Physics and Astronomy Colloquium

semester, 2018


Thursday, January 18th 2018
3:35 pm:
Speaker: Harvey Brown, Philosophy of Physics, University of Oxford
Subject: Quantum Bayesianism (QBism): the way to understand the quantum world

The recent philosophy of Quantum Bayesianism, or QBism, represents an attempt to solve the traditional puzzles in the foundations of quantum theory by denying the objective reality of the quantum state. Einstein had hoped to remove the spectre of nonlocality in the theory by also assigning an epistemic status to the quantum state, but his version of this doctrine was recently proved to be inconsistent with the predictions of quantum mechanics. In this talk, I present plausibility arguments, old and new, for the reality of the quantum state, and expose what I think are weaknesses in QBism as a philosophy of science. (The talk is based on this paper: http://philsci-archive.pitt.edu/12978/


Thursday, January 25th 2018
3:35 pm:
There will be no colloquium this week

Thursday, February 1st 2018
3:35 pm:
Speaker: Mark Bell, University of Minnesota
Subject: Nuclear Weapons and International Politics Today

Nuclear weapons are back in the news. This talk provides an overview of the most important and pressing current issues relating to nuclear weapons and international politics, including ongoing US-North Korea tensions, US nuclear modernization and the US Nuclear Posture Review, the extent of presidential authority over nuclear weapons, the risk of nuclear proliferation by U.S. allies and adversaries, and the recent nuclear ban treaty. The talk places these current issues within a broader historical context and discusses the extent to which today's nuclear concerns represent continuity or change from previous eras.

Faculty Host: Robert Lysak

Thursday, February 8th 2018
3:35 pm:
Speaker: Cristian Batista, University of Tennessee
Subject: Skyrmions and Vortices in Magnetic Systems

The history of magnetism dates back to earlier than 600 b.c., but it is only in the twentieth
century that scientists have begun to understand it, and develop technologies based on this
understanding. The new experimental techniques that were developed over twentieth century
allowed physicists to discover new forms of magnetism that they called “antiferromagnets”.
Unlike ferromagnets, the magnetic moments of antiferromagnets point along different directions
in such a way that the magnetic unit cell has no net magnetic moment. Typical configurations of
antiferromagnets are spiral orderings arising from competing exchange interactions or from the
Ruderman–Kittel–Kasuya–Yosida (RKKY) interaction between magnetic moments embedded
in metallic environments.
The new century started with the observation of a new generation of antiferromagnets
comprising more exotic magnetic textures, such as skyrmion and vortex crystals [1-3]. These
textures were unveiled thanks to the enormous progress made in real and reciprocal space
visualization techniques. We will discuss a few attractive properties of these novel phases and
the simple principles that should guide the experimental search. For instance, we will see that an
external magnetic field can induce a skyrmion crystal phase in hexagonal lattices (lattices with
six equivalent orientations for the spiral ordering) with easy-axis anisotropy [4-10]. Moreover,
we will see that magnetic skyrmions behave as mesoscale particles, which can order in different
three-dimensional structures, such as face centered tetragonal and hexagonal closed packed
crystals [10].
References
[1] U. Rößler, A. Bogdanov, and C. Pfleiderer, Nature 442, 797 (2006).
[2] A. N. Bogdanov and D. A. Yablonskii, Sov. Phys. JETP 68, 101 (1989).
[3] A. Bogdanov and A. Hubert, Journal of Magnetism and Magnetic Materials 138, 255 (1994).
[4] S. Hayami, S.-Z. Lin, and C. D. Batista, Phys. Rev. B 93, 184413 (2016).
[5] A. O. Leonov and M. Mostovoy, Nature Communications 6, 8275 (2015).
[6] Shi- Zeng Lin, Satoru Hayami and C. D. Batista, Phys. Rev. Lett. 116, 187202 (2016).
[7] C. D. Batista, S-Z. Lin, S. Hayami and Y. Kamiya, Reports on Progress in Physics, Volume 79, 8
(2016).
[9] Satoru Hayami, Shi-Zeng Lin, Yoshitomo Kamiya, and Cristian D. Batista, Phys. Rev. B 94, 174420.
[10] Shi-Zeng Lin and C. D. Batista, arXiv:1707.05818v1.

Faculty Host: Natalia Perkins

Thursday, February 15th 2018
3:35 pm:
Speaker: Mark Saffman (University of Wisconsin)
Subject: Quantum computing with simple and complex atoms
Refreshments in atrium after the Colloquium.

