The Relativistic Heavy Ion Collider at Brookhaven National Laboratory has revealed evidence of a remarkable new state of hot, dense matter that behaves more like a viscous-free liquid than gas.

It’s a simple question: What happens to matter when it’s crammed into the smallest volumes and heated to the highest temperatures? We’re not talking a mere few thousand degrees here, such as the temperature of the earth’s core, or a few million degrees, as found in the sun’s center. To reach the most extreme temperatures of a searing thousand billion degrees Celsius, physicists must smash atomic nuclei into one another to replicate the conditions present a few microseconds after the Big Bang itself, 13.7 billion years ago.

Enter the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory (Brookhaven) on Long Island, New York. Since 2000, this machine, 2.4 miles in circumference, has been accelerating gold and other heavy nuclei close to the speed of light in opposite directions before smashing them head-on inside house-sized detectors positioned at four points around the ring. Under these wild conditions, the protons and neutrons inside nuclei melt, laying bare the fundamental constituents of matter: quarks and gluons.

“From the viewpoint of the man on the street, measuring the properties of these extreme forms of matter is a crucial aspect of understanding how our universe has evolved,” said Brookhaven associate director Steven Vigdor.

RHIC physicists aim to study an elusive form of matter called quark-gluon plasma (QGP). The strong nuclear force normally confines quarks in tight bundles such as protons and neutrons. But unlike gravity and electromagnetism, which get stronger as the distance between particles shrinks, the strong force counterintuitively gets very weak. This property, known as “asymptotic freedom,” would appear to keep quarks and gluons from interacting with each other when the temperature is hot enough to melt many protons and neutrons inside a volume as small as a nucleus, producing a dense gas of free particles—the QGP. Were RHIC’s collisions hostile enough to allow this exotic phase of matter to be studied in the lab?

Brookhaven’s quest for the QGP began in 1986 with experiments at the Alternating Gradient Synchrotron, in tandem with similar experiments in Europe at CERN’s (The European Organization for Nuclear Research) Super Proton Synchrotron. Just as RHIC fired up in 2000, CERN announced hints of an unusual QGP-like state. But by colliding beams of heavy nuclei head-on at much higher energies, three years later RHIC’s detectors had found the most compelling evidence to date that a new form of quark-gluon matter exists at extreme temperatures.

While it may seem as though the search for the QGP is simply a matter of banging things together at high energies and waiting for the “eureka moment” when the detectors pick it up, nothing could be farther from the truth. To understand what is happening in the microscopic fireball that exists for a mere trillionth of a trillionth of a second when the gold nuclei pile through one another, RHIC’s researchers have to work backwards from the debris recorded by RHIC’s four giant detectors—STAR, PHENIX, PHOBOS and BRAHMS—and meticulously compare this data to sophisticated theoretical models.

In 2005, this painstaking process produced a stunning surprise. Instead of behaving like a gas of free particles, the fleeting RHIC state behaved more like a liquid. It got even better. The patterns among the thousands of emerging particles revealed that the liquid flows with almost zero resistance or viscosity, making it the most “perfect” liquid ever observed.

Brookhaven’s announcement of the perfect liquid at the 2005 American Physical Society meeting in Tampa was the first time string theory—a candidate “theory of everything” that invokes six or seven extra dimensions to unite all nature’s forces, including gravity—had been linked to a hard experimental result. Although the strong force is described beautifully by a 35-year old theory called quantum chromodynamics (QCD), its equations are extremely difficult to solve in situations where the interactions between quarks and gluons are strong—as they must be to produce the perfect liquid behavior observed at RHIC. Bizarrely, the mathematics of string theory has offered a complementary approach by connecting strongly coupled QCD-like forces to weakly coupled gravitational forces, correctly predicting the almost zero viscosity measured in the RHIC collisions.

“String theory may provide a method for extracting predictions in a regime where traditional field theory approaches are difficult to use,” said Thomas Schaefer of North Carolina State University. “But currently its predictions are for a supersymmetric cousin of QCD, so it cannot really be quantitative.”

Claiming outright discovery of the QGP is difficult, since it is now clear that the constituents of extreme nuclear matter behave much more cooperatively than expected. An upgrade to RHIC will see the collision rate increase ten-fold by 2012, by which time CERN’s Large Hadron Collider will be colliding heavy ions at even higher energies, to reach higher temperatures. The story of how ordinary matter transforms under these wild conditions, and therefore how the first instants of the universe unfolded, is far from over. RHIC remains optimally positioned to characterize this transformation in detail.

“The ‘perfect liquid’ is a very interesting result that came as a surprise to most—perhaps all— theorists on the subject,” said Frank Wilczek of the Massachusetts Institute of Technology, who shared the 2004 Nobel Prize for the discovery of asymptotic freedom. “The RHIC results don’t in any way undermine the validity of QCD as the theory of the strong interaction, but they challenge us to understand how the theory behaves at high temperatures.”