this reformulation clarifies several old puzzles in quantum gravity.  Although the subject material may sound difficult, the talk should be accessible to beginning graduate students.

"Entropic Unjamming of Emulsions"

February 8, 2018

Rodrigo E. Guerra, NYU
Host: Seth Fraden

Abstract: Like many yield-stress fluids, the mechanical properties of emulsions are controlled by two seemingly irreconcilable energy scales. In a compressed emulsion, droplets are pressed together by an external osmotic pressure and held in place by the balance of repulsive contact forces: forming solids with elastic moduli proportional to the ratio of interfacial tension to droplet size, σ/R ​​. In dilute emulsions, by contrast, droplets are free to slide past each other and diffuse: composing viscoelastic fluids with osmotic moduli proportional to 3 kB T/{ 4π R3}​​. Naively, the transition between a dense fluid and an amorphous solid should mark the balance between these energy scales; however, since σ/R is typically 106 to 1010 times larger than 3 kB T/4π R3 ​​, it is not clear how such an enormous gap is bridged. Ignoring thermal fluctuations entirely or treating them as a perturbation reduces this question to a geometric packing problem that can be studied in simulation. Here we show, however, that thermal fluctuations play a fundamental role in this transition, and that the balance of elasticity and thermal agitation defines a critical osmotic pressure, Π*​​, below which the solid is too weak to resist the thermal excitations of its constituent droplets. Measurements of the elastic moduli of barely-compressed emulsions confirm values of Π*​​ predicted by a simple balance of thermal fluctuations and local yielding, and show that the fragility and softness of emulsions amplify effects of thermal fluctuations: requiring values of Π* ∼ 105 ⋅ 3 kB T/4π R3 ​​ to contain them.

"The elastic Leidenfrost effect: coupling vapor release and elastic deformations to power sustained bouncing"

February 13, 2018

Scott Russell Waitukaitis, AMOLF
Host: Seth Fraden

Abstract: The Leidenfrost effect occurs when an object near a hot surface vaporizes rapidly enough to lift itself up and hover. Although well-understood for liquids and stiff sublimable solids, little is known about the interaction of vaporizable soft solids with hot surfaces.  In this talk, I will introduce a new phenomenon that occurs with vaporizable soft solids: the elastic Leidenfrost effect. By dropping vaporizable hydrogel spheres onto hot surfaces I will show that, rather than hovering, they energetically bounce several times their diameter for minutes at a time. With high-speed video during a single impact, one sees high-frequency microscopic gap dynamics at the sphere-substrate interface.  Solving for the dynamics of a simplified system with numerical simulations, I will reveal how these otherwise-hidden agitations constitute work cycles that harvest mechanical energy from the vapor and sustain the bouncing.  Quite literally, the hydrogel sphere behaves like a soft engine, where nearly all of the components are embedded into a single object made from a single material.  These findings suggest a novel strategy for injecting mechanical energy into soft materials, with potential relevance in fields ranging from soft robotics to active matter.

"Active nematic liquid crystals in biological materials: from multicellular tissues to active bio-polymers"

February 15, 2018

Guillaume Duclos, Brandeis
Host: Seth Fraden

Abstract:  Active nematics describes a phase of matter where active particles that consume energy to produce mechanical work assemble at high density in a state with orientational order but no positional order. In this talk, I will show how the active nematic framework allows us to better understand aspects of the collective behaviors that emerge in biological materials.

I will focus on two model systems:

  1. A tissue-based liquid crystal composed of elongated cells
  2. A bio-polymer liquid crystal composed of elongated viruses, microtubules and molecular motors

First, I will describe the emergence of nematic order in multicellular tissues composed of elongated cells and show how the interplay between confinement, topological defects and activity controls the self-organization and the emergence of collective flows in 2D active nematics. In a second part, I will present our recent efforts to describe the emergence of flows in biomimetic 3D active gels and 3D active liquid crystals. Although most out-of-equilibrium collective phenomena in living cells and their potential engineering applications take place in complex 3D environments, majority of the experimental and theoretical work exploring self-organization of active biological materials has been restricted to 2D systems. Here, I will explore how active fluids composed of biological polymers and molecular motors behave and self-organize in 3D. I will first describe the generic bend instability that emerges in a flow-aligned 3D active gel and show how the interplay between activity, nematic elasticity and confinement controls the wavelength of this activity driven instability. I will then present current work on the emergence of flows and topological defect loops in 3D with a system composed of a passive colloidal liquid crystal doped with active microtubules.

"The physics, biology, and technology of resonance energy transfer"

February 27, 2018

Philip Nelson, University of Pennsylvania
Host: Jané Kondev

Abstract: Resonance energy transfer has become an indispensable experimental tool for single-molecule and single-cell biophysics, and a conceptual tool to understand bioluminescence and photosynthesis. Its physical underpinnings, however, are subtle: It involves a discrete jump of excitation from one molecule to another, and so we regard it as a strongly quantum-mechanical process. And yet its first-order kinetics differ from what many of us were taught about two-state quantum systems; quantum superpositions of the states do not seem to arise; and so on. The key step involves acknowledging quantum decoherence.
Ref: P C Nelson, Biophys J in press (2018).

CANCELLED: "Cosmology with the Hyper Suprime-Cam (HSC) survey"

March 13, 2018

Rachel Mandelbaum, Carnegie Mellon University
Host: Marcelle Soares-Santos

Abstract: Hyper Suprime-Cam (HSC) is an imaging camera mounted at the Prime Focus of the Subaru 8.2-m telescope operated by the National Astronomical Observatory of Japan on the summit of Maunakea in Hawaii. A consortium of astronomers from Japan, Taiwan and Princeton University is carrying out a three-layer, 300-night, multiband survey from 2014-2019 with this instrument. In this talk, I will focus on the HSC survey Wide Layer, which will cover 1400 square degrees in five broad bands (grizy), to a 5 sigma point-source depth of r~26. We have covered 240 square degrees of the Wide Layer in all five bands, and the median seeing in the i band is 0.60 arcseconds. This powerful combination of depth and image quality makes the HSC survey unique compared to other ongoing imaging surveys. In this talk I will describe the HSC survey dataset and the completed and ongoing science analyses with the survey Wide layer, including galaxy studies, strong and weak gravitational lensing, but with an emphasis on weak lensing. I will demonstrate the level of systematics control, the potential for competitive cosmology constraints, some early results, and describe some lessons learned that will be of use for other ongoing and future lensing surveys.

"How 10 Years of Education Research Revealed My 40 Years of Bad Assumptions"

March 20, 2018

David Pritchard, Massachusetts Institute of Technology
Host: Matthew Headrick

Abstract: Once upon a time, I thought our final exam measured what students should learn. But further investigations of exactly what students learned and what they learned it from, how much they remembered as seniors, the role of homework copying, the limitations of partial credit grading, and the disparity between what physics teachers want to teach and what their students want to learn have been disquieting. I shall discuss how we can help students learn what they should learn, and describe a new classroom pedagogy that helps students to become more expert. Then I’ll describe how education research, development, and online learning might be combined to spread better learning universally.

