Physics & Astronomy Colloquium


Thursdays, 4:00-5:00 pm

Virtual Colloquium Meetings are held via Zoom. Meeting information will be sent in email. You may watch past presentations by clicking the title link when available.

For more information, contact Robijn Bruinsma.

Click here for the 2019-2020 archived Physics & Astronomy Colloquium.


Fall 2020
Thursday, October 8, 2020

Nuclear Femtography - A new frontier of science and technology
Jianwei Qiu
Jefferson Lab

The proton and neutron, known as nucleons, are the fundamental building blocks of all atomic nuclei that make up essentially all the visible matter in the universe, including the stars, the planets, and us. Nucleons have a complex internal structure. Within Quantum Chromodynamics, nucleons emerge as strongly interacting and relativistic bound states of quarks and gluons. Both theory and experimental technology have now reached a point where we are capable of exploring the inner structure of nucleons and nuclei at sub-femtometer distance, leading to the newly emerging science of nuclear femtography. In this talk, I will demonstrate that the newly upgraded CEBAF facility at Jefferson Lab and the Electron-Ion Collider, which the US Department of Energy recently approved for construction at Brookhaven National Lab, will be two complementary and necessary facilities for exploring the science of nuclear femtography. They are powerful tomographic scanners and/or microscopes able to precisely image the inner structure of nucleon and nuclei with a sub-femtometer resolution. They will help us address the most compelling unanswered questions about the elementary building blocks of our visible world, and are capable of taking us to the frontier of the Standard Model.


Thursday, October 15, 2020

Controlling the quantum states of atoms to probe fundamental physics
Paul Hamilton
University of California, Los Angeles

Modern techniques to control the quantum states of atoms have enabled measurements with an unprecedented precision and accuracy. This ability makes atomic systems attractive for a range of applications including quantum sensing, quantum computation, and quantum simulation. I will discuss ongoing experiments at UCLA harnessing this control of atoms to make novel gravitational, rotational, and magnetic sensors, and their application to searches for particles and fields beyond the Standard Model including sterile neutrinos, dark matter, and dark energy.


Thursday, October 22, 2020

The renaissance of jet physics
Zhongbo Kang
University of California, Los Angeles

The particle collisions observed in high energy colliders are dominated by the phenomenon of jets. These are collimated sprays of particles that result directly from quantum chromodynamics (QCD). Following advances in both experimental techniques and theory, the study of jets has become a powerful tool for the exploration of fundamental properties of QCD under different conditions, and for the search for new phenomena in high-energy collisions. Jets can now be characterized not just by their overall direction and energy but also by their internal substructure. Jet physics is at the forefront of phenomenology studies at the Large Hadron Collider (LHC) and at the future Electron Ion Collider (EIC). In this talk, I will highlight novel experimental opportunities and new theoretical studies of the physics of jets, how they affect probes of QCD at the LHC and studies of the quantum imaging of protons at the EIC.


Thursday, October 29, 2020

Investigating the energy frontier of Particle Physics while analyzing 40 million proton collisions per second in real time
Michalis Bachtis
University of California, Los Angeles

The Large Hadron Collider has recently completed its second run collecting an enormous dataset of proton collisions at the center of mass energy of 13 TeV. The new dataset provides a unique opportunity to search for heavy new particles that are predicted by several theoretical models and could not be produced in the energies achieved before. Recent results on those searches performed by the UCLA group will be presented. In parallel with data analysis, an established new instrumentation effort towards the upgrade of the CMS experiment will be presented, featuring high throughput processors built at UCLA that can analyze more than 3 Tb/s of data in real-time. Finally an extension of this instrumentation program will be presented, where similar technology targeting 5G wireless is used to perform real-time RF signal processing with applications in particle accelerators and other areas of experimental physics.


