gravity

Seminars filled my research time today. At lunch, Sergei Dubovsky (NYU) talked about conjectures for the fundamental action of strings in string theory. He gave a tiny bit of motivation for why 26 dimensions is the preferred dimensionality of spacetime in string theory. Apparently the 26 dimensions are expected to be 25 spacelike and one timelike, which seems odd to me, but causality is a bitch (as they say). Causality featured prominently in his talk, because he is experimenting with different causal structures for the dynamics on the worldsheet of the fundamental string.

In the afternoon, Michele Vallisneri (JPL) told us about detecting gravitational radiation with LISA-like missions. They face there the same issue that Lang and I have been talking about for astronomical imaging: It is no longer possible to think about there being the catalog of sources from the data. There will always be many qualitatively different explanations of the data stream, and always secure scientific conclusions will have to be based on marginalizations over that catalog-space probability distribution function. He said interesting things about the hardware injections of false events into the LIGO data stream to stress-test the analyses and about the software engineering aspects of these huge projects.