The Physics Department Colloquia are held on Tuesdays at 4 p.m. via Zoom unless otherwise noted. Please contact Dawn Mitchell <email@example.com> for Zoom passwords.
September 15, 2020
Howard Stone, Princeton University
Host: Guillaume Duclos
Abstract: Many modern research themes in science and engineering introduce new questions, some of which can be understood using fundamental concepts. We will provide examples of research discoveries from the subjects of thin film dynamics and cellular biology and in each case illuminate the phenomenon using fundamental ideas in fluid dynamics, with experiments and theory.
In the first example, we will document experimentally the time and (three-dimensional) space variations of the shape of a falling film near the edge of a vertical plate and rationalize the quantitative features using a similarity solution. This example seems particularly unusual since we are able to theoretically show that the shape is described by a nonlinear partial differential equation, involving three independent variables, yet the equation can be reduced by a similarity.
September 22, 2020
Rob Phillips, California Institute of Technology
Host: Jané Kondev
Abstract: 2019 marked the 75th anniversary of the publication of Erwin Schrodinger's ``What is Life?'', a short book hailed by Roger Penrose as ``among the most influential scientific writings'’ of the 20th century'’. In this talk, I will briefly review the long argument made by Schrodinger as he mused on how the laws of physics could help us understand ``the events in space and time which takes place within the spatial boundary of a living organism.'' Though often chronicled for its influence on some of the titans of the rise of molecular biology, my talk takes a different tack. I pivot to the idea of Schrodinger’s classic as a timeless manifesto. Rather than a dated historical curiosity, I argue that ``What is Life?'' is full of stylish approaches to understanding the mysterious livingworld around us and instead, might be viewed as a call to arms to tackle the unfinished business of the scientific study of living matter. To make those points, I will focus on two key case studies from my own laboratory that pursue several of the key threads laid out in Schrodinger’s book: i) how does the control of hereditary information buried within the genome look from the point of view of physics and ii) how do the spatiotemporal structures (mitotic spindle, organelles, tissues, etc.) so familiar in biology arise as the playing out of underlying laws of non equilibrium physics?
September 29, 2020
October 6, 2020
Henriette Elvang, University of Michigan, Ann Arbor
Host: Brian Swingle
Abstract: There is a wide range of physical phenomena -- from boiling water to pion scattering -- that can be described theoretically in a common mathematical language, namely that of quantum field theory. In recent years, there has been a resurgence of new powerful approaches to explore this theoretical framework. In the talk, without assuming any prior knowledge of field theory, I will present some of these ideas and give examples of their application in contexts that range from ferromagnets to nonlinear electrodynamics and quantum gravity.
October 13, 2020
Reto Trappitsch, Brandeis University
Abstract: Nuclear astrophysics and the evolution of elements in the Milky Way are topics generally associated with theory, large-scale nuclear physics experiments, and astronomical observations. Primitive meteorites, which are Solar System rocks that represent the original composition of the solar nebula, however preserved micrometer-sized, bona fide stardust grains. These grains formed in the death throes of dying stars and subsequently travelled through the interstellar medium until they were incorporated into the first solids that formed in the solar nebula. Each stardust grain represents a fingerprint of its parent star. Therefore, by analyzing these samples we can directly probe the stellar environment they formed in.
In my talk I will give an overview of stellar nucleosynthesis and galactic chemical evolution and discuss how stardust grain measurements done in the lab have contributed to our understanding of these fields. Additionally I will discuss how open questions in these fields pose challenges that need to be addressed in the future with a multi-disciplinary approach combining theoretical and experimental nuclear physics, nucleosynthesis modeling, and stardust grain analyses.
October 20, 2020
Asimina Arvanitaki, Perimeter Institute
Host: Brian Swingle
Abstract: While there is undisputed evidence for Dark Matter, its nature and properties remain one of the biggest questions of our time. What is Dark Matter(DM)? How is it produced? Does it have interactions other than gravitational? In this talk, I will describe how a large class of bosonic particles can account for the DM of the Cosmos. These particles can be much lighter than those of the Standard Model with Compton wavelengths that are bigger than the size of our solar system or smaller than a millimeter. In the presence of attractive self-interactions, there is a parametric resonance effect in the early universe that can cause growth of structure at small scales, an effect so dramatic that can cause structures to collapse well before matter-radiation equality. The signatures of this effect span several experiments and orders of magntude in parameter space. When the DM boson is heavy, the dense DM halos can alter the optimal search strategies in direct detection experiments. When the DM boson is light, these halos may leave their imprint in searches for dark matter substructure, primordial gravitational waves and alter the star formation history of the universe.