"Schrodinger's clowder: entanglement in many-body physics"

March 27, 2018

Anushya Chandran, Boston University
Host: Matthew Headrick

Abstract: Nearly 80 years after Schrodinger introduced his famous cat, quantum entanglement has fuelled a conceptual revolution in many-body quantum physics. What Einstein, Podolsky and Rosen once found anathema, we now understand as a quantitative tool for classifying the allowed quantum phases of matter, analyzing the performance of quantum simulations and diagnosing thermalization. In this talk, I will re-introduce you to entanglement from this many-body point of view. We will see how the entanglement entropy organizes quantum phases of matter in the absence of local order parameters and how the entanglement spectrum probes the bulk-boundary correspondence in such quantum phases. Finally, we will turn to the dynamics of entanglement and how it has offered a phenomenological understanding of the newly discovered many-body localized phase.

"Mapping the string landscape"

April 10, 2018

Washington Taylor, Massachusetts Institute of Technology
Host: Matthew Headrick

Abstract: The number of vacuum solutions to string theory is extremely large, and is currently estimated to be at least 10^{300,000}.  In the context of inflationary cosmology this "string landscape" is naturally populated and to some extent resolves the cosmological constant problem.  Each of these solutions, however, gives rise to different low-energy physics, which has raised questions about the predictability of the theory.  This talk will describe recent progress in attaining a global description of the (supersymmetric) string landscape.  This global picture suggests that certain types of gauge groups associated with forces of nature arise naturally and generically, while other gauge groups require extensive fine-tuning.  This global picture also sheds light on the  extent to which string theory is a unique theory of quantum gravity, the possible nature of dark matter, and what kinds of constraints string theory may place on physics at low (sub-Planck) energies.

"Dissipation induced transitions in soft matter systems"

April 17, 2018

Suri Vaikuntanathan, University of Chicago
Host: Aparna Baskaran

Abstract: Stochastic thermodynamics provides a useful set of tools to analyze and constrain the behavior of far from equilibrium systems. In my talk, I will report an application of ideas from stochastic thermodynamics to problems of self assembly and organization far from equilibrium. In the first part of the talk, I will consider systems that under non-equilibrium growth conditions undergo compositional or morphological transformation. I will show how ideas from stochastic thermodynamics can be used to phenomenologically describe and constrain morphological and compositional changes excited during a non-equilibrium growth process.

Minimal models of active and driven particles have recently been used to elucidate many properties of nonequilibrium systems. However, the relation between energy consumption and changes in the structure and transport properties of these nonequilibrium materials remains to be explored. In the second part of the talk, I will discuss this relation in a minimal model of a driven liquid that settles into a time periodic steady state. Using concepts from stochastic thermodynamics and liquid state theories, I will show how the work performed on the system by various nonconservative, time-dependent forces—this quantifies a violation of time reversal symmetry—modifies the structural, transport, and phase transition properties of the driven liquid.

"The Dark Side of the Cosmic Dawn"

April 24, 2018

Tracy Slatyer, Massachusetts Institute of Technology
Host: Marcelle Soares-Santos

Abstract: Dark matter constitutes more than 5/6 of the matter in the universe, but its nature and interactions remain one of the great puzzles of fundamental physics. Dark matter collisions or decays, occurring throughout the universe's past, have the potential to produce high-energy particles; such particles may already have reshaped the history of our cosmos, leaving traces of their existence in ionization and heating of the intergalactic medium. I will discuss possible signatures of new dark matter physics in cosmological observations, from the cosmic dark ages to the epoch of reionization, and future directions in both theory and observation.

Fall 2017 Colloquia

"Protein self-assembly in the cell: Spatial and temporal control through membrane localization"

September 5, 2017

Margaret Johnson, Johns Hopkins University
Host: Michael Hagan

Abstract: Cell division, endocytosis, and viral budding would not function without the localization and assembly of protein complexes on membranes. What is poorly appreciated, however, is that by localizing to membranes, proteins search in a reduced space that effectively drives up concentration. We have derived an accurate and practical analytical theory to quantify the significance of this dimensionality reduction in regulating protein assembly on membranes. We define a simple metric, an effective equilibrium constant, that allows for quantitative comparison of protein-protein interactions with and without membrane present. We find that many of the protein-protein interactions between pairs of proteins involved in clathrin-mediated endocytosis in human and yeast cells can experience enormous increases in effective protein-protein affinity (10-1000 fold) due to membrane localization. By developing new methods for reaction-diffusion simulation of protein structures, we further characterize the non-equilibrium dynamics of these assembly processes, with reaction parameters defined from experiment. Both theory and simulations highlight the power of membrane localization in triggering robust protein-protein binding, suggesting that it can play an important role in controlling the timing of endocytic protein coat formation.

"How to control the size of a self-assembling, self-filling shell"

September 12, 2017

Michael Hagan, Brandeis University
Host: Physics Department

Abstract: The self-assembly of a protein shell around a cargo is a common mechanism of encapsulation in biology. For example, many viruses assemble an icosahedral protein shell (capsid) around the the viral nucleic acid. Some viruses then acquire an additional exterior coating by budding through a cell membrane. Similarly, bacterial microcompartments (BMCs) are large icosahedral protein shells that assemble around collections of enzymes to act as organelles inside of bacteria.

In this talk I will use coarse-grained computational models and simple scaling calculations to illuminate the factors that control such a self-assembly process. I will particularly focus on how material properties (such as nucleic acid electric charge, membrane bending modulus, or enzyme cohesive forces) control assembly pathways and the size of the assembled shell.

"We Have No Idea"

September 19, 2017

Daniel Whiteson, University of California, Irvine & Jorge Cham, PHD Comics
Host: Bjoern Penning

Abstract: Jorge and Daniel will talk about the big unsolved mysteries of the Universe, including dark matter, dark energy, and the behavior of cats. A fun presentation that combines science, humor, and live drawing, inspired by their new book.

"Mechanics of Epithelial Tissues: Structure, Rigidity and Fluidity"

September 26, 2017

Dapeng Bi, Northeastern University
Host: Bulbul Chakraborty

Abstract: Cells must move through tissues in many important biological processes, including embryonic development, cancer metastasis, and wound healing. Often these tissues are dense and a cell's motion is strongly constrained by its neighbors, leading to glassy dynamics. Although there is a density-driven glass transition in particle-based models for active matter, these cannot explain liquid-to-solid transitions in confluent tissues, where there are no gaps between cells and the packing fraction remains fixed and equal to unity. I will demonstrate the existence of a new type of rigidity transition that occurs in confluent tissue monolayers at constant density.  The onset of rigidity is governed by a model parameter that encodes single-cell properties such as cell-cell adhesion and cortical tension. I will also introduce a new model that simultaneously captures polarized cell motility and multicellular interactions in a confluent tissue and identify a glassy transition line that originates at the critical point of the rigidity transition. This work suggests an experimentally accessible structural order parameter that specifies the entire transition surface separating fluid tissues and solid tissues.

