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

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

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

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, 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.

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 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.