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Colloquium

Effective Fall 2021, unless stated otherwise, we are back to holding our colloquia in person. All physics and astronomy colloquia will take place on  Thursday afternoon  from  4 to 5 pm  in room 4327 Stevenson Center, but we will also have a hybrid Zoom option available for those who would prefer to attend remotely

For details please contact Reina Beach, by email (reina.beach@vanderbilt.edu) or phone (615 322-7284).

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Fall 2022

Thursday September 1st 2022 4:00 PM

No colloquium: Fall Faculty Assembly

Thursday September 8th 2022 4:00 PM

GUY and REBECCA FORMAN LECTURE

Ashok Goel, Georgia Tech

Using AI for Transforming Adult Learning and Online Education    (show abstract)

NSF has established a National AI Institute for Adult Learning and Online Education (AI-ALOE). The AI-ALOE Institute seeks to address the societal challenges of lifelong learning, workforce development, and reskilling and upskilling of millions of American workers each year. Online education offers an affordable medium for taking education to workers where they live. However, there remain serious questions about the quality of online education. Fortunately, online education offers access to unprecedented amounts of data about learners and learning. AI-ALOE is developing a novel technology infrastructure for collecting, analyzing, and sharing this data. It is also developing new AI assistants that use this data to enhance cognitive engagement, teacher presence, and social interaction. In addition, it is conducting foundational AI research on personalization of learning at scale, interactive machine teaching to enable teachers to develop their own AI assistants, and mutual theory of mind to enable learners and AI assistants to better understand one another. I will describe ALOE’s vision, program, and projects, using examples from my own research for illustration.

Host: Shane Hutson

Thursday September 15th 2022 4:00 PM

Josh Caldwell, Vanderbilt School of Engineering

Strong Coupling and Extreme Anisotropy in Infrared Polaritonic Media   (show abstract)

The field of nanophotonics is based on the ability to confine light to sub-diffractional dimensions. In the infrared, this requires compression of the wavelength to length scales well below that of the free-space values. While traditional dielectric materials do not exhibit indices of refraction high enough in non-dispersive media to realize such compression, the implementation of polaritons, quasi-particles comprised of oscillating charges and photons, enable such opportunities. Two predominant forms of polaritons, the plasmon and phonon polariton, which are derived from light coupled with free carriers or polar optic phonons, respectively, are broadly applied in the mid- to long-wave infrared. However, the short scattering lifetimes of free-carriers results in high losses and broad linewidths for the former, while the fast dispersion and narrow band of operation for the latter result in significant limitations for both forms. Here we will discuss the opportunity to implement polaritonic strong coupling between different media in an effort to dictate the polaritonic dispersion relation, and thus, the propagation and resonant properties of these materials. Further, by employing the extreme anisotropy of crystals ranging from two-dimensional materials such as hexagonal boron nitride and transition metal dichalcogenides to low-symmetry monoclinic to triclinic materials, novel optical phenomena such as hyperbolicity and shear polaritons are observed. The talk will highlight ultra-strong coupling between both forms of polaritons in the context of infrared emitters, as a means to control planar propagation using hyperbolic polaritons, and modifying thermal dissipation at ultrafast time scales.

Host: S.T. Pantelides

Thursday September 22nd 2022 4:00 PM

Co-hosted by VandyGRAF

Leo Stein, University of Mississippi

Gravitational waves from black hole mergers: The big picture and subtle details   (show abstract)

Extracting science from gravitational wave observations requires theoretical modeling, both analytical and numerical. The current state of these models is sufficient for present-day detectors, but next generation detectors bring the promise and challenge of much higher signal-to-noise ratios. This demands we understand the fine details: our modeling errors must be smaller than statistical noise. I'll give an overview of modeling gravitational waves from binary black hole mergers, then go into some of the subtle details: (i) The beginnings of our numerical waveforms need to be stitched to analytical approximations; and (ii) The late portion of our numerical waveforms can be modeled as "quasinormal ringing." Getting both of these right is harder than it sounds!