"Unlocking the mysteries of the Universe with the CMS experiment at the Large Hadron Collider"

October 10, 2017

Tulika Bose, Boston University
Host: Bjoern Penning

Abstract: The discovery of the Higgs Boson at the Large Hadron Collider (LHC) in 2012 was a ground-breaking event in particle physics history. The LHC has restarted recently at an unprecedented center of mass energy of about 13 TeV and the data collected by the CMS experiment is expected to help fully understand the nature of electroweak symmetry breaking and potentially discover new physics. In this talk, I will review recent results from the CMS experiment with special focus on searches for physics beyond the Standard Model.

"Extragalactic Jets as Probes of Clusters of Galaxies"

October 17, 2017

Elizabeth Blanton, Boston University
Host: John Wardle

Abstract: I will present multi-wavelength (X-ray, optical, infrared, and radio) observations of clusters of galaxies, including in-depth study of nearby objects and a survey of distant systems. Cooling of the hot, gaseous intracluster medium in cluster centers can feed the supermassive black holes in the cores of the dominant cluster galaxies leading to active galactic nucleus (AGN) outbursts. This AGN feedback can reheat the gas, stopping cooling and large amounts of star formation. Most relaxed, cool core clusters host powerful AGN in their central galaxies and these AGN can significantly affect the distribution of e.g., temperature and abundance on cluster scales. AGN heating can come in the form of shocks, buoyantly rising bubbles that have been inflated by radio jets and lobes, and sound wave propogation. Sloshing of the cluster gas, related to minor, off-center interactions with galaxy sub-clusters or groups also affects the distribution of temperature and abundance on large scales. This sloshing gas can interact with the AGN's radio-emitting jets and lobes causing them to bend. This bending is also found in AGN jets and lobes embedded in clusters undergoing major, head-on cluster, cluster mergers. Since this bending is a signature of interaction within clusters, bent, double-lobed AGN observed in the radio can be used as beacons for clusters of galaxies at high redshifts. I will describe our large sample of high-redshift, bent-double radio sources that were observed in the infrared with the Spitzer Space Telescope and in the optical with the Discovery Channel Telescope and that have yielded approximately 200 new, distant clusters of galaxies. The clusters in our survey ("COBRA," Clusters Occupied by Bent Radio AGN) will serve as important laboratories for studying galaxy evolution.

"The Jazz of Physics: The Link Between Music and The Structure of the Universe"

October 24, 2017

Stephon Alexander, Brown University
Host: Bjoern Penning

Abstract: In this talk Alexander revisits the interconnection between music and the evolution of astrophysics and the laws of motion. He explores new ways music, in particular jazz music, mirrors modern physics, such as quantum mechanics, general relativity, and the physics of the early universe. Finally, he discusses ways that innovations in physics have been and can be inspired from "improvisational logic" exemplified in Jazz performance and practice.

"Mechanics of Cell-Nanomaterials Interaction"

October 31, 2017

Huajian Gao, Brown University
Host: Michael Hagan

Abstract: Nanomaterials, including various types of nanoparticles, nanowires, nanofibers, nanotubes, and atomically thin plates and sheets have emerged as candidates as building blocks for the next generation electronics, microchips, composites, barrier coatings, biosensors, drug delivery, and energy harvesting and conversion systems. There is now an urgent societal need to understand the biological interactions and environmental impact of nanomaterials which are being produced and released into the environment by nearly a million tons per year. This talk aims to discuss mechanics as an enabling tool in this emerging field of study. The discussions will touch on some of the recent experimental, modelling and simulation studies on the mechanisms of cell uptake of low-dimensional nanomaterials and their effects on subcellular vesicles and damage.

"Flexibility and Form: Emergence of Shape in Thin Sheets"

November 7, 2017

Narayanan Menon, UMass Amherst
Host: Michael Hagan

Abstract: Thin sheets assume a rich diversity of shapes in the natural world, ranging from folds on the earth’s crust, to the wavy shapes of leaves and flowers, down to more microscopic biomembranes and synthetic thin films. We have used thin polymer films floating on the surface of a fluid as a venue in which to study the emergence of complex shapes via successive elastic instabilities. Understanding these patterns required new notions of ‘thinness’ or bendability of a sheet, which define regimes in which textbook theories of post-buckling fails. I will end by describing opportunities for wrapping and encapsulation in this new regime of highly-bendable materials.

"Decision Analysis Methods Used to Make Appropriate Investments in Human Exploration Capabilities and Technologies"

November 14, 2017

Julie Williams-Byrd, NASA Langley Research Center
Host: Anique Olivier-Mason

Abstract: NASA is transforming human spaceflight. The Agency is shifting from an exploration-based program with human activities in low Earth orbit (LEO) and targeted robotic missions in deep space to a more sustainable and integrated pioneering approach. Through pioneering, NASA seeks to address national goals to develop the capacity for people to work, learn, operate, live, and thrive safely beyond Earth for extended periods of time. However, pioneering space involves daunting technical challenges of transportation, maintaining health, and enabling crew productivity for long durations in remote, hostile, and alien environments. Prudent investments in capability and technology developments, based on mission need, are critical for enabling a campaign of human exploration missions. There are a wide variety of capabilities and technologies that could enable these missions, so it is a major challenge for NASA’s Human Exploration and Operations Mission Directorate (HEOMD) to make knowledgeable portfolio decisions. It is critical for this pioneering initiative that these investment decisions are informed with a prioritization process that is robust and defensible. It is NASA’s role to invest in targeted technologies and capabilities that would enable exploration missions even though specific requirements have not been identified. To inform these investments decisions, NASA’s HEOMD has supported a variety of analysis activities that prioritize capabilities and technologies. These activities are often based on input from subject matter experts within the NASA community who understand the technical challenges of enabling human exploration missions.

This paper will review a variety of processes and methods that NASA has used to prioritize and rank capabilities and technologies applicable to human space exploration. The paper will show the similarities in the various processes and showcase instances where customer specified priorities force modifications to the process. Specifically, this paper will describe the processes that the NASA Langley Research Center (LaRC) Technology Assessment and Integration Team (TAIT) has used for several years and how those processes have been customized to meet customer needs while staying robust and defensible.

"Sloppy Models, Differential Geometry, and How Science Works"

November 27, 2017

Monday, November 27 to Wednesday, November 29, 2017
Eisenbud Lecture Series in Mathematics and Physics
James Sethna, Cornell University
Host: Bulbul Chakraborty

Monday, November 27, 2017 (Lecture I), 4:00pm, Gerstenzang 121
"Sloppy Models, Differential Geometry, and How Science Works"
James P. Sethna, Katherine Quinn, Archishman Raju, Mark Transtrum, Ben Machta, Ricky Chachra, Ryan Gutenkunst, Joshua J. Waterfall, Fergal P. Casey, Kevin S. Brown, Christopher R. Myers

Abstract: Models of systems biology, climate change, ecosystems, and macroeconomics have parameters that are hard or impossible to measure directly. If we fit these unknown parameters, fiddling with them until they agree with past experiments, how much can we trust their predictions? We have found that predictions can be made despite huge uncertainties in the parameters – many parameter combinations are mostly unimportant to the collective behavior. We will use ideas and methods from differential geometry to explain what sloppiness is and why it happens so often. We show that physics theories are also sloppy – that sloppiness may be the underlying reason why the world is comprehensible.

