The David and Edith Harris Physics Colloquium Series

SPRING 2019 Schedule

Thursdays // Socials: 3:30pm in 4-349 (Pappalardo Room) // Talk: 4:00pm in 10-250 (unless otherwise noted)

FEBRUARY 7, 2019
FREDERICK P. SALVUCCI
MIT
Host: Peter Fisher

"Thinking globally, while acting locally at MIT" 

For at least the past fifty years, MIT has been in a location shaped by major changes in transportation and environmental policy. MIT has been a major factor in the local and national economy, and significant beneficiary of many of these transportation and environmental changes. The future is posing new challenges in transportation land use, environment, and climate change for the Boston area, which can be successfully confronted, but it is important for MIT to conduct itself as a good neighbor, and build alliances with its neighbors in the Boston Metropolitan area to help develop a more equitable and successful metropolitan community, as well as local environment conducive to an enjoyable experience for students, faculty, researchers and neighbors.

Time: 4:00 pm
Place: Room 34-101 *NOTE ROOM CHANGE
Refreshments @ 3:30 pm in 4-349 (Pappalardo Community Room)

FEBRUARY 14, 2019
DOUGLAS STANFORD
IAS/Stanford University
Host: Daniel Harlow

"Black holes, chaos, and random matrix statistics"

Theoretical black holes are a useful model system for quantum chaos. In particular, they exhibit the butterfly effect in a simple way. However, there are aspects of chaos that are not so easy to see in the black hole setting. In particular, a quantum chaotic theory should exhibit random matrix statistics in the spectrum of its energy eigenvalues. Verifying this for a black hole is a challenge in quantum gravity.

This talk will review the relationship between black holes and chaos, and describe an attempt to obtain random matrix statistics from the spacetime geometry of a black hole.

Time: 4:00 pm
Place: Room 10-250
Refreshments @ 3:30 pm in 4-349 (Pappalardo Community Room)

FEBRUARY 21, 2019
JOEL FAJANS
UC Berkeley
Host: Miklos Porkolab

"Fundamental Tests with Antihydrogen Atoms"

Motivated by the baryogenesis problem (the scarcity of antimatter in the University), CERN's ALPHA collaboration has been studying the properties of antihydrogen atoms. Since first trapping antiatoms in 2010, we have learned to routinely trap over 1000 antiatoms simultaneously, and keep the antiatoms trapped for many tens of thousands of seconds. We have been able to measure the 1S-2S and hyperfine bandwidths to the 10kHz level, which, on some scales, exceeds the accuracy of the best CPT tests. In addition, we have measured the Lyman-alpha line as a precursor to laser cooling the antiatoms, and measured the antiatom charge to 0.7ppb. We are constructing a new apparatus designed to measure the antimatter g to 1%; this will be a test of the weak equivalence principle. This talk will describe how we trap antihydrogen, and discuss our physics results.

Time: 4:00 pm
Place: Room 34-101 *NOTE ROOM CHANGE
Refreshments @ 3:30 pm in 4-349 (Pappalardo Community Room)

FEBRUARY 28, 2019
ANDREA YOUNG
UC Santa Barbara
Host: Ray Ashoori

"Designer topological ground states of electrons in van der Waals heterostructures."

Two dimensional materials are characterized by a large asymmetry between covalent or ionic intralayer and van der Waals interlayer bonding.  As a result, individual atomically thin layers can be mechanically isolated from the sample bulk and subsequently recombined into 'van der Waals heterostructures' consisting of stacked layers of different two dimensional crystals.  I will discuss how we can use van der Waals heterostructuring of graphene to design electronic ground states, focusing on those distinguished by their topology.  In the first part of the talk, I will focus on topological properties of noninteracting electrons, where topology is encoded in the conductance of a two dimensional device.  Absent interactions, band insulators are characterized by a topological invariant directly related to the number of edge states they host on the boundary, with a topological classification that depends on the presence or absence of time reversal symmetry.  In the presence of time reversal symmetry, realizing topologically nontrivial states requires spin-orbit coupling; I will show how such states can be realized in graphene by enhancing the spin orbit coupling via proximity of the graphene layer to a semiconducting transition metal dichalcogenide. In the second part of the talk, I will describe our efforts to realize topologically ordered states that rely on strong electron correlations. In this case, topology is encoded in the quantum statistics of the emergent quasiparticles.  I will show how ultra-clean graphene heterostructures allow us to observe robust versions of well-known topologically ordered phases within the fractional quantum Hall effect, while enabling new experiments to detect their unusual quasiparticle statistics.  Finally, I will describe how moiré superlattices generated by the interplay of the slightly mismatched lattices of graphene and hexagonal boron nitride substrate allow us to realize new types of topologically ordered states in which fractional charges are localized on a lattice, opening new directions for the detection and control of nonabelian statistics.

