A more recent direction of research uses string theory to think about systems in condensed matter physics, the physics of solids and liquids. At first sight this appears bizarre: string physics takes place on tiny scales! How could string theory possibly model systems of many atoms with energies determined by the ambient temperature?
What makes this feasible â€“ although not necessarily doable â€“ is theÂ AdS/CFT correspondence.Â This is aÂ dualityÂ which relates gravitational systemsÂ with weak interactionsÂ to field theories with strong interactions. Condensed matter systems are commonly described by strongly interacting field theories.
AdS/CFT provides a handy new tool: the properties of condensed matter field theories can be calculated using the equivalent gravitational theory where computations are easier. Unfortunately there’s no guarantee that a system you can construct using AdS/CFTÂ compares well with real condensed matter!
Indeed genuine solids and liquids involve real atoms, while the AdS/CFT model requires esoteric particles and abstract mathematics. Nevertheless there are two hopes for this type of research.
Sometimes quantum field theories exhibitÂ universalÂ behaviour â€“ many different theories behave in the same way. A set of theories that all function similarly is called aÂ universality class. Researchers hope that a model constructed via the AdS/CFT correspondence would be in the same universality class as a real system. They would then be justified in their use of AdS/CFT simplifications.
The more optimistic physicists dream of finding an AdS/CFT construction that leads directly to a real-world example. In this case we would instantly have an effective description of a genuine condensed matter system,Â despite not knowing exactly why our efforts succeeded.
One popular research track employs AdS/CFT to model the properties of a super-hot gas calledÂ a quark-gluon plasma. This was first produced at the Relativistic Heavy Ion Collider in Brookhaven by colliding large nuclei together, typically those of gold atoms.