In the latest RHIC results, four different experiments show evidence of the quark-gluon plasma. Moreover, in RHIC physicists have drawn the stunning conclusion that this plasma has properties of a liquid, rather than the gas that had been predicted, and that this liquid has no viscosity, making it the most perfect ever observed (Viscosity is resistance to flow—materials like honey have high viscosity.).

A simulation of a gold-gold collision at RHIC. Due to relativistic effects, the colliding nuclei appear like pancakes as they approach each other. Shortly after they collide, the quark-gluon plasma (purple) forms, but it rapidly cools and condenses into elementary particles (green), which themselves decay into the other particles shown in the last frame on the right. (image courtesy of Brookhaven National Laboratory)

A detailed analysis of the motion of the particles of the plasma—the quarks and gluons—shows that they move together, with their paths coordinated. This tendency for parts of a fluid to flow together, in the same direction, characterizes a zero-viscosity fluid, whereas in a viscous material like honey, parts of the fluid resist flow, so different parts move in very different ways.

Equally striking is the possible connection with string theory (see An Especially Elegant Universe). In string theory, particles can be described as vibration modes of tiny strings, much too small to be detected themselves but, according to string theorists, manifest in elementary particles and gravitation. In fact, string theory was invented to bring together these two areas of research, which so far have been successful only separately. Although an active area of investigation in theoretical physics, string theory has been repeatedly demeaned as incapable of being tested by experiment.

This image shows the products of a RHIC gold-gold collision. The white and yellow dots indicate where particles were detected, and the blue-green lines show calculated particle tracks. (image courtesy of Brookhaven National Laboratory)

It turns out that a string theory set up in a ten-dimensional space can describe mutually interacting quarks, and in fact string theory methods can be applied to predict the measured viscosity of the quark-gluon liquid.

Many physicists are cautious, however, because there is some question that string theory methods should even apply in this case.

Whether or not the RHIC results are explained by string theory, this new evidence for the quark-gluon plasma, with properties of a perfect liquid, is an astonishing result. Much work remains to characterize the plasma and especially its transition to the elementary particles that appear in profusion when the plasma freezes out. This is physics at its most fundamental, investigating both the matter at the beginning of the universe and the force— the “strong interaction”—that holds nuclei and quarks together.