CHICAGO--One is a seething soup of particles 150,000 times hotter than the center of the sun, a temperature not seen in this universe since a microsecond after the Big Bang. The other is a cloud of atoms trapped in a bowl of light and supercooled to a few billionths of a degree.

These exotic forms were created in very different laboratories under very different conditions, but strangely enough, they both act like perfect liquids with no peer when it comes to an easy flow. Even stranger, string theory may be just the thing to explain the unusual behavior of both materials.

In a symposium at the 2009 AAAS Annual Meeting, physicist Peter Steinberg of Brookhaven National Laboratory said the quest for the perfect fluid was unexpected, one in which three distinct physics communities came together unexpectedly. "It surprised the heck out us," he admitted. "None of us ever saw this coming."
At Brookhaven's Relativistic Heavy Ion Collider, better known as RHIC, researchers hurl gold ions together at tremendous speeds--99.99% of the speed of light--to produce collisions hot enough to dissolve the nuclei of the ions into tiny particles called quarks and gluons.

 In 2005, the RHIC researchers using that "atom smasher" found evidence that they had succeeded in their goal, producing a  quark-gluon plasma or "quark soup' that behaved in strange ways, said physicist Barbara Jacak of Stony Brook Unviversity.

The soup's particles flying away from the crash site didn't disperse individually, as might be expected in a hot gas. Instead, they seemed to move together in a highly coordinated fashion, like a school of fish.

The soup acted like a liquid with very low or no viscosity. Viscosity is a measure of how well a liquid resists flow--a poured puddle of viscous honey soon stops rippling outward, even as a puddle of less viscous water continues to spread. The RHIC soup seemed to flow without any resistance--so well, in fact, that its discoverers dubbed it "the perfect liquid."

It was an astonishing discovery, but its creation left "a huge raging debate," said Jacak. "Just what is interacting inside this hottest, densest stuff in the universe?"

Three years earlier and 19 orders of magnitude cooler," joked physicist John Thomas of Duke University, his lab team saw a similar phenomenon unfold. They were trapping cold lithium atoms within a laser-shaped bowl of light, then applying magnetic fields to cause "the atoms to bump into each other and start evaporative cooling," a chill similar to the one produced by rubbing alcohol on skin, said Thomas.

Just like the RHIC quark soup, the supercooled lithium atoms expanded in a unique, sideways fashion, pouring through space like a liquid with near zero viscosity. The "superfluid"  defied expectations in other ways, Thomas said. Unlike an ice skater, who draws in her arms and narrows her body to produce a faster spin, the atom cloud seemed to rotate faster as it expanded.

Two different materials, one strange behavior: what was the connection?

Thomas remembers getting a call from one of his colleagues one day, asking: "Do you know the string theorists are citing your paper?"

"And I didn't know if that was good or bad," he admitted.

String theory--the idea that the fundamental particles of the universe are tiny strings vibrating in multiple dimensions--has had its ups and downs as a "theory of everything" in nature, said Clifford Johnson, a physicist at the University of Southern California. But string theorists were seeing something in the perfect fluid experiments that piqued their interest.

The quark soup and the atom cloud are both examples of strongly coupled systems, where the individual parts stick together through interactions with multiple partners. High temperature superconductors and neutron stars are highly coupled systems--and some researchers think the behavior of gravity near a black hole is also an example of a highly coupled system.

Johnston said string theory calculations describing the black hole scenario may explain how quarks move in a quark-gluon plasma, and therefore offer a testable way to measure the viscosity of these new liquid states of matter.

"This is the first time that string theory has been used as a tool to guide experiments. And these experiments may shape string theory in ways we haven't thought of yet," he said.

All of the scientists expressed hope that the path paved by the cloud, the soup, and the string will yield more unexpected finds "and tell us much more about physics in ultra-extreme conditions in the universe," said Johnson.