Host: A. Lupsasca

Thursday September 29th 2022 4:00 PM

Co-hosted by VandyGRAF

Charles Gammie, University of Illinois, Urbana-Champaign

Portrait of a Black Hole   (show abstract)

The Event Horizon Telescope images of M87* and Sgr A* have opened a new era of resolved imaging of black holes at the event horizon scale. I will describe how EHT measurements are made, how they can be interpreted with the aid of state-of-the-art models, what they teach us about black holes, and what the prospects are for future experiments.

Host: A. Lupsasca

Thursday October 6th 2022 4:00 PM

ALICE CLAIR KIMBLER HANKLA LECTURE

Monica Pate, New York University

Celestial Holography   (show abstract)

The formulation of a complete theory of quantum gravity remains a fundamental open problem in modern physics. Meaningful progress has arisen from concrete realizations of the holographic principle, which is a conjectured equivalence between systems of quantum gravity and ordinary non-gravitational systems in fewer dimensions. At present, this principle is best understood in the context of negatively-curved spacetimes. Celestial holography -- the subject of this lecture -- is a novel extension of the holographic approach to the asymptotically flat context. More specifically, it is a proposed equivalence between gravitational scattering in asymptotically flat spacetimes and a conformal field theory living on the celestial sphere. Remarkably, the process of so-reformulating the physics has unearthed new structure that underlies the gravitational scattering problem. Notably, gravitational scattering admits an infinite-dimensional symmetry, which has been now identified as the w(1+infinity) symmetry that has previously appeared in other two-dimensional physical systems. These symmetries imply an infinite number of constraints on scattering amplitudes, which physically enforce universal behavior in the infrared.

Host: S. Hutson and A. Lupsasca

Thursday October 13th 2022 4:00 PM

FALL BREAK

Thursday October 20th 2022 4:00 PM

Shane Larson, Northwestern University

The landscape of multi-messenger astronomy in the LISA era   (show abstract)

Multi-messenger astronomy has a long and colorful history as a tool for understanding the Cosmos. The recent simultaneous detection of gravitational waves with a short gamma-ray burst has galvanized the attention of astronomers, illuminating the tremendous scientific gains that can come from multi-messenger astronomy. In the next decade, the LISA gravitational-wave observatory will open up a new era of astronomy, increasing the number of known sources in the gravitational-wave catalog by several orders of magnitude. Among those sources will be thousands of multi-messenger systems that astronomers can mine for information about the Universe. In this talk, we will examine the complementary of gravitational-wave observations with other techniques. We'll discuss the power of combining both gravitational wave and electromagnetic observations, and give several vignettes of how the two observational mediums may be used in concert to probe a variety of different astrophysical phenomena.

Host: S. Taylor

Thursday October 27th 2022 4:00 PM

Shane Hutson, Vanderbilt University

Something's wrong in the cellular neighborhood   (show abstract)

Biological tissues and organs are typically surrounded by a boundary layer of cells known as an epithelium (or endothelium). When an epithelium is injured, surrounding cells respond in a distance-dependent manner to reseal the wound. So, how do the surrounding cells “know” that there is a wound nearby, i.e., that something is wrong in the cellular neighborhood? Across a wide range of organisms, the first response of epithelial cells to local wounds is a dramatic increase in cytosolic calcium. We have investigated this process in fruit flies using fast and reproducible laser wounds. Our results show that this increase occurs quickly – calcium floods into damaged cells within 15 milliseconds, moves into adjacent cells over ~20 s, and appears in a much larger set of surrounding cells via a delayed second expansion over 40-300 s – but calcium is nonetheless a reporter: cells must detect wounds even earlier. We will discuss how measurements of laser-tissue interactions can be combined with quantitative image analysis and the genetic tools available in fruit flies to tease apart wound detection in terms of its physical and biochemical mechanisms. We will discuss the experimental evidence and a corresponding computational model developed to match experimental observations, test the plausibility of hypothesized mechanisms, and make experimentally testable predictions. This work supported by NIH Grant 1R01GM130130.