Thursday, November 5, 2020

Quantum control of spins in silicon
Mark Eriksson
Wisconsin Quantum Institute and University of Wisconsin-Madison

Quantum computing is based on the manipulation of two-level quantum systems, or qubits. In most approaches to quantum computing, qubits are as much as possible isolated from their environment in order to minimize the loss of qubit phase coherence. The use of nuclear spins as qubits is a well-known realization of this approach. In a radically different approach, quantum computing is also possible for strongly coupled multi-electron spin 1/2 systems, as realized in silicon-based devices. In this talk I will present both a historical overview of how quantum manipulation in silicon has developed, as well as the latest results from both our group at Wisconsin and from around the world. I will also discuss an interesting scientific case study, comparing some limiting cases: qubits composed of single-spins, be they electron or nuclear, where magnetically-driven manipulation (possibly effective rather than direct) is required, and qubits composed of multiple electrons, for which case direct electric-field manipulation is possible. I will end with a brief discussion of how silicon fits into the broad quantum science and technology ecosystem, which is growing at an astounding rate.

This article in Physics Today discusses closely related material.


Thursday, November 12, 2020

Results from LIGO-Virgo’s third observing run
Jess McIver
University of British Columbia

In less than five years, the field of gravitational wave astronomy has grown from a groundbreaking first discovery to revealing new populations of stellar remnants through distant cosmic collisions. Advanced LIGO and Advanced Virgo’s third observing run, from April 2019 to March 2020, potentially added dozens more known compact object mergers to the eleven confident detections from the first two Advanced-era observing runs. I'll summarize recent results from LIGO-Virgo and their implications, including the recently announced discovery of a 142 solar mass black hole, and discuss challenges for LIGO, Virgo, and KAGRA in this new era of multi-messenger astronomy with gravitational waves.


Thursday, November 19, 2020

Power and Privilege
Sherard Robbins
Visceral Change, LLC

This one-hour presentation has been designed to offer an in depth dialogue around the systems of power and privilege and how they impact our everyday lives, personally, professionally, and academically. This session offers discussion time to define privilege and how those systems serve as predicates for how we interpret normalcy and establish a standard of justification. Through the exploration of power and privilege, participants will gain a better understanding of how to promote inclusivity and social justice among people of all identities.


Thursday, November 26, 2020

Thanksgiving holiday, no colloquium


Thursday, December 3, 2020

Click link to view: Non-reciprocity in collective phenomena: pattern-formation, synchronization and flocking
Vincenzo Vitelli
University of Chicago

The interaction between a peregrine falcon and a dove is visibly non-reciprocal. Unlike the dogma preached by Newton’s third law, the actions they exert on each other are by no means equal and opposite. What happens to the well-established framework of phase transitions in non-reciprocal systems far from equilibrium?

In this talk, I will answer this question by looking at three archetypal classes of self-organization out of equilibrium: synchronization, flocking and pattern formation. Simple demonstrations with robots will be presented along with naturally occurring phenomena from various domains of science that share a common feature: reciprocity has no reason to exist. In all these cases, the emergence of unique time-depend foundation for a general theory of critical phenomena in non-reciprocal matter.


Thursday, December 10, 2020

Click the link to view: The Modern Amplitudes Program: Supercolliders, Fluid Dynamics, and Black Holes
Clifford Cheung
Caltech

Scattering amplitudes are fundamental observables encoding 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. In particular, I describe how gravity serves as the "mother of all theories" whose amplitudes secretly unify, among others, all gluon and pion amplitudes. This fact has far-reaching theoretical and phenomenological connections, e.g. to fluid mechanics and to new approaches to the black hole binary inspiral problem.


Winter 2021
Thursday, January 7, 2021

Click the link to view: Accelerating our understanding of the multi-scale dynamics of
high-energy plasmas

Paulo Alves
University of California, Los Angeles

At the core of some of the most important problems in plasma physics – from controlled nuclear fusion to the acceleration of the most energetic particles in the Universe – is the challenge of capturing the intricate interplay between microscopic plasma processes and global plasma dynamics, which are separated by many orders of magnitude in spatial and temporal scales. State-of-the-art first-principles simulations are beginning to capture a sufficiently large dynamical range to probe fundamental aspects of this interplay. Advances in experimental capabilities are further allowing us to closely validate theoretical/computational models, and even probe beyond the range of scales accessible to our largest simulations. Moreover, the increasing quantity and quality of plasma data being produced is creating new opportunities for innovation in the way we tackle these long-standing challenges.

