Seminar Archive
Fall 2008
F07-S08
F06-S07

String theory is primarily aimed at unifying quantum field theory and general relativity. Moreover, the invention of the notion of gauge/gravity duality has opened the door for a deep understanding of the relation between these two theories. Over the last decade, applications of this duality has shed light on the possibility of using string theory (or the supergravity approximation) to compute some quantities that are relevant for real life applications. In this talk, I will show how one can use the non-extremal 3-brane solution of type II-B supergravity to calculate the shear viscosity of the strongly coupled {\cal N}=4 Yang-Mills theory in the large 't Hooft limit. These calculations set a lower bound on the ratio between shear viscosity and entropy density. Although there is no rigorous field theoretic calculations that explain why this bound should be respected, all known substances, including liquid helium, are found to respect this bound. Such findings can open a new era for using string theory in a wider range of applications.

1. Gubser et. al., Gauge theiry correlators from non-critical string theory, arxiv:hep-th/9802109.
2. Kovtun et. al., Viscosity in strongly interacting quantum field theories from black hole physics, arxiv:hep-th/0405231.
3. Policastro et. al., From AdS/CFT correspondence to hydrodynamics, arxiv:hep-th/0205052.
4. Kovtun and Strainets, Quasinormal modes and holography, arxiv:hep-th/0506184.

I will go over some of the basic theory and examples of minimal and constant mean curvature surfaces.

There are many situations in physics and mathematics when it is useful to work with the model of a distributional source, such as charge concentrated on a sheet, line, or point. In general relativity the situation is more complicated, since a distributional metric does not give a well defined curvature tensor. In this seminar we will review the problems, and the general "fix" for co-dimension one sources. We will also discuss more recent, and more partial, work on distributional co-dimension two sources.

1. Co-dimension one -- Geroch and Traschen ("Strings and Other Distributional Sources in GR" , 1987)
2. Co-dimension two--Traschen ("Co-dimension Two Branes and Distributional Curvature", 2008)

The electromagnetic field is described by a 1-form potential, associated to a 2 form field strength. In analogy with the electromagnetic case, one can define a 3-form potential and a corresponding 4-form field strength. I will discuss how the different tensorial structure makes this field typically uninteresting for applications that concern "local" physics. However, four-forms can have a relevance in cosmology - in particular due to their contribution to the cosmological constant - and have recently been object of intense research.