The Physics Department Colloquia are held on Tuesdays at 4 p.m. in Abelson 333 and on Zoom unless otherwise noted. To request a Zoom link, please email firstname.lastname@example.org.
August 31, 2021
Arnab Datta, Brandeis University
Host: John Wardle
"Energy cost of protein gradient formation in cells"
Abstract: Cells make protein gradients for various purposes, such as establishing position information in development or defining cell polarity in the process of cell division. Two classes of mechanisms for maintaining protein gradients in cells have been reported in the literature: i. those that combine protein diffusion and degradation, and ii. mechanisms that involve active transport of proteins by molecular motors. An example of the first mechanism is the Bicoid protein gradient in the Drosophila embryo, which provides positional information to the nuclei during development . A Smy1 gradient along actin cables in budding yeast cells regulates cable length and is formed by active transport of the proteins by myosin motors to the bud neck . Establishing and maintaining these protein gradients require cells to expend energy. In this talk I examine different mechanisms of gradient formation in cells and estimate the energy costs associated with them. I also consider the scaling of the energy expenditure with cell size for the two different models of gradient formation and discuss when one mechanism is energetically less costly than the other.
 O. Grimm, M. Coppey, and E. Wieschaus, Development. (2010) 137:2253-64.
 J. A. Eskin, A. Rankova, A. B. Johnston, S. L. Alioto, and B. L. Goode, Mol Biol Cell. (2016) 27:828-37.
Daichi Hayakawa, Brandeis University
Host: John Wardle
"Programming the self-assembly of tubules using DNA origami triangles as building blocks"
Abstract: DNA origami is a method by which a single-stranded DNA scaffold is folded into some prescribed shape by hundreds of user-designed DNA ‘staple’ strands. Historically, this process has been used to make intricate 3D nanostructures with sub-nanometer precision. In this talk, I will discuss a new route for using DNA origami to make colloidal particles that then self-assemble into well-defined geometrical structures. More specifically, I use DNA origami to make triangular subunits and control their binding angles to program the assembly of nanotubes. The nanotubes are assembled from one type of triangle whose three edges bind to themselves at prescribed dihedral angles and are programmed by the DNA sequence design. I show that DNA origami triangles, each roughly 50 nanometers in size, can assemble into rigid tubules reaching tens of micrometers in length. Interestingly, I find that there is a distribution in width and the chirality of the assembled tubes, suggesting that our DNA origami colloids could be flexible or that the kinetic pathway toward tube closure plays a vital role in determining the final structure. Further, I discuss a method for limiting such tube type distributions by increasing the number of subunit types involved in the assembly.
September 7, 2021
September 14, 2021
James Cho, Flatiron Center for Computational Astrophysics and Brandeis University
Host: Albion Lawrence
Abstract: 'Exoplanets' is an exciting, new field of astrophysics. The field has grown rapidly since the first discovery of an exoplanet around a Sun-like star, only ~25 years ago – for which the Nobel prize in physics was recently awarded. With thousands of exoplanets now detected, accurate characterization of their atmospheres – in particular, their composition, weather, and climate – has become the next critical step in understanding them. The characterization is not only crucial for understanding current observations, it is crucial for ultimately assessing whether planets can harbor life. In this talk, how physics and mathematics are used to address these complex problems as well as what we currently understand about the problems are presented. The presentation will focus on the structure and evolution of ‘exo-storms’ and the variability they induce that could be observed with current missions and future ones that are soon to come online.
September 21, 2021
September 27, 2021
*Please note that this colloquium takes place on a Monday
Brian Swingle, Brandeis University
The physics of quantum chaos is relevant for a broad range of problems, from the origin of hydrodynamics in quantum systems to the nature of black holes in quantum gravity, and it is increasingly experimentally accessible thanks to rapid developments in highly-controlled quantum systems. However, this breadth has led to a myriad of different characterizations of chaos, with the interrelations between them often still mysterious. I will describe progress towards a more unified framework by highlighting an emerging set of connections between three key manifestations of chaos: information scrambling, fluctuating hydrodynamics, and random matrix theory.
