Physicists from around the world are using the Relativistic Heavy Ion Collider to explore some of Nature's most basic -- and intriguing -- ingredients and phenomena. Here's a look at the physics of RHIC in plain English. If you need a further guide to physics terms and concepts, visit the physics primer page. 

RHIC's main physics mission is to collide heavy ions together, creating the conditions that physicists are interested in studying.

RHIC is the first machine in the world capable of colliding heavy ions, which are atoms which have had their outer cloud of electrons removed. RHIC primarily uses ions of gold, one of the heaviest common elements, because its nucleus is densely packed with particles.

RHIC collides two beams of gold ions head-on when they're traveling at nearly the speed of light (what Einstein called relativistic speeds). The beams travel in opposite directions around RHIC's 2.4-mile, two-lane "racetrack." At six intersections, the lanes cross, leading to an intersection that looks like this:

When ions collide at such high speeds fascinating things happen. Here's one simulation of what such a collision looks like.

If conditions are right, the collision "melts" the protons and neutrons and, for a brief instant, liberates the quarks and gluons (third image). Just after the collision, thousands more particles form as the area cools off (fourth image). Each of these particles provides a clue as to what occurred inside the collision zone. Physicists will sift through those clues for interesting information.

To understand why RHIC collisions are scientifically interesting, it is important to know that scientists believe that all protons and neutrons are made up of three quarks, along with the gluons that bind them together. Theory holds that for a brief time at the beginning of the universe there were no protons and neutrons, only free quarks and gluons. However, as the universe expanded and cooled, the quarks and gluons bound together and, for the next 13 billion years, remained virtually inseparable. RHIC is the first instrument humans have built that can take us "back in time" to see how matter behaved at the start of the universe.

Physicists around the world are interested in RHIC collisions. The information found at RHIC can be applied in nuclear physics (the study of the atom), particle physics (the study of the atom's parts), astrophysics (the study of stars and planets), condensed matter physics (the science of solid matter) and cosmology (the study of the universe).

RHIC collisions occur thousands of times per second. Each one acts as a microscopic pressure cooker, producing temperatures and pressures more extreme than exist now even in the cores of the hottest stars. In fact, the temperature inside a RHIC collision can exceed 1,000,000,000,000 degrees above absolute zero -- about ten thousand times the temperature of the sun.

But since the heavy ions in RHIC collisions are so small (see physics primer), the actual impact of the speeding ions on each other is about the same as the impact of a mosquito hitting a screen door on a summer evening. And, RHIC collisions last only a few billionths of a second.

In other words, RHIC collisions may be super-fast and super-hot, which makes them interesting to physicists, but they're too small and too brief to be dangerous.

On to the quark-gluon plasma >