Physics and Astronomy Colloquium

semester, 2019

Thursday, January 24th 2019
3:35 pm:
Speaker: Roger Steuwer, U of Minnesota
Subject: From the Old to the New World of Nuclear Physics, 1919-1939
Refreshments in atrium after the Colloquium.

These two interwar decades, as I discuss in my new book, The Age of Innocence: Nuclear Physics between the First and Second World Wars (Oxford University Press. 2018), saw the nascent field of nuclear become the dominant field of experimental and theoretical physics owing to an international cast of gifted physicists. Prominent among them were Ernest Rutherford and James Chadwick, George Gamow, the husband and wife team of Frédéric and Irène Joliot-Curie, John Cockcroft and Ernest Walton, Ernest Lawrence, Enrico Fermi, Niels Bohr, Gregory Breit and Eugene Wigner, and Lise Meitner and Otto Robert Frisch. Their fundamental discoveries and pioneering inventions arose from a quest to understand nuclear phenomena; none were motivated by a desire to find a practical application for nuclear energy. In this sense, they lived in an “Age of Innocence.” They did not, however, live in isolation. Their research reflected their idiosyncratic personalities; it was shaped by the physical and intellectual environments of the countries and institutions in which they worked; and it was buffeted by the turbulent political events after the Great War--the harsh postwar treaties, the runaway inflation in Germany and Austria, and the intellectual migration from Germany and later from Austria and Italy.

Faculty Host: Michel Janssen

Thursday, January 31st 2019
3:35 pm:
Physics and Astronomy Colloquium in Physics Tate B50
Speaker: Boris Shklovskii, University of Minnesota
Subject: Coulomb gap here, there, and everywhere

In 1975 Alexei Efros and myself discovered that due to electron-electron interactions the density of localized electron states vanishes near the Fermi level as quadratic function of the energy distance to the Fermi level. We named this phenomenon the Coulomb gap and showed that it leads to the variable range hopping conductivity which depends on temperature T as \exp[-(T_ES/T)^1/2]. This ES law was observed in hundreds of experimental papers, where in many cases it describes 10^6 times variation of conductivity. After reminding the history and physics of this discovery I will review many new applications of ES law beyond lightly doped semiconductors, among which the Quantum Hall Effect is the most prominent. I will also dwell on McMillan-Shklovskii conjecture on the Coulomb gap evolution across an Insulator-Metal transition and related question of screening of the Coulomb gap.

Thursday, February 7th 2019
3:35 pm:
Physics and Astronomy Colloquium in Physics Tate B50
Speaker: Marcelo Magnasco, The Rockefeller University
Subject: Shifting attention to dynamics: Self-reconfiguration of neural networks

In recent years, considerable evidence has accrued indicating that brain function can be flexibly reconfigured on the fly: certain brain areas are capable of carrying out a variety of different functions and are able to switch between those functions in a context-sensitive, dynamic fashion. Some evidence has also emerged that ongoing brain activity, the ceaseless background brain dynamics, may be implicated in setting and controlling these dynamic functions as well as attentional gain control. A crucial link between them is dynamics. I will discuss both the accumulating evidence as well as a theoretical outline of dynamical mechanisms for functional self-reconfiguration of neural networks, including reconfiguration of logic function, reconfiguration of information routing, and poising at critical points to reconfigure dynamics.

Faculty Host: Jorge Vinals

Thursday, February 14th 2019
3:35 pm:
Physics and Astronomy Colloquium in Physics Tate B50
Speaker: Reinhard Schwienhorst, Michigan State University
Subject: The Tevatron legacy and the LHC through the lens of single top-quark production

Single top-quark production has provided flagship top-quark measurements at the Tevatron and LHC. These electroweak production modes have small production cross-sections and final states with large backgrounds, challenging the understanding of the detector, while also demanding innovations in analysis techniques. Single top measurements constrain models of new physics directly and indirectly and provide a direct determination of the CKM matrix element Vtb. All this makes single top an excellent representation of the core Tevatron and LHC programs. I will review the history of single top-quark measurements and discuss their role in future LHC runs.

