The strongly coupled quark gluon plasma and the AdS/CFT correspondence

Ordinary matter is made of neutrons and protons which are themselves
made of confined quarks and gluons. At very high temperatures and/or
very high densities, quarks and gluons are deconfined and form a phase
of matter called the quark gluon plasma. This phase of matter is
thought to have existed in the early universe, a few microseconds
after the big bang. In order to form and probe this state of matter,
physicists have collided heavy ions at very high energies in
accelarators. One property of this phase that has emerged is that it
is a very good liquid. As a measure of its perfectness, one can
indirectly measure the ratio of shear viscosity to entropy density. It
turns out to be smaller than any other liquid known to
us. Theoretically, it is very difficult to compute this ratio from
first principles. It is also hard to explain and predict other
properties of this phase of matter. This is where gauge/gravity
duality in the form of the AdS/CFT correspondence may have a role to
play. Using this correspondence, we can probe properties of strongly
coupled plasmas.  Although these plasmas are not the QCD plasma, some
of their properties work out to be quite similar. It is an ongoing
program to see how far we can stretch the correspondence to reproduce
qualitative and quantitative properties of the QCD plasma. I will
review and discuss what the current status of this program is.