Physics & Astronomy Colloquium

Thursdays, 4:00-5:00 pm

Virtual Colloquium Meetings are held via Zoom, in-person events in PAB 1-434. Meeting information will be sent in email. You may watch past presentations by clicking the title link when available.

For more information, contact Katsushi Arisaka.

Click here for the 2021-2022 archived Physics & Astronomy Colloquium.

Click here for the 2020-2021 archived Physics & Astronomy Colloquium.

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

Fall 2022
Thursday, September 29, 2022

Early Science from the James Webb Space Telescope
Tommaso Treu

In just two months, JWST has revolutionized our understanding of the universe. Its unprecedented sensitivity and angular resolution have given us a new view of the cosmos, enabling new discoveries in many areas of astronomy. I will describe the first results from my GLASS-JWST Early Release Science Program. The primary goal of the program is to understand how the first galaxies form, grow, and cause the last major phase transition of the universe, called "cosmic reionization". The second major goal of GLASS-JWST is understanding how heavy elements such as Carbon and Oxygen were formed out of the pristine gas leftover from the Big Bang and dispersed into the cosmos. Highlights include: i) the discovery of a hitherto unknown population of luminous galaxies at z>10, indicating that galaxies formed earlier and faster than previously thought; ii) the discovery of strongly inverted metal gradients in low mass galaxies, unexpected in galaxies formation models. I will also describe a number of other discoveries and surprises, including a highly magnified star billions of light years away, a very cool brown dwarf, and a supernova at z>3.

Thursday, October 6, 2022

The Beginning of Helioseismology and Studies of Solar Dynamics
Roger K. Ulrich

Helioseismology began with the 1962 report by Leighton, Noyes, and Simon announcing the discovery of the 5-minute oscillations. The discovery happened before I was a professional astronomer. I was a graduate student at UC Berkeley working on convection in Louis Henyey's group studying stellar structure and evolution. As I was nearing completion of the thesis, John Bahcall visited and suggested we should compute a solar model. I computed a solar model, sent it to John and was greeted by return mail with a list of things that had to be fixed. Fellow students and I did the fixes and published a paper. Meanwhile, I got an invitation from John to come to Cal Tech and work on computing solar models.

My interest in the solar 5-minute oscillations began with observations by fellow graduate student Ed Frazier which showed that the oscillatory motion was disrupted by convection cells rather than being generated. Based convective envelope which showed that the on that clue, I did a modal analysis of a model oscillatory power should be restricted to frequencies that depend on the horizontal wavelength. This prediction eventually led to the development of Helioseismology which in turn led to Asteroseismology. I will review the high points instruments of the prediction verification and subsequent development of the space-based now serving as the basis for helioseismology.

After 1985 I accepted the responsibility for managing the program of observations at the 150-foot solar tower telescope on Mt. Wilson. Consequently, after that date I stopped doing helioseismology and began studying solar surface dynamics and magnetic fields. These studies continue and I have recently found a new variations of velocities near the rotation which have timing closely related to the solar cycle.

Thursday, October 13, 2022

Quantum Simulation, Sensing, and Computation With Ultracold Atoms
Dan Stamper-Kurn
UC Berkeley

Abstract: Ultracold atomic gases are perhaps the coldest matter in the universe, reaching temperatures below one nano-kelvin. At these low temperatures, noise is ironed out and the quantum mechanical properties of atoms, not only of their internal atomic states but also of their center-of-mass motion, become accessible and visible. I will describe applications of this ultracold quantum material in the areas of quantum simulation, sensing, and computation. Specifically, I will show how quantum gases far from equilibrium allow us to probe geometric singularities in band structure, a quantum simulation of condensed matter. I will describe how single atoms, trapped tightly within optical tweezers, can be serve as quantum sensors within a scanning-probe microscope of optical fields. Finally, I will explain how cavity-enhanced detection allows us to make mid-circuit measurements within an atoms-based quantum computing platform, a step toward quantum error correction. And what's next? Feedback control of quantum systems? Electromagnetic vacuum fluctuations serving as a chemical catalyst? Telecom-frequency optical clocks? Simulation of flat-band ferromagnetism? Perhaps all of the above.

