Eric Sembrat's Test Bonanza

Abstract

Certain non-equilibrium systems, including growing microbial colonies, amorphous solids under oscillatory shear, turbulent liquid crystals, and avalanches undergo dynamical phase transitions across which we observe fluctuating, active regions of the system either propagate and grow with time, or go extinct, forcing the system into an absorbing state. 

We will focus on such transitions in two very different systems: a microbial colony in which a fit strain irreversibly converts to a less fit one (leading to the possibility of strain extinction), and a dense, amorphous solid under oscillatory shear. In the case of the microbial colony, we show that the spatial distribution and geometry of the colony profoundly impacts the phase transition, with spatial fluctuations driving extinction of the fit strain. 

In the driven amorphous solid, we show that the dynamical phase transition competes with another phase transition at which the solid loses rigidity: the jamming point.  We show that as the jamming point is approached, the absorbing states associated with reversible, quiescent dynamical behavior become more and more complex. 

Abstract

This presentation will describe an arc in the mathematical/theoretical physics research of the presenter that has traversed concept spaces from equations, to graphical imagery, to coding theory error-correction, and pointing toward evidence of an evolution-like process possibly having acted on the mathematical laws that describe reality.

About the Lecturer

Professor Gates was appointed Ford Foundation Professor of Physics at Brown in 2017, and also holds an appointment in the Department of Mathematics. He first joined the Brown community in fall 2016, as an inaugural Provost Visiting Professor. Prior to this, he was Distinguished University Professor, University Regents Professor, John H. Toll Professor of Physics and Director of the Center for Particle and String Theory at the University of Maryland. Gates was awarded the 2011 National Medal of Science, is a member of the National Academy of Sciences, the American Academy of Arts and Sciences, the American Philosophical Society, and is a fellow of both the American Association for the Advancement of Science and the American Physical Society. He served on the Maryland State Board of Education and was a member of the President’s Council of Advisors on Science and Technology (PCAST). As a member of the PCAST, he was co-chair of its working group on STEM preeminence for the nation and co-authored a report to the President: ”Prepare and Inspire K-12 Education in Science, Technology, Engineering and Math (STEM) for America’s Future.”

About Frontiers in Science Lectures

Lectures in this series are intended to inform, engage, and inspire students, faculty, staff, and the public on developments, breakthroughs, and topics of general interest in the sciences and mathematics. Lecturers tailor their talks for nonexpert audiences.

Abstract

Eukaryotes package and maintain their genetic code in chromatin fibers. The fundamental unit of chromatin is the nucleosome, a complex of approximately equal mass of protein and DNA molecules.  By altering the physical and biochemical properties of the nucleosome, the cell regulates the structure and stability of chromatin and thus tunes gene expression. 

 In this talk, I will discuss efforts by our group to use molecular dynamics simulations in conjunction with data from NMR, SAXS, and Cryo-EM experiments to understand the processes by which chromatin remodeling factors alter the structure and dynamics of single and poly-nucleosomal arrays.  I will focus on the effects of histone variants, post-translational modifications, and linker histones. 

In addition, I will discuss how we can use Bayesian inference to rigorously determine a minimal ensemble of states of flexible biomolecular complexes to describe the results of small angle X-ray scattering experiments from enhanced sampling simulations.   

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).

Abstract:

In this talk, I’ll discuss climatic issues faced by LGBT people in Physics, informed by findings of the recent American Physical Society report on the status of LGBT people in Physics. This report was prepared for the APS by an ad hoc committee of physicists, spanning a range of institutions types and career stages, and included information from focus groups held at APS meetings, the first ever climate survey of LGBT people in physics, and a set of in-depth interviews with individuals who self-identify as LGBT. Driven by this evidence, I’ll discuss implications for pedagogy, as well as collective and individual actions the community can take to alleviate the issues identified. 

Bio:

Tim is Associate Professor of Physics at Tufts University. He received his PhD in Physics in 2007 from the University of Exeter in the UK where he studied frustration phenomena in liquid crystals as part of the Electromagnetic Materials group with Professor Roy Sambles FRS. He then spent two years as a postdoctoral scholar at Case Western Reserve University in the Rosenblatt group contributing to a diverse range of projects: from the Rayleigh-Taylor instability to direct imaging of liquid crystalline order via the technique of Optical Nanotomography. He joined the faculty of Tufts University in the Department of Physics and Astronomy in Fall 2011. 

