Gravity Measured, or Not

Gravity makes apples drop, keeps clouds from flying into space, and stops people from floating up. That much even schoolchildren know. But gravity’s more dynamic features are not known. In theory, gravity travels through space in the form of subatomic particles called gravitons, which move at the speed of light. But no one was able to confirm this. Then, in January, physicist Sergei Kopeikin of the University of Missouri announced he had. Other scientists soon said he had not.

Kopeikin, with the help of astronomer Edward Fomalont of the National Radio Astronomy Observatory in Charlottesville, Virginia, used an array of radio telescopes to measure the deflection of radio waves coming from a distant quasar as they passed near Jupiter. Kopeikin estimated that Jupiter caused only a tiny amount of deflection—less than 15 billionths of an arc second, or the thickness of a human hair as seen from a distance of 400 miles. After tinkering with Einstein’s general relativity equations to put in a new correction factor, he used the data from the experiment to calculate gravity’s speed: 1.06 times that of light, give or take an error of 20 percent. That supported Einstein’s calculations.

Other physicists disagreed. Kopeikin and Fomalont’s experiment, they said, was merely an inaccurate measurement of light’s velocity. Gravity may indeed be deflecting the quasar’s waves, they said, but the effect is too small to measure with present-day instruments.

“It’s a cool idea,” says theoretical physicist Clifford Will of Washington University in St. Louis. “The only other way to measure the speed of gravity is through gravitational waves,” he points out, “which involves multimillion-dollar satellites. Still, my calculations show the effect just isn’t there.”

Kopeikin claims that his opponents have made “mathematical mistakes,” but Will disagrees. “Too frequently, the public perceives science as a matter of opinion,” he says. “However, in a lot of cases, especially in physics, there is an objective reality that is accessible either by calculation or experiment. In this case the reality is that Sergei is flat wrong.”

—Kathy A. Svitil 

Quantum Computing Makes a Giant Leap
 

Photons, electrons, and other elementary particles have the bizarre ability to interact even when miles apart. Einstein called this “spooky actions at a distance,” but today’s physicists have a more sober term for it: entanglement. Such spookiness, they’ve found, is essential to quantum computing, which would use tiny particles to store and process information. In March physicist Roberto Merlin of the University of Michigan and his colleagues laid the foundation for a workable quantum computer when they announced that they had entangled three electrons, using a system that could someday be scaled up to involve many more. Previous quantum engineers had never reliably linked more than two.

Merlin and his team created a semiconductor “quantum well,” doped it with impurities that gave off free electrons, then placed it within a magnetic field. They then zapped the electrons in the well with pulses of laser light, each 100 million billionths of a second long and covering a spot 16/100 of an inch across. The pulses created temporary particles, known as excitons, on the well’s surface. Nearby electrons interacted with the excitons and then became entangled. The result was an unearthly harmony: As the electrons became entangled, their spinning created energy peaks within the magnetic field and harmonics on top of those peaks. The more electrons, the more harmonics.

Although the researchers linked only three electrons, Merlin says they could entangle many more: “In principle, you could come up with a laser that entangles electrons A, B, and C, and then another laser that entangles C and D, and then D, E, and F, and so on. It is like creating a chain.” Merlin believes that such linkages will lead to a quantum computer in just a few years. “The method works,” he says. “The main problem is a materials problem.”

—Kathy A. Svitil

Particles and Theory Collide

The weird world of particle physics got weirder in April, when physicists announced the discovery of a new subatomic particle with properties that defy conventional theory. Researchers using the BaBar Detector at the Stanford Linear Accelerator Center in California have spent the past four years smashing together electrons and their antimatter counterparts—positrons—to explore one of the greatest mysteries in the universe: Why is everything made from matter, rather than antimatter? In the debris from one collision they found a previously unknown particle they called Ds(2317).

The new particle is thought to be a short-lived union between a charm quark and a strange antiquark. Quarks are ethereal particles that make up protons and neutrons—the building blocks of atoms—and other bits of subatomic matter. They come in six varieties: up, down, top, bottom, strange, and charm. Each has an antimatter counterpart. Although particle accelerators routinely produce unusual configurations of quarks and antiquarks, Ds(2317) was peculiar because its mass is at least 9 percent lower than expected.

In the world of subatomic particles, finding a 9 percent mass discrepancy is like seeing an elephant do a disappearing act. Surprisingly, some researchers suggest that the low mass might be because Ds(2317) is actually not a charm-antistrange composite but a quark “molecule,” built out of four quarks. No such particle has ever been seen; however, a five-part pentaquark was discovered in July (see “New Matter Detected at Japanese Accelerator,” page 45).

BaBar team leader Marcello Giorgi, a physicist from the University of Pisa in Italy, thinks Ds(2317) may be a harbinger of a paradigm shift in the world of subatomic physics. Mass and energy are equivalent at these small scales, so Giorgi and his colleagues reason that they can get the mass of Ds(2317) to fall within the right range by tinkering with the strength of the strong nuclear force that binds the charm quarks and the strange antiquarks. If experiments now in the works prove them right, it means that previous calculations of the strong nuclear force, one of the most fundamental forces in the universe, may be wrong. “We would have to revisit all the knowledge that we have about the force that binds elementary quarks to produce matter,” Giorgi says. “That would be a very big deal.”

—Kathy A. Svitil

Physicists Find Suspected Flaw in Cosmic Symmetry

In general, symmetry is the rule in the universe—the world makes perfect sense if seen in a mirror, for instance—but in April, physicist Edward Stephenson at Indiana University found a flaw in the balance of nature that researchers have been seeking for decades. Scientists have assumed this imbalance, called charge-symmetry breaking, had to exist because without it there would be no hydrogen, and hence no galaxies, planets, or people.

“There was a point about one second after the Big Bang when neutrons and protons condensed out of the underlying mixture of particles,” Stephenson says. “The neutrons decayed into protons, but the protons remained stable. After 10 or 20 minutes, there was an enormous amount of the subatomic materials needed to form hydrogen, which is the building block of stars and galaxies. It is all a consequence of charge symmetry breaking down.” The effects of charge-symmetry breaking are still evident today. Neutrons are measurably more massive than protons, which have an electric charge but are otherwise identical, because of a bias built into the laws of physics.

Until recently, all of this remained theory. Stephenson put it to the test at the Indiana University Cyclotron Facility. He and his colleagues slammed a beam of heavy hydrogen atoms into a cloud composed of more heavy hydrogen. Most of the time, the encounter obliterated the atoms. One time in 10 billion, however, two heavy hydrogen nuclei fused to make a helium atom and a particle called a pion, which helps bind an atomic nucleus together. That reaction can occur only by breaking charge symmetry. Physicists at Ohio University observed similar evidence of symmetry violation by colliding neutrons and protons to form heavy hydrogen and pions. They also announced their results in April.

The big question now is why particles can occasionally evade laws that apply the rest of the time. Stephenson plans further experiments to measure the rate of symmetry violation, which may help piece together this puzzle.

—Kathy A. Svitil