Nature Journal Club

Jan Zaanen

Leiden University, the Netherlands

A theoretical physicist journeys to a hairy black hole’s horizon.

Rumour has it that Steven Spielberg is producing the ultimate science fiction movie, using state-of-the-art general-relativity simulations to create a realistic image of the warped space-time near a black hole. But wouldn’t it be great to see such worlds in real life? In fact, you can: by extending your eyesight with ‘AdS/CFT’, a mathematical result of string theory that describes a ‘through the looking-glass’ experience that would embarrass the imagination of Lewis Carroll.

AdS/CFT states that information about the strange world of the black hole is, in a very indirect way, encoded in or ‘imaged’ by the properties of certain quantum-weird forms of matter. Scientists realized recently that these ‘quantum critical’ states of matter are routinely produced in condensed-matter laboratories. But a particular prediction of AdS/CFT made the string theorists nervous: the event horizon of the special black hole that is imaged by the quantum critical electrons seems to imply that the latter should show a macroscopic entropy at zero temperature. It has further been predicted that the black hole would be unstable and would eventually suck up ‘stuff’ from its surroundings, covering its horizon with ‘hair’ (S. A. Hartnoll et al. J. High Energy Phys. 2008, 015; 2008). In the electron system, out of the blue and at a quite low temperature, some unexpected order will set in that removes the ground-state entropy, giving it a unique ground state.

Intriguingly, I learned the other day that condensed-matter experimentalists, unaware of the string theorists’ nervousness, are now in the grip of the same idea. The latest thermodynamic experiments on quantum-critical electrons are suggestive (albeit inconclusive) of a developing zero temperature entropy — for the experimentalists, a catastrophe — interrupted at a very low temperature by the onset of an exotic quantum liquid crystalline order (Z. Fisk Science 325, 1348–1349; 2009). It may be that we don’t need spacecraft or Spielberg to visit black holes, just a little patience with the condensed-matter experimentalists.

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