每日科技报告 第55期 Law of Nature May Have Been Broken -- Briefly

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For One Tiny Instant, Physicists May Have Broken a Law of Nature

For a brief instant, it appears, scientists at Brook haven National Laboratory on Long Island recently discovered a law of nature had been broken.

This image of a full-energy collision between gold ions shows the paths taken by thousands of subatomic particles produced during the impact.

Action still resulted in an equal and opposite reaction, gravity kept the Earth circling the Sun, and conservation of energy remained intact. But for the tiniest fraction of a second at the Relativistic Heavy Ion Collider (RHIC), physicists created a symmetry-breaking bubble of space where parity no longer existed.
Parity was long thought to be a fundamental law of nature. It essentially states that the universe is neither right- nor left-handed -- that the laws of physics remain unchanged when expressed in inverted coordinates. In the early 1950s it was found that the so-called weak force, which is responsible for nuclear radioactivity, breaks the parity law. However, the strong force, which holds together subatomic particles, was thought to adhere to the law of parity, at least under normal circumstances.
Now this law appears to have been broken by a team of about a dozen particle physicists, including Jack Sandweiss, Yale's Donner Professor of Physics. Since 2000, Sandweiss has been smashing the nuclei of gold atoms together as part of the STAR experiment at RHIC, a 2.4-mile-circumference particle accelerator, to study the law of parity under the resulting extreme conditions.
The team created something called a quark-gluon plasma -- a kind of "soup" that results when energies reach high enough levels to break up protons and neutrons into their constituent quarks and gluons, the fundamental building blocks of matter.
Theorists believe this kind of quark-gluon plasma, which has a temperature of four trillion degrees Celsius, existed just after the Big Bang, when the universe was only a microsecond old. The plasma "bubble" created in the collisions at RHIC lasted for a mere millionth of a billionth of a billionth of a second, yet the team hopes to use it to learn more about how structure in the universe -- from black holes to galaxies -- may have formed out of the soup.
When the gold nuclei, traveling at 99.999% of the speed of light, smashed together, the plasma that resulted was so energetic that a tiny cube of it with sides measuring about a quarter of the width of a human hair would contain enough energy to power the entire United States for a year.
It was the equally gargantuan magnetic field produced by the plasma -- the strongest ever created -- that alerted the physicists that one of nature's laws might have been broken.
"A very interesting thing happened in these extreme conditions," Sandweiss says. "Parity violation is very difficult to detect, but the magnetic field in conjunction with parity violation gave rise to a secondary effect that we could detect."
Sandweiss and the team -- which includes Yale physics research scientists Evan Finch, Alexei Chikanian and Richard Majka -- found that quarks of a like sign moved together: Up quarks moved along the magnetic field lines, while down quarks traveled against them. That the quarks could tell the difference in directions suggested to the researchers that symmetry had been broken.
The results were so unexpected that Sandweiss and his colleagues waited more than a year to publish them, spending that time searching for an alternative explanation. The physicist is still quick to point out that the effect only suggests parity violation -- it doesn't prove it -- but the STAR collaboration has decided to open up the research to scrutiny by other physicists.
"I think it's a real effect, but we'll know more in the upcoming years," Sandweiss says.
Next, the team wants to test the result by running the experiment at lower collision energies to see if the apparent violation disappears when there is not enough energy to create the necessary extreme conditions.
If the effect proves to be real, it could help scientists understand a similar asymmetry that led to one of physics' most fundamental mysteries -- namely, why the universe is dominated by ordinary matter today when equal amounts of matter and antimatter were created by the Big Bang.
Sandweiss, for one, is looking forward to some answers. "I'd really like to see this evolve and find out exactly what's going on," he says.

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