Mar

26

2012

The homotopy theory of topological defects in ordered media fails to completely characterize systems with broken translational symmetry. We argue that the problem can be understood in terms of the lack of rotational Goldstone modes in such systems and provide an alternate approach that correctly accounts for the interaction between translations and rotations. Dislocations are associated, as usual, with branch points in a phase field, whereas disclinations arise as critical points and singularities in the phase field. We introduce a three-dimensional model for two-dimensional smectics that clarifies the topology of disclinations and geometrically captures...

Mar

26

2012

With recent advances in experimental imaging, computational methods, and dynamics insights it is now possible to start charting out the terra incognita explored by turbulence in strongly nonlinear classical field theories, such as fluid flows. In presence of continuous symmetries these solutions sweep out 2- and higher-dimensional manifolds (group orbits) of physically equivalent states, interconnected by a web of still higher-dimensional stable/unstable manifolds, all embedded in the PDE infinite-dimensional state spaces. In order to chart such invariant manifolds, one must first quotient the symmetries, i.e. replace the dynamics on M by an equivalent, symmetry reduced flow on M/G, in...

Mar

22

2012

Gamma-Ray Bursts (GRBs) are the brightest light sources in the Universe, as well as the most distant sources known. These characteristics, combined with their powerlaw spectra, make them ideal cosmological probes. In this talk I will discuss how GRBs are impacting several areas of extragalactic astrophysics and cosmology. In particular, I will show how they can be used to trace the evolution of the mean density and clumpiness of the interstellar medium with redshift, and the properties of dust in high-z galaxies. Detection of GRBs at very high redshifts can help set constraints on the small-scale power spectrum of density fluctuations. High-resolution observations of long GRBs...

Mar

19

2012

In the natural world, complex behaviors such as learning, aggression and sleep are regulated by interconnected networks of genes and their products. Owing to their nontrivial topology and large number of components, most of these networks are poorly understood and consequently, our knowledge of how diverse behaviors arise remains limited. In this talk, I will argue that the fruit fly circadian clock, a genetic circuit that signals to and modulates several key behavioral networks, is an ideal system with which to dissect the fundamental principles that govern organismal...

Mar

15

2012

Most of the baryons in the present universe are missing. This talk gives a historical review of the issue, followed by some highlights of current theoretical and observational effort to understand it.

Mar

14

2012

Engineered biological circuits expressed in living cells are becoming increasingly attractive as a technology, with applications...

Mar

13

2012

This talk will describe new results on the properties of colloidal crystals, both on their solidification and on their melting. It will describe how hard-sphere like colloids crystallize, and will explore the huge discrepancy between the nucleation rates predicted by theory and measured in simulation and those measured experimentally. The discrepancy can be as large as 150 orders of magnitude! A simple modification to the theory, suggested by experiment, is able to account for this behavior and to rectify the discrepancy....

Mar

12

2012

The viruses that infect bacteria have a hallowed position in the development of modern biology, and once inspired Max Delbruck refer to them as "the atom of biology". Recently, these viruses have become the subject of intensive physical investigation. Using single-molecule techniques, it is actually possible to watch these viruses in the act of packing and ejecting their DNA. This talk will begin with a general introduction to viruses and their life cycles and will then focus on simple physical arguments about the forces that attend viral DNA packaging and ejection, predictions about the ejection process and single-molecule measurements...

Mar

10

2012

In this exciting event, three lectures will be presented from world renown Chef Jose Andres and Harvard Physics Professors Michael P. Brenner and David A. Weitz. Awards will also be presented to the top Dekalb County high school student...