Time: 4:00 pm
Place: Room 10-250
Refreshments @ 3:30 pm in 4-349 (Pappalardo Community Room)

MARCH 7, 2019
GIANLUCA GREGORI
Oxford University
Host: Nuno Loureiro

"Magnetic field generation and amplification in laboratory experiments and in the Universe"

Magnetic fields are ubiquitous in the Universe and they are an essential player in the dynamics of the luminous matter. However, understanding why there is a magnetic field and why it is so strong has remained a difficult question to answer. The standard theoretical model for the origin of strong magnetic fields is through the amplification of tiny seed fields via turbulence to the level consistent with current observations. We have conducted experiments at large scale laser facilities demonstrating that turbulence is indeed capable of rapidly amplifying magnetic fields. Turbulence also affects cosmic rays diffusion through space. Here we show, using the same laboratory experiments discussed above, how energetic-particle propagates through a random magnetic field.

Time: 4:00 pm
Place: Room 10-250
Refreshments @ 3:30 pm in 4-349 (Pappalardo Community Room)

MARCH 14, 2019
NILANGA LIYANAGE
University of Virginia
Host: Or Hen

"Proton Radius puzzle and new results from Jefferson Lab Proton Radius (PRad) experiment"

The proton charge radius (rp) is one of the important bench-mark quantities in physics. Precise knowledge of its value is critically important for the understanding of the underlying quark-gluon structure of the nucleon as well as in atomic physics, especially in the spectroscopy of atomic hydrogen. The recent result from Lamb shift measurements from the muonic hydrogen: rp = 0.8409(4) fm [1, 2], with its unprecedented less than 0.1% precision, is currently up to eight standard deviations smaller than the average value from all previous experiments. This result triggered the well-known “proton radius puzzle" in nuclear and atomic physics. So far, all theoretical efforts and more precise simulations have failed to explain this discrepancy on the value of this fundamental quantity. This situation urgently requires new high precision measurements to understand the source of the discrepancy. The Jefferson Lab Proton Radius (PRad) experiment collected data in 2016 for a high precision determination of the proton charge radius through the electron-proton elastic scattering process using a novel non-magnetic-spectrometer method. Scattered electrons were detected in a high resolution, large acceptance electromagnetic calorimeter, as well as in a pair of large area Gas Electron Multiplier (GEM) detectors which provided precision coordinate determination. A windowless hydrogen gas flow target was used to ensure that there was no target window background. Systematic uncertainty in the ep cross section measurement was controlled by normalizing it to the simultaneously measured well known Moller cross section. The precision of the ep cross section results from the experiment is expected to be better than 1%. Data analysis is nearing completion now. The preliminary results will be presented in this talk. 

References
[1] The size of the proton, R. Pohl et al., Nature 466, 213 (2010).
[2] Proton Structure from the Measurement of 2S-2P Transition Frequencies of Muonic Hydrogen, A. Antognini et al., Science 339, 417 (2013)

Time: 4:00 pm
Place: Room 10-250
Refreshments @ 3:30 pm in 4-349 (Pappalardo Community Room)

MARCH 21, 2019
GREGORY EYINK
Johns Hopkins University
Host: Hong Liu

"Surprising Turbulent Phenomena and their Explanation — An Untold Story"

