Researchers colliding gold particles at Brookhaven National Laboratory found stronger
interactions among quarks and gluons (force lines shown in the simulation at right) than
had been expected (left). The matter is now being described as a nearly “perfect liquid.”
conditions similar to those a microsecond after the birth of
the universe. But instead of a gaslike plasma, the physicists
reported in 2005, RHIC served up a hot quark soup, behaving more like a liquid than a plasma or gas.
“It’s given us a certain amount of consternation about
what to call this stuff,” says Barbara Jacak of the RHIC team.
“It certainly shows liquidlike properties.”
An ordinary plasma’s electrically charged particles
should block the path of light, for example, just as a thick
fog dampens the beams of a car’s headlights. But light
passes right through RHIC’s quark-gluon soup, says Jacak.
And free-flying quarks would easily be able to zip through
the rarefied molecules of a gas, like a bowling ball scattering
any pins in its way. But even the heaviest quarks get stuck
in the soup.
“That is really astounding,” Jacak says. “It’s as if these
bowling pins stopped the big giant bowling ball, and the only
way they could do that is if they are somehow tied together
Soon after the RHIC experiments, string theorists
realized that their strings might be tying the bowling pins
together, explaining the odd liquidlike behavior of the
quark-gluon plasma. That was a spectacular realization in
itself. But around the same time, another branch of physics found itself dipping into a perfect liquid, this time made
from cold lithium atoms.
In 2002, physicists at Duke University first created what
they called a stable, strongly interacting gas of cold atoms,
using the isotope lithium- 6. Using laser beams to confine
and cool the lithium atoms, researchers produced an atomic
cloud with a temperature lower than a tenth of a millionth
kelvins — barely above absolute zero.
Curiously, when researchers released the cigar-shaped
cloud from its laser prison, it expanded at its sides, but not
at the tips. Such an odd “elliptical flow” also described the
expanding cloud of quarks and gluons produced at RHIC.
“It’s quite remarkable that we have such different systems,
yet we have this common behavior,” says Duke’s John E.
Thomas, who also spoke at the Chicago meeting.
Such similar flow seemed especially surprising given the
wide disparity of the two systems, with a temperature difference of 19 orders of magnitude separating them. In both
cases, the flow seemed to signal the features of a liquid — and
a liquid with extremely low resistance to flow. Both cases
constituted what physicists call a “strongly coupled” system,
in which the particles exhibit collective behavior.
Strongly coupled systems are like a baseball stadium with
a big crowd, where the fans can perform the wave, rather
than a poorly attended game with the crowd so “weakly
coupled” that nobody else notices if one fan stands up. In
strongly coupled systems, string theory–based calculations
A strongly interacting ultracold gas created by Duke University researchers expands and
contracts at its sides but not its tips (as shown by the series of snapshots). The atoms
exhibit collective behavior, so the gas behaves like a nearly “perfect liquid.”