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 compartmenttwo for themselves and two for their scientific gearthey 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 globeone 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.*