Quantum computing is a few decades old and is currently an area where there is great excitement, and rapid developments. A handful of distinct approaches have shown the capability of on demand generation of entanglement and execution of basic quantum algorithms.

One of the daunting challenges in developing a fault tolerant quantum computer is the need for a very large number of qubits. Neutral atoms are one of the most promising approaches for meeting this challenge. I will give a snapshot of the current status of quantum computing in general and atomic quantum computing in particular. The atomic physics underlying our ability to control neutral atom qubits will be described, and I will show how one of the most complicated atoms in the periodic table may lead to some simple solutions to hard problems.

Faculty Host: Paul Crowell

Thursday, February 22nd 2018
3:35 pm:
Speaker: Erez Berg (University of Chicago)
Subject: Critical Metals: Lessons from quantum Monte Carlo studies

Critical phenomena are one of the cornerstones of classical statistical mechanics. Quantum critical points (i.e., continuous phase transitions at zero temperature) in insulating materials are relatively well understood, by analogy with classical critical points in one spatial dimension higher. In contrast, the theory of quantum critical behavior in metals is still, to a large degree, open. Such metallic critical points are believed to play an important role in the physics of several "strongly correlated" materials, such as high temperature superconductors. Fortunately, many classes of metallic quantum critical points can be simulated efficiently using quantum Monte Carlo without the notorious "sign problem", which often hinders numerical simulations of fermionic systems. I will describe some recent progress along these lines, and how it sheds new light on some of the outstanding puzzles in the field.

Faculty Host: Rafael Fernandes

Thursday, March 1st 2018
4:00 pm:
Speaker: Sara Seager, MIT
Subject: Mapping the Nearest Stars for Habitable Worlds
Joint Colloquium with Earth Sciences (Nier Lecture). Note later start time.

Bio:

"Sara Seager is a planetary scientist and astrophysicist at the Massachusetts Institute of Technology where she is a Professor of Planetary Science, Professor of Physics, Professor of Aerospace Engineering, and holds the Class of 1941 Professor Chair. She has pioneered many research areas of characterizing exoplanets with concepts and methods that now form the foundation of the field of exoplanet atmospheres. Her present research focus is on the search for life by way of exoplanet atmospheric “biosignature” gases. Professor Seager works on space missions for exoplanets including as: the PI of the CubeSat ASTERIA; the Deputy Science Director of the MIT-led NASA Explorer-class mission TESS; and as a lead of the Starshade Rendezvous Mission (a space-based direct imaging exoplanet discovery concept under technology development) to find a true Earth analog orbiting a Sun-like star. Among other accolades, Professor Seager was elected to the US National Academy of Sciences in 2015, is a 2013 MacArthur Fellow, is a recipient of the 2012 Sackler Prize in the Physical Sciences, and has Asteroid 9729 named in her honor."

Abstract:

Thousands of exoplanets are known to orbit nearby stars and small rocky planets are established to be common. The ambitious goal of identifying a habitable or inhabited world is within reach. But how likely are we to succeed? We need to first discover a pool of planets in their host star’s “extended” habitable zone and second observe their atmospheres in detail to identify the presence of water vapor, a requirement for all life as we know it. Life must not only exist on one of those planets, but the life must produce “biosignature gases” that are spectroscopically active, and we need to be able to sort through a growing list of false-positive scenarios with what is likely to be limited data. The race to find habitable exoplanets has accelerated with the realization that “big Earths” transiting small stars can be both discovered and characterized with current technology, such that the James Webb Space Telescope has a chance to be the first to provide evidence of biosignature gases. Transiting exoplanets require a fortuitous alignment and the fast-track approach is therefore only the first step in a long journey. The next step is sophisticated starlight suppression techniques for large ground-and space-based based telescopes to observe small exoplanets directly. These ideas will lead us down a path to where future generations will implement very large space-based telescopes to search thousands of all types of stars for hundreds of Earths to find signs of life amidst a yet unknown range of planetary environments. What will it take to identify such habitable worlds with the observations and theoretical tools available to us?

Nier Info:

Professor A.O. Nier

A.O. Nier served as a highly distinguished faculty member of the Physics Department for 42 years starting in 1938. He was actively involved in research up to the time of his death in 1994. A firm believer in “pursuits of knowledge - in areas which cross traditional lines” he had an enormous impact on the geological sciences by his pioneering work on isotope abundances and measurements of many elements which are used in radiometric age determinations of geologic materials. He received many national and international awards in recognition of his discoveries and contributions to Physics, Geological Sciences and many other fields.


