Department Colloquia
The Physics Department Colloquia are held at 11:30 am on Tuesdays in Abelson 131.
Spring 2025
April 22, 2025
Abstract: Ecological communities can display a wide range of dynamics, including alternative stable states, oscillations, and chaotic fluctuations. Although tremendous progress has been made in characterizing natural communities, the lack of experimentally tractable model systems has made it difficult to discern the principles governing community stability and function. In this talk I will describe our progress using laboratory microcosms to understand how interactions within a community can lead to diverse dynamical behaviors. Consistent with predictions from theory, we find that microbial communities first experience species extinction and then later lose stability when we experimentally increase either the number of species or the strength of interspecies interactions. Before losing stability, we find that communities reach a unique stable state independent of initial species abundances, whereas beyond the instability transition communities display alternative stable states and persistent fluctuations in species abundance. These results argue that coarse-grained features of communities can provide predictive insight into the emergent dynamics of diverse ecological communities.
April 1, 2025
Abstract: Astronomical observations suggest pervasive, dynamically important magnetic fields in our Galaxy and the intracluster medium. Their origin remains a long-standing question in astrophysics and cosmology. It is widely believed that such fields first arose as weak 'seeds' generated by cosmic batteries, and were subsequently amplified by the turbulent plasma flows to current levels via the 'dynamo' process. However, a complete understanding of these processes in a weakly collisional plasma is still lacking. Our first-principles numerical and theoretical study provides a unified paradigm for understanding the origin and evolution of cosmic magnetism by taking into account the effects of nonequilibrium micro-physics of collisionless plasmas on macroscopic astrophysical processes. We apply an external mechanical force to a weakly collisional, initially unmagnetized plasma. The driven large-scale motions are subject to strong phase mixing, which leads to the development of thermal pressure anisotropy. This anisotropy triggers the Weibel instability, which produces filamentary 'seed' magnetic fields on plasma-kinetic scales. The plasma is thereby magnetized, enabling efficient stretching and folding of the fields by the plasma motions and the development of Larmor-scale kinetic instabilities such as the firehose and mirror. The scattering of particles off the associated microscale magnetic fluctuations provides an effective viscosity, regulating the field morphology and turbulence. During this process, the seed field is further amplified by the fluctuation dynamo until energy equipartition with the turbulent flow is reached. By demonstrating that equipartition magnetic fields can be generated from an initially unmagnetized plasma through generic large-scale turbulent flows, this work has important implications for the origin and amplification of magnetic fields in the intracluster and intergalactic mediums.
March 18, 2025
Abstract: Limiting global warming requires dramatically reducing CO2 emissions, but also necessitates implementing carbon dioxide removal (CDR) technologies. A promising avenue is marine CDR through ocean alkalinity enhancement (OAE). OAE involves deposition of alkaline substances in the surface ocean, lowering the partial pressure of CO2 , and leading to carbon uptake from the atmosphere. The process of air-sea gas exchange leading to re-equilibration may take years, and is superimposed onto the background ocean dynamics. Here, we consider OAE as a tracer transport problem, and focus on the role of various scales of ocean turbulence in leading to vertical and lateral transport of alkalinity, thus modulating OAE efficiency. We begin by looking at climate model simulations, allowing us to create a global atlas of OAE efficiency across geographic regimes and seasonal variability. We then move to a hierarchy of ROMS configurations simulating OAE deployments off the California coast at three increasingly high resolutions, and spanning the four seasons. Our experiments yield insights into the influences of seasonality, model resolution, and ultimately ocean turbulence on carbon uptake. We find that OAE efficiency is significantly influenced by turbulent flow features such as mesoscale eddies, submesoscale instabilities, and the background seasonal and interannual flow variability characterizing the time and place of interest. This work raises many additional questions on the roles of geophysical turbulence in setting vertical tracer transport and mixing in the context of OAE.
March 4, 2025
Abstract: Current ideas about quantum gravity often involve scenarios where notions of space-time are not fundamental: rather they are emergent, approximate descriptions of more fundamental structures. One class of such ideas involves metamorphosis of internal degrees of freedom into physical space dimensions. This talk traces the conceptual origins of these ideas in the theory of strong interactions, their evolution into descriptions of quantum gravity and their connection to quantum entanglement. We will discuss some recent attempts to understand entanglement of internal degrees of freedom and its relevance to the question of locality.
February 25, 2025
Abstract: Einstein's General Relativity says gravity is a manifestation of dynamic spacetime. There are, however, hints—particularly from string theory—that spacetime may be an emergent concept, analogous to fluids arising from large collections of molecules at macroscopic scales.
Entanglement, which describes intrinsic quantum correlations without classical counterparts, was recognized early by Einstein, Podolsky, Rosen, and Schrödinger in the 1930s, but its significance in quantum many-body systems only gained prominence since the 2000s. Concurrently, von Neumann's work in the late 1920s and early 1930s on the mathematical foundations of quantum mechanics led to the development of operator algebras (now called von Neumann algebras), which have found wide applications in mathematics, but not so much in physics.
These three seemingly disparate subjects—emergence of spacetime, entanglement, and von Neumann algebras—are now understood to be intricately connected, providing new languages and powerful tools for characterizing the emergence of spacetime.
February 4, 2025
Abstract: Understanding the nature of dark matter is the most important task for contemporary particle physics. While the parameter space remains vast, searches at particle accelerators are sensitive to a wide range of plausible dark matter scenarios. In this talk I will focus on a well-motivated scenario where dark matter is accompanied by a massive “dark photon” that can appear in laboratory experiments. I will describe searches for the dark photon at the Large Hadron Collider with the CMS experiment making use of real time analysis techniques, as well as a new effort to search for the dark photon using the SpinQuest experiment located at the Fermilab Main Injector. I will describe the computing challenge that the HL-LHC poses for future dark photon searches, and the proposed DarkQuest upgrade to SpinQuest that will greatly improve its sensitivity.