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physics-astronomy

Physics and Astronomy Colloquium, 2009-2010

Colloquia are held on Thursdays at 4pm in room 4327 (building 4) of the Stevenson Science Center unless otherwise noted. Click here for directions, or phone the department. A reception with the speaker is held at 3:30pm in Stevenson 6333.

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Fall 2009

Thursday, August 27

Enectali Figueroa-Feliciano, MIT

Towards Direct Detection of Dark Matter with the Cryogenic Dark Matter Search   (show abstract)

After 20 years of hard work, we've come to the realization that we don't know what 96% of the universe is made of. About 23% of the universe is made up of dark matter, a mysterious substance that has played the dominant role in the formation of structure in our universe. The Cryogenic Dark Matter Search has attained world-leading sensitivity in the race to directly detect dark matter in the laboratory. I will review dark matter science, our experiment, our current results, and the next steps in the hunt to solve one of the most pressing questions in physics today.

Host: A. Berlind

Thursday, September 3, Guy & Rebecca Forman Lecture

Wolfgang Christian, Davidson College

Implementing A Computer Rich Modeling-Cycle Pedagogy    (show abstract)

Over the past dozen years the Davidson College Physics Department has produced some of the most widely used interactive computer-based curricular materials for the teaching of introductory and advanced physics courses. These materials are based on Java applets called Physlets and on new Open Source Physics programs and authoring tools. This talk outlines the pedagogical and technical features of our material and describes our current effort to create new material using the Easy Java Simulations modeling tool and to distribute it via the comPADRE National Science Digital Library. The Open Source Physics collection is available on the comPADRE website at http://www.compadre.org/osp/. Partial funding for this work was obtained through NSF grant DUE-0442581

Host: R. Scherrer

Thursday, September 17

Shane Hutson, Vanderbilt University

How does a fly make itself? Dissecting morphogenesis with laser-microsurgery   (show abstract)

During the development of an organism, sheets of epithelial cells dynamically expand, contract and bend. These movements generate organismal form in a process known as morphogenesis – a process driven by cell-generated forces. The generation, distribution and regulation of these forces have been explored in multiple mathematical and computational models; however, few attempts have been made to test the validity of these models in vivo. I will present a method for probing morphogenetic forces in vivo using laser microsurgery. My group currently focuses on laser hole-drilling – a quantitative method borrowed from the engineer’s toolbox for residual stress analysis – in which a single laser pulse is used to rapidly ablate a subcellular hole clean through a one-cell thick epithelium. The surrounding cells recoil away from this hole, relaxing the pre-ablation, morphogenetic stresses. By carefully tracking the recoils (on ms time scales, with sub-µm precision and for dozens of embryos), one can estimate how stress is distributed. By staging the embryos, one can infer how this stress distribution changes during development. I will present results from fruit fly (Drosophila) embryos during two stages of embryonic development – germband retraction and dorsal closure. I will also discuss finite-element models that reproduce the observed recoil behavior and their implications for the microscopic construction and dynamic maintenance of an embryonic epithelium.

Host: R. Scherrer

Thursday, October 15

Tom Rogers

Bad Movie Physics from the Perspective of Art and Science   (show abstract)

A discussion of the bad physics portrayed in Hollywood movies. How it detracts from the art of moviemaking while reinforcing physics misconceptions and how to turn it around as an effective teaching tool. To see some examples of Insultingly Stupid Movie Physics, visit http://www.intuitor.com/moviephysics/

Host: R. Scherrer

Thursday, October 22

October Break, no colloquium

Thursday, October 29

Duco Jansen, Biomedical Engineering, Vanderbilt University

Stimulating neurons with light: current state and future challenges   (show abstract)

A novel method that employs infrared laser pulses to induce electrical activity (EP/AP) in neurons will be presented. This method has been shown to have several fundamental advantages over traditional electrical stimulation, including the spatial precision of stimulation that can be achieved in a non-contact fashion, and the lack of a stimulation artifact on the recording electrodes in classic stimulation-recording experiments. In this seminar I will present an overview of the concepts and applications of optical nerve stimulation. Characterization of optical stimulation and physiological validation will be shown. The underlying biophysical mechanisms of optical stimulations appear to be thermally mediated. I will present our work on mechanistic studies as well as on the applications in the peripheral nervous system (stimulation of motor neurons and stimulation of sensory nerves in the cochlea), stimulation in the CNS, and the development of a stand-alone optical nerve stimulator.

