Einstein's `time warp' theory. It has been reconfirmed in a new experiment

There's something about the time warps predicted by Albert Einstein's special theory of relativity which defies common sense. Clocks running slow just because they are moving fast. An astronaut returning home younger than her stay-behind twin just because she has been on a high-speed jaunt. Such things have perplexed even some professional physicists. Yet when so-called relativistic time dilation has been put to precise experimental test, it has emerged more firmly established than ever. Now three Danish physicists have demonstrated its validity with a whole new order of accuracy.

John Maddox, editor of the premier scientific journal Nature, has welcomed this development with a sense of relief. He notes that, since Einstein published his theory in 1905, ``the most robust torrent of correspondence between Nature and its readers has largely served to illustrate the widespread disbelief in Einstein's proposition.'' He adds that such disbelief ``should be banished by the [Danish] results.''

This may be wishful thinking. Time warping can seem too paradoxical for skeptics to give in that readily. However, the Danish work is impressive.

At the Institute of Physics at the University of Aarhus, Matti Kaivola, Ove Poulsen, and Erling Riis have used the transitions of neon atoms between certain energy states as their clock. The frequency of radiation absorbed or emitted by such transitions can be used as a time standard. Neon atoms in a stationary container and in a fast-moving beam respectively represent the resting and traveling clocks. Two separate lasers had their frequencies locked to those of the radiation emitted and absorbed by the two sets of neon atoms.

If the moving clock represented by the neon beam did indeed run more slowly than the clock at rest, this would show up as a change in the characteristic radiation frequency. The experimenters looked for this by measuring the difference in frequency of the lasers which tracked that of the two sets of neon atoms.

The moving clock slowed exactly as special relativity predicts. Here ``exactly'' means to an accuracy of one part in 25,000.

That's a 20-fold improvement over the last major test of time dilation. It was made at the European Center for Nuclear Research (CERN) at Geneva in 1977 as a subatomic particle analogue of the twins. The particles, called muons, were sent racing around a storage ring at 99.94 percent of the speed of light. They lived 29.33 times longer than do muons at rest, which last about 2.2 millionths of a second. In other words, a clock racing along the muon beam would appear to observers in the laboratory to run 29.33 times more slowly than their own clocks. This verified the prediction of special relativity to one part in 1,000.

While such accuracy may strengthen the status of Einstein's theory, it won't relieve the perplexity of those who wonder who, after all, does the traveling? Since the motion is relative, the astronaut could consider that her sister did the traveling while her spaceship remained at rest. Why wasn't her sister the younger one?

This seeming paradox, which has sparked much puzzlement over time warps, involves a misconception. The motions of the twins are not strictly relative. The spaceship must decelerate, turn around, and reaccelerate to return home. In the language of relativity theory, the astronaut changes her reference frame while her twin does not. This is an absolute difference between them. It unambiguously picks out the space traveler as the twin who will be younger. As with other effects of special relativity, time dilation only shows up when relative speeds are close to that of light or when high precision is involved. That is why it is counterintuitive enough to puzzle even some scientists.

Nevertheless, relativistic effects do have practical consequences. Particle accelerators are designed to take account of them. Together with gravitational time effects predicted by Einstein's general theory of relativity (clocks run slower under stronger gravity), special relativity's time dilation is important for the new United States military navigational system, which also is available for civilian purposes. The system uses satellite-carried atomic clocks. Its precision is such that, were relativity neglected, navigational errors of several miles could accumulate within a day.

Relativity's vision of time and space as a single space-time entity may involve effects that occasionally baffle common sense. But as Einstein said, the facts have forced physicists to bring their concepts of time and space ``down from the Olympus [of the absolute] . . . in order to adjust them and put them in a serviceable condition.'' The new facts from Denmark only add to that imperative.

A Thursday column. Robert C. Cowen is the Monitor's natural science editor. -- 30 --{et

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