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.