  The Relativistic Heavy Ion Collider (RHIC) was built to re-create and study
in the laboratory the extremely hot and dense matter that filled our entire
universe during its first few microseconds. Its operation since June 2000 has
been extremely successful, and the four large RHIC experiments have produced an
impressive body of data which indeed provide compelling evidence for the
formation of thermally equilibrated matter at unprecedented temperatures and
energy densities -- a "quark-gluon plasma (QGP)". A surprise has been the
discovery that this plasma behaves like an almost perfect fluid, with extremely
low viscosity. Theorists had expected a weakly interacting gas of quarks and
gluons, but instead we seem to have created a strongly coupled plasma liquid.
The experimental evidence strongly relies on a feature called "elliptic flow"
in off-central collisions, with additional support from other observations.
This article explains how we probe the strongly coupled QGP, describes the
ideas and measurements which led to the conclusion that the QGP is an almost
perfect liquid, and shows how they tie relativistic heavy-ion physics into
other burgeoning fields of modern physics, such as strongly coupled Coulomb
plasmas, ultracold systems of trapped atoms, and superstring theory.