Faculty Host: Roger Rusack

Thursday, February 21st 2019
3:35 pm:
Physics and Astronomy Colloquium in Physics Tate B50
Speaker: Flavio Cavanna, Yale
Subject: Lifting the lid on DUNE, the new international mega-science project in the US

The experimental discoveries of the last half century have placed neutrinos in the spotlight to unlock the mysteries of the matter's abundance unbalance in the Universe and of the ultimate fate of the stars. The lack of direct observations of proton decays, on the other hand, keeps at bay the dream that the forces of nature were unified at the beginning of time.
The Deep Underground Neutrino Experiment (DUNE) is the new leading-edge, international mega-science experiment for neutrino science and proton decay search.
DUNE will consist of two paired neutrino detectors placed in the world’s most intense neutrino beam. One massive detector will be installed deep underground at the SURF laboratory, in South Dakota — 1,300 kilometers away from FERMILAB, where the second detector will be positioned just downstream the neutrino source. Detecting the energetic beam neutrinos at the far site and comparing with those detected at the near site can give insight about our matter dominated universe. The underground location of the far detector, screened from the overwhelming cosmic ray flow, may allow to detect tiny signals from neutrinos originated by a core-collapse supernova in the Milky Way and thus possibly peer inside a newly-formed neutron star. And finally, the extra-large amount of mass of the detector may allow primordial symmetries to occasionally resurface inside a proton and spontaneously morphing a quark into a lepton, with the proton instantly falling apart into a detectable flash of radiation.
But not only large mass and far distance matter to pursue this ambitious discovery plan: unprecedented detection technologies and a worldwide effort to build the detector are required. DUNE will use the state-of-the-art Liquid Argon TPC technology to instrument deep underground 70.000 tons of Liquid Argon at 87K, with millimeter scale 3D precision.
A 1 kTon precursor of the far LAr-TPC detector has been constructed and recently activated at the CERN Neutrino Platform, and is now taking data. A first look of the spectacular events collected will be shown.

Faculty Host: Roger Rusack

Thursday, February 28th 2019
3:35 pm:
Physics and Astronomy Colloquium in Physics Tate B50
Speaker: Joerg Schmalian, Karlsruhe Institute of Technology
Subject: Failed Theories of Superconductivity

Superconductivity is one of the most fascinating quantum states of matter. Almost half a century passed between the discovery of superconductivity by Kamerlingh Onnes and the theoretical explanation of the phenomenon by Bardeen, Cooper and Schrieffer (BCS). During the intervening years the brightest minds in theoretical physics tried and failed to develop a microscopic understanding of the effect. A summary of some of those unsuccessful attempts to understand superconductivity not only demonstrates the extraordinary achievement made by formulating the BCS theory, but also illustrates that mistakes are a natural and healthy part of scientific discourse, and that inapplicable, even incorrect theories can turn out to be interesting and inspiring.

Faculty Host: Rafael Fernandes

Thursday, March 7th 2019
3:35 pm:
Physics and Astronomy Colloquium in Physics Tate B50
Speaker: Marc Pinsonneault, Ohio State University
Subject: Asteroseismology in the Gaia Era

Stellar oscillations are powerful tools for understanding the structure and evolution of stars. With the advent of time domain space missions they can now be measured for large samples of evolved cool stars. The combination of this asteroseismic data, astrometry from Gaia, and large spectroscopic surveys is transforming our understanding of stellar populations and stellar physics. In this talk I review the current state of the art in red giant asteroseismology: both how well we can measure stellar properties using it and how it has changed our understanding of stellar populations and the theory of stellar structure and evolution. I will also discuss the powerful combination of asteroseismology and Gaia, providing two examples: testing the parallax zero point in Gaia with asteroseismology and testing asteroseismic scaling relations with Gaia.

Faculty Host: Evan Skillman

Thursday, March 14th 2019
3:35 pm:
Physics and Astronomy Colloquium in Physics Tate B50
Speaker: Alexander Grosberg, New York University
Subject: Statistical Mechanics of Active Particles

Active particles are the ones having a source of energy to drive them, in addition to the usual Brownian motion. It could be swimming bacteria, or artificial swimmers of various kinds. Statistical mechanics of such out-of-equilibrium systems presents many steep challenges and features many unexpected phenomena. While energy barriers is a staple in physics (Boltzmann limit), force barriers are important for active particles (Sisyphus limit), leading to rectification of random walks, repulsive depletion, etc. Activity can also cause separation of active particles from passive ones even when there is no energetic preference for segregation. The latter effect is particularly strong for polymers, promising interesting applications in the physics of cell nucleus.

