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