Abstract:

One of the remarkable features of spins in the solid state is the enormous range of time-scales over which coherent manipulation is possible. If one considers gate-controlled manipulation of nuclear spins at one extreme, and strongly-interacting multi-electron qubits at the other extreme, coherent control of spins in semiconductors has been demonstrated with over 9 orders of magnitude variation in the manipulation time. Remarkably, confining three electrons in two neighboring quantum dots enables all electrical control and measurement of spin dynamics on time scales less than one nanosecond.

 In this talk I will discuss the interesting commonalities and contrasts between the two limiting cases: qubits composed of a single-spin, be it electron or nuclear, where magnetically-driven manipulation is required, and qubits composed of multiple electrons, for which case direct electric-field manipulation is possible.

Bio:

Mark A. Eriksson is Vilas Distinguished Achievement Professor in the Department of Physics at the University of Wisconsin-Madison.  Prior to joining the University of Wisconsin in 1999, he received his Ph.D. from Harvard University in 1997 and was a postdoctoral member of technical staff at Bell Labs for two years from 1997-1999. Currently he leads a multi-university team studying semiconductor-based quantum computing and focusing on the development of spin qubits in silicon/silicon-germanium gate-defined quantum dots.  Eriksson is a fellow of the American Physical Society and the American Association for the Advancement of Science.

*This talk is based on part of the following paper:

Michel Janssen, "Arches and Scaffolds: Bridging Continuity and Discontinuity in Theory Change." Forthcoming in: Alan C. Love and William C. Wimsatt (Eds.), Beyond the Meme. Development and Structure in Cultural Evolution. Minneapolis: University of Minnesota Press, in preparation. The paper can be downloaded at: https://drive.google.com/file/d/1UpnsJ9uagoBjPcrtGmPVXsq-2dseX5o4/view

Abstract:

In early 1927, Paul Dirac and Pascual Jordan, independently of one another, published their versions of a general formalism tying the various forms of the new quantum theory together and giving the theory’s statistical interpretation in full generality. This formalism has come to be known as the Dirac-Jordan (statistical) transformation theory.  A few months later, in response to these publications, John von Neumann published his Hilbert space formalism for quantum mechanics.

The relation between the two formalisms can be captured in terms of a metaphor of arches and scaffolds that I have argued fits a number of instances of theory change in physics. What is unclear in this case is whether the story is best told with Hilbert space playing the role of the arch built on transformation theory as a scaffold to be dismantled once the arch could support itself, or with transformation theory playing the role of the arch and Hilbert space providing the scaffold built to prevent Jordan’s mathematically unsound arch from collapsing. Either way, a narrative for this episode in the history of quantum mechanics based on the arches-and-scaffolds metaphor illustrates the promise of borrowing ideas from the approach to evolutionary biology known as evodevo for reconstructing genealogies of theories rather than species.

 

Abstract: Scientists and journalists have similar, but not identical interests in getting information about their work to the public in an appealing, but accurate way. James Gorman will draw on his experience as a New York Times reporter and editor to talk about these common and competing interests and what works in translating technical information for a popular audience.

Bio: James Gorman is a science writer at large for The New York Times and the host and writer of the regular video feature “ScienceTake.” He has been at the Times since 1993, as an editor on The New York Times Magazine, deputy science editor, editor of a personal technology section, outdoors columnist, science columnist and editor of Science Times. Over the course of his career at the Times and elsewhere, Mr. Gorman has written about everything from the invention of flea collars to the nature of consciousness. Most recently he has covered neuroscience and the lives of animals in and out of scientific research. Before joining The Times, Mr. Gorman wrote books on penguins, dinosaurs, the Southern Ocean and hypochondria. His most recent book is “How to Build a Dinosaur,” 2009, written with the paleontologist Jack Horner. He also writes humor, which he has contributed to The New Yorker, The Atlantic, the New York Times Magazine and other publications. He has taught science writing at New York University, Fordham University and online in Stanford University’s Continuing Studies program. In the fall of 2011, he was the McGraw Visiting Professor of Writing at Princeton University. Mr. Gorman graduated from Princeton in 1971 with a bachelor's degree in English literature.

 Abstract:

Ions in water are typically understood to be hydrated, i.e. the appropriate water dipolar orientations point toward the charge (which is here taken to be of spherical symmetry).  Depending on the specific charge, these oriented waters of hydration are tightly bound in up to three aqueous monolayers.  The effective “hard core” radius of the ion together with its tightly bound water molecules is in the nanometer range.  

Because dipoles have zero net charge, the hydrated ion complex is generally viewed to have the samec harge as the unhydrated ion.  We apply the Hone, Pincus¹ view of hydrogen bonding in water to suggest that this may not be precisely correct and that there may be an effective charge which differs from the nominal charge.  It is this effective charge that would be implicated in such phenomena as Debye screening.