Clocking Einstein

None of Einstein's ideas have so fascinated the public and provoked such controversy among physicists as the so-called "clock paradox." One of the major predictions of the great physicist's Special Theory of Relativity, the paradox is based on the assumption that time passes more slowly for an object in motion than one at rest. Thus, if Einstein was correct, an astronaut traveling at extremely high speeds--say to a distant star and back--would age less during his trip than a twin brother who had remained on earth. Depending on the length of his mission, the astronaut could, upon his return, actually be years younger than his twin.

Aging Slowly. Man has not yet advanced far enough technologically to stage such a test of relativity. But Physicist Joseph C. Hafele of Washington University in St. Louis and Astronomer Richard Keating of the U.S. Naval Observatory have apparently verified the clock paradox in a less dramatic fashion. Last October, carrying four extremely precise atomic clocks, they set off on two successive round-the-world plane trips to check the validity of Einstein's prediction (TIME, Oct. 18). Their scheme was elegantly simple. On the eastbound flight, their plane was traveling in the direction of the earth's rotation. Thus to an observer in outer space the airborne clocks would appear to be moving faster (their air speed added to the rotational velocity of the earth's surface) than a reference clock back in Washington; hence, the flying clocks would lose a little time -- or, like the astronaut, "age" a little more slowly. On the westbound trip, when they were flying against the earth's rotation, the airborne clocks would seem to the same observer to be traveling more slowly than the Washington clock. (Their air speed would be subtracted from the rotational velocity of the earth.) Thus the Washington clock would appear to slow down, and the airborne clocks would gain time in relation to it. To be sure, the differences in time, which become significant only at speeds approaching the velocity of light (186,000 miles a second), would be extremely small at slow jet speeds -- only billionths of a second. But Hafele and Keating figured that their Hewlett-Packard atomic clocks would be up to so delicate a test. The intricate timekeepers are governed by the natural frequency of the cesium atom, which, when electrically excited, vibrates precisely 9,192,631,770 times a second.

Final Calculations. Hafele and Keating constantly had to monitor such subtle environmental effects on their clocks as temperature, cabin pressure and magnetic fields. Later, on their return to Washington, they had to feed into their final calculations the effect of the varying speeds, altitudes and flight paths of their planes. Yet all the time-consuming work paid off handsomely. According to theory, the four clocks should have lost 40 billionths of a second on the eastbound trip and gained 275 billionths of a second on the westbound. In fact, the actual results were only 5% off on the eastbound and no more than 30% on the westbound flight. Although the results may not be accurate enough to convince all skeptics, Hafele is satisfied. "The experiment," he says, "was successful beyond our best expectations."

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