Over the decades,
experimental condensed matter physicists have discovered a number of exotic states of matter, including
superconductors and
superfluids. These states are described using the formalism of quantum field theory, but some phenomena are difficult to explain using standard field theoretic techniques. Some condensed matter theorists including
Subir Sachdev hope that the AdS/CFT correspondence will make it possible to describe these systems in the language of string theory and learn more about their behavior. So far some success has been achieved in using string theory methods to describe the transition of a
superfluid to an
insulator. A superfluid is a system of
electrically neutral atoms that flows without any
friction. Such systems are often produced in the laboratory using
liquid helium, but recently experimentalists have developed new ways of producing artificial superfluids by pouring trillions of cold atoms into a lattice of criss-crossing
lasers. These atoms initially behave as a superfluid, but as experimentalists increase the intensity of the lasers, they become less mobile and then suddenly transition to an insulating state. During the transition, the atoms behave in an unusual way. For example, the atoms slow to a halt at a rate that depends on the
temperature and on the Planck constant, the fundamental parameter of quantum mechanics, which does not enter into the description of the other
phases. This behavior has recently been understood by considering a dual description where properties of the fluid are described in terms of a higher dimensional black hole. == Criticism ==