For the past 30 years nearly all theoretical physicists have believed that quantum field theories based on Einstein's general relativity necessarily must be ill-defined in the ultraviolet. This is the well-known nonrenormalizability problem of gravity. But is this actually true in general? We describe recent calculations that cast doubt on this simple picture and show that story of quantum gravity is much more interesting than seemed possible. The new calculations are made possible by recent enormous advances in our ability to compute scattering amplitudes based on unitarity, a new relationship between gravity and gauge theories, as well as advanced integration methods. The results show that in certain highly supersymmetric supergravity theories ultraviolet divergences do not appear at the locations predicted by symmetry considerations, at least as far as has been calculated. In less supersymmetric theories, ultraviolet divergences do occur but their structure can be bizarre and appear to be linked to anomaly-like behavior. This is even true in what was thought to be a well understood case: pure Einstein gravity with no matter. Potential explanations and prospects for the future are described.

Held Tuesdays at 4:30 pm in the William R. Hewlett Teaching Center, room 201.

Refreshments in the lobby of Varian Physics at 4:15 pm.

*Autumn 2015/2016, Committee: A, Linde (Chair), S. Chu, P. Hayden, M. Schnitzer, L. Senatore*