Assumptions of Physics

Christine Aidala, Professor, Department of Physics

The Assumptions of Physics project seeks to find a minimal set of physical assumptions from which the basic laws of physics can be rigorously rederived.  It consists of two complementary approaches.  The first, Reverse Physics, analyzes known theories to identify those physical principles and assumptions that can be taken as their conceptual foundation.  The second, Physical Mathematics, seeks to identify and construct mathematical structures that can be rigorously justified from physical requirements and are therefore physically significant.  Results from both approaches will be presented, as well as future plans and opportunities to contribute.

West Hall 335 ---- 13:00h - 14:00h EDT --- 06/08/2023.

Bringing Hot and Cold Atoms from Academic Labs to Real-World Applications

Alisher Duspayev, Sixth Year, Department of Physics

Using various properties of atoms as a basis for novel technologies has attracted a lot of interest in recent years. Such properties include large electric-dipole moments, shifts of quantized energy levels in external fields, ultranarrow linewidths of transitions between quantum states, etc. Prominent examples of already-existing atom-based technologies include room-temperature Rydberg-atom-based field sensors, portable atomic clocks and quantum simulators based on cold atoms or trapped ions. In this talk, I will overview the concurrent efforts to realize novel atom-based technologies and discuss the relevant physics behind these ideas. In the second part, I will focus on what is being done in this direction by our group on the sub-basement level of the Physics Department at UMich. In particular, I will discuss the recent demonstration of how Rydberg atoms can be used to monitor electric fields within cold-ion sources. The talk will be concluded with a brief outlook for the ongoing effort to realize a novel type of atom interferometer for inertial sensing.      

West Hall 335 ---- 13:00h - 14:00h EDT --- 06/15/2023.

Exploring Biophysics using the toolbox of Nonequilibrium Thermodynamics

Gabriela Fernandes Martins, Fifth Year, Department of Physics

The development of Thermodynamics was a stepping-stone to understanding the properties of complex systems, allowing to constrain which types of processes are physically realizable. However, classical Thermodynamics does not describe nonequilibrium systems, which represent a large range of phenomena observed in nature, including life itself. Examples range from the transport of cargo by molecular motors and gene transcription, to bacterial chemotaxis and beyond. Achieving a unified theoretical description of all such processes is challenging, and in the last years, physicists have teamed with biologists to apply the tools of Nonequilibrium Thermodynamics to study such phenomena. In this talk, I will provide a gentle introduction to Stochastic Thermodynamics, a field of Nonequilibrium thermodynamics. Special focus will be given to Markov-jump processes and its direct applications to describing biophysical systems. I will finish by showing how this toolbox can be used to derive fundamental operational limits of a general biochemical motif consisting of a ligand binding to a receptor.

West Hall 335 ---- 13:00h - 14:00h EDT --- 06/22/2023.

Solitons: An Unexpected Journey Through First-Year Graduate Level Physics 

Matthew Mitchell, Fifth Year, Department of Physics

Over the last half-century, there has been increased interest in many integrable nonlinear evolution equations, such as the Nonlinear Schrodinger, Sine-Gordon, Korteweg-de Vries, and Benjamin-Ono equations, due to their ubiquity in describing physical systems exhibiting weakly nonlinear and dispersive behavior along with the remarkable fact that all of these equations have the same qualitative solution method, known generally as an Inverse Scattering Transform. A peculiar phenomenon also shared by many of these equations has to do with a particular set of solutions known as solitons, which have a distinctly particle-like behavior: a spatially localized profile propagating at a fixed speed when alone, obeying a "nonlinear superposition principle" so many individual solitons can be present in a solution at once, and maintaining their individual identities after interacting. In order to demystify these peculiar solutions, we'll embark on a mathematical journey that will take us to some unexpected places. To survive, you'll need your wits about you; specifically, most of your knowledge from your first-year graduate physics courses. You have been warned!

West Hall 335 ---- 13:00h - 14:00h EDT --- 06/29/2023.

Holographic Renormalization Group Flows Across Dimensions

Robert Saskowski, Fifth Year, Department of Physics

Renormalization group (RG) flows are commonly used in quantum field theory to describe the changes in degrees of freedom as we "flow" from very high to very low energies. In particular, there have been a number of proofs of the irreversibility of RG flows in various dimensions, i.e., the statement that the number of high-energy degrees of freedom is larger than the number of low-energy degrees of freedom. In this talk, I will discuss RG flows across dimensions, which one can think of as probing compact extra dimensions. We will do so using the powerful tool of holography, which allows us to recast this field theoretic problem as a geometric flow in anti-de Sitter space.

West Hall 335 ---- 13:00h - 14:00h EDT --- 07/06/2023.

All-Infrared Electromagnetically Induced Transparency with Rydberg Atoms

Ryan Cardman, Sixth Year, Department of Physics

When light is resonant with an electronic transition of matter, it will be absorbed and attenuated as it propagates through the medium. Entirely counterintuitive, a second laser, known as the coupler or control beam, resonant with another electronic transition of the medium can inhibit this absorption and allow the light to pass through unattenuated. This phenomenon is an example of quantum interference and is known as electromagnetically induced transparency (EIT). EIT spectroscopy of room-temperature, alkali-metal vapors in Pyrex cells provides an in-situ and nondestructive Doppler-free readout of Rydberg-state energies and their perturbations from background electric fields and many-body interactions. In this talk, I will describe the quantum dynamics of EIT and the implications of adding a third infrared beam known as the dressing laser. Applications of EIT in the sensing of RF/microwave field and phase, noble-gas pressures, and plasma fields are also introduced.