Faculty Host: Boris Shklovskii

Thursday, March 21st 2019
3:35 pm:
Physics and Astronomy Colloquium in Physics Tate B50
There will be be no colloquium this week due to spring break

Thursday, March 28th 2019
3:35 pm:
Physics and Astronomy Colloquium in Physics Tate B50
Speaker: Jason Hogan, Stanford University
Subject: Atom interferometry for fundamental physics and gravitational wave detection

In recent years, atom interferometry and atomic clocks have made impressive gains in sensitivity and time precision. The best atomic clocks have stability corresponding to a loss of less than one second in the lifetime of the universe. Matter wave interferometers have achieved record-breaking coherence times (seconds) and atomic wavepacket separations (over half a meter), resulting in a significant enhancement in accelerometer and gravity gradiometer sensitivity. Leveraging these advances, atomic sensors are now poised to become a powerful tool for discovery in fundamental physics. I will highlight ongoing efforts to test aspects of general relativity and quantum mechanics, and search for new fundamental interactions. A particularly exciting direction is gravitational wave detection. I will describe the Mid-band Atomic Gravitational wave Interferometric Sensor (MAGIS) proposal, which is targeted to detect gravitational waves in a frequency band complementary to existing detectors (0.03 Hz – 10 Hz), the optimal frequency range to support multi-messenger astronomy. Finally, I will discuss MAGIS-100, a 100-meter tall atomic sensor being constructed that will serve as a prototype of such a detector, and will also be sensitive to proposed ultra-light dark matter (scalar and vector couplings) at unprecedented levels.

Faculty Host: Roger Rusack

Thursday, April 4th 2019
3:35 pm:
Van Vleck Colloquium in Physics Tate B50
Speaker: Rob Kennicutt, Texas A&M University
Subject: The Schmidt Law at Sixty

Sixty years have passed since Maarten Schmidt's conjecture that star formation in galaxies was closely coupled to gas density, and since that time the Schmidt law has become an indispensable tool for interpreting, modeling, and simulating large-scale star formation in galaxies. Despite its success as a sub-grid "recipe" for the star formation rate, however, we remain far away from an ab initio theory of star formation, or even a clear understanding of the observed scaling laws themselves. This talk will review the current state of our observational understanding of star formation in galaxies, and the complexity which lies beneath the surface of the observed SFR scaling relations. We are witnessing an observational and theoretical renaissance in the subject, as multi-wavelength observations reveal the multi-scale nature of the star formation process and the complex interactions which are taking place between cosmological, gravitational, interstellar, and stellar feedback processes on these different scales. The picture which emerges is one in which the superficially simple star formation scaling laws are manifestations of a highly dynamic, complex, and self-regulating ecosystem in galactic disks.

Faculty Host: Ronald Poling

Thursday, April 11th 2019
3:35 pm:
Speaker: Dimitar Sasselov, Harvard
Subject: Stellar UV Light and the Origins of Life's Building Blocks on Earth and on Exoplanets
Faculty Host: Evan Skillman

Thursday, April 18th 2019
3:35 pm:
Physics and Astronomy Colloquium in Physics Tate B50
Speaker: Ian Tregillis, Los Alamos National Laboratory
Subject: Verification & Validation of a Richtmyer-Meshkov Instability Based Ejecta Source Model, or, Why Should Los Alamos Care About a Freshman Physics Problem?

Computational physicists are commonly faced with the task of resolving discrepancies between the predictions of a complex, integrated multi-physics numerical simulation and corresponding experimental datasets. Such efforts commonly require a slow iterative procedure. However, a different approach is available in cases where the multi-physics system of interest admits closed-form analytic solutions. In this situation, the ambiguity is broken into separate consideration of theory-simulation comparisons (issues of verification) and theory-data comparisons (issues of validation). We demonstrate this with the specific example of a fluid- instability based ejecta source model (“RMI+SSVD”) under development at Los Alamos National Laboratory and implemented in FLAG, a Los Alamos continuum mechanics code. For a specific (but wide-ranging) class of explosively driven metal coupon experiments, the ejecta model prediction inherently reduces to a one-dimensional vacuum kinematics problem. This enables us to compute, purely analytically, piezoelectric ejecta mass measurements suitable for “apples-to-apples” comparisons to both simulated and measured datasets. Thus, studying the solution to a very simple yet overlooked problem yields rich and concrete insights into performance of the model, its strengths and shortcomings, as well as strategies for improving it. These conclusions are made quantitative through the introduction of a straightforward yet rigorous “compatibility score” metric incorporating published measurement uncertainties on relevant experimental parameters.