October 27, 2020
Mark Bowick, University of California, Santa Barbara
Host: Brian Swingle
Abstract: Thermalized two-dimensional metamaterials, such as graphene at room temperature, exhibit novel mechanical properties by virtue of being bending, rather than stretching, dominated. A wide variety of responses can be obtained simply by varying purely geometrical properties such as size and shape. Furthermore, by slicing and dicing to change the topology of the sheet, one can realize long sought-after phenomena such as the thermal crumpling of elastic surfaces and multiple origami-like folding. I will discuss recent developments in this area including theory, simulations and experimental tests and challenges.
November 3, 2020
November 10, 2020
Ana Maria Rey, University of Colorado, Boulder
Host: Brian Swingle
Abstract: Non-equilibrium quantum many-body systems can display fascinating phenomena relevant for various fields in science ranging from physics, to chemistry, and ultimately, for the broadest possible scope, life itself. The challenge with these systems, however, is that the powerful formalism of statistical physics, which have allowed a classification of quantum phases of matter at equilibrium does not apply. Precisely engineered ultracold gases are emerging as a powerful tool to shed light on the organizing principles and universal behaviors of dynamical quantum matter. One emerging paradigm where ultracold atomic systems have shown to be a powerful resource is the case of dynamical phase transitions (DPTs). DPTs share many aspects of standard equilibrium phase transitions but differ in many fundamental ways with them. They also allow for the generation of metrologically useful steady states. I will discuss the observation of DPTs in different but complementary systems: trapped quantum degenerate Fermi gases, trapped bosonic gases and long lived arrays of atoms in an optical cavity. I will show how these systems can be used to simulate iconic models of quantum magnetism with tunable parameters and to probe the dependence of their associated dynamical phases on a broad parameter space. Besides advancing quantum simulation, our studies pave the ground for the generation of metrologically useful entangled states which can enable real metrological gains via quantum enhancement.
November 17, 2020
Jay Deep Sau, University of Maryland
Host: Brian Swingle
Abstract: Majorana zero modes are fermion-like excitations that were originally proposed in particle physics by Ettore Majorana and are characterized as being their own anti-particle. In condensed matter systems Majorana zero modes occur as fractionalized excitations with topologically protected degeneracy associated with such excitations. For over a decade the only candidate systems for observing Majorana zero modes were the non-Abelian fractional quantum Hall state and chiral p-wave superconductors. In this talk, I will discuss a recent set of proposals for realizing Majorana zero modes in a large class of spin-orbit coupled, time-reversal symmetry broken superconducting systems. The simplicity of this class of systems has resulted in several experimental attempts, which have successfully observed preliminary evidence for the Majorana zero modes in the form of zero-bias conductance peaks and the fractional Josephson effect. Following this I will review recent experimental progress on spin-orbit coupled superconductors. I will end by describing what a topological quantum computer based on Majorana modes might look like in the future.
November 24, 2020
Hannah Yevick, Massachusetts Institute of Technology
Host: Jané Kondev
Abstract: Tissue folding is a ubiquitous shape change event during development whereby a cell sheet bends into a curved 3D structure. In this talk, I will show that this mechanical process is remarkably robust, and the correct final form is almost always achieved despite internal fluctuations and external perturbations inherent in living systems. While many genetic and molecular strategies that lead to robust development have been established, much less is known about how mechanical patterns and movements are ensured at the population level. I will describe how quantitative imaging, physical modeling and concepts from network science can uncover collective interactions that govern tissue patterning and shape change.
Actin and myosin are two important cytoskeletal proteins involved in the force generation and movement of cells. Both parts of this talk will be about the spontaneous organization of actomyosin networks. First, I will present how out-of-plane curvature can trigger the global alignment of actin fibers and a novel transition from collective to individual cell migration in culture. I will then describe how tissue-scale cytoskeletal patterns can guide tissue folding in the early fruit fly embryo. I will show that actin and myosin organize into a network that spans a large region of the embryo. Redundancy in this supracellular network encodes the tissue’s intrinsic robustness to mechanical and molecular perturbations during folding. Moving forward, deciphering the physical basis of robustness in living systems will inspire new ways to engineer and control biological tissues and bioinspired active materials that can spontaneously fold into predetermined complex 3D shape.
December 1, 2020
Zoom link: https://brandeis.zoom.us/j/91486577615
Juan Perilla, University of Delaware
Host: Michael Hagan
Abstract: The essential conundrum of modern biology, namely the question of how life emerges from myriad molecules whose behavior is governed by physical law alone, is embodied within a single cell—the quantum of life. The rise of scientific supercomputing has allowed for the study of the living cell in unparalleled detail, from the scale of the atom to a whole organism and at all levels in between. In particular, the past three decades have witnessed the evolution of molecular dynamics (MD) simulations as a “computational microscope”, which has provided a unique framework for the study of the phenomena of cell biology in atomic (or near-atomic) detail. Here I present an overview of our efforts to determine the molecular details during the life-cycle of multiple infectious diseases using the computational microscope.