Thursday, October 20, 2022

Magnetic Chirality
Sang-Wook Chenog

Chirality with all broken mirror symmetries, combined with any spatial rotations, matters ubiquitously from DNA functionality, vine climbing, to the piezoelectricity of quartz crystals. Chirality does not necessarily involve the presence of screw-like twisting, and magnetic chirality means chirality in spin ordered states or mesoscopic spin textures. Magnetic chirality does not change with time reversal operation, and chirality prime () means that time reversal symmetry in addition to all mirror symmetries, combined with any spatial rotations, are broken. In the case of , there exist two kinds: type-I with unbroken “space inversion ⊗ time reversal” and type-II with broken “space inversion ⊗ time reversal”. Four examples of magnetic chirality will be discussed: helical spin state, magnetic toroidal moment combined with canted moment, magnetic quadruple moment combined with alternating canted moments, and Bloch-type skyrmions. We will also discuss a few examples of type-I type-, and the emergent physical phenomena of and such as self-inductance, magnetooptical Kerr effect, anomalous Hall effect and linear magnetoelectricity. Some of these exotic phenomena have been recently observed, and many of them need to be experimentally confirmed in the future.

Thursday, October 27, 2022

How Our Immune System Learns From a Changing Experience
Shenshen Wang

Our adaptive immune system is able to learn from past experience to better fit an unforeseen future. This is made possible by a diverse and dynamic repertoire of cells expressing unique antigen receptors and capable of rapid Darwinian evolution within an individual. However, naturally occurring immune responses exhibit limits in efficacy, speed and capacity to adapt to novel challenges. In this talk, I will discuss theoretical frameworks we developed to (1) explore functional impacts of non-equilibrium antigen recognition, and (2) identify conditions under which natural selection acting local in time can find adaptable solutions favorable in the long run, through exploiting environmental variations and physical constraints. Using these examples, I show that a generalized landscape theory provides a unifying framework for understanding non-equilibrium processes across scales. In light of coevolution, I will discuss a broader scope of our work and its implications for vaccine strategies. I hope to convey that physicists can make a unique contribution to understanding systems as complex as the immune system – it is an exciting time to do so.

Thursday, November 3, 2022

Next Generation Control and Acquisitions Platforms for Studying Large-Scale Neural-Behavioral Dynamics in Freely-Behaving Animals
Daniel Aharoni

Next generation control and acquisitions platforms for studying large-scale neural-behavioral dynamics in freely-behaving animals A major challenge in neuroscience is to uncover how defined neural circuits in the brain encode, store, modify, and retrieve information. Adding to this challenge is the fact that neural function does not operate in isolation from but rather within living, behaving animals. Great technological advances over the past decades have allowed researchers to begin to optically measure and modulate neural activity but these approaches are often limited to head-fix animals when studying neural function at spatial and temporal scales relevant to internal neural circuit dynamics. While a great deal of scientific and technological progress has been made, there is still much to learn concerning complex neural function, especially within the context of natural behavior. To tackle this challenge, significant advancement of neural, behavioral, and computational tools is needed along with new experimental approaches to enable the detailed study of neural circuits within the context of complex behavior and naturalistic, ethologically relevant environments. Here, I will present our ongoing work leading the UCLA Miniscope Project, an open-source collaborative effort aimed at accelerating innovation of neural-behavioral technology while also democratizing access to these transformative tools. This talk will specifically focus on what I hope the future of neural-behavioral research looks like, the critical role engineering and physics will play in reaching this future, and projects my lab is currently working on to address underlying obstacles within modern neural-behavioral research.

Thursday, November 10, 2022

Aspects of Symmetry in Quantum Field Theory — a Modern Update
Thomas Dumitrescu

Abstract: Symmetry has long been a transformative notion in physics, starting with classical and quantum mechanics, but it really comes into its own in quantum field theory (QFT). QFT is a powerful framework for describing interacting systems with many degrees of freedom that serves as a unifying language across many areas of modern physics, including particle physics and string theory, as well as condensed matter and statistical physics. I will present a modern perspective on the notion of symmetry in QFT, with an eye toward recent developments that have greatly expanded this notion. I will then show how symmetries (both old and new) can be applied to gain deeper insight into the dynamics of interesting, strongly-coupled QFTs in diverse dimensions.

Thursday, November 17, 2022

What Does Optimal Teaching and Learning Look Like?
Joshua Samani

Abstract: Improving teaching and learning is a hard, complex problem that benefits from knowledge of the science of human learning. I'll present some of the most robust, generalizable effects from the known science of how people learn and how those effects can inform university-level teaching and learning. I'll then discuss a big, largely-unanswered question in the field followed by work at UCLA attempting to contribute to answering it.