Events Archive

Nov
20
2012
Percolation, Phase Transitions and Speciation on a Neutral Landscape
The problem of speciation and species aggregation on a neutral landscape, subject to random mutational fluctuations without selective drive, has been a focus of research since the seminal work of Kimura on genetic drift. This problem, which has received increased attention due to the recent development of a neutral ecological theory by Hubbell, bears comparison with mathematical problems such as percolation and...
Nov
19
2012
How worms wiggle
Directed locomotion requires coordinated motor activity throughout an animal’s body. The nematode C. elegans, with only 302 neurons, offers an opportunity to understand how locomotion is organized by an entire motor system. We discovered that the mechanism that organizes undulatory locomotion in C. elegans is a novel form of sensory feedback within the motor circuit. Stretch-sensory feedback simply compels each body segment to bend in the same direction and shortly after the bending of the adjacent anterior segment. Remarkably, the entire...
Nov
17
2012
When galaxies collide, what happens to their supermassive black holes?
Galaxy collisions and mergers are a common consequence of the structure formation in the universe. We know that they happen because we see a number of beautiful examples on the sky through the "eyes" of many astronomical observatories. It is also thought that almost every galaxy (including our own, the Milky Way) harbors a supermassive black hole at its center. I will discuss the "knowns" and "unknowns" in the evolution of supermassive black hole pairs that form in collisions of their host galaxies and end their cosmic journey when the two black holes merge due to the emission of gravitational waves....
Nov
15
2012
Physics Graduate & Undergraduate In-reach Series: Computational Neuroethology: A physicist’s journey into wet lab studies of the brain’s neural code
Dr. Robert Liu trained as a condensed matter experimentalist working on quantum transport and noise in mesoscopic semiconductors, but always believed that the true value of a physics doctorate was in learning how to think through and solve problems. As a graduate student at Stanford, he became fascinated with questions about the brain and memory, and decided to apply his quantitative training from physics to the study of neuroscience. He moved to the University of California at San Francisco’s Sloan-Swartz Center for Theoretical Neurobiology to begin postdoctoral work on studying the neural...
Nov
14
2012
Interrogating a granular material: force networks, state variables, and acoustics
Granular materials exhibit large spatial variations in their response to external loading, whether static or dynamic. As such, continuum models of properties such as the shear modulus and sound speed often fail. A promising alternative is to build an understanding of bulk behaviors from measurements at the particle scale, by analogy with the statistical mechanics of thermal systems. I will describe experiments in which we utilize photoelastic particles and piezo-embedded 'smart' particles to explore how two familiar properties -- temperature equilibration and...
Nov
13
2012
Immunophysics of host-pathogen interactions: An immune-cell perspective
Motile immune cells like neutrophils (the most abundant type of white blood cell) track down invading microbes by chemotaxis, keep hold of them by adhesion, and neutralize them by phagocytosis.  We integrate concepts and tools from immunobiology and biophysics to examine the mechanistic underpinnings of this amazing, cross-disciplinary feat.  Single-live-cell experiments using micropipette manipulation, optical tweezers, and a new type of horizontal atomic force microscope allow us to assess the nano-to-microscale ingredients of one-on-one encounters between human neutrophils and their targets (such as opsonized...
Nov
08
2012
A Century of Superfluidity: from Mercury to Neutron Stars, from Nuclei to Ultra-cold Atoms
Nov
08
2012
Analytic Modeling of z>=6 Interstellar Medium
The statistics of newly discovered z>~6 galaxies---their abundance and clustering---have allowed us to study their ensemble properties, but the difficulties inherent in detecting, resolving, and obtaining spectra of these distant, faint objects has prevented a deeper empirical exploration of their internal physics. However, in advance of observatories like GMT and TMT, multi-wavelength observations can be a useful probe of the high-redshift ISM. I will present a novel analytic model for the internal physics of z>~6 galaxies that, when combined with information from galaxy statistics, probes an enormous range of distance scales---from the tens of megaparsec regions over which...
Nov
08
2012
Superflow decay via quantum phase slips in one-dimensional Bose gases
In recent years, experiments with ultracold atoms [1,2,3,4] have investigated transport properties of one-dimensional (1D) Bose gases in optical lattices and shown that the transport in 1D is drastically suppressed even in the superfluid state compared to that in higher dimensions. Motivated by the experiments, we study superfluid transport of 1D Bose gases. In 1D, superflow at zero temperature can decay via quantum nucleation of phase slips even when the flow velocity is much smaller than the critical velocity predicted by mean-field theories. Using instanton techniques, we calculate the...
Nov
07
2012
Event-Based Optimal Control of Neurons
Motivated by issues related to treating certain neurological diseases such as Parkinson’s disease by a method called electrical deep brain stimulation, we consider applying optimal control methods to both mathematical models of neurons and in vitro neurons. Patients suffering from Parkinson’s disease experience involuntary tremors that typically affect the distal portion of their upper limbs. It has been hypothesized that these tremors are associated with simultaneous spiking of a cluster of neurons in the thalamus and basal ganglia regions of the brain. In a healthy situation, the periodic firing of neurons is not synchronized, but they can engage in a pathological synchrony...

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