Host: S. Hutson

Thursday, November 5

Charles Ahn, Yale University

The Materials Physics of Complex Oxides   (show abstract)

Complex oxide materials exhibit a tremendous diversity of behavior encompassing a broad range of functional properties, such as magnetism, ferroelectricity, and superconductivity. As diverse as this behavior is, an even richer spectrum of possibilities becomes available if one starts to combine different complex oxides together with atomic-scale precision to create new artificially structured, heterogeneous systems. In these nanostructured materials, cross-coupling between the functionalities of the individual component materials allows one material to modify the properties of the other constituent. This talk describes experiments on the electrostatic modulation of magnetism and superconductivity using intense electric fields in these materials, along with applications of this approach to address current challenges in the modern electronics industry.

Host: R. Scherrer

Thursday, November 12

Teresa Montaruli, University of Wisconsin

In Search of Extraterrestrial High Energy Neutrinos   (show abstract)

I will review the search for astrophysical neutrinos and the status and results of neutrino telescopes in operation and decommissioned. I will describe the methods used for data analysis, and background discrimination. I will give emphasis to recent results of IceCube and ANTARES. I will interpret these results and consider their impact on theoretical predictions of neutrino fluxes correlated with measurements using other messengers, specifically gammas and ultra-high energy cosmic rays.

Host: T. Weiler

Thursday, November 19

Craig Hogan, Fermilab

Holographic Noise in Michelson Interferometers: a Direct Experimental Probe of Unification at the Planck Scale   (show abstract)

Classical spacetime and quantum mass-energy form the basis of all of physics. They become inconsistent with each other at the Planck scale, 5.4 times 10^{-44} seconds, which may signify a need for reconciliation in a unified theory. Although proposals for unified theories exist, a direct experimental probe of this scale, 16 orders of magnitude above Tevatron energy, has seemed hopelessly out of reach. However in a particular interpretation of holographic unified theories, derived from black hole evaporation physics, a world assembled out of Planck-scale waves displays effects of unification with a new kind of uncertainty in position at the Planck diffraction scale, the geometric mean of the Planck length and the apparatus size. In this case a new phenomenon may measurable, an indeterminacy of spacetime position that appears as noise in interferometers. The colloquium will discuss the theory of the effect, and our plans to build a holographic interferometer at Fermilab to measure it.

Host: R. Scherrer

Thursday, November 26

Thanksgiving Holidays, no colloquium

Thursday, December 3

Anna Roe, Psychology, Vanderbilt University

Perception and the brain: from physics to psychophysics   (show abstract)

Physicists deal with measurements of the physical world. The accuracy of these measurements are subject to the fidelity and accuracy of their experimental instruments. Theories are developed to explain these measurements in a parsimonious and elegant fashion. Psychophysicists deal with measurements of the physical world as perceived through the brain. Their purpose is to understand and characterize mental representations. Currently, there are no theories to explain how internal mental representations are generated. As a neuroscientist, I strive towards understanding the circuitry in the brain which gives rise to the mental representations. Currently, what is known is that there are multiple stages of cortical processing that lead to what we know as perception. My research focuses on identifying what these stages are and, within each stage, what the elemental units of function are. The methods I use include optical imaging, electrophysiological, and behavioral methods in awake, trained animals. These studies have identified cortical elements representing simple object features, higher order complex objects, and mechanisms underlying visual attention and short-term working memory. My hope is that some day we will have an algorithm in hand that transforms the world of physics into the world of psychophysics.

Host: S. Hutson

Thursday, December 10

Chris Mihos, Case Western Reserve University

Using Intracluster Light to Probe Galaxy Clusters   (show abstract)

The life of a cluster galaxy is a violent one. As galaxy clusters form and evolve, their member galaxies frequently collide and interact both with other galaxies and galaxy groups, and with the cluster as a whole. Over time, gravitational tides strip stars from their host galaxies and spread them throughout the cluster to form a diffuse "intracluster light" (ICL). Using numerical simulations, we can explore the formation and structure of the ICL, and use it as a tracer of the dynamical evolution of galaxy clusters. We are using deep wide-field imaging to search for this extremely faint light in galaxy clusters, and have discovered a remarkably complex web of ICL in the nearby Virgo cluster.

