Until the first gold atoms started making their 13-microsecond laps around RHIC's 2.4-mile-long perimeter, physicists thought they had a pretty good idea of what to expect from the collisions. The gold nuclei were supposed to shatter and form a hot gas, or plasma, of quarks and gluons. For physicists, watching the collisions in RHIC would be like watching the Big Bang unfolding before their eyes, but running in reverse—instead of a seething cloud of gluons and quarks settling down to form protons and neutrons, they would see the protons and neutrons burst open in sprays of quarks and gluons.
The universe has been around now for about 400 quadrillion seconds, but for physicists like Tom Kirk, the former associate lab director for high-energy and nuclear physics at Brookhaven, the first millionth of a second was more intriguing than any that followed.
"You may remember Steven Weinberg's book The First Three Minutes," says Kirk, referring to a classic account of the physics of the early universe. "Steve said after those first three minutes, the rest of the story is boring. Well, we could say after that first microsecond, everything else was pretty boring."
Kirk smiles and raises his eyebrows slightly, gauging whether I'm sympathetic to his remark, made, perhaps, only half in jest. When that first microsecond of eternity ended, the remainder of cosmic history unfolded with stolid inevitability. Once quarks finished clumping together as protons and neutrons, it was only a matter of time—and gravity—before the first simple atoms gathered in vast clouds to form stars and galaxies, which eventually begot us. (For an elaboration of your personal relationship with quarks, see "The Big Bang Within You," below.)
RHIC was designed to observe directly, for the first time, how quarks behave when freed from their nuclear prisons. The initial results, announced in 2005, stunned physicists everywhere. The particles released by the high-speed smashups were not bounding around freely the way atoms in a gas do but moving smoothly and collectively like a liquid, responding as a connected whole to changes in pressure within the fireball. The RHIC physicists describe their creation as a near "perfect" fluid, one that has extremely low internal friction, or viscosity. By the standards physicists use, the quarks and gluons make a much better liquid than water.
Since the similarity of quarks and gluons to water is not readily apparent to me, I take the subway to Columbia University to meet Zajc, the leader of one of RHIC's main experimental groups, hoping he'll enlighten me. "So how does one calculate the viscosity of those quarks and gluons?" he asks rhetorically. I sit silently, clueless, hiding my befuddlement behind a vigorous show of note taking. "It turns out there's a connection here to black-hole physics." That connection could be the first, long-awaited sign that string theory—which is in desperate need of evidence, any evidence, to support its ambitious claims to truth—is on the right track. The implausible-sounding connection between droplets of quarks and black holes may also vindicate a theory that once had 200 of the world's leading theorists jubilantly dancing the macarena.
Ehhhh! Maldacena!
M-theory is finished,
Juan has great repute.
The black hole we have mastered,
QCD we can compute.
Too bad the glueball spectrum
Is still in some dispute.
Ehhhh! Maldacena!
So goes Jeffrey Harvey's über-geek version of the once-ubiquitous 1996 hit. Harvey, a theoretical physicist at the University of Chicago, wrote the lyrics to honor Juan Maldacena, a young Argentine string theorist now at the Institute for Advanced Study in Princeton.
It was the summer of 1998, and Maldacena had just published a paper that to physicists bordered on the miraculous. He proposed an unexpected link between two ostensibly different theories of fundamental physics: string theory and quantum chromodynamics. String theory purports to describe all the elementary components of matter and energy not as particles but as vanishingly small vibrating strings. Photons, protons, and all the other particles are, according to this theory, just different "pitches" of vibration of these strings. If it is right, string theory would unify gravity and quantum mechanics in a single overarching framework—a goal that physicists have pursued for more than half a century. The problem is that there is no shred of experimental evidence that string theory is correct; all the arguments in its favor have been made entirely on the basis of its sophisticated mathematical structure. Direct experimental tests of string theory have thus far proved impossible, in part because strings are predicted to be so small that no conceivable particle accelerator could ever reach the energies needed to produce them.
Quantum chromodynamics, or QCD, on the other hand, is backed by decades of experiments. It describes the interactions of quarks and gluons. (Quarks come in three "colors," analogous to electric charge; hence the "chromo" in chromodynamics.) Unfortunately, unlike string theory, QCD says nothing about gravity, so physicists know they need a broader, more complete theory if they want to explain all of physics. Moreover, the equations of QCD are notoriously difficult to work with.
