2024-2025 Department Colloquia
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.
Fall 2024
November 12, 2024
Abstract: One of the last great unknowns in our history of the universe is when and how the first galaxies emerged after the Big Bang. These galaxies transformed the cosmos – they illuminated the invisible scaffolding of dark matter that underpins the universe, they ionized the intergalactic reservoirs of hydrogen, and they synthesized the elements that would one day seed life on Earth. Thanks to JWST, these enigmatic galaxies are finally coming into view. In this talk I will present latest results on these sources, and preview ongoing experiments I am leading. I will discuss new classes of galaxies being revealed at the highest redshifts such as remarkably luminous early systems, a surprisingly abundant population of obscured black holes (``Little Red Dots"), and extremely metal-poor (perhaps metal-free?) sources. I will describe novel strategies to hone in on the elusive protagonists of cosmic reionization, the last large-scale process that touched almost every baryon in the universe. Throughout, I will outline how in the coming years JWST, Roman, and the upcoming ELTs promise a once-in-a-generation expansion of the astrophysical frontier to the brink of the Big Bang.
October 29, 2024
Abstract: Active processes in living systems create a novel class of nonequilibrium matter composed of many interacting components that individually consume energy and collectively generate motion or mechanical stress. In this talk, I will discuss experimental tools and conceptual frameworks we develop to uncover laws governing fluctuations, order, and self-organization in systems in which individual components break time reversal symmetry. I will describe how such frameworks provide powerful insight into dynamics of nonequilibrium living systems across scales, from the emergence of thermodynamic arrow of time to spatiotemporal organization of signaling protein patterns and discovery of odd elasticity.
October 15, 2024
Abstract: Symmetries provide a powerful organizing principle in physics. However, there are reasons to believe that exact global symmetries do not exist in consistent quantum-gravitational theories. Thus, we expect all symmetries in the world around us to be at least slightly broken. The extent and nature of symmetry breaking phenomena can offer important clues about fundamental physics. In this talk, I will discuss both phenomenological aspects of broken symmetries, including a look at high-precision experiments searching for flavor- and CP-violating effects; and theoretical aspects, including efforts to better understand the constraints quantum gravity imposes on approximate symmetries.
October 1, 2024
September 17, 2024
Abstract: Particle colliders and their experiments have played a crucial role in discovering the fundamental building blocks of our universe and the forces between them. I will give a glimpse into the many ideas for future accelerators that are being discussed before outlining the feasibility study under way for the next major facility at CERN. Its goal is to study the latest discovered particle - the Higgs boson - as well as the overall consistency of the theoretical framework we call the Standard Model of particle physics in more depth to address some of the unsolved mysteries of our universe.
September 3, 2024
Abstract: The hydrodynamics of many living systems -- from morphogenesis to mitosis -- involve, in part, a dynamic competition between bulk active stresses and interfacial tension. We mimic this competition experimentally to understand how activity and capillarity compete to generate functionalized structures. A microtubule-driven active liquid is embedded in a binary, polymeric liquid that phase separates, forming active droplets surrounded by a passive background. Fluctuations in active stress near the boundary induce spontaneous curvature changes in the soft interface. The complex coupling between the bulk active fluid and deformable interfaces creates several exciting phenomena. First, we characterize a nonequilibrium analog of the classic Saffman-Taylor instability using both experiment & theory. Second, we describe how nonreciprocal coupling between the active fluid and interface drives asymmetric fluctuations. Our work supports the claim pressure in active systems is generally not a state variable but rather depends on mesoscopic details. We end with an outlook on time-reversal asymmetry in active systems. Altogether our work demonstrates active droplets exhibit rich hydrodynamic and thermodynamic behavior that can help us better understand the fundamental nature of nonequilibrium living systems.