April 26, 2005
By crashing the nuclei of gold atoms into each other at 99.995 percent of the speed of light, physicists at Brookhaven National Laboratory in Upton, New York, have created a new state of matter — a nearly perfect liquid of strongly interacting quarks and gluons, the fundamental components of matter. The liquid's surprising properties are challenging theorists and may shed light on the universe's earliest moments.
Scientists reported the findings last Monday at a meeting of the American Physical Society in Tampa, Florida.
"This is our first opportunity to view this phase of matter in the laboratory, rather than just knowing it must have existed in the Big Bang," Scott Pratt, a nuclear theorist at Michigan State University, told Astronomy.
The emergence of liquid-like drops from the enormously energetic collisions between gold nuclei surprised most physicists. They expected amorphous quark-gluon plasma, a hot gas that quarks could zip through easily. Instead, they found a substance that flowed and responded to pressure like a liquid, and stripped energy from speeding particles extremely quickly.
"It was surprising for it to have such strongly, obviously, liquid-like properties," said Samuel Aronson, Brookhaven's associate director for high energy and nuclear physics.
Two features proved the liquid nature of the "mini bangs," created within Brookhaven's huge Relativistic Heavy Ion Collider, which recreates the extreme conditions that existed a millionth of a second after the Big Bang.
The initially football-shape droplets expanded faster in width than length, something amorphous plasma could not do. "The explosions had a lot of asymmetry," Pratt noted, "and this can only happen if the system is expanding according to the hydrodynamics of an almost perfect fluid."
Also, experimenters detected far fewer jets of energetic particles than expected. They calculated that particles trying to escape the collision zone were losing energy at an enormous rate — more than a billion billion times faster than charged particles moving through ordinary matter.
"Even the most high-energy particles lose more energy than people thought," Pratt said. "It's like even Shaquille O'Neal has trouble getting through the crowd."
While this new liquid phase of matter is surprising, it doesn't threaten quantum chromodynamics (QCD), the standard theory that describes the universe's fundamental forces and particles. "Nothing in these blobs violates QCD," said John Cramer, a heavy-ion researcher at the University of Washington. "But starting from QCD, nobody knows how to predict in advance that they should behave like a liquid."
According to Aronson, the ball is now in the theorists' court. They'll have to get better at applying QCD theory to show why matter under these extreme conditions — a billion times hotter than the Sun — behaves the way experimenters found. "Now we can hold the theorists' feet to the fire," he said.
Although the researchers believe they've recreated the conditions that existed in the first microsecond of the Big Bang, they still don't know enough about their quark-gluon liquid, or just how it "freezes" into protons and neutrons to make cosmological inferences.
Pratt explained that if the primordial liquid stores enough latent heat to undergo a dramatic phase change, like water boiling, "bubbles and drops could grow into seeds for later fluctuations in the universe, such as galaxies." Still, he cautioned, very preliminary results seem to show a more gradual transition.
Cramer focused on the liquid itself. "What it's telling us is that in the first microsecond after the Big Bang, the universe was a liquid that had a high compressibility. That would tend to damp out any fluctuations," he said. "So it may change the way people try to evolve the universe from the Big Bang into galaxies and stars."
It may, agreed Aronson. But, he added, "It's too early to know yet."
Robert Adler is a freelance science writer living in Santa Rosa, California. He is the author of Science Firsts: From the Creation of Science to the Science of Creation (Wiley & Sons, 2002).