October 5, 2021
Lee Roberts, Boston University
Host: Aram Apyan
Abstract: The Standard Model provides a very precise prediction of the muon’s magnetic anomaly aμ = (gμ - 2)/2, the deviation from 2 of the gyromagnetic ratio gμ. In his seminal 1926 paper, P.A.M. Dirac predicted that for electrons ge = 2, but experiments then revealed that ge was slightly larger than 2. The reason was to be found in Quantum Mechanics, and the first radiative correction to ge, calculated by Julian Schwinger, explained a deviation of order 0.1 %. Today, the Standard Model predicts the value of aμ to a precision of ± 0.36 parts per million (ppm). Dedicated experiments have measured aμ to ± 0.35 ppm precision. Therefore, precision measurements of the anomaly provide a stringent test of the Standard Model’s completeness, since Nature knows about all forces that could contribute to the muon’s magnetism, including those from New Physics that has not yet been discovered.
I will briefly review the intellectual history that began with the discovery of spin and the g-factor of the electron and its role on the development of Modern Physics. I will then focus on the new measurement of the muon magnetic anomaly that was recently reported by Fermilab experiment E989. The result determined from the first data set collected in 2018 has a precision of 0.46 ppm, and agrees well with the previous result obtained at Brookhaven National Laboratory at the beginning of this century. The combined experimental value exhibits tension with the Standard Model value.
October 12, 2021
Aram Harrow, Massachusetts Institute of Technology
Host: Aram Apyan
Abstract: The appeal of quantum computing is based on the fact that simulating N quantum systems on a classical computer takes time exponential in N. This exponential hardness is known to hold even for shallow quantum circuits, meaning unitary dynamics that run for a constant amount of time. We show that when the quantum circuits are made of random gates on a 2D geometry, they are not always exponentially hard to simulate. Instead, we give evidence for a phase transition in computational difficulty as the depth and local dimension are varied. Our evidence consists of (1) fast classical simulations of random circuits on a 400x400 grid of qubits, (2) a mapping to the order/disorder transition in an associated stat mech model, and (3) a proof that some circuit families are easy to simulate approximately but hard to simulate exactly. Our algorithms are based on tensor network contraction and mapping the 2D random unitary circuit to a 1D process consisting of alternating rounds of random local unitaries and weak measurements.
October 19, 2021
Special Division of Science Seminar
*Please note that this seminar will take place in Gerstenzang 123.
Chanda Prescod-Weinstein, University of New Hampshire
October 26, 2021
Piyush Grover, University of Nebraska, Lincoln
Host: Seth Fraden
Abstract: The geometrical framework of dynamical systems theory was originally developed by Poincare to study the chaotic dynamics of the gravitational three-body problem. In this talk, we will first discuss the application of this theory to explain the (sometimes puzzling) motion of celestial bodies, as well as to design non-intuitive fuel-efficient space missions to moon and beyond. A typical particle meanders through the phase space of an N-body problem by travelling on invariant manifolds that connect different equilibria and periodic orbits. These invariant manifolds, created by the competing gravitational forces, act as `interplanetary superhighways'. Next, we will discuss the far reaching generalizations of this framework in the context of hydrodynamics, including our recent work in 2D active nematics. Hydrodynamics equations give rise to a deterministic nonlinear dynamical system evolving in an infinite dimensional phase space. The dominant flow structures are understood in terms of Exact Coherent Structures (ECS) and the invariant manifolds connecting them. An ECS is (generically unstable) stationary, periodic, quasiperiodic, or traveling wave solution of the hydrodynamic equations. A finite set of ECS, together with their invariant manifolds, constitutes a reduced-order but exact characterization of the global phase space. Though each ECS is non-turbulent, this representation has been shown to be adequate for describing high Reynolds number turbulent flows of passive fluids, which appear as chaotic trajectories meandering through the phase space and visiting the neighborhoods of different ECS in a recurring fashion. We provide evidence that the ECS and their invariant manifolds also act as an organizing template for the complicated spatiotemporal motion of active fluid turbulence.
November 2, 2021
Philip Harris, Massachusetts Institute of Technology
Host: Aram Apyan
November 9, 2021
Martin Bazant, Massachusetts Institute of Technology
Host: Aram Apyan
November 16, 2021
Colin Hill, Columbia University
Host: Aram Apyan
November 23, 2021
November 30, 2021
December 7, 2021