Thursday, March 8th 2018
3:35 pm:
Tate Grand Opening

Thursday, March 15th 2018
3:35 pm:
Subject: There will be no colloquium this week due to Spring Break

Thursday, March 22nd 2018
3:35 pm:
Speaker: Pablo Jarillo-Herrero (MIT)
Subject: Magic Angle Graphene: a New Platform for Strongly Correlated Physics

The understanding of strongly-correlated quantum matter has challenged physicists for decades. Such difficulties have stimulated new research paradigms, such as ultra-cold atom lattices for simulating quantum materials. In this talk I will present a new platform to investigate strongly correlated physics, based on graphene moiré superlattices. In particular, I will show that when two graphene sheets are twisted by an angle close to the theoretically predicted ‘magic angle’, the resulting flat band structure near the Dirac point gives rise to a strongly-correlated electronic system. These flat bands exhibit half-filling insulating phases at zero magnetic field, which we show to be a Mott-like insulator arising from electrons localized in the moiré superlattice. Moreover, upon doping, we find electrically tunable superconductivity in this system, with many characteristics similar to high-temperature cuprates superconductivity. These unique properties of magic-angle twisted bilayer graphene open up a new playground for exotic many-body quantum phases in a 2D platform made of pure carbon and without magnetic field. The easy accessibility of the flat bands, the electrical tunability, and the bandwidth tunability though twist angle may pave the way towards more exotic correlated systems, such as quantum spin liquids.

Faculty Host: Vlad Pribiag

Thursday, March 29th 2018
3:35 pm:
Speaker: Barry Mauk, APL
Subject: New perspectives on Jupiter’s novel space environment and aurora from NASA’s Juno mission

B. H. Mauk, The Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland, USA (Barry.Mauk@jhuapl.edu)

Jupiter’s uniquely powerful auroras are thought to be symptoms of Jupiter’s attempt to spin up its space environment and shed angular moment (albeit minuscule amounts). The processes involved connect together such disparate phenomena as the volcanoes of Jupiter’s moon Io and the Jupiter-unique synchrotron emissions imaged from ground radio telescopes at Earth. While the power sources for auroral processes at Earth and Jupiter are known to be very different, it has been expected that the processes that convert that power to auroral emissions would be very similar. NASA’s Juno mission, now in a polar orbit at Jupiter, is dramatically altering this view about how Jupiter’s space environment operates. Auroral processes are much more energetic than expected, generating beams of electrons with multiple MeV energies and with directional intensities that can be more intense than the electrons within Jupiter’s radiation belts. The most intense auroral emissions appear to be generated by processes that have no precedent within Earth auroral processes. And, the auroral generation processes are poorly correlated, unexpectedly, with any large-scale electric currents thought necessary to regulate the interactions between Jupiter’s spinning atmosphere and space environment. These and other findings are discussed, along with presentation of Juno’s broader mission and discoveries.

Faculty Host: Robert Lysak

Thursday, April 5th 2018
3:35 pm:
Speaker: Alessandra Corsi, Texas Tech
Subject: Multi-messenger time-domain astronomy: GW170817 and the future

On 2017 August 17, the field of gravitational-wave (GW) astronomy made the big leagues with a dazzling discovery. After several GW detections of black hole (BH)-BH mergers with no convincing electromagnetic counterparts, advanced LIGO and Virgo scored their first direct detection of GWs from a binary neutron star (NS) merger, an event dubbed GW170817. Soon after the GW discovery, GW170817 started gifting the astronomical community with an electromagnetic counterpart spanning all bands of the spectrum. In this talk, I will review what we have learned from GW170817, what questions remain open, and what are the prospects for future EM-GW studies of the transient sky.

Faculty Host: Vuk Mandic

Thursday, April 12th 2018
3:35 pm:
Speaker: Doug Glenzinski, Fermilab
Subject: A Rare Opportunity - the Mu2e Experiment at Fermilab

The muon, a heavy cousin of the electron, was discovered in 1936. Since
that time they have only ever been observed to do one of two things: 1)
scatter or 2) decay into final states that include a combination of
charged leptons and neutrinos. A new experiment at Fermilab - the Mu2e
experiment - is going to look for a third thing: a muon trapped in atomic
orbit that interacts with the nucleus to produce an electron and nothing
else. This is a process that's predicted to occur very very rarely, maybe
once every 10^15 decays,(or less!). But this very rare decay will probe
new physics mass scales up to 10,000 TeV/c^2 and may hold the key to
understanding physics at its most fundamental level. The Mu2e experiment
is an ambitious endeavor whose goal is to observe this very rare
interaction for the first time - a discovery that could help reveal a new
paradigm of particle physics.