In his famous undergraduate physics lectures, Richard Feynman remarked about the problem of fluid turbulence: "Nobody in physics has really been able to analyze it mathematically satisfactorily in spite of its importance to the sister sciences.” This statement was already false when Feynman made it. Unbeknownst to him, Lars Onsager decades earlier had made an exact mathematical analysis of the high Reynolds-number limit of incompressible fluid turbulence, using a method that would now be described as a non-perturbative renormalization group analysis and discovering the first “conservation-law anomaly” in theoretical physics. Onsager’s results were only cryptically announced in 1949 and he never published any of his detailed calculations. Onsager’s analysis was finally rescued from oblivion and reproduced by the speaker in 1992. This “ideal turbulence” theory has subsequently been intensively developed in the mathematical PDE community, where deep connections emerged with John Nash’s work on C1 isometric embeddings. Even more importantly, the theory has led to a fundamental new physics concept of “spontaneous stochasticity”, a breakdown of Laplacian determinism for classical dynamics, which explains the strong mixing properties and intrinsic unpredictability of turbulent flows. This “ideal turbulence” theory has recently been successfully extended to other important cases, including compressible fluid turbulence, relativistic fluid turbulence, and kinetic plasma turbulence, yielding many new testable predictions. It should be regarded as the current “standard model” of high Reynolds-number turbulence. This talk will review Onsager’s exact analysis of high-Re incompressible fluid turbulence, subsequent developments, and future challenges.

Time: 4:00 pm
Place: Room 10-250
Refreshments @ 3:30 pm in 4-349 (Pappalardo Community Room)

APRIL 4, 2019
MARIN SOLJAČIĆ
MIT
Host: TBA

"Enabling novel light phenomena at the subwavelength scale"

By nano-structuring materials at length scales smaller than the wavelength of light, one can create effective materials, exhibiting optical properties unparalleled in any naturally occurring materials. This talk will present our work in three areas of research that have recently been of particular interest to the nanophotonics community: plasmonics, topology, and artificial intelligence. First, via plasmonics, one can spatially confine light by orders of magnitude compared to light confinement in regular materials: this enables new ways in which light can interact with matter. Second, many of the exciting phenomena in topological physics can also be observed in nanophotonics, including: Chiral Edge States, Weyl points, Fermi arcs, etc. Third, numerical simulations of nanophotonics phenomena can often reproduce real experiments very closely: this makes nanophotonics a great training-ground for developing and studying new AI algorithms for science.

Time: 4:00 pm
Place: Room 34-101 *NOTE ROOM CHANGE
Refreshments @ 3:30 pm in 4-349 (Pappalardo Community Room)

APRIL 11, 2019
ANGELA OLINTO
University of Chicago
Host: Jacqueline Hewitt

"Space Probes of the Highest Energy Particles: POEMMA & EUSO-SPB"

Basic questions regarding ultrahigh energy cosmic rays (UHECRs) remain unanswered: What cosmic objects generate such extremely energetic particles that reach above 10^20 eV (100 EeV)? What is this extreme acceleration mechanism? What are the corresponding neutrino fluxes from sources and propagation? How do particles interact at extreme energies? Giant ground observatories, such as the Pierre Auger Observatory and the Telescope Array, have shown that UHECRs are extragalactic and have a surprising composition trend. Hints of anisotropies begin to appear at energies above ~60 EeV, just when data become very limited. We are designing and building space and sub-orbital missions to increase the statistics of UHECR observations at the highest energies. Our international collaboration built the Extreme Universe Space Observatory (EUSO) on a super pressure balloon (SPB) to detect UHECR fluorescence from above. EUSO-SPB1 flew in the Spring of 2017. We are now building EUSO-SPB2 that will also observe Cherenkov from UHECRs and inform the design of the POEMMA (Probe Of Extreme Multi-Messenger Astrophysics) space mission to discover the sources of UHECRs and observe neutrinos from transients above ~20 PeV. POEMMA will add neutrino observations to transient multi-messenger events.

Time: 4:00 pm
Place: Room 10-250
Refreshments @ 3:30 pm in 4-349 (Pappalardo Community Room)

APRIL 18, 2019
ALEXANDER GROSBERG
New York University
Host: Arup Chakraborty

"To Knot or Not to Knot" 

While topological ideas are widely popular in physics, topology of classical linear threads of polymers presents steep mathematical and conceptual challenges, with applications in both biopolymers and materials.  I will start with the simplest phenomenon in the field: if you try to walk a dog on (unreasonably) long leash, it is likely that leash will soon be heavily wound around your legs.  Viewing topological constraints as a form of quenched disorder, I will formulate a few known classical results about “knot entropy” and minimal thermodynamic work needed to untie all knots.  Continuing with increasingly sophisticated models and phenomena, I will review several more recent theoretical and experimental achievements, and conclude with the discussion of a controversial concept of “topological glass”.  