"Crackling Noise"

November 28, 2017

Monday, November 27 to Wednesday, November 29, 2017
Eisenbud Lecture Series in Mathematics and Physics
James Sethna, Cornell University
Host: Bulbul Chakraborty

Tuesday, November 28, 2017 (Lecture II), 4:00pm, Abelson 131
"Crackling Noise"
James P. Sethna
Abstract: A piece of paper or candy wrapper crackles when it is crumpled. A magnet crackles when you change its magnetization slowly. The earth crackles as the continents slowly drift apart, forming earthquakes. Crackling noise happens when a material, when put under a slowly increasing strain, slips through a series of short, sharp events with an enormous range of sizes. There are many thousands of tiny earthquakes each year, but only a few huge ones. The sizes and shapes of earthquakes show regular patterns that they share with magnets, plastically deformed metals, granular materials, and other systems. This suggests that there must be a shared scientific explanation. We shall hear about crackling noise and that it is a symptom of a surprising truth: the system has emergent scale invariance – it behaves the same on small, medium, and large lengths.

"Normal Form for Renormalization Groups: The Framework for the Logs"

November 27, 2017

Monday, November 27 to Wednesday, November 29, 2017
Eisenbud Lecture Series in Mathematics and Physics
James Sethna, Cornell University
Host: Bulbul Chakraborty

Wednesday, November 29, 2017 (Lecture III), 10:00am, Abelson 333
"Normal Form for Renormalization Groups: The Framework for the Logs"
James P. Sethna, Archishman Raju, Colin Clement, Lorien Hayden, Jaron Kent-Dobias, Danilo Liarte, and Zeb Rocklin
Abstract: Ken Wilson’s renormalization group solved for the behavior of phase transitions by mapping statistical mechanics into a differential equation in the space of all Hamiltonians, as we examine them on different length scales. This mapping from complex physical systems to simple differential equations has allowed us to explain scale invariance that emerges in everything from crackling noise to the onset of chaos. The results of the renormalization group are commonly advertised as the existence of power law singularities near critical points. This classic prediction is often violated, with logarithms and exponentials that pop up in the most interesting cases. Mathematicians have developed normal form theory to describe the likely behaviors of differential equations. We use normal form theory to systematically group these seeming violations into universality families. We recover and explain the existing literature, predict the nonlinear generalization for universal homogeneous functions, and show that the procedure leads to a better handling of the singularity even for the classic 4-d Ising model.

"Neutrinos: from zeros to heroes?"

December 5, 2017

Roxanne Guenette, Harvard University
Host: Bjoern Penning

Abstract: The Standard Model, that describes extremely well the particles and their interactions, predicts that neutrinos are massless and only interacts via weak interaction. These properties made neutrinos some of the least interesting particles of the model... until the discovery that they oscillate. This groundbreaking result implies that neutrinos are massive particles and opens the door to physics beyond the Standard Model- the holy grail of particle physicists. In addition, it seems that neutrinos could hold the key to many great mysteries of physics, such as the imbalance in the Universe between matter and anti-matter, and these are now within the reach of the next generation of neutrino experiments. After reviewing the intriguing properties of neutrinos, I will present the open questions in neutrino physics and describe how current and future neutrino experiments, focusing on Liquid Argon experiments, can bring new answers.

Spring 2017 Colloquia

Clusters of Galaxies: Laboratories for Probing the Interplay between Baryons and Dark Matter

January 17, 2017

Esra Bulbul (MIT)
Host: John Wardle

Abstract: As the most massive collapsed structures in the Universe, galaxy clusters are unique laboratories for studying the evolution of baryons in concert with dark matter particles, as well as for exploring the nature of dark matter, and dark energy. The thermodynamical state of the intra-cluster gas has been extensively studied through multi-wavelength X-ray and radio (Sunyaev Zel'dovich effect) observations. I will highlight the most recent measurements of a) the evolution of baryons in the deep potential wells of clusters, b) the potential of utilizing clusters in cosmological studies and indirect searches for dark matter with a particular focus on the candidate 3.5 keV emission line.

Discovering a new approach to cosmology with the Dark Energy Survey and Gravitational Waves

January 24, 2017

Marcelle Soares Santos (FermiLab, Illinois)
Host: John Wardle

Abstract: Motivated by the exciting prospect of new wealth of information that will arise from observations of gravitational and electromagnetic radiation from the same astrophysical phenomena, the Dark Energy Survey (DES) Collaboration has performed a broad range follow-up program for LIGO/Virgo events using its Camera (DECam). In this talk, I present an overview of this effort, including results of searches for signatures of the first two LIGO-triggered binary black hole mergers in the 2015-2016 observing campaign and status of the ongoing 2016-2017 campaign. I will also discuss plans for upcoming seasons and long term prospects for this exciting emerging field: multi-messenger cosmology with gravitational waves and optical data.

The Past, Present, and Future of 21cm Cosmology

January 26, 2017

Adrian Liu (UC Berkley)
Host: John Wardle

Abstract: Despite tremendous recent progress, gaps remain in our knowledge of our cosmic history. For example, we have yet to make direct observations of Cosmic Dawn or the subsequent Epoch of Reionization. Together, these represent the important period when the first stars and galaxies were formed, dramatically altering their surroundings in the process. Radio telescopes targeting the 21cm line will open up these crucial epochs to direct observations in the next few years, filling in a missing chapter in our cosmic story. I will review our recent results from the Precision Array to Probe the Epoch of Reionization (PAPER) experiment. These results have begun to place limits on heating processes during reionization. I will also motivate unconventional ideas in experiment design that have been proposed and implemented to deal with the unique technical challenges of 21cm cosmology. Cognizant of "lessons learned" from the current generation of instruments, I will describe our recently commenced Hydrogen Epoch of Reionization Array (HERA), including its forecasted promise to provide exquisite constraints on reionization astrophysics as well as on fundamental parameters such as the neutrino mass.

The Extraordinary Lives of Supermassive Black Holes at the Centers of Galaxies

January 31, 2017

Francisco Muller-Sanchez (University of Colorado, Boulder)
Host: John Wardle

Abstract: Some of nature's most powerful objects are well-fed supermassive black holes at the centers of galaxies. Weighing up to billions of times the mass of our sun, they usually outshine the stellar emission of their host galaxies. The discovery of a number of black hole – galaxy relations has shown that the growth of supermassive black holes is closely related to the evolution of galaxies. This evidence has opened a new debate in which the fundamental questions concern the interactions between the central black hole and the interstellar medium within the host galaxy, and can be addressed by studying two crucial processes: feeding and feedback. With the help of NASA’s space observatories and the largest optical telescopes in the world, I will explain how supermassive black holes are fed and how they influence their host galaxies. I will focus on my recent results for a large sample of luminous nearby galaxies, which also includes galaxies from the Sloan Digital Sky Survey with dual/offset emission lines to identify outflows and distinguish them from dual/binary black holes. The prospects for JWST/TMT observations of nearby galaxies will also be discussed, including the feeding of the black hole at sub-parsec scales, the starburst-black hole connection and the launching mechanisms of black hole-driven outflows.