Host: A. Berlind

Spring 2010

Thursday, January 14

Pankaj Mehta, Princeton University

From biological networks to complex behaviors   (show abstract)

It is now clear that Phil Anderson’s famous maxim “More is Different” holds true even in biology. For example, microbiologists now agree that bacteria commonly engage in complicated collective behaviors that require individual cells to receive, interpret, and respond to information from one another and their environment. Underlying these behaviors are complex biological signaling networks. Understanding these signaling networks poses interesting new physics problems. In this talk, I will discuss two examples from my own research: 1) how the identification of transcription factor binding sites naturally leads to fascinating questions about the “inverse” statistical mechanics of hard rods in a disordered potential and 2) how we can use methods from information theory and statistical physics for quantifying the information processing capabilities of bacterial signaling networks.

Host: R. Scherrer

Thursday, January 21

Naomi Ginsberg, Lawrence Berkeley National Laboratory

Ultrafast physics in photosynthesis: Mapping sub-nanometer energy flow   (show abstract)

In the first picoseconds of photosynthesis, photoexcitations of chlorophyll molecules are passed through a network of chlorophyll-binding proteins to a charge transfer site, initiating the conversion of absorbed energy to chemical fuels. The remarkably high quantum efficiency of this energy transfer relies on near-field coupling between adjacent chlorophyll molecules and their interaction with protein phonon modes. Using two-dimensional electronic spectroscopy, we track the time-evolution of energy flow in a chlorophyll-protein complex, CP29, found in green plants. The results from these nonlinear four-wave mixing experiments elucidate the role of CP29 as a light harvester and energy conduit by drawing causal relationships between the spatial and electronic configurations of its chlorophyll molecules. Through independent control of experimental light pulse polarizations, we have furthermore developed a technique to determine the relative angles between the transition dipole moments responsible for energy transfer. This work not only yields tools for structural and spectral molecular characterization, but also deepens our understanding of how photosynthetic systems have evolved to optimize the conversion of light to biomass.

Host: R. Scherrer

Monday, January 25 at 3:00 pm, (Note special day and time)

Thomas Angelini, Harvard University

Forces in Collective Cell Motion   (show abstract)

Individual living cells generate forces and direct their motion in well known ways. For example, planktonic bacteria swim through fluids by rapidly turning their flagella, and individual tissue cells migrate across surfaces in a cyclic process of expansion, adhesion, and retraction. These canonical types of motion, however, are not characteristic of cells within large, dense aggregates, such as bacterial colonies or the tissues of complex organisms. In this talk I will discuss tools and concepts of condensed matter physics that I have adapted to study the collectively generated forces that control multi-cellular motion within enormous cell aggregates. I will present research on bacterial biofilms, showing how they can spread by generating molecular gradients throughout the colony. I will also discuss collective motion within two-dimensional confluent sheets of mammalian tissue cells, showing how sub-cellular motions as well as multi-cellular forces, transmitted across long distances, each influence collective migration in different ways.

Host: R. Scherrer

Thursday, February 4

Amy C. Rowat, Harvard University

Single cell studies using microfluidic devices   (show abstract)

Cells that are genetically identical can exhibit differences in phenotype, however, such variation remains masked in bulk measurements. To capture variability among individual cells, as well as the behavior of subpopulations of cells, requires studies with single cell resolution. Here I will describe a new class of microfluidic devices that enables studies at the single cell level. First, I will describe a microfluidic device that enables measurements of the mechanical properties of individual cells. The ability of cells to deform through narrow spaces is central in physiological contexts ranging from immune response to metastasis. To elucidate the effect of nuclear shape on the deformability of neutrophil cells, I manipulate levels of a structural protein in the nucleus, and show this alters both nuclear shape and the ability of cells to deform through the narrow channels. These results help to elucidate the mechanism underlying nuclear shape transitions, and have implications for understanding changes in the physical properties of cancer cells. Second, I will describe a microfluidic device that enables tracking lineages of single cells. In most cases, heritable phenotypic variation arises from differences in DNA sequence, yet even cells that are genetically identical can exhibit variation in phenotype, which are critical during differentiation and development, and possibly in response to environmental stress. By studying the expression patterns of three, naturally regulated proteins in lineages deriving from single yeast cells, I show that the timescale of phenotypic variation differs markedly among the observed proteins; this is an essential step towards understanding the timescales of phenotypic variation, and correlations in phenotype among single cells within a population.

