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.

Enter Juan Maldacena. He developed a theory nearly identical to standard QCD, with the major difference being that in his version quarks come in many different colors rather than the usual three. Even though his theory does not fully apply to the universe we know—it doesn't correctly describe quarks—physicists frequently use such so-called toy models to get a handle on otherwise impossibly difficult problems. And Maldacena's theory had a remarkable feature, the one that inspired his colleagues to hit the dance floor. He proved that his pseudo-QCD and string theory are not in fact different theories at all; mathematically, they are entirely equivalent.

It was this power to shift problems from the QCD perspective to a string theory view that first led some physicists to see a link between the quarks and gluons at RHIC and the equations describing a black hole. Dam Thanh Son, a physicist at the University of Washington in Seattle, was one of them. I called him to ask about what seems, on the face of it, an extraordinarily unlikely comparison. What could quarks and gluons possibly have in common with nature's ultimate trash compactors—ultradense concentrations of matter whose gravitational field is so powerful it curves space-time around itself, trapping anything that crosses its surface?

Son insists that black holes, quarks, and gluons really do have a big thing in common: They can be described by equations that govern the behavior of liquids. Then he explains that black holes—and quarks and gluons—are really no stranger than a cup of water.

"If you have a cup of water, and you disturb the water—say, you drop a pebble into it—the disturbance will not last forever. The water will come to rest. If you take a cup of honey, the motion ceases more quickly than in water; the more viscous the fluid, the quicker the perturbation of the system decays with time."

When something falls into a black hole, Son says, the surface of the black hole is disturbed, just like the water in a cup. "The black hole will wiggle for some time and come to rest. In these two processes"—disturbances in black holes and in water—"there is a connection at the mathematical level. The equation that describes the evolution of the stirring of water in a cup is similar in form to the equations that describe the evolution of the surface of a black hole. When I deform a black hole, it goes back and forth and then comes to rest. To describe that I use equations that are similar to equations used for any fluid."

As word spread that RHIC had created a quark-gluon fluid, Son and a number of other theorists began to wonder if they could use Maldacena's sleight of hand and substitute the equations of a black hole for the ones normally applied to quarks and gluons. The switch would make calculating the properties of the primordial particle soup much easier. Compared with a trillion-degree ruck of quarks and gluons, black holes are simple objects. (Which is why the lyrics to the Maldacena macarena go: "The black hole we have mastered, QCD we can compute.")

One property of the quarks and gluons that Son and his colleagues wanted to calculate was viscosity. Using a black-hole model, they predicted that quarks and gluons should have almost zero viscosity. When experimentalists at RHIC finally crunched through all their data, they confirmed that the quark-gluon fluid indeed had a low viscosity, at or near the theoretical minimum value predicted by the five-
dimensional black-hole model.

"Talk about a shot out of the blue," Zajc says. "Who would have thunk it? It is the most fascinating thing I've been involved with, to see this completely unexpected connection emerge and start having an impact on our field."

So does this success bolster the idea that string theory is the right way to unify all of physics?

"Absolutely," says Horatiu Nastase of Brown, who has also sought to understand RHIC's results in terms of a black hole. "At least that's my interpretation and the interpretation of other people. My understanding is that one is experimentally testing, in this indirect way, string theory."

Zajc and many other physicists aren't so sure. "I've thought an awful lot about this," he says. "But I'm not ready yet to claim that this validates string theory. Even the string theorists will tell you the viscosity result depends only on ordinary quantum mechanics—it's just that string theory gives you a snazzy way to calculate it."

In any event, the black hole under consideration is not the sort that could swallow Long Island. It's an entirely different animal. According to string theory, the universe may contain as many as 10 dimensions. Most of them are hidden, curled up on scales so small that we cannot sense or even detect them. The black hole in Son's calculation dwells in a theoretical world of five dimensions, where the effects of gravity drop so precipitously with increasing distance that a five-dimensional black hole poses no threat—if it even exists at all. Some physicists consider the five-dimensional black hole to be a mathematical convenience, a way to tackle a complex physical system. Others are open to a far more radical interpretation, however.

"What we think of as atomic nuclei, quarks, and gluons may really be objects that are projections, in a sense, on a screen," says Miklos Gyulassy of Columbia, sounding more like Plato philosophizing than like the theoretical physicist that he is. "We are on the screen. It looks to us like there are photons and these other particles, but they might really be manifestations, projections, from a higher-dimensional space, of objects that are more conveniently described in our world by saying, 'There is a photon,' or 'There is a gluon.' So the very hot quarks and gluons at RHIC may really be a hologram of some nasty black hole somewhere."

All of these issues and more will continue to be studied at RHIC and at an even more powerful accelerator nearing completion in Switzerland. The Large Hadron Collider, as the new accelerator is called, will be almost 17 miles in circumference and will reach energies 27 times higher than RHIC's.

"One question that screams out to be answered is whether we'll see the same sort of perfect fluid that we see at RHIC," Zajc says, "or whether we'll see something like an ideal gas where the quarks and gluons are essentially free. I think it will continue to be a perfect fluid, or very nearly so. But we've been surprised before in this field."

As to whether Maldacena's ideas will further strengthen string theory or prove a theoretical dead end is anyone's guess. The data, says Zajc, are simply too raw.

"This is what new discoveries look like from the inside," he says. "If you'll allow me to mix metaphors, it's sort of a Mixmaster of swirling ideas that may gradually be distilled into something elegant and nice. But at the moment we're watching the sausage-making process."