Using the “COSMOS” to Understand Black Hole and Galaxy Co-evolution

February 2, 2017

Francesca Civano (Harvard)
Host: John Wardle

Abstract: Observations indicate that supermassive black holes (SMBHs, 10**6-10**9 Msun) dwell at the centers of most local galaxies. Scaling relations between SMBH mass and several large-scale properties of the host galaxies point to a co-ordindated growth of galaxies and their central engines over cosmic time: they "co-evolve". Who is the leading actor on the the cosmic stage: the black hole or the galaxy? Is black hole activity triggering star-formation or suppressing it? Does the galaxy control the black hole growth? To address these questions, in this talk I will present my work testing this co-evolution scenario, focusing on SMBH growth mechanisms, accretion and mergers. I will use the extraordinarily rich multiwavelength dataset of the Cosmic Evolutionary Survey (COSMOS). I will concentrate on the highest energy data available, the X-ray ones, from the surveys I have led using both the Chandra and NuSTAR NASA satellites. These data provide us with a unique and powerful tool to find and study accreting SMBHs in the distant Universe.

The Merger-Free Co-Evolution of Galaxies and their Supermassive Black Holes

February 7, 2017

Brooke Simmons (UC San Diego)
Host: John Wardle

Abstract: Supermassive black holes and galaxies co-evolve over cosmic time, but despite more than a decade of research the engine(s) for this co-evolution are still not fully understood. The typical co-evolution picture invokes major galaxy mergers to both drive material toward the centre of the gravitational potential and trigger star formation, growing both galaxy and black hole together. This is certainly a plausible explanation for many observed systems, but there is growing evidence from both local ground-based and higher-redshift HST observations that supermassive black holes often grow in systems that cannot have had a major merger. This talk will review the field of black hole-galaxy co-evolution from 0 < z < 3 and across many orders of magnitude in the AGN luminosity and black hole mass function, and discuss the relative importance of both mergers and completely calm, "secular" evolution on black hole growth. New evidence from recent years suggests merger-free process may contribute significantly to both the overall growth of supermassive black holes and their co-evolution with their host galaxies.

How to Find Dark Matter

February 28, 2017

Bjoern Penning (University of Bristol)
Host: John Wardle

Abstract: Dark Matter (DM) is a long standing puzzle in fundamental physics and the goal of a diverse research program. I will review the evidence for DM and how to search for it. Underground and astrophysical searches attempt to detect DM particles in the cosmos directly or by searching for their decay products while particle colliders attempt to produce DM in the laboratory. Each of these detection methods probe different parts of the parameter space with complementary sensitivity in mass, interaction type, and uncertainties. I will show the connection between these searches, theoretical developments that connect their search strategies and how an interdisciplinary effort can probe the entire natural phase space in the near term future.

Assessing and Reducing the Risks of Solar Geoengineering

March 7, 2017

David Keith (Harvard)
Host: W. Benjamin Rogers

Abstract: I will discuss new results suggesting it may be possible to implement solar geoengineering using stratospheric aerosols without ozone loss while significantly reducing some other important side effects. Estimates of the risks and efficacy of solar geoengineering are deeply uncertain. Accurate physically-based models along with laboratory and in situ experiments will be needed to improve estimates of the efficacy and risks of proposed solar geoengineering methods. As an example I will discuss our ongoing laboratory experiments and plans for small perturbative outdoor experiments. Governance poses the greatest challenge for solar geoengineering: I will review some recent work on governance of research and deployment of solar geoengineering and argue in favor of an international open-access and interdisciplinary research program.

Hydrodynamic Quantum Analogs

March 21, 2017

John Bush (MIT)
Host: W. Benjamin Rogers

Abstract: A decade ago, Yves Couder and coworkers discovered that droplets walking on a vibrating fluid bath exhibit several features previously thought to be exclusive to the microscopic, quantum realm. These walking droplets propel themselves by virtue of a resonant interaction with their own wavefield, and so represent the first macroscopic realization of a pilot-wave system of the form proposed for microscopic quantum dynamics by Louis de Broglie in the 1920s. New experimental and theoretical results in turn revealand rationalize the emergence of quantization and quantum-like statistics from this hydrodynamic pilot-wave system in anumber of settings.

Physics Explains How a Virus Motor Stuffs a Long DNA into a Small Shell

March 28, 2017

Steve Harvey (U Penn)
Joint Quantitative Biology/Department of Physics Colloquium
Host: Michael Hagan

Abstract: Many viruses that infect bacteria (bacteriophages) have similar structures: an icosahedral shell (the capsid) surrounding the double-stranded DNA genome (dsDNA), and a tail structure designed to recognize the target bacterium, penetrate the bacterial cell wall, and deliver the DNA into the bacterial cell through a channel in the tail. The assembly of a new virus has three steps: (1) the capsid assembles spontaneously, nucleated by a protein complex (the motor) at one vertex of the icosahedron; (2) the motor captures dsDNA and drives it into the empty capsid, using the energy of ATP hydrolysis to overcome the strong electrostatic forces that resist DNA confinement; and (3) the tail assembly replaces one of the motor's components, forming the mature virus. We are investigating the second step, asking how the motor drives the DNA into the capsid. Many models have been proposed, most based on the belief that DNA is passive substrate, gripped by the motor and propelled forward with lever-like motions. I have proposed that DNA is an active component of force generation. In the original "scrunchworm" model (1), the DNA is driven cyclically between the standard A-DNA and  B-DNA conformations. A-DNA is shorter than B-DNA, and the cycle of DNA shortening and lengthening is coupled to a protein-DNA grip-and-release cycle that rectifies the motion, generating DNA translocation. Our first computer simulations on DNA within the channel of a viral portal complex provided some support for this model (2). We have now completed simulations on DNA-portal interactions in four different complexes. Those simulations have confirmed the role of the DNA shortening and lengthening motions. But they also show that the proposed mechanism, based on transitions between A-DNA and B-DNA, is not correct. Instead, the DNA conformational changes are the consequence of a surprisingly simple physical mechanism (3).

This talk is the first public presentation of these results.
(1) SC Harvey (2015), J Struct Biol 189:1-8.
(2) JT Waters, HD Kim, JC Gumbart, X-J Lu, and SC Harvey (2016), J Phys Chem B 120:6200-6207.
(3) KS Sharp, SC Harvey, et al. (manuscript in preparation)
How ocean eddies support the growth of phytoplankton

April 4, 2017

Amala Mahadevan (Woods Hole Oceanographic Institution)
Host: Albion Lawrence

Abstract: Oceanic turbulence is strongly affected by the earth’s rotation and by density stratification. Flow at large scales is mostly two dimensional and vertical velocities are suppressed. This talk will examine how the eddy field in the ocean supports the growth of phytoplankton by inducing the vertical supply of nutrients in some regions, and enhancing their exposure to light in other regions. The coupling between physical processes and phytoplankton growth will be explored in the context in different oceanic regimes.

