A Question of Time

To most of the passengers on Pan American Flight 106 from Washington's Dulles International Airport, it was simply a routine trip to London. But for Physicist Joseph C. Hafele and his companion, Astronomer Richard Keating, it was the beginning of a journey into the most esoteric realms of modern science. Occupying four seats in the big 747's tourist compartment--two for themselves and two for their scientific gear--they were setting off on an extraordinary round-the-world odyssey: an expedition to test Albert Einstein's controversial "clock paradox," which, stated simply, implies that time passes more slowly for a rapidly moving object than for an object at rest.

Atomic Clocks. The paradox, which stems from Einstein's 1905 Special Theory of Relativity, is difficult for the layman to comprehend and even harder for scientists to prove. It means that time itself is different for a speeding automobile, for example, than for one parked at the curb. The natural vibrations of the atoms in the engine of the moving auto, the movement of the clock on the dashboard and even the aging of the passengers occur more slowly than they do in the parked car. These changes are imperceptible at low terrestrial speeds, however, and according to the theory become significant only when the velocity of the moving object approaches the speed of light.

Einstein's prediction has since been backed by indirect experimental evidence. The existence of short-lived sub-atomic particles, for example, seems to be extended when they are speeded up in atom smashers. But there has never been a satisfactory test of the prediction with a clock actually traveling through space. To conduct that test, Hafele, a physicist at Washington University in St. Louis, persuaded the U.S. Naval Observatory to lend him four extremely accurate atomic clocks, each valued at $17,000 and weighing 60 lbs. In addition, the Navy agreed to foot the bill ($7,400) for two round-the-world jet flights for Hafele, Keating (a member of the observatory) and the atomic clocks.

Hafele's idea was relatively simple. He would make two circumnavigations of the globe--one in an easterly direction and the other in a westerly one. On the eastbound trip, the airborne clocks would be moving faster (by the speed of the jet) than a reference clock on the surface of the earth, which at the equator spins in an easterly direction at about 1,000 m.p.h. Thus, by Einstein's clock-paradox equation, the clocks on board should lose about one-hundred billionths of a second compared with another extremely accurate atomic clock left behind in Washington. During the westbound flight, however, the plane will be flying against the earth's rotation. To an observer in distant space, the clock in Washington would appear to be moving faster than its four counterparts in the air and thus would slow down in relation to them. As a consequence, the airborne clocks should gain about three-hundred billionths of a second relative to the Washington clock during the second trip.*

Star Treks. This Einsteinian concept of "time contraction," if proved to be a real physical effect, eventually could help man conquer the vast distances to the stars. Aging more slowly at high speeds, astronauts could make trips that would take longer than their normal terrestrial life spans. If their spacecraft traveled close to the speed of light (186,000 miles per sec.), as a matter of fact, so little time would elapse for the astronauts compared with the experience of people back on earth that they might return home to meet their own great-grandchildren.

Some theorists have refused to accept such fantastic consequences of the clock paradox and have sought to disprove it. They have even used the paradox in an effort to challenge all of relativity; for Einstein himself admitted that if only one part of his theory proved wrong, its whole finely structured mathematical edifice would crumble. In the September issue of Physics Today, Physicist Mendel Sachs takes a different tack. He contends that the Einstein theory and equations are correct, but that Einstein misinterpreted the equations in stating the clock paradox. A relativity theorist himself at the State University of New York in Buffalo, Sachs argues that the equations suggest that the difference between a clock aboard a spacecraft and one on the ground is observational rather than real. It is, he says, an effect similar to that experienced by an observer on a station platform who hears a change in pitch of the whistle of a passing train--when no change has actually occurred.

Sachs will thus have more than normal interest in the outcome of the experiment by Hafele and Keating, who by week's end returned to Washington from their eastbound flight. During their 58-hr. 5-min. trip, they had been kept busy taking temperature, pressure and magnetic readings inside the plane; any variations that could affect the experiment will have to be calculated into the results. It will take them another flight, scheduled for this week, and perhaps a month of complex analysis of their data before they come to any firm conclusions. But they are hopeful that the final results will help resolve one of the most enduring debates in 20th century physics.

* Actually, the final figures may be different since the scientists must take into account the varying speeds, altitudes and flight paths of their different planes.

This file is automatically generated by a robot program, so reader's discretion is required.