Physics Colloquia Spring 2010

(usually Fridays 1:00 PM in PS 109)

Titles link to the abstracts.

Date Speaker Title
Jan 22
Douglas Cheyne (University of Toronto)
Feb 5
Chris Beetle (FAU)
Matthew Duez (Cornell University)
Hui Wang (University of North Carolina)
Henry Fu (Brown University)
Sukanya Chakrabarti (UC Berkeley)
Feb 19
Sean Xing (Syracuse University)
Graziano Vernizzi (Northwestern University)
Jonathan Engle (Uni Erlangen-N rnberg)
Feb 26
Rajesh Menon (University of Utah)
Ghanim Ullah (Penn State University)
Mar 19
Konstantin Yakunin (FAU)
Apr 2
Korey Sorge (FAU)
Apr 16
Dora Leventouri (FAU)

Colloquium Abstracts

Combining MEG and beamformers to study the cortical dynamics of human motor control
Douglas Cheyne (Hospital for Sick Children and University of Toronto), Jan 22
Neuroimaging studies based on fMRI or PET imaging methods have been successful in identifying neural structures implicated in the performance of various types of motor tasks but offer little information regarding the precise timing of these activations. EEG-ERP studies on the other hand have provided insight into the temporal dynamics of the cortical control of movement in humans, but pose difficulties in terms of the precise localization of the cortical areas involved. Whole-head MEG recordings combined with advanced source reconstruction methods provides high temporal resolution measurements of cortical activity during motor tasks, but with greater ability to identify the underlying cortical sources. Although movement-related magnetic fields have been recorded since the early 1980s, traditional source localization approaches used in MEG (e.g., equivalent current dipole models) have been problematic for studies of the motor system due to the complex patterns of cortical activity that can accompany even very simple movements. I will present results from our laboratory using spatial filtering techniques based on minimum-variance beamforming to measure patterns of cortical activity accompanying simple motor tasks. These results demonstrate the successful application of beamforming to study both movement-locked (event-related) and induced oscillatory activity in human motor areas, including recent findings of high-frequency oscillatory activity in primary motor areas during voluntary movements. I will also present preliminary results from a speeded forced choice response task showing activation of ipsilateral motor cortex and frontal midline regions during response switching (within and between hand) and response errors reflecting the role of these brain areas in response selection and inhibition. These findings demonstrate the advantages of spatiotemporal imaging methods using MEG to study the neural control of both simple and complex movements.
Black Hole Angular Momentum and Constrained Ricci Flows
Christopher Beetle (FAU), Feb 5
General relativity predicts that spacetime generically contains regions, black holes, of such strong gravity that even light cannot escape from them to infinity. These regions attract other bodies gravitationally much like ordinary stars and planets, and observational evidence suggests that black holes do in fact play an important role in the dynamics of our universe. However, unlike ordinary stars and planets, it has neither been possible to locate the center of mass of a general black hole nor to define physically useful quantities like its angular momentum in a physically meaningful way. This talk will present work in progress aimed at resolving these issues. The work extends previous results that were limited to the unphysical case of axial symmetry using a technique based on R. Hamilton's Ricci flow. The Ricci flow defines a more or less unique way to continuously deform an arbitrary sphere geometry (i.e., not round) to the standard round sphere by solving a non-linear reaction- diffusion equation. I will argue that these techniques potentially open a valuable, if currently impractical, mathematical window on the physical dynamics and interactions of black holes.
What happens when black holes and neutron stars merge? An astrophysical experiment in strong gravity and nuclear physics
Matthew Duez (Cornell University), TUESDAY Feb 9, 2:00PM, BS 303
Black hole-neutron star binary mergers are fascinating and violent events that combine strongly curved spacetimes, relativistic speeds, and supernuclear-density matter. They are also promising sources of gravitational waves and potential causes of short-duration gamma-ray bursts. The gravitational wave signals of such mergers contain information about neutron star matter, particularly the unknown high-density equation of state; the waves also contain signatures of strong-gravity relativistic effects, such as those associated with black hole spin. Extracting this information requires theoretical waveforms provided by numerical models. These models are also needed to discover if such mergers can, indeed, power gamma-ray bursts. In this talk, I describe numerical simulations of black hole-neutron star mergers undertaken by the Cornell-Caltech-CITA Relativity Group. I focus particularly on our numerical investigations of the effects of black hole spin and neutron star equation of state. We consider effects both on the gravitational waveform and on the size and structure of the post-merger accretion disk.
Quantitative understanding of protein dynamics and thermodynamics in biological systems
Hui Wang (University of North Carolina), THURSDAY Feb 11, 2:00PM, BS 303