Probing the Dark Universe

April 25, 2017

James Battat (Wellesley)
Host: Gabriella Sciolla

Abstract: Dark energy has a commanding influence on the energy budget of our Universe, forcing the Universe to expand at an ever-growing rate. Dark matter, in the form of an exotic new particle, dominates the matter budget. It's fair to say that there is plenty of opportunity for growth in our understanding of both of these constituents. I will describe two different experiments designed to pull back the curtain on this dark sector. On the one hand, I'll explain how lunar laser ranging can measure the Moon's orbit with millimeter precision to test General Relativity and other fundamental theories. On the other, I'll describe a detector designed to sense the wind of dark matter produced as our Solar System speeds through the galaxy. Together, these complementary measurements aim to shed light on the dark universe.

The Restoration of Early Sound Recordings using Optical Metrology and Image Analysis

May 2, 2017

Carl Haber (Lawrence Berkeley National Laboratory)
Host: Gabriella Sciolla

Abstract: Unlike print and latent image scanning, the playback of mechanical sound carriers has been an inherently invasive process. Some of the earliest sound recordings contain material of great historical interest, may be in obsolete formats, and are damaged, decaying, or are now considered too delicate to play. We will discuss the use of optical metrology and numerical methods to acquire and analyze high resolution digital images of the original media. The results will be illustrated with sounds and images.

Fall 2016 Colloquia

Active Materials : Applying the soft materials paradigm to Biology

September 6, 2016

Aparna Baskaran (Brandeis)
Host: Department of Physics

Abstract: In this talk I will introduce and discuss a recently developed class of microscopically driven materials that have been termed active materials. Drawing lessons from both biology and in vitro experimental systems, I will discuss theoretical challenges and different approaches that have proved fruitful so far. In particular, I will discuss the physics of active brownian particles and active nematics.

Cosmic inflation and quantum gravity

September 13, 2016

Albion Lawrence (Brandeis)
Host: Department of Physics

Abstract: A new generation of cosmic microwave background (CMB) experiments are poised to test “high scale” models of cosmic inflation which are highly sensitive to quantum gravitational effects.  In this talk I will review basic aspects of inflation and its imprint on the CMB, and then discuss the difficulties in constructing high-scale models which are not spoiled by quantum gravity.  I will describe a specific class of models which use nontrivial quantum field theory dynamics to evade these difficulties.

Improving student understanding of quantum mechanics

September 20, 2016

Chandralekha Singh (University of Pittsburgh)
Host: Prof. Matthew Headrick

Abstract: Learning quantum mechanics is challenging, in part due to the non-intuitive nature of the subject matter. Our research shows that the patterns of reasoning difficulties in learning quantum mechanics are often universal similar to the universal nature of reasoning difficulties found in introductory physics. Our research also shows that students often have difficulty in monitoring  their learning while learning quantum mechanics. To help improve student understanding of quantum concepts, we are developing quantum interactive learning tutorials (QuILTs) as well as tools for peer-instruction. The goal of QuILTs and peer-instruction tools is to actively engage students in the learning process and to help them build links between the formalism and the conceptual aspects of quantum physics without compromising the technical content. I will discuss the effectiveness of these learning tools based upon assessment data.

The impact of cell volume and molecular crowding on cell mechanics and gene expression

September 27, 2016

Ming Guo (MIT)
Host: W. Benjamin Rogers

Abstract: Cells alter their mechanical properties in response to their local microenvironment; this plays a role in determining cell function and can even influence stem cell fate. In this talk, I will show a robust and unified relationship between cell stiffness and cell volume. As a cell spreads on a substrate, its volume decreases while its stiffness concomitantly increases. The reduction of cell volume is a result of water efflux which leads to a corresponding increase in intracellular molecular crowding. We find that bulk modulus, cortical shear modulus and cytoplasmic shear modulus of cells all scale with cell volume, and possibly reflect the change in molecular crowding. Moreover, we have directly measured the equation of state of living mammalian cells, and find that it can be described by a hard-sphere equation of state. Finally, we find that changes in cell volume and hence stiffness alter stem-cell differentiation, regardless of the method by which these are induced. These observations reveal a surprising, previously unidentified, relationship between cell stiffness and cell volume which strongly influences cell biology, and highlight the impact of molecular crowding.

Fingers, toes and tongues: the anatomy of interfacial instabilities in viscous fluids

October 11, 2016

Irmgard Bischofberger, (MIT)
Host: W. Benjamin Rogers

Abstract: The invasion of one fluid into another of higher viscosity is unstable and produces complex patterns in a quasi-two dimensional geometry. This viscous-fingering instability, a bedrock of our understanding of pattern formation, has been characterized by a most-unstable wavelength that sets the characteristic width of the fingers. We have shown that a second, previously overlooked, parameter governs the length of the fingers and characterizes the dominant global features of the patterns.Because interfacial tension suppresses short-wavelength fluctuations, its elimination would suggest an instability producing highly ramified singular structures. Our experimental investigations using miscible fluids show the opposite behavior – the interface becomes more stable even as the stabilizing effect of interfacial tension is removed. This is accompanied by slender structures, tongues, that form in the narrow thickness of the fluid. Among the rich variety of global patterns that emerge is a regime of blunt structures, “toes”, that exhibit the unusual features characteristic of proportionate growth. This type of pattern formation, while quite common in mammalian biology, was hitherto unknown in physical systems.

Fault-tolerant quantum computation in the 21st century

October 18, 2016

Daniel Gottesman (Perimeter Institute)
Host: Matthew Headrick

Abstract: Experimentalists are getting better and better at building qubits, but no matter how hard they try, their qubits will never be perfect. In order to build a large quantum computer, we will almost certainly need to encode the qubits using quantum error-correcting codes and encode the quantum circuits using fault-tolerant protocols. This will eventually allow reliable quantum computation even when the individual components are imperfect. I will review the current state of the art of quantum fault tolerance and discuss progress towards answering the most important questions that will enable large fault-tolerant quantum computers.

Localization: Moving Beyond Statistical Mechanics

November 1, 2016

Christopher Laumann (Boston University)
Host: Albion Lawrence

Abstract: The central assumption of statistical mechanics is that interactions between particles establish local equilibrium. Isolated quantum systems, however, need not equilibrate; this happens, for example, when sufficient quenched disorder causes localization. The many-body localized (MBL) phase transports neither heat nor charge; may possess orders disallowed in equilibrium; and, may exhibit quantum coherence even when highly excited. We will review the emerging understanding of how quantum localization can lead to new quantum phenomena even in highly excited states. I will give some theoretical intuition about how this might be used to build a better quantum computer and also review some of the latest experiments investigating localization.

