Dualities led to the discovery of D-branes as solutions to string theory. 

At large N_c, one finds a decoupled set of states near coincident D-branes with two different descriptions. (i) as SU(N_c) SYM with N=4, and (ii) IIB string theory on AdS_5 \times S_5. Maldacena proposed that these are equivalent. 

Large N_c is a classical limit in the sense that the correlation functions factorize in a way similar to classical correlations (different from sending \hbar to zero).

This proposal was the start of the AdS/CFT era. One application is to analyze strongly coupled field theories. So some condensed matter theorists are learning GR!

This has a strange aspect to it — rather like Jeopardy. Example from Horowitz talk at String 2013.

Answer: A charged black hole in asymptotically AdS space has a frequency dependent optical conductivity that varies as X.

Question: What is the intermediate frequency dependence of the optical conductivity of the high T_c superconductor Y?

Harvey says that this matching is not well-justified. It may be correct if there are certain universal behaviors that can be captured by crude approximations.

Postmodern aspect: Here’s the calculation, let’s match it to the system…