West Hall 335 ---- 13:00h - 14:00h EDT --- 07/13/2023.

A Short Guide to Gravitational Wave Cosmology

Isaac McMahon, Department of Physics

Gravitational waves have emerged as a new method for measuring the Hubble constant. With recent upgrades to active detectors and future instruments in the works, this new measurement should soon rival the canonical late and early universe measurements in precision and introduce another facet to the Hubble tension. In this talk, I'll walk through the current process of how gravitational wave detection and observational astronomy interlace to make multimessenger cosmology. I'll talk about how gravitational waves are used to infer properties of compact object collisions, how telescopes all over the world are employed to search for a visual signal from the collisions, and then how these signals are combined to measure cosmological parameters.

West Hall 335 ---- 13:00h - 14:00h EDT --- 07/20/2023.

Statistical physics of uncovering and assessing network structures

Maximilian Jerdee, Fourth Year, Department of Physics

Networks are everywhere in science and data. The patterns in which collections of atoms, people, animals, or even concepts interact often carry rich structure that informs complex collective behaviors. In this talk, we focus on two classes of questions often asked of these networks. First, community detection — the art of extracting meaningful groups from network structure, for example sorting neurons into functional groups. Second, ranking — inference of a hierarchy of the participants, for example finding the pecking order of chickens from their pecks. In both cases, we will discuss how ideas and techniques from statistical physics have transformed how these questions are framed and answered.

West Hall 340 ---- 13:00h - 14:00h EDT --- 07/27/2023.

Ultrafast Time-Resolved Spectroscopy Using Optical and X-ray Probes Elucidates the Structural and Electronic Evolution of Photoexcited Coenzyme B12

Taylor McClain, Fifth Year, Department of Biophysics

Coenzyme B12, also known as adenosylcobalamin (AdoCbl), is an important enzymatic cofactor in prokaryotic and eukaryotic species. This molecule consists of a central cobalt coordinated equatorially to a corrin ring ligand, with a lower axial dimethylbenzimidazole α-ligand and an upper axial deoxyadenosyl β-ligand. AdoCbl’s ground state reactivity has been studied in detail, but it was only recently that its role as chromophore in the bacterial photoreceptor protein CarH showed a biological usage of the molecule’s inherent light-sensitivity. The light-induced response of AdoCbl can be characterized using time-resolved spectroscopies. Presently, our group has performed studies with transient absorption (TA) UV-Vis spectroscopy, X-ray absorption spectroscopy, and X-ray emission spectroscopy. TA spectroscopy provides electronic information about the valence orbitals of the system. The X-ray spectroscopies are element- specific probes of the electronic and atomic structure around the central cobalt atom of the AdoCbl. These studies reveal a distinct solvent-dependence of AdoCbl photochemistry as well as a dynamic dance of structural distortions immediately following light excitation. Time- dependent density functional theory (TD-DFT) and finite difference method (FDM) calculations are used to simulate the X-ray absorption and emission spectra. This information is crucial for understanding the CarH protein more deeply as well as bringing cobalamin-based light-mediated drug delivery and optogenetic applications closer to reality.

West Hall 340 ---- 13:00h - 14:00h EDT --- 08/03/2023.

Higher Order Threshold Dynamics Schemes for Motion by Mean Curvature.

Jiajia Guo, Fourth Year, Department of Mathematics

Threshold dynamics is well known as a popular algorithm for simulating the motion of interfaces, nowadays, it's broadly used in image segmentation and solid dewetting. The original version of the algorithm, which is only first-order accurate in time in the two-phase setting, was proposed by Merriman, Bence, and Osher in 1992. Since then, many extensions of the algorithm have been given, for instance, to multiphase mean curvature motion, where it has proven particularly useful and flexible. There have also been high-order accurate versions of the algorithm proposed in several previous studies. Our goal is to take a step toward providing more accurate versions of threshold dynamics with nice properties, for example, monotonicity, which respects the comparison principle of the exact evolution. We'll also introduce our recent finding: the connection between threshold dynamics and the median filter, which is an elegant, monotone discretization of the level set formulation of motion by mean curvature. This results in a new level set method for multiphase mean curvature motion that allows locating the interface via interpolation and enforces the correct junction condition at the free boundaries, at the generality demanded by applications.

West Hall 340 ---- 13:00h - 14:00h EDT --- 08/17/2023.

Using Single-Molecule Microscopy to Probe Actin-Membrane Interactions in Live Cells

Adam Decker, Third Year, Department of Biophysics

The plasma membrane and actin cytoskeleton both serve various roles in regulating the morphology and function of mammalian cells. Here, we seek to quantify the interaction between these structures in real-time and explore their co-regulatory impact in the context of immune cell signaling. We have implored a chimeric fluorescent protein probe, known as membrane proximal actin (MPAct), and single molecule microscopy methods to observe the remodeling and dynamics of near-membrane actin in B cells. Our version of MPAct consists of an inner-leaflet membrane anchor, a photoswitchable fluorophore, and the actin-binding domain of f-tractin. Using MPAct, we have quantified the remodeling of near-membrane actin during B-cell receptor (BCR) activation, showing that actin transiently decouples from the plasma membrane upon stimulation and later recovers to co-localize with BCR clusters. Our results apply a novel single-molecule probe to characterize the dynamics of actin in live cells highlighting the role of actin in regulating the assembly of BCR signaling domains. Future studies will investigate the signaling proteins that coordinate these interactions and explore the role of actin in stabilizing lipid domains in the plasma membrane. 

West Hall 335 ---- 13:00h - 14:00h EDT --- 08/24/2023.