  The Relativistic Heavy Ion Collider (RHIC) studies nuclear matter under a
variety of conditions. Cold nuclear matter is probed with deuteron-gold
collisions, while hot nuclear matter(possibly a quark-gluon plasma (QGP)) is
created in heavy-ion collisions. The distribution of spin in polarized nucleons
is measured with polarized proton collisions, and photoproduction is studied
using the photons that accompany heavy nuclei.
  The deuteron-gold data shows less forward particle production than would be
expected from a superposition of $pp$ collisions, as expected due to
saturation/shadowing. Particle production in $AA$ collisions is well described
by a model of an expanding fireball in thermal equilibrium. Strong hydrodynamic
flow and jet quenching shows that the the produced matter interacts very
strongly. These phenomena are consistent with new non-perturbative interactions
near the transition temperature to the QGP.
  This writeup will discuss these results, and their implications for
cosmic-ray physicists.