Host: R. Scherrer

Thursday November 3rd 2022 4:00 PM

GUY and REBECCA FORMAN LECTURE

Randy Knight, California Polytechnic State University

What do we know about the teaching and learning of physics?   (show abstract)

Forty years of physics education research – from community colleges to Harvard and around the world – have established a solid understanding of how students learn introductory physics and how students respond to different types of teaching methods and materials. This talk will distill four decades of research into 10 key bullet points and will look at the implications for physics instruction.

Host: S. Hutson

Thursday November 10th 2022 4:00 PM

Co-hosted by VandyGRAF

Robert Pisarski, Brookhaven National Lab

The Ugly Duckling and the Swan in Heavy Ion Collisions   (show abstract)

I begin with a review of gauge theories as they apply to the modern theory of nuclei, which is Quantum ChromoDynamics, or QCD. I discuss why we expect QCD to form a Quark-Gluon Plasma, or QGP, at temperatures of about a trillion degrees, and the possible phase transitions associated with deconfinement and chiral symmetry breaking. The QGP appears to have been created in the collisions of nuclei at ultra-relativistic energies, at both Brookhaven and CERN. I discuss two notable signals: the success of nearly ideal hydrodynamics, and the quenching of jets in a QGP. I then discuss the new frontier, which is going down to moderate collision energies, where the baryon chemical potential is significant. This also makes contact with results from the binary collisions of neutron stars. I immodestly suggest that the region of low temperature and high chemical potential in QCD is one of the great problems for physics in the 21st century.

Host: J.-F. Paquet

Thursday November 17th 2022 4:00 PM

FRANCIS G. SLACK LECTURE

Chris G.van de Walle, University of California, Santa Barbara

Role of point defects in quantum technologies   (show abstract)

Quantum technologies have become a top research priority. However, the underlying hardware still requires major development. Point defects in semiconductors and insulators play a key role. On the one hand, they can be detrimental since they cause decoherence. On the other hand, they can act as functional centers, providing a platform that combines the environmental isolation necessary to maintain the coherence of quantum states with the ability to perform electrical and optical manipulation. I will discuss first-principles analyses of a prototype quantum defect, the nitrogen-vacancy (NV) center in diamond, that elucidate its properties and form a foundation for predicting which centers in other materials might exhibit similarly favorable properties. Building on the general methodology for performing point-defect calculations, we have developed the capability to predict transition energies, lineshapes, and radiative and nonradiative rates. These developments enable us to analyze, identify, and predict quantum point defects, as will be illustrated with examples for a number of materials.

Host: S. Pantelides

Thursday November 24th 2022 4:00 PM

THANKSGIVING HOLIDAY

Thursday December 1st 2022 4:00 PM

Co-hosted by VandyGRAF

Michael Johnson, Harvard-Smithsonian Center for Astrophysics

Black Hole Shadows and Photon Rings: Experimental Relativity with Radio Interferometry   (show abstract)

By combining global networks of radio telescopes, very long baseline interferometry provides the sharpest images in astronomy. This technique has recently culminated in the first images of a black hole, produced using the Event Horizon Telescope (EHT). These images revealed dark "shadows" encircled by bright and unresolved rings for the nuclear supermassive black holes in M87 and the Milky Way. I will discuss these results, including their implications for astrophysical theories of black hole accretion and jet formation. I will then describe our efforts to develop the next-generation EHT (ngEHT) and a space-enhanced EHT over the coming decade, which will improve the dynamic range of current EHT images by two orders of magnitude and will enable studies of horizon-scale dynamics through black hole movies. These efforts can resolve the fractal substructure from unstable photon orbits near a black hole that is predicted to appear within the blurry EHT ring, and they will ultimately measure the masses of thousands of supermassive black holes across cosmic history.

