If a pebble is thrown into a black hole, an observer outside the black hole will see waves of energy diffuse outwards along the event horizon. The equations obeyed by this diffusion process turn out to be precisely those that describe the flow of energy in what are known as `strongly correlated media’. These are states of matter — which include the quark-gluon plasma that permeated the early universe, as well as the interacting electrons in a high temperature superconductor — that are very difficult to study using conventional techniques and intuition, because all of the particles in the medium interact strongly with each other. The connection between black holes and strongly correlated media has been made precise in recent years by a framework known as the holographic correspondence. Researchers at SITP have successfully used this correspondence to discover several new physical processes in strongly correlated electronic media by studying the dynamics of black holes.
In a recent example of this research, Professor Sean Hartnoll, in collaboration with Professor Aristomenis Donos at Durham university in the UK, found that by varying the amount of `bumpiness’ of the black hole event horizon, the corresponding electronic system underwent a dramatic `metal-insulator’ transition. These are abrupt transitions in which the medium changes from conducting electricity to being an insulator. Many of the most interesting and incompletely understood metals, such as high temperature superconductors, are on the verge of such metal-insulator transitions. By opening a new perspective on metal-insulator transitions, based on properties of seemingly unrelated black hole event horizons, this work offers a completely new angle of attack on these challenging materials.