Introduction to the physics
This page provides an introduction to the recent progress in applying string theory techniques to help understand heavy ion physics. This is an exciting field because it has brought string theory in close contact with a data-rich experimental program. It's not going to prove or disprove the claim that string theory is a "theory of everything." But it may demonstrate that string theory can provide quantitative insight into the way quarks and gluons interact at high temperatures and energy densities.
This page is intended for the non-specialist. No effort has been made to provide a scholarly list of references. A collection of references (by no means complete) can be found at the end of this talk delivered by S. Gubser at the DNP 2007 conference in Newport News.
String theory and gauge theory
String theory is a decades-old effort to describe physical phenomena in terms of strings. The most widely emphasized aspect of string theory is the idea that all the fundamental constituents of matter might be different vibrational states of the same object: a string. Starting with this idea plus quantum mechanics, one can derive gravity as described by General Relativity as well as electromagnetism as described by Maxwell's equations. One can go further and derive quasi-realistic descriptions of the Standard Model of particle physics. Experiments at the Large Hadron Collider (LHC) at the European Center for Nuclear Research (CERN) may provide some hints about whether a string theory description of the world, along the lines just described, is right.
A strong current in the development of string theory since the late 1990's has been the gauge-gravity duality, according to which a gauge theory (some variant of electromagnetism) admits a dual gravitational description in a negatively curved spacetime. Often called AdS/CFT (an abbreviation for "Anti-de Sitter / Conformal Field Theory correspondence"), this duality emerged in part from efforts to enumerate black hole microstates in string theory. Using AdS/CFT, one can relate calculations in gravitational theories to physical phenomena in gauge theories. One of the most interesting gauge theories is quantum chromodynamics (QCD), which is the part of the Standard Model that describes the strong interactions among quarks and gluons inside the proton.
Relativistic heavy ion collisions
Relativistic heavy ion collisions are a type of experiment at the interface between nuclear physics and high-energy physics, where atomic nuclei are stripped of their electrons and slammed together as hard as possible. As of 2009, the leading facility for this type of experiment is the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory (BNL). Soon the LHC will begin to probe even higher energies than those accessible at RHIC. When heavy ions, like gold nuclei, are slammed together hard enough, the quarks and gluons inside them are liberated (or "deconfined"). It's believed that the resulting quark-gluon plasma (QGP) thermalizes before it blows itself apart. What heavy-ion experiments see is the detritus that comes out of such a collision. With a lot of hard work, experimentalists can infer properties of the quark-gluon plasma. It seems to have properties that are hard to explain using the standard toolkit of quantum field theory. For example, it seems that the viscosity is small, and the thermalization time is short, compared at least to naive expectations from perturbative QCD. To take another example: heavy quarks moving through the QGP seem to lose energy more quickly than was originally anticipated from perturbative QCD.
Why discuss string theory and heavy ion collisions in the same breath?
There is little doubt that QCD provides a correct description of the QGP. But QCD is hard to solve. Perturbative methods, based on Feynman diagrams, are reliable only for "hard" processes, meaning that a lot of momentum is exchanged between the participating particles. Lattice methods (basically, solving QCD with a big computer) are not well-adapted to describing real-time processes in a thermal medium. Hydrodynamic approximations work in many circumstances, but it's not entirely clear why that's true, plus there are free parameters in hydrodynamic descriptions.
String theory, and in particular AdS/CFT, offers an alternative view of strongly coupled gauge dynamics which may be useful in understanding what goes on in relavitistic heavy ion collisions. What AdS/CFT can do is recast questions about thermal states of gauge theories in terms of black holes in a negatively curved spacetime. The QGP is probably thermally equilibrated and strongly coupled, and so may admit a description in terms of a black hole. The big caveat is that the gauge theories that one can understand most readily via AdS/CFT are not QCD, but supersymmetric relatives of it. So, when working in this field, one is always forced to ask to what extent properties of strongly coupled gauge theories should be expected to be universal. In the absence of a really compelling answer to this question, the most useful response is to do one's best with the calculational tools one has and make the most careful comparisons one can between string theory and real-world quark-gluon plasmas---keeping in mind that things don't have to agree, and if they do it could be for the wrong reasons.
Efforts have been made in string theory to compute various physical quantities, including viscosities, diffusion constants, the thermalization time, and aspects of energy loss and photon production in the QGP (more precisely, in its supersymmetric relatives.) The results are encouraging enough to merit further work.