Maldacena's realization raised an enormous question: If string theory and his slightly altered take on QCD are essentially one and the same beast, does that mean there is a way to connect string theory to the physics of the real world? For the next few years, Maldacena's tour de force remained largely a plaything for theorists, who almost immediately found intriguing ways to use it. Most important, his theory simplified the grueling calculations of QCD by offering a way to translate certain QCD problems into the more tractable mathematics of string theory. "Things that are hard to calculate in QCD are easy in string theory, and vice versa," says Horatiu Nastase, a string theorist at Brown University.
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."