In this talk, I will discuss how state-of-the-art kinetic simulations are beginning to unveil the physics interplay between small-scale kinetic plasma processes and global plasma dynamics in the context of magnetic field generation and particle acceleration in relativistic astrophysical outflows. I will also discuss how techniques from the fields of Artificial Intelligence and Machine Learning can help us take full advantage of the data from high-fidelity numerical simulations and experiments to accelerate the development of computationally efficient descriptions of microscopic plasma processes, and improve the accuracy of multi-scale plasma models for a broad range of applications.


Thursday, January 14, 2021

Click the link to view: Affinity maturation of antibodies and the puzzle of HIV spikes
Mehran Kardar
Massachusetts Institute of Technology

Affinity maturation (AM) is the process through which the immune system evolves antibodies (Abs) which efficiently bind to antigens (Ags), e.g. to spikes on the surface of a virus. This process involves competition between B-cells: those that ingest more Ags receive signals (from T helper cells) to replicate and mutate for another round of competition. Modeling this process, we find that the affinity of the resulting Abs is a non-monotonic function of the target (e.g. viral spike) density, with the strongest binding at an intermediate density (set by the two-arm structure of the antibody). We argue that, to evade the immune system, most viruses evolve high spike densities (SDs). This is indeed the case, except for HIV whose SD is two orders of magnitude lower than other viruses. However, HIV also interferes with AM by depleting T helper cells, a key component of Ab evolution. We find that T helper cell depletion results in high affinity antibodies when SD is high, but not if SD is low. This special feature of HIV infection may have led to the evolution of a low SD to avoid potent immune responses early on in infection. Our modeling also provides guides for design of vaccination strategies against rapidly mutating viruses.


Thursday, January 21, 2021

Grand unification of quantum algorithms
Isaac Chuang
Massachusetts Institute of Technology

The three main branches of quantum algorithms, for simulation, search, and factoring, hold historically disparate origins. Today, we can now understand and appreciate all of these as being instances of a single framework, recently created by Gilyen, Su, Low, and Weibe, based on two key ideas: (1) the transformability of singular values by quantum evolution, and (2) the nonlinearity available to process two-level quantum signals. This remarkable unified framework opens doors to new quantum algorithms and opportunities for quantum advantage.


Thursday, January 28, 2021

Click the link to view: Pushing the limits of hydrodynamics
Pavel Kovtun
University of Victoria

Hydrodynamics is a well-established field with a venerable history. In this talk, I will focus on foundational aspects of hydrodynamics which came to light in recent years. Do the equations of hydrodynamics even make sense? To what degree can the crudeness of hydrodynamics be improved? What about the phenomena that hydrodynamics should describe but fails to? And what about the phenomena that hydrodynamics shouldn't describe, but does?


Thursday, February 4, 2021

Click the link to view: Wormholes and entanglement
Juan Maldacena
Princeton University

Wormholes are spacetime geometries that connect distant regions of spacetime. We will review the simplest such wormhole which results from the analytic extension of the original black hole solution. This is a non-traversable wormhole that can be interpreted as an entangled state of two black holes. We will then discuss the type of traversable wormholes that are allowed by basic physical principles. We will discuss concrete traversable wormhole solutions in four dimensions.


Thursday, February 11, 2021

Introduction to the physics basis for burning plasmas in tokamaks: turbulent-transport
Anne White
Massachusetts Institute of Technology

The new strategic plan developed by the US scientific plasma physics and fusion community moves aggressively toward the deployment of fusion energy. The mature physics basis for net-energy tokamaks, recent technological innovations, and a burgeoning $2 billion-dollar industry have opened the door for the US to build a fusion pilot plant by the 2040s. In particular, the ability to predict turbulent-transport in tokamak plasmas has improved dramatically in just the last five years. This is thanks to new modeling tools, but also, to a multi-decade-long vision that emphasized direct fluctuation measurements and comparisons with state-of-the-art first-principles simulations, with leadership from UCLA. This seminar will introduce the physics basis for burning plasmas in tokamaks. A brief review of the nuclear physics and plasma physics relevant for net-energy fusion devices, and the fundamentals of tokamak confinement, will be presented in a manner accessible to advanced undergraduate students and first-year grad students. Then several exemplary transport model validation efforts led by students and scientists at UCSD, MIT and General Atomics will be described in detail, to illustrate how such studies directly influence the development of high-fidelity reduced transport models that are being used to predict fusion performance in the ITER and SPARC tokamaks; and will ultimately be used to help design a fusion pilot plant in the US.


