Hottest soup in the universe

Scientists say the tiny bubbles of plasma they've created in a "big bang machine" are the hottest dollops of soup ever seen in the universe, reaching temperatures of several trillion degrees.

What's more, the weird properties of that soup may help scientists create a new breed of electronic devices - and figure out why the universe didn't blow itself up as soon as it came into being.

The proton-sized soup bubbles, known more formally as quark-gluon plasma, were created about a billion times in the Relativistic Heavy Ion Collider, or RHIC, a particle accelerator at Brookhaven National Laboratory in New York. Like the larger, more recently constructed Large Hadron Collider, RHIC is capable of creating the conditions that existed in the universe a millionth of a second after the big bang.

RHIC (pronounced like "Rick") does that by accelerating gold ions in a 2.4-mile-round magnetic ring and smashing them together at nearly the speed of light. In 2005, RHIC's researchers said the collisions were so energetic that they liberated quarks and gluons from subatomic particles, creating a free-flowing plasma that acted like a nearly perfect liquid.

Five years ago, the researchers weren't yet able to determine just how hot the quark-gluon soup had gotten. Today, they announced new findings that pegged the temperature at 4 trillion degrees Celsius (7.2 trillion degrees Fahrenheit). That's 250,000 times hotter than the center of our sun, and roughly 40 times hotter than the core of a Type II supernova, said Steven Vigdor, Brookhaven's associate lab director for nuclear and particle physics.

"This is the hottest matter ever created in the laboratory" and qualifies as the "highest temperature known in our present universe," he told journalists during a teleconference organized in conjunction with this week's American Physical Society meeting in Washington.

The temperature is also about twice as high as what's required to melt protons and neutrons into their constituent quarks and gluons - which confirms that RHIC actually did create quark-gluon plasma, Vigdor said.

How the temperature was taken

Making the measurement wasn't easy, said Barbara Jacak, a physicist at Stony Brook University who is the scientific spokesperson for RHIC's PHENIX detector team. She compared the feat to judging the temperature of molten metal in a bronze foundry by analyzing its color and brightness.

For molten metal, the color changes from red to orange to yellow to white-hot - but for the blast created in a particle accelerator, the color goes way beyond the visible spectrum, into the gamma-ray region.

Physicists analyzed the light emissions from RHIC's blast of subatomic particles, and carefully separated out the emissions from the gold-on-gold collision. They came up with the 4-trillion-degree estimate by matching their data against theoretical models for the plasma's expansion, Jacak said.

Details of the findings are to be published in Physical Review Letters.

Now that the temperature has been characterized, researchers can be confident that the meltdown of subatomic particles really does create a liquid that flows with almost no frictional resistance. Such an ordered state does not mesh with the picture physicists previously had about the expected behavior of free-flying quarks and gluons.

"All we want to do now is find out why," Jacak said.

The LHC is scheduled to go back into operation later this week, and the schedule calls for the LHC's ALICE experiment to begin smashing lead ions together this fall. ALICE's collisions could be two to three times more energetic than RHIC's maximum level. Some physicists speculate that at those energies, quark-gluon plasma would stop acting like a liquid and start acting like a gas.

Breaking the rules of symmetry

RHIC's experiments have already shown that "this is not your father's quark-gluon plasma," Vigdor said - and not just because of its liquid properties.

Another experiment analyzed how free quarks interacted with the ultra-strong magnetic fields produced by the gold-on-gold ion collisions. That experiment, involving RHIC's STAR detector, found that the quarks didn't behave as expected: Positively charged quarks tended to emerge from the collision parallel to the magnetic field, while negatively charged quarks went in the opposite direction.

That runs counter to the idea that the interactions of quarks and gluons should exhibit mirror symmetry, or parity. Physicists had thought there'd be just as much chance for the quarks to emerge one way as the other way, regardless of charge. But the quark-gluon soup broke that symmetry - and that may point the way toward other examples of symmetry-breaking at high temperatures.

As a matter of fact, physicists are counting on finding those examples.

"Even though we all like symmetry, it is really imperfection that we owe our existence to," Dmitri Kharzeev, head of Brookhaven's nuclear theory group, said during the teleconference.

The best-known and most intriguing example has to do with the balance of matter and antimatter in the universe. Theoretically, the emergence of the universe should have given rise to equal amounts of matter and antimatter ... which would have annihiliated themselves immediately in a burst of energy. But for some reason, our universe gave matter preferential treatment - perhaps due to symmetry-breaking at the time of the big bang. Physicists would love to find out how that happened.

Previous experiments have found evidence of symmetry-breaking for matter and antimatter, and the question is to be addressed head-on by the Large Hadron Collider's LHCb experiment. Kharzeev said the symmetry-breaking behavior seen in quark-gluon plasma could help the the antimatter sleuths crack their more mysterious case.

Symmetry-breaking is also thought to play a role in the mystery of the Higgs boson, the so-called "God particle" that would explain why some particles have mass while others are massless. Identifying the Higgs boson is a prime objective for the LHC as well as the Tevatron at Fermilab in Illinois.

The path to spintronics?

All this may sound like a purely theoretical exercise - but Kharzeev said symmetry-breaking could have some down-to-earth implications.

For years, Kharzeev and his colleagues at Brokhaven have been trying to develop devices that take advantage of a technology known as "spintronics." Today's electronic devices are based on the properties of electric charge, but spintronic devices would be designed to take advantage of an electron's magnetic spin.

The research team has been working on the patents for a spintronic device made from ultra-thin sheets of graphene. Under the right conditions, such gadgets could perform calculations at the speed of light, as if they were working with photons rather than electrons.

If researchers could figure out a way to create asymmetric magnetic interactions in graphene, like the asymmetric interactions in the quark-gluon soup, that would bring them a big step closer to manipulating spin and putting spintronics to work, Kharzeev said.

"This is a very practical thing," he told journalists. "It's real, and this connection - no matter how far it may seem from real life - is helping us and other people develop practical applications."

Update for 3:45 p.m. ET Feb. 16: Some commenters have wondered how it is that any particle collider could endure that much heat. The answer is that the quark-gluon plasma existed for much less than a billionth of a trillionth of a second before it cooled and turned into the more commonly seen spray of subatomic particles. Also, each dollop of quark-gluon soup was only as big as a proton - so the experiment really didn't have a chance to set anything ablaze. However, the collider can give off some pretty nasty radiation, as is the case with most particle experiments - so you wouldn't want to be standing next to the collision point with a soup spoon.

More about quark-gluon soup:

• Inside Science: Hottest temperature in universe measured
• LiveScience: Big bang conditions created in lab
• Reuters: Hottest temperature ever heads scientists to big bang 
• Discover Magazine: The planet's hottest science experiment
• New Scientist: Atom smasher shows vacuum of space in a twist

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