Faculty Host: Thomas W. Jones

Thursday, April 25th 2019
3:35 pm:
Physics and Astronomy Colloquium in Physics Tate B50
Speaker: Sanjay Reddy, University of Washington
Subject: Neutron stars come of age

About 50 years after their discovery, neutron stars are poised to take center stage in this era of multi-messenger astrophysics. In the not-so-distant-future (10-20 years) it is likely that next generation gravitational wave observatories will detect gravitational waves from hundreds of mergers involving neutron stars every year. I will highlight advances in theory and mention some key observations that have already provided fundamental new insights about neutron star properties and their central role in nuclear astrophysics. I will discuss how neutron stars, and extreme phenomena involving them, can serve as laboratories to study phase transitions in dense matter, nucleosynthesis, and dark matter in the coming decades.

Faculty Host: Joseph Kapusta

Thursday, May 2nd 2019
3:35 pm:
Speaker: Jack Harris (Yale University)
Subject: Quantum optomechanics with superfluid helium
Note change of room for this week only. Refreshments in atrium after the Colloquium.

To observe quantum effects in the motion of macroscopic objects typically requires high-precision readout, low temperature, and low optical and mechanical loss. Superfluid helium offers many advantages in these regards. I will describe two optomechanics experiments based on superfluid helium. In the first, the superfluid fills a Fabry-Perot optical cavity. The cavity is used to monitor the quantum fluctuations of the superfluid's acoustic modes. This system is amenable to single photon/phonon detection schemes, and so may provide a route to more exotic quantum effects in massive objects. The second experiment uses magnetic levitation to suspend a mm-scale drop of superfluid in vacuum. I will describe preliminary measurements of the drop's formation, trapping, and evaporative cooling, and of the drop's mechanical resonances and optical resonances.

Faculty Host: Paul Crowell

Thursday, May 9th 2019
3:35 pm:
Physics and Astronomy Colloquium in Physics Tate B50
Speaker: David Weinberg, The Ohio State University
Subject: Decoding Chemical Evolution and Nucleosynthesis
Student Award ceremony at the beginning of colloquium. Refreshments served in B50 Atrium after colloquium

I will discuss insights from analytic and numerical models of
galactic chemical evolution and observations of Milky Way
elemental abundances from the SDSS APOGEE survey. Under
generic model assumptions, abundances and abundance ratios
approach an equilibrium in which element production from
nucleosynthesis is balanced by element depletion from star
formation and outflows. Reproducing solar abundances requires
outflows with mass-loading factors of 1-3, but one can evade
this conclusion by assuming low stellar yields or metal-enhanced
winds; the high observed deuterium abundance of the local ISM
argues against these alternatives and in favor of outflows.
Starbursts or other sudden transitions can produce temporary
boosts in alpha-to-iron ratios, and other surprising behavior
such as backward evolution of a stellar population from high
metallicity to low metallicity. APOGEE observations show
that the distributions of stars in (alpha,iron,age)-space change
steadily across the Milky Way disk. Given these distributions,
the behavior of other APOGEE abundance ratios can be explained
by changes in the ratio of core collapse to Type Ia supernova
enrichment. The separability of "multi-element cartography"
offers a route to empirically constraining supernova yields
in a way that is insensitive to uncertainties in other aspects
of chemical evolution.

Faculty Host: Evan Skillman

Thursday, May 16th 2019
3:35 pm:
There is no colloquium this week.

Thursday, October 3rd 2019
3:35 pm:
Physics and Astronomy Colloquium in Physics Tate B50
Speaker: Jeffrey Crelinsten, The Impact Group @Centre for Social Innovation
Subject: TBD
Faculty Host: Michel Janssen

Thursday, November 14th 2019
3:35 pm:
Physics and Astronomy Colloquium in Physics Tate B50
Speaker: Ana Maria Rey, University of Colorado - Boulder
Subject: TBD
Faculty Host: Rafael Fernandes

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