Physics

Year In Science

Sunday, January 13, 2002
RELATED TAGS: SUBATOMIC PARTICLES, LIGHT

Quantum Leap
In January, two teams of Harvard physicists announced they had done the impossible by stopping a beam of light dead in its tracks. The work, completed independently by the two groups, was a crucial step to developing superfast, ultrasecure, hack-proof quantum computers. (For more details on the experiment itself, see "Trapping Light" in the April 2001 issue of Discover.) A quantum computer would use light to transport bits of data, but to be successful it would have to be able to temporarily store that light. By year's end both teams had succeeded in preserving a light pulse when it was stopped and letting it loose again.

Physicist Ronald Walsworth of the Harvard-Smithsonian Center for Astrophysics initially brought light to a standstill by trapping it inside a glass bottle filled with warm gas atoms. His team was able to get about half the pulse to stop on the first try—the other half escaped in the fraction of a second before measurements could be made. But later that spring Walsworth and his colleagues discovered they could get light waves to oscillate in concert—peaks lined up with peaks and troughs with troughs—by nudging them with gentle pulses of a magnetic field, "which allows us," he says, "to preserve all the information contained in the pulse." Someday, with more sophisticated equipment, researchers may be able to stop a light pulse completely and reverse it at will, another essential step down the road to quantum computing.
— Kathy A. Svitil


Little Bang

A 3-D digital camera tracks the paths of the thousands of new subatomic particles created when two gold ions are smashed together in a collider at Brookhaven National Laboratory.
Photograph courtesy of Brookhaven National Laboratory
If you are reading this article, then the new Relativistic Heavy Ion Collider at Brookhaven National Laboratory on Long Island in New York, did not destroy Earth. Some scientists argued that the collider had the potential, once geared up to full power, to create a planet-swallowing black hole. Now that hurdle has been jumped, and Thomas Kirk, a physicist at Brookhaven, reports, "We don't expect we'll make a black hole."

However, researchers at the lab did announce in January that they might have accomplished something almost as unprecedented: the creation of a form of matter not seen since the first tick of the history of the universe. That form of matter, a superdense state called a quark-gluon plasma, has long been a goal of particle physicists. In this plasma, the protons and neutrons that make up atomic nuclei are shattered into a cloud of quarks and gluons, particles that carry the force that normally keeps quarks together. The last time quarks had been rolling around loose rather than bound up in protons, neutrons, or other subatomic particles was only one millisecond after the Big Bang, when the whole universe was a toasty 1.8 trillion degrees Fahrenheit. But the evidence that they had created the plasma was not conclusive, and it may be another year before Brookhaven physicists are confident enough to declare success.

Inaugurated in 2000, the collider was designed to create novel states of matter by smashing together larger bits of matter at higher speeds than had been achieved before. These big bits of atoms, such as gold or silicon atoms stripped of their electrons, are much more complex than the protons that physicists have been accustomed to studying. That complexity means physicists aren't terribly sure what they will see when they smash things together.

After a shakedown period of several months, the collider began running at full energy for the first time in July with a series of experiments that continued until year-end. But even at two-thirds power, physicists say, the collider provided evidence of phenomena that are difficult to explain. The number of particles spawned by a collision of two atomic nuclei of gold is larger than what one would get by smashing together the individual particles that make up those nuclei. "We have a kind of new math," says Jens Jorgen Gaardhoje, a physicist from the University of Copenhagen in Denmark who is working at the collider. Where those additional particles come from is anyone's guess.

For now, the big prize is the quark-gluon plasma. Physicists running a competing experiment at the CERN lab outside Geneva announced that they had created such a plasma in 2000. Their report was quickly criticized as premature, and Kirk does not want to repeat that mistake. Before his team declares they have proof of a quark-gluon plasma, they want to make sure every potential source of error is found and accounted for. "It's an important announcement," Kirk says. "And though we might not have bullet-proof evidence, we want it to be bullet-resistant."
— Jeffrey Winters


Two and Two are Five
The Rockies may crumble, Gibraltar may tumble, but some physical constants are set in stone, aren't they? The speed of light is the same everywhere in the universe and has been since the Big Bang. The same is true for the charge of an electron and the gravitational attraction between two masses. In fact, astronomers and physicists can only talk with any certainty about the Big Bang and what they see in other galaxies if they can assume the physics they see through their telescopes is the same as that which they see in their labs.

But that assumption was challenged in August by a team of astronomers and physicists in Australia, England, and the United States. The researchers found evidence that, over the past 10 billion years, the strength of the bond between an atomic nucleus and its surrounding electrons has changed by one part in 100,000. If the finding holds up, says team member Chris Churchill of Pennsylvania State University, "We're going to have to put a lot of physics in the garbage can."

The speed of light 10 billion years ago is impossible to measure directly, so researchers focused on what is known as the fine-structure constant. The constant is actually made up of three other constants—the speed of light, the charge of the electron, and Planck's constant (the ratio between the energy and frequency of radiation). When all three are combined so that their units of measurement cancel one another out, what's left is a number: approximately 1/137.

To see if the fine-structure constant—or any of its constituent parts—had changed, Churchill and his colleagues gathered measurements of light frequencies from 72 distant quasars. They then looked for instances where intervening clouds of gas between Earth and the quasar absorbed some of the light. The results were unsettlingly clear: The more distant the gas cloud, the more the usual pattern of light absorption was altered. Clearly, the atoms in those clouds behaved differently from those closer to home.

Lennox Cowie, an astronomer at the University of Hawaii in Honolulu, had looked for just the same change in absorption in the mid-1990s and did not find it. He hesitates, therefore, to read too much into the new results: "They are so surprising that you really have to nail it to the wall before you believe it." Measurements of such precision, stretching over so many light-years, can be thrown off by the smallest errors, Cowie points out. But if that's the case, says Churchill, why aren't the differences more random? Why did they all change in the same direction? Cowie and Churchill agree that more observations must be performed before most physicists will accept the change in the constant.

An inconstant constant may seem contradictory, but some physicists believe it may be the only way to explain puzzling phenomena such as the accelerating expansion of the universe. Even if the findings hold up, however, it will be tricky to find out which component of the fine-structure constant has changed. "We can't say if it's the speed of light or the charge of the electron or all of them," Churchill says. Physicists are scrambling to find measures with which to check each component independently, he adds. "But there are a lot of theories out there."
— Jeffrey Winters

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