All cellular processes are facilitated by proteins through physical and biochemical interactions. Thus, understanding the dynamics and thermodynamics of proteins is a critical first step in uncovering the underlying mechanisms of biological processes. At the cell level, proteins dynamically form spatial and temporal patterns that are important for cell growth, division, movement, and establishment of cell polarity. At the molecule level, the thermodynamic and mechanical properties of single proteins are strongly influenced by the thermal and chemical conditions of the environment. By utilizing computational techniques and statistical physics, a quantitative understanding of these events can be obtained. We proposed a non-equilibrium, Ising-like model to investigate the clustering mechanisms of the chemotaxis receptors in the bacteria E. coli. We have demonstrated that the experimentally observed periodic spatial patterns of the chemotaxis clusters can be successfully explained. To investigate the dynamics of individual proteins, I will introduce a computational method for a Flexibility And Stability Test (FAST) on three-dimensional protein structures. This method is able to calculate the four-dimensional free energy landscape of a protein defined by temperature and three order parameters in matter of minutes. Our method offers an efficient approach to study many of the thermodynamic properties based on the calculated free energy landscape, including stability curves and heat capacity over experimentally accessible temperatures.
Swimming microorganisms in viscoelastic media
Henry Fu (Brown University), MONDAY Feb 15, 2:00PM, BS 303
While the basic principles of swimming in Newtonian fluids are well understood, in many cases the natural environments which microorganisms navigate are non-Newtonian fluids or even gels, which result in different swimming behavior. For example, mammalian sperm swim through mucus in the female reproductive tract. First, I discuss how viscoelastic response affects swimming shapes and speeds of flexible swimmers such as sperm. Second, microorganisms in nonlinearly viscoelastic fluids have additional swimming mechanisms that are not available in Newtonian fluids. Third, I describe issues that arise for swimmers moving through viscoelastic gels. Swimming through solids such as gels requires altered boundary conditions on the swimmer, and unlike incompressible fluids, a gel can have compressional modes with relative motion between polymer and solvent fractions. In addition, many biological gels are heterogeneous on the lengthscale of swimming microorganisms, necessitating non-continuum models that treat the gel microstructure and swimmer on equal footing. Using simple models of swimmers, I illustrate the consequences of these effects for viscoelastic swimming.
Deciphering the Dynamical Impact of Cold Dark Matter Sub-Structure
Sukanya Chakrabarti (UC Berkeley), THURSDAY Feb 18, 2:00PM, BS 303
The Cold Dark Matter (CDM) paradigm is very successful at explaining the growth of structure on large scales. However, it predicts an excess of structure on sub-galactic scales. The over-abundance of CDM sub-structure in simulations relative to observations of Local Group dwarf galaxies is currently one of the most outstanding problems in astrophysics and cosmology. Motivated by this discrepancy, we ask the question if dark galaxies (or dim dwarf galaxies) can be discovered by their tidal gravitational effects on the gas disks of galaxies. I will focus most of my talk on my recent work (Chakrabarti & Blitz 2009; Chakrabarti et al. 2010) where I analyze observed perturbations in the outskirts of the gas disk of the Milky Way to infer and characterize a dark sub-halo that tidally interacted with our galaxy. By comparing the Fourier amplitudes of a large set of high resolution Smoothed Particle Hydrodynamics simulations of the Milky Way tidally interacting with perturbers, I show that the best fit to the simulations occurs for a 1:100 mass ratio perturber with a pericentric approach distance of ~5 kpc. I will also demonstrate a fundamental property of parabolic orbits that allows us to break the degeneracy between the mass of the perturber and the distance of closest approach in the evaluation of the tidal force. Next, I will show recent results that allow us to extend this tidal analysis method and apply it in generality. I will end by discussing results obtained using my radiative transfer code RADISHE to calculate the emergent spectral energy distributions and images of simulated galaxies along the course of their time evolution to compare with multi-wavelength observations.
Rubber elasticity: complex matter and simple theories
Sean Xing (Syracuse University), Feb 19
The classical theory of rubber elasticity models a rubbery solid as a collection of independent Gaussian polymer chain with Hookean elasticity. It is well known that the classical theory fails in the regime of large deformations. This failure is generally attributed to entanglement effects. In this talk I will discuss how thermal elastic fluctuations modify the elasticity rubbery materials at large deformation regime, and how these fluctuations can be captured by a simple field theoretic calculation. After these fluctuations are taken into account, there is no experimental evidence supporting entanglement theories.
Self-assembly at the nanometer scale: symmetries and electrostatics
Graziano Vernizzi (Northwestern University), MONDAY Feb 22, 2:00PM, BS 303
Controlling features of self-assembled structures of viral size and shape has eluded scientists for decades. We show how electrostatic interactions can induce the formation of chiral charged patterns on the surface of cylindrical nano-fibers. Moreover, on spherical geometries (viral shells, micelles, vesicles, fullerenes) electrostatic self-assembly may lead to faceting. A model that describes the interplay between icosahedral symmetry and icosahedral shaped faceting is presented and discussed. Such shells appear in oppositely charged molecules co-assembled into membranes or adsorbed onto interfaces forming emulsions.
Gravity, quantum, and loops: An introduction to loop quantum gravity and some recent developments.
Jonathan Engle (Uni Erlangen-N rnberg), THURSDAY Feb 25, 2:00PM, BS 303
