Physics and Astronomy Colloquium

All future

Monday, September 24th 2018
Speaker: Natalia Perkins, University of Minnesota
Subject: The pursuit of Majorana fermions in Kitaev materials
Refreshments in atrium after the Colloquium.

The 1937 theoretical discovery of Majorana fermions has since impacted diverse problems ranging from neutrino physics and dark matter searches to fractional quantum Hall effect, superconductivity and quantum spin liquids (QSLs). In my talk I will focus on the hunting for Majorana fermions in Kitaev materials, which believe to harbor a variety of QSLs. These Kitaev QSLs exhibit two types of fractionalized quasiparticle excitations - itinerant Majorana fermions and gapped Z2 fluxes. In recent years, a remarkable theoretical and experimental progress has been achieved in understanding that these fractionalized quasiparticles and, in particular, Majorana fermions can be effectively probed by conventional spectroscopic techniques such as inelastic neutron scattering, Raman scattering with visible light, and resonant inelastic X-ray scattering.

Thursday, September 27th 2018
3:35 pm:
Physics and Astronomy Colloquium in Physics Tate B50
Speaker: Raymond Jeanloz, UC Berkeley
Subject: Pressure as a Probe for Atoms, Molecules and Planets
Refreshments in atrium after the Colloquium

Experiments using diamond anvils and the largest lasers in the world can exceed the atomic unit of pressure, profoundly changing the nature of atom structure and chemical bonding. Laboratory studies of the metallic hydrogen and helium that dominate large-planet (Jupiter, Saturn) and stellar interiors, and of new forms of ice that comprise icy giant planets (Neptune, Uranus), are complemented by experiments converting simple metals into ionic salts and taking elements to the statistical-atom (Thomas-Fermi-Dirac) regime. Interesting in their own right, and as validation of first-principles quantum simulations, these experiments also inform us about the origins and evolution of planets.

Faculty Host: Cynthia Cattell

Thursday, October 4th 2018
3:35 pm:
Speaker: Professor Nergis Mavalvala, Massachusetts Institute of Technology
Subject: Gravitational wave detectors: past, present and future
Refreshments in atrium after the Colloquium.

The Laser Interferometer Gravitational-wave Observatory (LIGO) detected gravitational waves for the first time in 2015, and has continued to make discoveries. I will discuss the instruments that made these discoveries, the science so far, and plans for future improvements and upgrades to LIGO.

Monday, October 8th 2018
Speaker: Fiona Burnell, University of Minnesota
Subject: To be announced
Refreshments in atrium after the Colloquium.

Thursday, October 11th 2018
3:35 pm:
Speaker: Joel Moore, UC Berkeley
Subject: Topology and the electromagnetic responses of quantum materials
Refreshments in atrium after the Colloquium.

This talk starts by reviewing the remarkable theoretical and experimental
progress in topological materials over the past decade. Three-dimensional
topological insulators realize a particular electromagnetic coupling known as
“axion electrodynamics”, and understanding this leads to an improved
understanding of magnetoelectricity in all materials. We then turn to how
topological Weyl and Dirac semimetals can show unique electromagnetic
responses; we argue that in linear response the main observable effect
solves an old problem via the orbital moment of Bloch electrons, and how
in nonlinear optics there should be a new quantized effect, at least approximately.

Thursday, October 18th 2018
3:35 pm:
Speaker: Terry Hwa, University of California, San Diega
Subject: Bacterial growth laws and the origin of dimensional reduction
Refreshments in atrium after the Colloquium.

A cell is the smallest unit of a freely living system. Our understanding of cells is measured by our ability to predict cellular behaviors in response to environmental changes and genetic manipulations. Traditionally, researchers strive to gain insights into cellular behaviors through characterizing the underlying molecular interactions. This ‘bottom-up’ approach however requires a macroscopic number of largely inaccessible parameters in order to be predictive. I will describe a complementary ‘top-down’ approach based on quantitative phenomenology. Extensive quantitative experiments establish that the model bacterium E. coli organizes many of its behavioral responses in very simple manners in accordance to the rate of cell growth. The existence of these simple empirical relations (growth laws) despite myriads of complex molecular interactions is a striking manifestations of a tremendous degree of dimensional reduction occurring in living cells. I will describe how the growth laws can be used to make accurate predictions of cell behaviors without fitting parameters. I will also discuss how the magical dimension reduction can be accomplished by cells through clever strategy of gene regulation.

Faculty Host: Elias Puchner

Thursday, October 25th 2018
3:35 pm:
Speaker: Steven Gubser, Princeton
Subject: Number theory and spacetime
Refreshments in atrium after the Colloquium.

