Quark Soup

September 24, 2008

Years before the dyspeptic Doomsday Brigade raised the (false) alarm about the supposed dangers associated with CERN's Large Hadron Collider, they had a different high-energy particle accelerator in their sights: the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory in Long Island, New York. As RHIC was preparing to fire up in 1999, rumors began swirling that the machine could potentially wipe out the Earth by creating mini-black holes (sigh), or possibly a cataclysmic chain reaction whereby an exotic form of matter called strangelets devoured the universe.

Sound familiar? It should -- both these scenarios figured in the most recent LHC hysteria (although mini-black holes dominated the coverage, because it takes too long to explain what a strangelet is in today's sound-byte obsessed news cycle). It's tough not to get a sense of deja vu reading those panicky headlines from 1999. "Big Bang Machine Could Destroy Earth!" screamed the Sunday Times of London, while the Chicago Sun-Times started running regular "public safety alerts" as RHIC's opening date drew near. Brookhaven officials actually received a call from a reporter asking -- in all seriousness -- whether the new collider could have created a black hole that swallowed the plane of John F. Kennedy, Jr. as it flew past Martha's Vineyard.

My favorite quote was from MIT physicist Robert Jaffe, who wearily told Newsday, "It's more likely that a spaceship is going to land in the middle of Texas, and that aliens are going to come out and tell us that the New York Yankees are all aliens." (No doubt there are actually some people out there who would consider this a perfectly plausible scenario, but these are the same people who think tinfoil hats offer viable protection as well as being a nifty fashion accessory.)

People panicked for pretty much the same reasons they panicked over the LHC: they didn't understand the underlying science, and thus freaked out over the the fact that the whole purpose of RHIC is to simulate the conditions that existed immediately after the big bang.  This is achieved by accelerating gold atoms to near the speed of light such that the nuclei collide with sufficient force as to produce enormous energies -- and very high temperatures, a million times hotter than the core of our sun.

The RHIC experiments were designed to test the theory that at those temperatures, a new for of matter would emerge, called a quark-gluon plasma (QGP). Standard cosmology hods that the QGP only existed briefly a mere ten millionths of a second after the big bang before cooling, expanding and coalescing into the familiar atomic nuclei (hadrons) we know and love. A QGP wouldn't last long enough to observe it directly, so -- as with most high-energy accelerators -- physicists can only determine if a QGP was produced by analyzing the types of particles that shower out from the collision.

The QGP is basically a primordial soup of the quarks and gluons that make up individual protons and neutrons -- except in a QGP, they would be bound tightly into protons and neutrons, but able to roam freely. It's a bit like how ice melts into water, except that RHIC is melting atoms. Scientists call this a phase transition, the process by which one form of matter turns into another when the conditions (temperature and pressure) are just right -- i.e., the QGP formed from the energy of the big bang then cooled into normal matter. RHIC seeks to reproduce this cosmological phase transition in reverse by melting normal everyday matter into the quark-gluon plasma.

In April 2005, RHIC's scientists from all four experimental groups made science news headlines when they announced they had succeeded in finding strong evidence for the QGP, with a surprising twist: it didn't behave so much like a plasma (ionized gas) of weakly interacting quarks and gluons, as a liquid of strongly interacting quarks and gluons -- a finding rather at odds with prevailing theoretical predictions, although still within the scientifically acceptable boundaries of prior calculations. Translation: they found it, and it wasn't quite what they were expecting, but what the heck -- it's still pretty darned interesting!

Further insight awaits once the LHC gets back on track (sometime in 2009, it now appears), since there are three new experiments devoted to studying the QGP's properties. In the meantime, University of Chicago physicist Sidney Nagel thinks we can learn something about the QGP by studying sand -- which also behaves like a liquid under specific circumstances.

Seeking to simulate RHIC collisions, Nagel's team fired jets of tiny glass or copper beads at a solid target and captured the spray patterns from the collision with high-speed photography. Those patterns were more consistent with how a liquid would behave. Apparently, if the density of particles in the jet is high enough, incoming particles start colliding with rebounding particles, exerting pressure and causing the "liquid" to squirt out perpendicularly to the jet. And that's pretty much how the QGP found at RHIC in 2005 behaved.

Nagel, for one, wasn't surprised to see grains of sand emulating subatomic particles: "All physics is related," he told Physics World last year. I guess we'll have to wait until 2009 and beyond to find out if the LHC confirms Nagel's experimental hunch. 

Photo: Illustration of fallout from the collision of two relativistically accelerated gold ions from STAR detector at RHIC. Source: Wikimedia Commons, via Brookhaven National Laboratory (public domain).