Thursday, February 18, 2021

Our Galactic Center: A Unique Laboratory for the Physics & Astrophysics of Black Holes
Andrea Ghez
University of California, Los Angeles

The proximity of our Galaxy's center presents a unique opportunity to study a galactic nucleus with orders of magnitude higher spatial resolution than can be brought to bear on any other galaxy. After more than a decade of diffraction-limited imaging on large ground-based telescopes, the case for a supermassive black hole at the Galactic center has gone from a possibility to a certainty, thanks to measurements of individual stellar orbits. The rapidity with which these stars move on small-scale orbits indicates a source of tremendous gravity and provides the best evidence that supermassive black holes, which confront and challenge our knowledge of fundamental physics, do exist in the Universe. This work was made possible through the use of speckle imaging techniques, which correct for the blurring effects of the earth's atmosphere in post-processing and allowed the first diffraction-limited images to be produced with these large ground-based telescopes.

Further progress in high-angular resolution imaging techniques on large, ground- based telescopes has resulted in the more sophisticated technology of adaptive optics, which correct for these effects in real time. This has increased the power of imaging by an order of magnitude and permitted spectroscopic study at high resolution on these telescopes for the first time. With adaptive optics, high resolution studies of the Galactic center have shown that what happens near a supermassive black hole is quite different than what theoretical models have predicted, which changes many of our notions on how galaxies form and evolve over time. By continuing to push on the cutting-edge of high-resolution technology, we have been able to capture the orbital motions of stars with sufficient precision to test Einstein’s General theory of Relativity in a regime that has never been probed before.


Thursday, February 25, 2021

Innovative Approaches in mm-Wavelength Cosmology: From Inflation to the Epoch of Reionization and Beyond
Abigail Crites
University of Toronto
Special seminar: 1:00 - 2:00pm

I will describe how I use mm-wavelength instruments (both spectrometers and photometers) to explore our universe across cosmic time and to probe fundamental physics. I will describe some of my primary science interests, the epoch of reionization, star formation across cosmic time, and cosmology using the cosmic microwave background (probing inflation and neutrino physics), and discuss the development of instrumentation and data analysis tools to study these areas. I will motivate these plans with a discussion of TIME, the pathfinder instrument I am leading for studying reionization with mm-wavelength line intensity mapping, and discuss the role that the expertise we have built in the development of new instrumentation and data analysis for scientific discovery can play in the success of future instruments such as TIME-EXT and CCAT-prime. I will discuss models for expected signals from current and future instruments and discuss what we can learn from combining data from mm-wavelength spectrometers with other instruments, such 21 cm instruments, ALMA, HST, and in the future, JWST, which probe similar epochs. I will also discuss CMB-S4, a next generation cosmic microwave background experiment that will probe cosmology, the early universe, and neutrino physics.


Thursday, February 25, 2021

Click the link to view: Rotating quantum gases under the microscope
Richard Fletcher
Massachussets Institute of Technology

When charged particles are placed in a magnetic field, the single-particle energy states form discrete, highly-degenerate Landau levels. Since all states within a Landau level have the same energy, the behaviour of the system is completely determined by the interparticle interactions and strongly-correlated behaviour such as the fractional quantum Hall effect occurs. In contrast to transport measurements in condensed matter systems which probe the behaviour of the entire sample, ultracold atomic quantum gases afford the ability to manipulate and observe the dynamics of single wavefunctions subject to a magnetic field, offering a complementary, microscopic insight into the individual building blocks of quantum Hall systems.

However, atomic quantum gases are electrically neutral, meaning one must engineer "synthetic" magnetic fields for the atoms. Here, we present recent experiments from MIT on high-resolution microscopy of a rotating Bose-Einstein condensate, in which the Coriolis force felt by a massive particle in a rotating frame plays the role of the Lorentz force felt by a charged particle in a magnetic field. Remarkably, in a magnetic field the X and Y coordinates of a particle do not commute, leading to a Heisenberg uncertainty relation between spatial coordinates. We directly observe this uncertainty, and the resulting zero-point motion of atoms which sets a fundamental limit on their position. In a second experiment, we investigate the purely interaction-driven behaviour of a Bose-Einstein condensate living entirely in the lowest Landau level, where all single-particle states are degenerate. We reveal a spontaneous crystallisation of the fluid, driven purely by the interplay of interactions and the magnetic field.