Host: R. Scherrer

Thursday, February 25

Jason Petta, Princeton University

Strong-arming electron spin dynamics   (show abstract)

A single electron spin in an external magnetic field forms a two-level system that can be used to create a spin qubit. However, achieving fast single spin rotations, as would be required to control a spin qubit, is a major challenge. It is difficult to drive spin rotations on timescales that are faster than the spin dephasing time and to individually address a single spin on the nanometer scale. In this colloquium, I will describe a new method for quantum control of single spins that does not involve conventional electron spin resonance (ESR). In analogy with an optical beam splitter, we use an anti-crossing in the energy level spectrum of our quantum dot “artificial atom” as a beam splitter for an incoming quantum state. The anti-crossing is used to prepare a superposition of singlet and triplet spin states, which then evolve according to the time-dependent Schrodinger equation. A return sweep through the anti-crossing results in quantum interference of the spin states. By changing the effective path length of one arm of our interferometer, we are able to achieve coherent rotations between a singlet state and a T+ triplet state. The rotations are nearly a factor of 100 faster than spin rotations achieved using conventional ESR and allow spins to be locally controlled using simple gate voltage pulses.

Host: R. Scherrer

Thursday, March 4

Mark Devlin, University of Pennsylvania

Where Did Half the Starlight in the Universe Go?   (show abstract)

We believe that approximately half of all the light from stars is absorbed and reprocessed by dust. The resulting emission is grey body with a temperature near 30 Kelvin. The COBE satellite made the first measurements of the resulting Far Infrared Background (FIRB), but since that time, we have been unable to resolve the background into individual galaxies. The Balloon-borne Large Aperture Submillimeter Telescope (BLAST) was designed to do this job. Its three bands at 250, 350, and 500 microns span the peak in emission for galaxies at z=1. I will discuss the BLAST experiment and present results from our measurements of resolved and unresolved galaxies.

A film about these experiments - Blast! - will be shown at 7 pm as part of the Vanderbilt International Lens series. Professor Devlin will host a Question and Answer session after the film.

Host: A. Berlind

Thursday, March 11

Spring Break, no colloquium

Thursday, March 18

Greg Bryan, Columbia University

The First Stars in the Universe: Birth, Death and Rebirth   (show abstract)

The first generation of stars in the Universe formed out of a nearly pure hydrogen and helium gas, ending the preceeding "dark ages". Remarkably, their formation appears easier to understand than present day star formation both because the initial conditions are well-prescribed and the relevant physical processes are simpler. Nevertheless, the range of scales is very large (from cosmological scales down to the radius of the sun) and simulating them required new techniques in computational physics. I will show results from numerical simulations of the formation of these objects indicating that they are likely to be considerably more massive than the sun. Such stars will produce both high-energy photons and energetic supernovae, and I will describe how we think these processes lead to the next generation of stars, and what their properties may be like. Finally, I will discuss current observational constraints and predictions for future observations.

Host: A. Berlind

Thursday, March 25, Seyfert Lecture

David Jewitt, University of California - Los Angeles

Primordial Ice Reservoirs of the Solar System   (show abstract)

We now know that primordial ice exists in at least three distinct Solar system reservoirs; the Oort cloud, the Kuiper belt and the asteroid main-belt. Continuing efforts to determine the nature of the ice and its distribution are important for several scientific reasons. First, the mere existence of the ice sets a limit to the degree of thermal processing of the objects in which it is found, and therefore constrains geophysical models of thermal evolution of ice rich bodies. Second, water ice, if in the amorphous form, can trap other volatiles from the protoplanetary disk of the Sun at high abundance. Their subsequent release upon crystallization can perhaps explain the anomalous activity observed in many comets and is a source of energy, since crystallization is exothermic. Third, water and other volatiles on the terrestrial planets seem likely to have been delivered, in part, from the ice reservoirs. The comets and ice-rich asteroids therefore may hold the key to understanding the origin of the oceans and atmosphere. In this talk, I will aim to provide a broad overview of our current knowledge (and lack of knowledge) of the primordial ice reservoirs. I will emphasize links to the formation epoch and draw connections for those interested in the origin of the oceans and the atmosphere and in the thermal evolution of asteroids and comets. I will try to do this in a way interesting to a non-specialist but scientific audience.