Time: 4:00 pm
Place: Room 10-250
Refreshments @ 3:30 pm in 4-349 (Pappalardo Community Room)

APRIL 25, 2019
HANS-WALTER RIX
Max Planck Institute for Astronomy
Host: Paul Schechter

"Reading physics from stellar spectra"

Extracting accurate and precise physical information from stellar spectra is a century-old effort. These spectra are known to contain the information to determine masses, ages, luminosities, velocities and element composition of stars: quantities that form the foundation for a host of astrophysical studies; but it is hard to get this information out.

Over the last decade, the explosive growth in the quantity and quality of stellar spectra has outpaced the advances in building ab initio models for stellar spectra. Consequently, much of the information content of stellar spectra in current sky surveys has gone untapped. I will lay out new ways to read physical quantities from stellar spectra that boost not only measurement precision, but also allow new kinds of quantities to be inferred.

I will show what this is starting to teach us about the formation history of our own Galaxy, when combined with the ultra-precise position measurements of the Gaia space mission.

Time: 4:00 pm
Place: Room 10-250
Refreshments @ 3:30 pm in 4-349 (Pappalardo Community Room)

MAY 2, 2019
FRANCESCA FERLAINO
University Of Innsbruck
Host: Martin Zwierlein

"The quantum phases of ultracold dipolar gases"

Superfluidity is a macroscopic quantum phenomenon that can occur in systems of very different size and interaction scales. Quantum matter as different as solids, liquid helium, and gases at ultralow temperatures can share remarkable similarities, fostering a flow of ideas across boundaries between such different fields. A paradigmatic case lies on the existence of “quasi- particles”, i.e. elementary excitations dressed by interactions. In superfluid helium, Laudau predicted two type of quasi-particles, the first ones being the well-known phonon mode. The second ones, much more bizarre and intriguing, is known as roton, i.e. a massive quasi-particle carrying large momentum and low energy. Manifesting itself as a pronounced local minimum in the system’s dispersion relation, this unusual excitation reveals the gas’s tendency to spontaneously build up short-wavelength density modulations, precursor of a crystallization instability that may eventually lead to the elusive and highly-debated supersolid phase.

For decades such phenomena have been bound to the peculiar behavior of superfluid helium. However, about fifteen years ago, a similar roton excitation has been predicted in dipolar quantum gases despite their much weaker interactions. Here, particles can be highly correlated due to the long-range and anisotropic character of the underlying many-body dipolar interactions. Thanks to the tremendous experimental progresses of the last few years, quantum gases of highly magnetic atoms are now available, opening the path to access such fascinating physics. This talk reports on the advances from our two Innsbruck experiments, including the first observation of roton quasiparticles and the creation of novel quantum phases such as quantum-stabilized droplets and long-lived states showing hallmarks of supersolidity in erbium and dysprosium ultracold quantum gases.

Time: 4:00 pm
Place: Room 10-250
Refreshments @ 3:30 pm in 4-349 (Pappalardo Community Room)

MAY 16, 2019
CLIFFORD CHEUNG
Caltech
Host: SPS/UWIP

"Unification from Scattering Amplitudes"

Scattering amplitudes are fundamental observables that encode the dynamics of interacting particles. In this talk, I describe how to systematically construct these objects without reference to a Lagrangian. The physics of real-world particles like gravitons, gluons, and pions are thus derived from the properties of amplitudes rather than vice versa. Remarkably, the expressions gleaned from this line of attack are marvelously simple, revealing new structures long hidden in plain sight. As an example, I describe how gravitons are in a very precise way equivalent to products of gluons---a fact with far-reaching theoretical and phenomenological applications, e.g. for new approaches to calculating quantities relevant to gravitational wave physics. Lastly, I show how gravity serves as the "mother of all theories" whose amplitudes secretly unify, among others, all gluon and pion amplitudes.

Time: 4:00 pm
Place: Room 10-250
Refreshments @ 3:30 pm in 4-349 (Pappalardo Community Room)

Last updated on April 23, 2019 10:50 AM