Host: A. Lupsasca

Thursday December 8th 2022 4:00 PM

WENDELL G. HOLLADAY LECTURE

Ron Mickens, Clark Atlanta University

Pitfalls, Perils, and Other Dangers for the Numerical Integration of Differential Equations   (show abstract)

Few differential equations, linear or nonlinear, that we create to model physical phenomena, can be exactly solved and expressed in terms of a finite combination of the elementary functions. This fact provides the main motivation for constructing techniques to calculate numerical solutions to these equations. However, as the title indicates, this process is fraught with a broad range of difficulties. Our goal is to consider some of these issues within the framework of "nonstandard finite difference (NSFD) discretization schemes," a methodology designed to resolve many of these difficulties. A number of important illustrative examples will be given.

Host: S. Hutson

Spring 2023

Thursday January 12th 2023 4:00 PM

Co-hosted by VandyGRAF

Jorge Noronha, University of Illinois, Urbana-Champaign

Host: J.-F. Paquet

Thursday January 19th 2023 4:00 PM

Sam Gralla, University of Arizona

Black holes: Seeing the Unseeable   (show abstract)

Black holes emit no light of their own, and absorb everything that crosses their event horizon. But they are not invisible, and thanks to some spectacular breakthroughs of recent years, now we can see them. I will tell the remarkable story of black hole imaging and explain the science in simple terms: What are we looking at in a black hole image, and what do we learn from it? I will also describe the exciting future of this new field, with new telescopes and space missions set to probe black holes in finer detail than ever before, putting our understanding of gravity to the ultimate test.

Host: A. Lupsasca

Thursday January 26th 2023 4:00 PM

Kimberly Boddy, University of Texas

Host:R. Scherrer

Thursday February 2nd 2023 4:00 PM

Chris Monahan, College of William and Mary

Host: R. Kunnawalkam Elayavalli

Thursday February 9th 2023 4:00 PM

Co-hosted by VandyGRAF

Veronica Dexheimer, Kent State University

Exotic matter in neutron stars   (show abstract)

The high densities achieved in neutron stars and the high densities and temperatures achieved in neutron-star mergers create ideal testing grounds in which to learn about exotic matter, namely hyperons and deconfined quarks. The presence of exotic matter can strongly affect the interior of neutron stars, but cannot be directly observed. New electromagnetic and gravitational-wave constraints have been slowly constraining the dense QCD equation of state, allowing us to learn important information about the strong interaction. Nevertheless, strong constraints on dense and hot matter depend on (a) the not yet observed post-merger period of gravitational-wave production from neutron-star mergers and (b) non-trivial comparisons with particle collision experimental data. In this talk, I discuss where we stand and what we expect to learn about dense matter in the near future.

Host: J. Velkvoska

Thursday February 16th 2023 4:00 PM

Karen Kasza, Columbia University

Host: S. Hutson

Thursday February 23rd 2023 4:00 PM

Duncan Lorimer, West Virginia University

Host: S. Hutson

Thursday March 2nd 2023 4:00 PM

Bill Press, University of Texas, Austin

Host: R. Scherrer

Thursday March 9th 2023 4:00 PM

James Dent, Sam Houston State University

Host: T. Kephart

Thursday March 16th 2023 4:00 PM

Spring Break- no Colloquium

Thursday March 23rd 2023 4:00 PM

Co-hosted by VandyGRAF

Andrew Strominger, Harvard University

Host:A. Lupsasca

Thursday March 30th 2023 4:00 PM

Co-hosted by VandyGRAF

Edgar Shaghoulian, University of California, Santa Cruz

Host: A. Lupsasca

Thursday April 6th 2023 4:00 PM

Travis Humble, Oak Ridge National Lab

Host: S. Hutson

Thursday April 13th 2023 4:00 PM

Co-hosted by VandyGRAF

Charles Gale, McGill University

Host: J.-F. Paquet

Thursday April 20th 2023 4:00 PM

Co-hosted by VandyGRAF

Mark Trodden, University of Pennsylvania

Host: R. Scherrer

Thursday April 27th 2023 4:00 PM

Co-hosted by VandyGRAF

Frans Pretorius, Princeton University

Host: A. Lupsasca