Our description of spacetime relies on the real numbers and hence is wedded to arbitrarily small intervals of length and time. But quantum theory hints at the existence of a smallest possible length, the Planck length. Number theory provides an alternative to the real numbers known as the p-adic numbers. Recent work has argued that quantum field theory defined over the p-adic numbers is holographically dual to a discrete spacetime. Constructions related to p-adic numbers also have a surprisingly prominent role in the early development of the renormalization group. I will explain what the p-adic numbers are and provide some intuition for what they are good for in string theory and beyond. The ultimate aim of using them to understand quantum gravity is ambitious indeed, but I will explain some first steps that give hope for the future.

Bio: Steve Gubser got his PhD from Princeton University in 1998, working with Igor Klebanov on what became the gauge-string duality. He did a post-doc at Harvard, then joined the faculty at Princeton. After a year at Caltech, he returned to Princeton and has been there ever since. He is now the Associate Chair for Undergraduates in Physics, and he is a recent recipient of a Simons Young Investigator award.

Faculty Host: Priscilla Cushman

Thursday, November 1st 2018
3:35 pm:
Speaker: Robert Kleinberg, Columbia University & Boston University
Subject: mK to km: How Millikelvin Physics is Reused to Explore the Earth Kilometers Below the Surface
Refreshments in atrium after the Colloquium.

Investigations of the superfluid phases of liquid helium-3 would seem to have little application to the study of rock formations thousands of meters below the surface of the earth. However, the physicist’s tool box is versatile, and techniques used in one field of study can be reused, with appropriate adaptation, in very different circumstances.

The temperature of liquid helium-3 in the millikelvin range can be measured using an unbalanced-secondary mutual inductance coil set designed to monitor the magnetic susceptibility of a paramagnetic salt. The loss signal is discarded by phase sensitive detection. Now consider the task of measuring the electrical conductivity, at centimeter scale, of the earth surrounding a borehole. Turn the mutual inductance coil set inside out, with secondary coils arranged to be unbalanced with respect to the rock wall. Instead of discarding the loss signal, use it to measure conductivity. A sensor based on this principle has been implemented in a widely deployed borehole geophysical instrument, used to estimate the prevailing direction of the wind millions of years ago, or to decide where to drill the next well in an oilfield.

Nuclear magnetic resonance may seem a very improbable measurement of the rock surrounding a borehole. Conventionally, we place the sample (which might be a human being) inside the NMR apparatus. In borehole deployment, the instrument is placed inside sample, the temperature is as high as 175C, pressure ranges to 140 MPa, and measurements must be made while moving at 10 cm/s. Apparatus with these specifications have been deployed worldwide, and are used to measure a number of rock properties, including the distribution of the sizes of pores in sedimentary rock, and the viscosity of oil found therein. They have also been used for geological and oceanographic studies in northern Alaska, and at the seafloor offshore Monterey, California.

Faculty Host: Shaul Hanany

Thursday, November 8th 2018
3:35 pm:
Speaker: Jason Petta, Princeton University
Refreshments in atrium after the Colloquium.

Thursday, November 15th 2018
3:35 pm:
Speaker: John Marko, Northwestern University
Subject: Physics of chromosome folding and disentanglement
Refreshments in atrium after the Colloquium.

All biological phenomena depend on genetic information which is encoded
into the base-pair sequence along the very long DNA molecules found in all
living cells. The DNAs in chromosomes, in addition to being biologically
important, are amazing physical objects, being 2 nanometers wide and (in
humans) several centimeters in length. I will explain how the cell takes
care of these long, fragile chromosomal DNAs and how it uses DNA itself as
a key mechanical component of the cell nucleus. Then, during and
following DNA replication, our cells face the gigantic challenge of
figuring out how to topologically separate those long polymers from one
another. I will discuss our current understanding of the "lengthwise
compaction" mechanisms underlying this process, focusing on the interplay
between "loop-extruding" SMC complexes (mainly condensin) and
DNA-topology-changing topoisomerase II.

Faculty Host: Elias Puchner

Thursday, November 29th 2018
3:35 pm:
Speaker: Marco Velli, UCLA
Refreshments in atrium after the Colloquium.
Faculty Host: Robert Lysak

Thursday, December 6th 2018
3:35 pm:
Speaker: Stacy McGaugh, Case Western
Refreshments in atrium after the Colloquium.
Faculty Host: J. Woods Halley

Thursday, January 31st 2019
3:35 pm:
Speaker: Roger Steuwer, U of Minnesota
Subject: From tFrom the Old to the New World of Nuclear Physics, 1919-1939he Old to the New World of Nuclear Physics, 1919-1939
Refreshments in atrium after the Colloquium.
Faculty Host: Michel Janssen

Thursday, May 2nd 2019
3:35 pm:
Speaker: Jack Harris (Yale University)
Subject: TBD
Refreshments in atrium after the Colloquium.
Faculty Host: Paul Crowell

The weekly calendar is also available via subscription to the physics-announce mailing list, and by RSS feed.