Eric Sembrat's Test Bonanza

I will give an overview of a program to map the space of configurations of binary compact coalescences, starting for preliminary indications a couple of years ago that the problem is amenable to reduced order modeling. I will then give an overview of Reduced Basis, our results obtained so far and a roadmap for the future.  Essentially, the core issue is how to chose the most relevant points in parameter space, in a nearly optimal way, and to select which configurations are "the most representative ones". This applies to the case in which the emitted gravitational waves can be simply evaluated using closed form approximations or obtained by solving simple equations but, most important, when deciding which configurations to solve for in numerical simulations of the full Einstein equations. I will also discuss ongoing work and the challenges associated with interpolating between high accuracy between the reduced basis solutions.  We have so far found that the representation error of the reduced basis decays exponentially with the number of elements in the basis, providing dramatic savings compared to traditional methods.

Sebastian Thrun recently left Stanford University and started Udacity. Udacity is an online university that seeks to reach the 99% of the world population that are presently excluded from high-quality higher education. The first class on Artificial Intelligence, jointly offered with Stanford, attracted 160,000 students, of which 23,000 graduated with Stanford-level qualifications. The speaker will discuss his experience with traditional forms of education, and brainstorm how higher education may look like in the digital age.

Thrun is co-founder of Udacity, a Google VP, and a Stanford Research Professor. At Google, he runs Google X, home to the Google self-driving car project and the recently announced Google Glass. Udacity is a XXI Century University, which seeks to democratize higher education. Thrun is one of the youngest people ever to be elected into the National Academy of Engineering; he published over 350 paper including 11 books. And he won the DARPA Grand Challenge.

Event starts at 2pm with refreshments; Talk starts at 2:30pm.

Extreme-mass-ratio binaries, composed of a small compact object (SCO) and supermassive black hole (SMBH), are eventual observational candidates for future space-based gravitational wave observatories.  Theoretically, these binaries are examples of the as-yet not thoroughly solved two-body problem in general relativity.  Extreme-mass-ratio inspiral (EMRI) calculations proceed by using black hole perturbation theory.  The gravitational field of the small mass affects its own orbit in the background geometry, leading to a radiation reaction that is formally divergent.  Regularization procedures tell how to compute the finite corrections to the SCOs orbit (the self force).  These require accurate knowledge of the perturbed metric.  We describe two, similar methods for computing the first-order gravitational perturbations and accurately determining the local perturbations in the metric.  Both involve use of mixed frequency domain/time domain methods.

Spin electronics in its broadest definition is the study of systems where both the charge and the spin of the electron play a role.  The term was originally intended as a new technological concept, where traditionally the electron’s charge is important because transistors rely on currents and voltages, while the electron’s spin is important only in magnetic materials used for memory; spin electronics represents a new hybrid system.  Examples range from technological developments such as MRAM (magnetic random access memory) that are based on magnetic tunnel junctions, to some forms of quantum computing.  More broadly, spin electronics can be viewed as the visibility of and strong interactions between charge and spin in highly correlated electron materials such as high Tc superconductors, colossal magnetoresistance manganites, and doped semiconductors near the metal-insulator transition. 

I will discuss why these materials show such unusual spin-charge properties, and efforts to introduce magnetic moments into semiconducting materials, focusing particularly on our work on amorphous Si doped with magnetic ions such as Gd or Mn.  These alloys possess dramatic magnetic and transport properties due to electron-electron and electron-local moment interactions, including enormous (many orders of magnitude) negative magnetoresistance.  These amorphous materials provide an important counterpart to the more traditionally studied crystalline magnetically-doped semiconductors.

As Goodyear discovered, when he first vulcanized rubber in 1839, a viscous liquid of macromolecules becomes an unusual, utterly random, solid, provided that enough chemical bonds are introduced between the molecules.  Perhaps surprisingly, given the randomness of their architectures, solids formed by the vulcanization process exhibit a number of rather simple and universal features -- both structural and elastic -- that are not exhibited by the apparently simpler, crystalline solids.  In this colloquium, I shall give an overview of current approaches to the physical properties of vulcanized matter and other random-network-forming media, paying special attention to their universal aspects.

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Particle scattering processes at experiments such as the Large Hadron Collider at CERN are described by scattering amplitudes. In quantum field
theory classes, students learn to calculate amplitudes using Feynman diagram methods. This is a wonderful method for a process like electron +
positron -> muon^- + muon^+, but it is a highly challenging for a process like gluon+gluon -> 5 gluons, which requires 149 diagrams even at the leading order in perturbation theory. It turns out, however, that the result for such gluon scattering processes is remarkably simple, in some cases it is just a single term! This has lead to new methods for calculating scattering amplitudes, and it has revealed that amplitudes have a surprisingly rich mathematical structure. The applications of these new methods range from calculation of processes relevant for LHC physics to theoretical explorations of quantum gravity. I will give a pedagogical introduction to these new approaches to scattering theory and their applications, not assuming any prior knowledge of quantum field theory or Feynman rules.

Gamma-ray bursts have been detected at photon energies up to tens of GeV, and there are reasons to believe that the sources emit at least up to TeV energies, via leptonic or/and hadronic mechanisms. I review some recent developments in the GeV photon phenomenology in the light of Fermi observations, as well as recent related theoretical work. I discuss then the expected production of gravitational waves, the possibility of accelerating cosmic rays resulting in high energy neutrinos, and recent observational constraints.

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 known results without the need to add compatibility conditions. Our work suggests natural generalizations of the two-dimensional smectic theory to higher dimensions and to crystals.

Some years ago an anomaly was noted in the decay of luminescence in certain doped alkali halides. The anomaly was eventually explained using a factor of a billion slowdown in lattice relaxation, a remarkable stretching of time scales. This slowdown was found to be caused by the creation of a ‘breather’ in the neighborhood of the dopant. Discrete breathers are nondispersive classical excitations that are known to be significant in many natural systems. In the talk I focus on the occurrence of breathers in doped alkali halides. Several more general properties of breathers have arisen from this study, among them is the quantum breather, a topic less fully explored than the classical theory because it does not yield easily to numerical simulation.