Thursday, March 4, 2021

Achievement of a burning plasma state
Omar Hurricane
Lawrence Livermore National Laboratory

It is widely agreed in the plasma physics community that the next major milestone in fusion research is the creation of a "burning plasma" – one in which the alpha-particles from the fusion reactions are the primary source of heating in the plasma, heating which is necessary to sustain the fusion reaction. A burning plasma is the last physically relevant step before "ignition" in inertial confinement fusion, or reactor-relevant high-gain plasmas in magnetic-confinement fusion – achieving a burning plasma has been the goal of fusion research for several decades.

In November 2020 two inertially confined fusion (ICF) experiments at the National Ignition Facility (NIF) facility in Livermore California, for the first time anywhere, broke into the burning plasma regime. In this talk, we describe the physics behind what a burning plasma is, how we diagnose it, as well as outline the related ICF physics principles and strategy that got us here.


Spring 2021
Thursday April 1, 2021

Biological Physics: Looking for new physics in the living world
Alex J. Levine
UCLA

Biological physics seeks not only to use physics to understand biological phenomena, but also to mine the living world for new physics. The latter goal is made possible by the fact that life creates uniquely ordered structures and drives them into particular, controlled, and long-lived nonequilibrium states. In this talk, I present two stories of looking for new physics in the living world. The first is based on studies of the filament networks that pervade both the interior of our cells (the cytoskeleton) and in the spaces between those cells in our tissues (the extracellular matrix). I will discuss the formation of filament bundles driven by thermal Casimir interactions between cross-linkers in filament bundles and the role of topologically-protected defects, such as braids in them. In the second story, I highlight the hair cell oscillators of the inner ear (as investigated by the Bozovic group) as an exemplar of controlled nonequilibrium states in biology. These cells make hearing possible. They also provide an interesting playground to study fluctuation-dissipation relations in nonequilibrium steady states under feed-back based control. Can we use the failure of various relations between fluctuations and response as a probe of not just nonequilibrium states in nature, but of particular classes of nonequilibrium states under different types of homeostatic control?


Thursday, April 8, 2021

High-precision physics and chemistry with ultracold molecules
Tanya Zelevinsky
Columbia University

Techniques for controlling the internal quantum states and motion of atoms have led to extremely precise clocks and state-of-the-art studies of degenerate gases. Extending such techniques to various types of molecules further enriches the understanding of fundamental physics, basic chemical processes, and many-body science. Samples of ultracold diatomic molecules can be created by binding laser-cooled atoms, or by direct molecular laser cooling. We explore both approaches and demonstrate a high-precision optical-lattice based molecular clock, as well as chemical processes in the quantum domain that manifest very differently than at room temperature.


Thursday, April 15, 2021

Electrons in Moiré Superlattices: A playground for strong correlations and topology
Ali Yazdani
Princeton University

Interactions among electrons and the topology of their energy bands can create novel quantum phases of matter. The discovery of electronic bands with flat energy dispersion in magic-angle twisted bilayer graphene (MATBG) has created a unique opportunity to search for new correlated and topological electronic phases. We have developed new scanning tunneling microscopy (STM) and spectroscopy (STS) techniques to probe the nature of electronic correlations and to detect the novel phases in this two-dimensional systems. Density-tuned STS studies have enabled us to study the properties of MATBG as function of carrier concentration revealing key and new properties of this novel material. These measurements establish that MATBG is a strong correlated system at all partial filling of its flat bands. [1] The strength of the interactions, which can be measured in our experiments, is found to be larger than the flat bandwidth in the non-interacting limit. We demonstrate that these interactions drive a cascade of transitions at each integer filling of these bands, creating likely the insulating states at low temperatures that are spin or valley polarized.[2] Most recently, we developed a new technique to detect topological phases and their associated Chern numbers and used it to show that strong interactions drive the formation of unexpected topological insulating phases in MATBG [3]. These phases, which are stabilized by a weak magnetic field, are rare examples of when topology emerges from interaction between electrons. I will describe these experiments, and other ongoing efforts, that illustrate the power of atomic scale experiments in revealing novel physics of electrons in moiré superlattices.

