Now that you're shrunk down to the size of an atom, let's begin. Let's take it apart to find out. Relativistic is a word that physicists use to mean things that are travelling at nearly the speed of light. Well, that's where the word comes from.
Heavy ions are large atoms without their electrons.
RHIC will use mostly gold , which is one of the heaviest common elements around. Collider , of course, means just what it sounds like. RHIC will collide things together, smashing them head-on and looking to see what comes out. This makes RHIC different from many other accelerators in the world, most of which smash a moving beam into a still target. RHIC and a few other machines are actually capable of aiming two beams of particles, each traveling at nearly the speed of light, and getting them to hit each other.