Host: D. Weintraub

Thursday, April 1

Katie Freese, University of Michigan

Dark Matter in the Universe   (show abstract)

One of the biggest unanswered questions in science is "What is the Universe made of?" Only 4% of the Universe consists of ordinary atomic matter; the remaining 96% is made of Dark Matter and Dark Energy whose nature is as yet unknown. This talk will examine the dark matter that comprises most of the mass of the Milky Way and all other galaxies. I will review the observational evidence for the existence of dark matter, and then turn to the hunt for the dark matter particle. A great deal of excitement currently pervades this field because of current and upcoming experiments that can find the dark matter, via both direct and indirect techniques. The best motivated dark matter candidates are Weakly Interacting Massive Particles such as those motivated by supersymmetry or extra dimensions. These particles have been powerful motivation for the Large Hadron Collider at CERN, underground experiments (e.g; XENON, CDMS), satellites such as GLAST or PAMELA, and neutrino detectors such as ICECUBE at the South Pole. The discovery of the dark matter particle will be an exciting milestone for particle physics, astrophysics, and for everyone interested in understanding the nature of our Universe.

Host: R. Scherrer

Thursday, April 8

G. Brian Stephenson, Argonne National Laboratory

Switching a ferroelectric film by asphyxiation   (show abstract)

Ferroelectrics are both fascinating and useful because their atomic-scale structure responds strongly to electric field and temperature. Large epitaxial strain and interfacial effects make ultrathin films behave very differently than bulk crystals. In particular, their polarization structure depends sensitively on the nature of the charge compensation at their interfaces. We have been using in situ surface x-ray scattering to monitor film growth and to observe the polarization structure of ultrathin epitaxial PbTiO3 films as a function of temperature, thickness, and environmental conditions. Recently we discovered that the effects of changing ionic surface compensation by applying a chemical potential are similar to the effects of changing electronic surface compensation by applying an electric potential. The direction of polarization in PbTiO3 / SrRuO3 heterostructures on SrTiO3 (001) substrates can be switched by changing the oxygen partial pressure (pO2) in equilibrium with the surface. Here we present results on the equilibrium polarization phase diagram and stability limits as a function of temperature, pO2, and film thickness. Results will be compared with a model for the interaction of surface chemistry with the ferroelectric phase transition. This opens up a rich new area for study, in which surface chemistry has a large interaction with the ferroelectric phase transition. Work supported by the U. S. Department of Energy under Contract No. DE-AC02-06CH11357.

Host: S. Pantelides

Thursday, April 15, Wendell Holladay Lecture

Jack C. Wells, Oak Ridge National Laboratory

Reflections on a Vanderbilt Physics Education and U.S. Competitiveness in Science and Technology   (show abstract)

The viability of the United States' economy in the 20th Century, and a great deal of our national security, has been in large part derived from the productivity of well-trained people and the steady stream of scientific and technical innovations they produce. Without high-quality, knowledge-intensive jobs and the innovation enterprises that lead to discover and new technology, our quality of life will suffer. Scientific research and graduate education in Science and Engineering have contributed in a critical way to this progress. However, in recent years, some have questioned the US commitment to policies supporting scientific research and the graduate schools and laboratories that enable the scientific enterprise. Recent policy statements, such as the National Academies' Report "Rising Above the Gathering Storm: Energizing and Employing America for a Brighter Economic Future", have emphasized the need to invest in education, research, and innovation to advance our brainpower advantage. Many of the recommendations from the "Gathering Storm Report" have been implemented in the America COMPETES Act of 2007. In this talk I will reflect upon lessons learned about scientific research while a graduate student in the Department Physics and Astronomy of Vanderbilt University and the alignment of these with the recent bi-partisan consensus on U.S. science and technology policy. Possible future challenges to this consensus will also be addressed.

Host: R. Scherrer

Thursday, April 22

This year's Slack Lecture by Professor David Awschalom of UCSB has been postponed until the fall semester.

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