Past Colloquia: 2014-2015
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.
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 stat" 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.
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.
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.
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.
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.
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 "reconnectio" 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.
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.
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.
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.
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.
April 14, 2015
Paul Townsend, Cambridge University
Host: Albion Lawrence
Abstract: In 1905 Einstein abolished the "luminiferous aethe" 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 constan" 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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
December 2, 2014
Peter Sarnak, Institute for Advanced Study, Princeton University
Dec. 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.
Dec. 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 wave. We discuss this in the context of the eigenfunctions on the modular surface — i.e "Mass For." and especially what can be proved about their nodal domains.
De. 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.