A joint Fermilab/SLAC publication

String theory may hold answers about quark-gluon plasma


String theorists describe the physics of black holes in five-dimensional spacetime. They found that these five-dimensional objects provide a good approximation of the quark-gluon plasma in one fewer dimension, a relationship similar to the one between a three-dimensional object and its two-dimensional shadow. Image: SLAC National Accelerator Laboratory

Recreating the conditions present just after the Big Bang has given experimentalists a glimpse into how the universe formed. Now, scientists have begun to see striking similarities between the properties of the early universe and a theory that aims to unite gravity with quantum mechanics, a long-standing goal for physicists.

“Combining calculations from experiments and theories could help us capture some universal characteristic of nature,” said MIT theoretical physicist Krishna Rajagopal, who discussed these possibilities at the recent Quark Matter conference in Annecy, France.

One millionth of a second after the Big Bang, the universe was a hot, dense sea of freely roaming particles called quarks and gluons. As the universe rapidly cooled, the particles joined together to form protons and neutrons, and the unique state of matter known as quark-gluon plasma disappeared.

In recent years, scientists have reproduced the quark-gluon plasma by smashing together heavy ions – first with gold nuclei at the Relativistic Heavy Ion Collider, and then with heavier lead ions at the Large Hadron Collider in 2010. The energy from the collisions at the LHC is nearly 14 times higher than those at RHIC, and produces temperatures more than 100,000 times hotter than the center of the sun, enough to melt the protons and neutrons from the ions down to their quark and gluon components.

The quark-gluon plasma created inside the experiments disappears almost instantaneously – billions of times faster than it did in the early universe – as the quarks and gluons reassemble into composite particles. These particles then fly into the surrounding detectors. Researchers study them to glean information about the quark-gluon plasma.

For example, in 2005 RHIC scientists discovered that the quark-gluon plasma didn’t behave like a gas of particles traveling randomly, but rather as a nearly perfect liquid. All the particles moved with a strong sense of coordination, much like a school of fish seems to move as a single object even though individual fish may be darting around in different directions.

Although researchers have since learned a great deal about the plasma, the collective movement among the quarks and gluons continues to be a stumbling block for explaining the behavior of the exotic fluid. The tools that normally describe particles and interactions on a subatomic scale don’t work, and physicists have set out to find a better measuring stick.

“We have to figure out how to get from understanding single particles to understanding the medium as a whole,” said CERN theoretical physicist Urs Wiedemann. Surprisingly, the answer may come from string theory.

Strung together

String theory is a hypothetical description of nature that accommodates both gravity and the quantum physics that describe the other three fundamental forces: electromagnetism and the strong and weak nuclear forces. Traditionally, gravity and quantum physics don’t play well together, but string theory uses extra dimensions of space to reconcile the two.

Theorists found that the mathematics of certain quantum theories and that of objects described by gravity in one extra dimension, such as black holes, look remarkably similar. Physicists are taking advantage of this duality by translating problems from the quark-gluon plasma into the language of gravity, where the equations become much simpler. Although the translation can’t provide precise calculations, solving a problem in one realm can give valuable insights into the other.

“On the theory side, black hole physics looks the same as quark-gluon plasma physics,” said physicist David Mateos, an ICREA research professor at the University of Barcelona in Spain, who also presented his work at the conference. “The fact that the same theory can be used to describe physics with gravity and physics without gravity is truly fascinating.”

String theory provides good approximations for certain properties of the quark-gluon plasma, such as its extremely low viscosity – the fact that it can flow without experiencing much resistance – or how certain particles are affected as they travel through the dense liquid.

“String theory is like a gift to us,” Rajagopal said. “We’re challenged with understanding the quark-gluon plasma as a liquid, and while string theory doesn’t give us precision, it can help us get a feel for the shape of the subject.”

Ultimately, physicists hope to understand how quarks and gluons bind together inside the fluid to form new particles, and what causes them to stay confined. Gravity-based explanations from string theory can also be used to describe phenomena that exhibit strong interactions similar to the quark-gluon plasma, such as superconductivity or supercold atomic gases.

In turn, experimental validations of these approximations could show how string theory best represents nature and point theorists toward avenues to explore further, Weidemann said. “But for now, string theory is just a useful tool for solving a current theory of reality.”

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