In the 90's arose the canonical quantization program of general relativity known as `loop quantum gravity.' By the end of the decade, a complete theory had been formulated, but the complexity of the dynamics made it difficult to use. The theory has nevertheless been successfully applied to the quantum theory of black holes and quantum cosmology, the latter of which leads to predictions which could be observable. In addition, a path integral version of the theory -- called spin-foams, has been developed, which should make more general predictions possible.
In this talk I review why we need a theory of quantum gravity, and introduce the motivations, basic ideas, and key results of loop quantum gravity, including a more recent result, in which I took part, completing for the first time a spin-foam model for LQG.
On breaking the far-field diffraction barrier in optical nanopatterning and nanoscopy
Rajesh Menon (University of Utah), Feb 26
A technique for creating deterministic structural complexity is essential to achieve high functionality at the nanoscale, whether in electronics, photonics, or molecular biology. Scanning-electron-beam lithography (SEBL) is the most widely used method in research, but it has a number of drawbacks. SEBL tends to be slow, expensive, prone to placement errors, and not compatible with organics and biological material. Ideally one would prefer to employ benign photons in the visible or near IR range for such patterning. However, the so-called far-field diffraction barrier limits the smallest feature achievable by wavelength, \lambda to ~ \lambda/4. In this presentation, I will describe a technique that circumvents this barrier by means of wavelength-selective chemistry. I call the technique Absorbance Modulation. I will also describe the application of absorbance modulation to scanning-optical nanoscopy. Significant excitement has been generated recently by several new techniques for optical live-cell imaging at the nanoscale. All these techniques rely on fluorescence to break the diffraction barrier. I will demonstrate, for the first time that the far-field diffraction barrier can be overcome without resorting to fluorescence. Finally, I will describe an alternative to absorbance modulation that exploits unique combinations of spectrally-selective reversible and irreversible photochemical transitions to achieve single-molecule resolution in 3 dimensions.
Modeling calcium signaling from single channel release events to intercellular waves
Ghanim Ullah (Penn State University), MONDAY Mar 1, 2:00PM, BS 303
Calcium (Ca 2+) signaling is one of the most important signaling mechanisms controlling e.g. the contraction of muscle cells, the release of neurotransmitters from neurons and astrocytes, transcription inside the nucleus and metabolic processes in the liver and pancreas. Ca 2+ signaling patterns exhibit a hierarchical structure varying from single-channel release events (10's of nanometers) to waves sweeping over entire organs, like the liver, to globally orchestrate the efficient release of enzymes. This multi-scale organization renders Ca 2+ signaling an ideal tool for studying basic concepts of pattern formation, especially since access to the most important experimental parameters is given. I will begin with a model that quantitatively describes the characteristics of elementary Ca 2+ elements (called Ca 2+- puffs) on the nano-scale as well as the organization of global waves and oscillations in the cell. I will show that the changes in spatial organization of Ca 2+ signaling effectors regulate the spatiotemporal features of the Ca 2+ signal as is observed, for instance, during oocytes maturation. Finally, I will give examples for the application of our model to multi-cellular systems.
Computational Models of Core-Collapse Supernovae
Konstantin Yakunin (FAU), Mar 19
Core-collapse supernovae are among the most energetic explosions in the universe marking the catastrophic end of massive stars. After rigorous studies for several decades, we are able to understand the essential components of the explosion mechanism such as convection, standing-accretion-shock instability (SASI), and the neutrino heating mechanism. In this talk, I present results of latest simulations of core-collapse supernova for different-mass progenitors using the CHIMERA code, with particular emphasis on the generation of gravitational waves. Gravitational radiation, together with neutrino radiation, is the most important observational tool for understanding the dynamics of the central region of core-collapse supernova explosions. I present physical foundations and the emission mechanism of gravitational waves from supernovae, compare GW signals from different models and discuss the possibility of their detection.
Magnetism of Nanoparticles in Fluids
Korey Sorge (FAU), Apr 2
The magnetic models used to study isolated magnetic spins and collective magnetic nanoparticles are well established. We can get behavior ranging from dilute spin glasses to heavily anisotropic ferromagnetic response. Physical pinning of highly anisotropic particles can lead to anisotropic magnetic behavior, but what happens if we can minimize or remove physical pinning? How does the problem change if both the particle and the magnetization vector can rotate? These questions may be studied by looking at particles in fluids. In this talk, we will look at how magnetic particles respond to probes in this fluid environment. In addition, we will see how the particle itself changes in this chemically reactive environment.
Hydroxyapatite based biomaterials: Crystal structure questions
Dora Leventouri (FAU), Apr 16
Hydroxyapatite: A fascinating biomaterial, the main mineral phase in our bones and teeth that still keeps some of its secrets very well, in spite of the enormous research over the years! I will discuss basic crystal structure questions and the methods we are using in trying to find the answers.