Friction and adhesion in colloids: Yielding, thickening, jamming

November 8, 2016

Jeffrey Morris (CUNY)
Joint IGERT/Physics Department Colloquium
Host: Bulbul Chakraborty

Abstract: In recent work, we have shown [1,2] that frictional interactions provide a rational basis for both continuous and discontinuous shear thickening in viscous suspensions. When the repulsive forces (such as those due to electrostatic or steric colloidal stabilization) are overwhelmed by shearing forces, contact is assumed to occur, and the system transitions from a low-viscosity (lubricated) to a high-viscosity (frictional) state. Contacting particles may experience both adhesive forces as well as friction. We will consider the influence of attractive forces at contact, in combination with the stabilizing repulsive forces. This combination of forces would be seen in the case of particles with van der Waals attraction in combination with colloidal stabilization. For sufficient attractive force a yield stress and shear thinning give way to the shear thickening response, a behavior observed in certain flocculated dispersions. At sufficient yield stress, the shear thickening is completely obscured, as the dispersions shear thins after yielding directly onto the high-viscosity (frictional) plateau. The suggestion that a material may exhibit both yielding at low stress and jamming at large stress [3] is explored. 

  1. R. Seto, R. Mari, J. F. Morris & M. M. Denn 2013 "Discontinuous shear thickening of frictional hard-sphere suspensions" Phys. Rev. Lett. 111 218301
  2. R. Mari, R. Seto J. F. Morris & M. M. Denn 2015 "Discontinuous shear thickening in Brownian suspensions by dynamic simulation" Proc. National Acad. Sci.  112. 15326
  3. N. J. Wagner & J. F. Brady 2009 "Shear thickening in colloidal dispersions" Phys. Today62, 27-32

November 15, 2016

Eisenbud Lecture Series in Mathematics and Physics
Nigel Hitchin (University of Oxford)

Abstract: Euler’s equations for a spinning top are well-known to be solvable by elliptic functions. They form the first example of a much wider range of equations, in particular Nahm’s equations, which are solvable using algebraic curves of higher genus. Nahm’s equations appear in various parts of differential geometry and physics, related to hyperk ahler geometry and magnetic monopoles in particular. Loosely speaking, the equations are linearized on the Jacobian of the curve. However, there are many situations where that curve is singular or non-reduced and this viewpoint is no longer valid. The talk will discuss the geometry of what happens in some of these cases.

November 16, 2016

Eisenbud Lecture Series in Mathematics and Physics, Lecture II
Nigel Hitchin (University of Oxford)

Abstract: The theory of Higgs bundles on a compact Riemann surface provided a natural setting for hyperbolic surfaces within the context of an SU(2)-gauge theory with a complex Higgs field. Replacing the group SU(2) by the group of symplectic diffeomorphisms of the two-sphere provides, thanks to work of Biquard, an infinite-dimensional gen eralization of Teichm ̈uller space, but it is as yet unclear what type of geometry, generalizing hyperbolic metrics, on the surface this parametrizes. The lecture will investigate some of the questions and features involved.

November 18, 2016

Eisenbud Lecture Series in Mathematics and Physics, Lecture III
Nigel Hitchin (University of Oxford)

Abstract: The moduli space of Higgs bundles on a curve, together with its fibration structure as an integrable system, forms a natural example to examine the predictions of mirror symmetry in the approach of Strominger, Yau and Zaslow. The mirror for gauge group G is regarded as being the moduli space for the Langlands dual group LG. Of particular interest is the how this manifests itself in the duality of “branes” on each side. We consider in the talk cases arising from noncompact real forms of complex groups, and also Lagrangians arising from the existence of holomorphic spinor fields.

The Physics of the Deep Ocean Circulation

November 29, 2016

Raffaele Ferrari (MIT)
Host: Albion Lawrence

Abstract: The deep ocean circulation is fed by waters that become dense enough to sink into the ocean abyss at high latitudes and return to the surface through convoluted three dimensional pathways. While the physics behind the sinking of dense waters is well understood, the physics that allows waters to rise back to the surface remains elusive. It is generally believed that small-scale turbulent mixing, such as is caused by breaking internal waves, drives upwelling of the densest ocean waters. However the observational evidence that the turbulent fluxes generated by small-scale turbulent mixing in the stratified ocean interior are more vigorous close to the ocean bottom than above implies that small-scale turbulent mixing converts light waters into denser ones, thus driving a net sinking of abyssal water. Using a combination of numerical models and observations, it will be shown that abyssal waters return to the surface along weakly stratified boundary layers, where the small-scale turbulent mixing of density decays to zero. The net ocean meridional overturning circulation is thus the small residual of a large sinking of waters, driven by small-scale turbulent mixing in the stratified interior, and a comparably large upwelling, driven by the reduced small-scale turbulent mixing along the ocean boundaries.

Shape From Mechanics: Designing and Understanding Self-Folding Origami

December 6, 2016

Chris Santangelo (UMass Amherst)
Host: W. Benjamin Rogers

Abstract: Origami, the ancient art of paper folding, has probably been around as long as paper. Yet, recent advances in materials have enabled the fabrication of self-folding origami structures, sometimes called “4d printing.” These self-folding structures promise to revolutionize the manufacture of complex structures on a variety of scales, yet realizing this has proven challenging. He will discuss recent work with collaborators on self-folding origami structures, focusing on the theory behind the mechanics of such structures, our limited understanding of how to design shape, and prospects for self-folding and responsive structures.

Spring 2016 Colloquia

"Genome in 3D: some physical considerations"

January 19, 2016

Leonid Mirny, MIT
Host: Zvonimir Dogic

Abstract: 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.

"Can Soft Signals Be Oncogenic"

January 26, 2016

Ravi Radhakrishnan, University of Pennsylvania
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.

"Spontaneous flow in active fluids"

February 2, 2016

Zvonimir Dogic, Brandeis

Abstract: 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.

"From Higgs Discovery to the Halls of Congress: How Scientists Can Engage In Public Policy"

February 9, 2016

Dan Pomeroy, MIT
Host: Craig Blocker

Abstract: 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.

"What should we do with a small quantum computer?"

February 23, 2016

Aram Harrow, MIT
Host: Matthew Headrick

Abstract: 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.

"The Dark Energy Survey and Gravitational Waves"

March 1, 2016

Marcelle Soares-Santos, Fermilab
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.

"Quantum Quenches"

March 8, 2016

Aditi Mitra, NYU
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.

"What's going on inside of a proton? The story of lattice QCD"

March 22, 2016

Paul Mackenzie, Fermilab
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.

"What does the Golden Ratio have to do with friction? An answer atom by atom"

March 29, 2016

Vladan Vuletic, MIT
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.

"Gravitational Wave Detection with Advanced LIGO"

April 5, 2016

Matthew Evans, MIT
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.

"Emergent Fine-tuning to Environmental Drives in a Random Chemical Mixture"

April 12, 2016

Jeremy England, MIT
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.

"Fiber-forming filaments figure out frustration: how impossible packing shapes the assembly of chiral filaments"

April 19, 2016

Gregory Grason, UMass Amherst
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.