[1] Y. Xie. et al. Nature 572, 101 (2019).
[2] D. Wong, et al. Nature 582, 198 (2020).
[3] K. Nuckolls et al. arXiv:2007.03810, to appear in Nature.


Thursday, April 22, 2021

The physics of virus self-assembly
Vinny Manoharan
Harvard University

Simple viruses consist of RNA and proteins that form a shell (called a capsid) that protects the RNA. The capsid is highly ordered, with the proteins being arranged in an icosahedral shell. Many simple viruses are self-assembled: you can mix the RNA and the capsid proteins in a test tube, and they will spontaneously form infectious viruses in high yield. This result suggests that we can understand RNA virus self-assembly from the perspective of statistical physics. The central question is how a random process like self-assembly can lead to a high yield of well-formed viruses. To address this question, we have developed an interferometric technique that allows us to measure the scattering of individual assembling viral particles (MS2 bacteriophage) on time scales ranging from 1 ms to 1000 s. By comparing the scattered intensity to that of the wild-type virus, we infer the mass of proteins that have attached to the central RNA as a function of time. We find that individual particles grow to nearly full size in a short time following a much longer delay period. The distribution of delay times suggests that the assembly follows a nucleation-and-growth pathway. I will discuss how such a pathway might allow the virus to assemble with such high yield.


Thursday, April 29, 2021

Revisiting and Repurposing the Double Helix
Taekjip Ha
Johns Hopkins University

DNA is an iconic molecule that forms a double helical structure, providing the basis for genetic inheritance, and its physical properties have been studied for decades. In this talk, I will present evidence that sequence dependent physical properties of DNA such as flexibility and self-association may be important for biological functions. In addition, I will present a new application of DNA where mechanical modulations of cell behavior can be studied at the single molecule level using rupturable DNA tethers.


Thursday, May 6, 2021

The Golden Age of Neutron Stars
Gordon Baym
University of Illinois Urbana-Champaign

Neutron stars were first posited in the early thirties, and discovered as pulsars in the late sixties; however we are only recently beginning to understand the matter they contain. I will describe the ongoing development of a consistent picture of the liquid interiors of neutron stars, driven by observational as well as theoretical advances. These include, in particular observations of now three heavy neutron stars of 2.0 solar masses and higher; very recent simultaneous inferences of masses and radii of neutron stars via the NICER telescope; and past and future observations of binary neutron star mergers, through gravitational waves as well as across the electromagnetic spectrum. Theoretically an understanding is emerging in QCD of how nuclear matter can turn into deconfined quark matter in the interior, and be capable of supporting heavy neutron stars, which I will illustrate with a discussion of modern quark-hadron crossover equations of state.


Thursday, May 27, 2021

Many-body localization and thermalization: some fundamentals of quantum statistical mechanics
David Huse
Princeton University

Most physical systems that contain many interacting degrees of freedom that are excited to energies well above of the ground state do act as a “bath” or “reservoir” for their own subsystems and thus go to thermal equilibrium under the system’s own dynamics, without any coupling to an external environment. This fundamental and long-studied process is called “thermalization”, and has been an active subject of recent research, motivated by atomic, condensed matter, and high energy physics. One class of systems that fail to thermalize are systems that are many-body localized (MBL), which is the interacting version of Anderson localization. Such MBL systems instead remain localized near their initial state. There is a novel dynamic quantum phase transition between many-body localization and thermalization. I will give an overview of these topics.

This talk will be given in hybrid format, with actual physical chalk, blackboard and speaker. Electronic and digital technology will only be used for the live video link.


Thursday, June 3, 2021

Some recent results in gravitational-wave science
Daniel Holz
University of Chicago

The past five years have witnessed the birth of the entirely new fields of gravitational-wave physics, astrophysics, and cosmology. We'll provide some background on this new way to probe the universe. We'll also discuss some recent results, focusing on constraints that come from a population of gravitational-wave events. Topics will include the lower (between neutron stars and black holes) and the upper (due to pair-instability supernovae) mass gaps, standard siren cosmology, and recent events including GW190814 (with a secondary object that might be a neutron star or a black hole; we're not sure which) and GW190521 (with two black holes potentially in the pair-instability mass gap).