Faculty Host: Kenneth Heller

Thursday, April 19th 2018
3:35 pm:
Speaker: Victoria Kaspi, McGill University.
Subject: Astronomy's Newest Extragalactic Mystery: Fast Radio Bursts!

Fast Radio Bursts (FRBs) are a newly discovered astrophysical phenomenon consisting of short (few ms) bursts of radio waves.FRBs occur roughly 1000 times per sky per day. From their dispersion measures,these events are clearly extragalactic and possibly generally at cosmological distances. One FRB is known to repeat and indeed has been localized to a dwarf galaxy at redshift 0.2. Nevertheless, the origin of FRBs, whether repeating or not, is presently unknown. In this talk I will review FRB properties as well as highlight efforts to find FRBs, including a new Canadian radio telescope,CHIME, that is predicted to make major progress on the FRB problem.


Thursday, April 26th 2018
3:35 pm:
Speaker: John Bush, MIT
Subject: Hydrodynamic quantum analogs

Droplets walking on a vibrating fluid bath exhibit several features previously thought to be exclusive to the microscopic, quantum realm. These walking droplets propel themselves by virtue of a resonant interaction with their own monochromatic wavefield, and represent the first macroscopic realization of a pilot-wave system of the form proposed for microscopic quantum dynamics by Louis de Broglie in the 1920s. New experimental and theoretical results allow us to rationalize the emergence of quantum-like behavior in this hydrodynamic pilot-wave system in a number of settings, and explore its potential and limitations as a quantum analog.

Faculty Host: J. Woods Halley

Thursday, May 3rd 2018
3:35 pm:
Speaker: Jeffrey Bub, Maryland
Subject: Discussions Over a Beer: Bohr, Einstein, Bell, and all that
Student awards will be distributed at the beginning of the Colloquium.

The Bohr-Einstein debate about the foundations of quantum mechanics is something physicists tend to think of as the sort of thing you might discuss over a beer after you’ve spent the day doing real physics. Following John Bell’s seminal 1964 paper on nonlocality, there’s a new game in town influenced by developments in quantum information. I’ll discuss the significance of this new wave in quantum foundations for the dispute that separated Bohr and Einstein. (Bring your own beer.)

Faculty Host: Michel Janssen

Thursday, May 10th 2018
3:35 pm:
There will be no colloquium this week.

Thursday, September 6th 2018
3:35 pm:
Speaker: Andrew Zangwill, Georgia Institute of Technology
Subject: Walter Kohn and the Creation of Density Functional Theory
Refreshments in atrium after the Colloquium.

Today's most popular method for calculating the electronic structure of atoms, molecules, liquids, solids, and plasmas makes no use of the Schrödinger equation or the many-electron wave function. Instead, it exploits a bold hypothesis: the electron density distribution completely characterizes the ground state of a many-electron system. This hypothesis was advanced in 1964-1965 by the theoretical physicist Walter Kohn and two young postdocs. This talk traces Kohn's educational trajectory (from Nazified Vienna to an internment camp in the Canadian forest to the University of Toronto to Harvard University), his professional trajectory (from applied mathematician to nuclear physicist to solid state physicist to the inventor of density functional theory), and the circumstances which led him to win a share of the 1998 Nobel Prize for Chemistry.

Faculty Host: Michel Janssen

Thursday, September 13th 2018
3:35 pm:
Speaker: Raman Sundrum, University of Maryland College Park
Refreshments in atrium after the Colloquium.
Faculty Host: Tony Gherghetta

Thursday, September 20th 2018
3:35 pm:
Speaker: Ali Yazdani, Princeton University
Refreshments in atrium after the Colloquium.

Thursday, September 27th 2018
3:35 pm:
Physics and Astronomy Colloquium in Physics Tate B50
Speaker: Raymond Jeanloz, UC Berkeley
Refreshments in atrium after the Colloquium
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.


Thursday, October 11th 2018
3:35 pm:
Speaker: Joel Moore, UC Berkeley
Refreshments in atrium after the Colloquium.

Thursday, October 18th 2018
3:35 pm:
Speaker: Terry Hwa, UCSD
Refreshments in atrium after the Colloquium.
Faculty Host: Elias Puchner

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

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