Physicists from around the world will use the Relativistic Heavy Ion Collider to explore some of Nature's most basic -- and most intriguing -- ingredients and phenomena.Here's a look at the physics of RHIC in plain English!If you need a guide to RHIC physics terms and concepts, visit the physics primer page. |
IntroductionHeavy Ion CollisionsRHIC's Ultimate Goal:
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RHIC main physics mission is to collide heavy ions together, creating the conditions that physicists are interested in studying.
RHIC will be the first machine in the world capable of colliding heavy ions, which are atoms without their outer cloud of electrons (see physics primer). RHIC will mostly use gold, one of the heaviest common elements around, because its nucleus is packed with particles.
RHIC will collide two beams of gold ions head-on when they're traveling at nearly the speed of light (what Einstein called relativistic speeds). The beams will be traveling in opposite directions around RHIC's 2.4-mile, two-lane "racetrack." At six intersections, the lanes will cross, leading to a "demolition derby" that looks like this:
When ions colide at such high speeds, physicists believe, fascinating things will happen. Here's one simulation of what such a collision might look like.
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If your browser and Internet connection permit, check out these MPEG animations of RHIC collisions by clicking on the images!
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. (see physics primer) 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 (give or take a few billion), remained virtually inseparable. RHIC will be 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 will occur thousands of times per second. Each one will act 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 might 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.
To learn more about RHIC and nuclear physics in general, click here to go to the home page of the National Research Council's new book, Nuclear Physics: The Core of the Matter, the Fuel of theStars. You can read it on-line for free, in either HTML or PDF formats.
While many RHIC collisions will produce interesting results, a rare few might create something even more special: a new form of matter.
Actually, it's not new to the universe, just to human eyes. It's thought to have existed ten millionths of a second after the Big Bang at the dawn of the Universe. It may also exist in the cores of very dense stars called neutron stars.
This form of matter is called quark-gluon plasma or QGP. Like its name suggests, QGP is a "soup", or plasma, of quarks and gluons. (see physics primer)
Physicists believe that RHIC collisions will compress and heat the gold nuclei so much that their individual protons and neutrons will overlap, creating an enormously energetic area where, for a brief time, a relatively large number of free quarks and gluons can exist. This is the quark-gluon plasma.
Below, you can see this phase transition (see physics primer) up close. The red, green and blue circles are quarks, connected by black lines representing gluons. At the beginning, trios of quarks and gluons are packaged in protons and neutrons, which are held together in the nucleus of an atom. As the pressure and temperature rise, new particles called pions (made of a quark and an anti-quark, shown in pastels) arise.
Finally, the conditions are just right for the phase transition to happen, and quark-gluon plasma is produced. Note that in the plasma, the quarks, gluons and anti-quarks are liberated from their usual bonds, and bond with one another freely.
If a RHIC collision produces a QGP, it will quickly cool, expand and coalesce into hadrons (see physics primer). Physicists will be able to determine if a QGP was produced not by observing it directly -- its lifetime is too brief -- but by looking at the particles that shower out from the collision.
A collision that produces QGP will send out different kinds and ratios of particles than a collision that doesn't produce QGP. QGP is predicted by the standard model of particle physics (Quantum Chromodynamics), so theoretical physicists can calculate what signals it should produce.
The next figure shows a timeline of how the universe is thought to have evolved as it cooled following the Big Bang. The top, purple band is the realm where QGP can exist, at very high temperatures above 1,000,000,000,000 degrees. Starting from the bottom left, you can track the evolution of the universe as it ages from 0.000000001 seconds to today.
As the universe aged and cooled, the plasma coalesced into protons and neutrons (hadronization), then nuclei (nucleosynthesis) and then atoms. Finally, the atoms came together into molecules, which allowed life -- and us! -- to arise.
As you can see by the purple arrow, RHIC will take us from today back to the same temperatures that existed at the dawn of the universe. By creating QGP over and over again under laboratory conditions, RHIC's physicists expect to gain a new understanding of the relationship between the most fundamental constituents of matter and the complex array of particles and nuclei that make up the world around us.
To learn more about RHIC and nuclear physics in general, click here to go to the home page of the National Research Council's new book, Nuclear Physics: The Core of the Matter, the Fuel of theStars. You can read it on-line for free, in either HTML or PDF formats.
In addition to colliding heavy ions, RHIC will also be able to collide single protons. While these collisions won't produce quark-gluon plasma, they're interesting to physicists for other reasons. Namely, scientists want to know more about a property of particles called spin.
Spin is really just the direction a particle is spinning around an axis as it travels -- just like the Earth spins on its axis as it travels around the sun. Each proton has a characteristic spin, which helps give it a characteristic magnetism.
In this picture of a RHIC proton-proton collision, the spin of the particles is shown as black arrows circling the spherical particles.
At RHIC, the proton beams will be "spin polarized." This means that all the protons in one beam will be spinning the same way, and that the other beam will contain protons all spinning in the opposite direction. RHIC is the first machine in the world capable of colliding such beams head-on.
Why is proton spin important to understand? Astronomers studying the universe use proton spin and magnetism as important measuring properties. Spin is also what allows doctors to use an MRI (Magnetic Resonance Imaging) machine to see inside our bodies and diagnose disease.
For these reasons and others, physicists want to measure and understand how different factors influence a proton's spin. Experiments elsewhere have shown that the spins of the quarks (and antiquarks, in some cases) inside particles such as protons accounts for only about 30% of the particle's overall spin.
RHIC spin exeriments should provide the first information on how much the spin of gluons contributed to the proton's spin, a contribution which recent theoretical work suggests may be large.
If the quark and gluon spins together still do not account for the proton's spin, the only remaining source available to "balance the books" is the movement of quarks and gluons relative to one another. Thus, RHIC's measurements of the spin substructure of the proton may lead us beyond our current, still rudimentary understanding of how quarks move inside protons and other particles.
To learn more about spin physics and particle physics in general, click here to go to the home page of the National Research Council's 1998 book, Elementary Particle Physics: Revealing the Secrets of Energy and Matter. You can read it on-line for free, in either HTML or PDF formats.
