RHIC has created a new state of hot, dense matter out of the quarks and gluons that are the basic particles of atomic nuclei, but it is a state quite different and even more remarkable than had been predicted. Instead of behaving like a gas of free quarks and gluons, as was expected, the matter created in RHIC's heavy ion collisions is more like a liquid.
Gluons and quarks
Ions about to collide
Just after collision
The "perfect" liquid
RHIC collisions reach a temperature up to 150,000 times hotter than the center of the sun and the energy density (energy per unit volume) predicted to be necessary for forming a plasma of quarks and gluons. But analysis of RHIC data reveals that the matter formed in RHIC's head-on collisions of gold ions is more like a liquid than a gas.
That evidence comes from measurements of unexpected patterns in the trajectories taken by the thousands of particles produced in individual collisions. These measurements indicate that the primordial particles produced in the collisions tend to move collectively in response to variations of pressure across the volume formed by the colliding nuclei. Scientists refer to this phenomenon as "flow," since it is analogous to the properties of fluid motion.
Some of the observations at RHIC fit with the theoretical predictions for a quark-gluon plasma (QGP), the type of matter postulated to have existed just microseconds after the Big Bang. Many theorists have concluded that RHIC has already demonstrated the creation of QGP, but some discrepancies remain between experimental data and early theoretical predictions of QGP formation.
Unlike ordinary liquids, in which individual molecules move about randomly, the hot matter formed at RHIC seems to move in a pattern that exhibits a high degree of coordination among the particles -- somewhat like a school of fish that responds as one entity while moving through a changing environment.
This is fluid motion that is nearly "perfect" meaning it can be explained by equations of hydrodynamics. These equations were developed to describe theoretically "perfect" fluids -- those with extremely low viscosity and the ability to reach thermal equilibrium very rapidly due to the high degree of interaction among the particles. RHIC scientists can infer from the flow pattern that, qualitatively, the viscosity is very low, approaching the quantum mechanical limit.
Together, these facts present a compelling case: the degree of collective interaction, rapid thermalization, and extremely low viscosity of the matter being formed at RHIC make this the most nearly perfect liquid ever observed.
Physicist Tim Hallman discusses the properties of the "perfect" liquid and plans for luminosity and detector upgrades to the Relativistic Heavy Ion Collider.
There is an emerging connection between the collider's results and calculations using the methods of string theory, an approach that attempts to explain fundamental properties of the universe using 10 dimensions instead of the usual three spatial dimensions plus time.
String theory seeks to unify the two great intellectual achievements of twentieth-century physics, general relativity and quantum mechanics, and it may well have a profound impact on the physics of the twenty-first century.
"What
have we learned from RHIC?," an article from Physics Today
by Thomas Ludlam and Larry McLerran
Operated by Brookhaven National Laboratory, the Relativistic Heavy Ion Collider is sponsored by the U.S. Department of Energy Office of Science, Office of Nuclear Physics.
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