Nature's First Liquid: The Quark Gluon Plasma

High energy collisions of nuclei recreate the high energy densities that existed a few microseconds after the Big Bang. Matter under such conditions is expected to be a plasma of quarks and gluons not confined inside hadrons. The Relativistic Heavy Ion Collider (RHIC) produces such matter in the laboratory, and its behavior has proven to be quite surprising. Examination of thousands of particles in the final state yields evidence for very rapid thermalization leading to a dense, opaque, collectively flowing system. The degrees of freedom cannot be hadrons and the interactions are not those expected for asymptotically free quarks. Rather, the matter behaves like a liquid with low viscosity, as may be expected for some plasmas. I will review experimental results and discuss insights into the properties of the produced matter.