Fall 2015 Colloquia

"The Rise of Jet Substructure: Boosting the Search for New Physics at the LHC"

September 8, 2015

Jesse Thaler, MIT
Host: Albion Lawrence

"Airway Surface Brush Sweeps Lungs Clean: Polymer Physics Helps Us Breathe Easier"

September 22, 2015

Michael Rubinstein, UNC
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.

"Measuring Entanglement Entropy in Synthetic Quantum Matter"

October 6, 2015

Markus Greiner, Harvard University
Host: Matthew Headrick

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.

"Understanding the Nature of Neutrinos: Recent Discoveries and Future Prospects"

October 13, 2015

Karsten Heeger, Yale
Host: Gabriella Sciolla

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.

Eisenbud Lectures in Mathematics and Physics

October 27, 2015

October 27 - October 29
Jeffrey Harvey, University of Chicago

October 27, 2015
"A physicist under the spell of Ramanujan and moonshine"

October 28, 2015
"Mock modular forms in mathematics and physics"

October 29, 2015
"Umbral Moonshine"

"Fingerprints of the Early Universe"

November 3, 2015

Cora Dvorkin, Harvard University
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.

"Geometry and symmetry in nanomaterial self-assembly"

November 10, 2015

Steve Whitelam, Lawrence Berkeley National Lab
Joint Quantitative Biology/Martin Weiner Lecture Series
Physics Department Colloquium
Host: Michael Hagan

"Hydrodynamics of Swimming Microorganisms in Complex Fluids"

November 17, 2015

Tom Powers, Brown University
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.

"Pattern formation in soft and biological matter"

December 1, 2015

Joern Dunkel, MIT
Host: Zvonimir Dogic

"Getting in Shape: how do microorganisms control their geometry?"

December 8, 2015

Ariel Amir, Harvard University
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.

Spring 2015 Colloquia

"S patiotemporal control of the active forces that shape living tissues"

February 5, 2015

Karen Kasza, Sloan-Kettering Institute

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.

"Examining the Mechanics of Dynamic Microtubule Networks"

February 10, 2015

Scott Forth, Rockefeller University

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.

"Active mechanics in living oocytes reveals molecular - scale kinetics"

February 12, 2015

Wylie Ahmed, Institute Curie

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.

"Non linear dynamics and phase space behavior of interacting biological components: From genes to ecosystems"

February 17, 2015

Al Sanchez, Harvard University

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.

"Cellular Forces Measured and Controlled by DNA-based Molecular Force Sensor & Modulator"

February 24, 2015

Xuefeng Wang, University of Illinois, Urbana-Champaign

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.

"Sculpting phase diagrams: freezing by heating, switchable crystals, and more"

February 26, 2015

Benjamin Rogers, Harvard University

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.

"Vortex Knots in Fluids and Superfluids"

March 9, 2015

Dustin Kleckner, University of Chicago

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.

"Measuring Elastic Membrane Properties in Computer Simulations"

March 10, 2015

Markus Deserno, Carnegie Mellon Physics
Host: 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.

"Manufacturing, actuation, sensing, and control for robotic insects"

March 17, 2015

Robert J. Wood, Harvard University
Host: 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.

"Flipping the classroom to improve STEM learning"

March 24, 2015

Sanjoy Mahajan, MIT
Host: Jané 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.

"Graphene Nanobolometers for Ultrasensitive Far-Infrared Detection"

March 31, 2015

Daniel Prober, Yale University
Host: 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.

"What happened to Einstein's Brane?"

April 14, 2015

Paul Townsend, Cambridge University
Host: Albion Lawrence

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.

"Dynamics and Nanostructure of Polymer Materials from NMR and Scattering Analysis"

April 21, 2015

Klaus Schmidt-Rohr, Brandeis University

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 2014 Colloquia

"What's the Matter with the Universe?"

September 2, 2014

Enectali Figueroa-Feliciano, MIT

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.

"Quantum entanglement and the geometry of spacetime"

September 9, 2014

Matthew Headrick, Brandeis University

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.

"Collective migration and cell jamming"

September 16, 2014

Jeffrey J. Fredberg, Harvard School of Public Health

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.

"Levitation by Casimir forces in and out of equilibrium"

September 30, 2014

Mehran Kardar, MIT

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.

"Modeling the toughness of metallic glasses"

October 7, 2014

Christopher Rycroft, Harvard University

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.

"Hydrodynamics and quantum anomalies"

October 14, 2014

Dam Thanh Son, University of Chicago

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.

"After the Higgs: What's Next for Particle Physics?"

October 21, 2014

Matthew Reece, Harvard University

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.

"That Spin 0 Boson Changes Everything--The Future of the Energy Frontier in Particle Physics"

October 18, 2014

Raymond Brock, Michigan State

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.

"The Quantum and the Continuum: Einstein's dichotomous legacies"

November 4, 2014

Parthasarathi Majumdar, Ramakrishna Mission Vivekanandan University

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.

"Testing Turing's Theory of Morphogenesis"

November 11, 2014

Seth Fraden, Brandeis University

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.

"Physics of Fusion Energy; What we know and what we don't know"

November 18, 2014

Ian Hutchinson, MIT

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.

Eisenbud Lectures in Mathematics and Physics

December 2, 2014

Peter Sarnak, Institute for Advanced Study, Princeton University

December 2, 2014
Lecture I: "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.

December 3, 2014
Lecture II "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.

December 4, 2014
Lecture III "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.

Spring 2014

"Universal models for coherent flow structures in turbulence"

January 14, 2014

Eric Brown, Yale University

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.

"Dark Energy and Cosmic Sound"

January 21, 2014

Daniel Eisenstein,The Harvard-Smithsonian Center for Astrophysics

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.

"Learning aerodynamics from insects and dreaming up new ways to fly"

January 28, 2014

Leif Ristroph, Courant Institute at New York University

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.

"From Black Holes to Fluid Holograms"

February 4, 2014

Veronika Hubeny, University of Durham

Abstract: 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.

"A Soft X-ray Spectropolarimeter Telescope"

February 11, 2014

Herman Marshall, MIT Kavli Institute

Abstract: 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.

"Weakly Interacting Particles at the LHC: Searches for new forces, symmetries and dark matter"

February 25, 2014

Christopher Rogan, Harvard University

Abstract: 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.

March 11, 2014

A Celebration of Leonard Eisenbud's 100th Birthday
Cumrun Vafa, Harvard University
Lecture 1: Strings and the Magic of Extra Dimensions

March 12, 2014
Lecture 2: Recent Progress in Toplogical Strings I
Lecture 3: Recent Progress in Topological Strings II

"The Physics of Sand: Emergent Behavior in the Macroworld"

March 25, 2014

Bulbul Chakraborty, Brandeis University

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.

"Performance through Deformation and Instability"

April 1, 2014

Katia Bertoldi, Harvard University

Abstract: 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.

"Swimming in Sand"

April 8, 2014

Daniel I. Goldman, Georgia Institute of Technology

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.

"Nanoscale Coaxial Probes with Optical, Solar and Sensing Utility"

April 29, 2014

Michael J Naughton, Boston College

Abstract: 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.