![]() He spent a year in fruitless attempts to get funding for such an experiment, until he was approached after a talk on the topic by Keating, an astronomer at the United States Naval Observatory who worked with atomic clocks. Louis, was preparing notes for a physics lecture when he did a back-of-the-envelope calculation showing that an atomic clock aboard a commercial airliner should have sufficient precision to detect the predicted relativistic effects. Hafele, an assistant professor of physics at Washington University in St. These experiments, however, used subatomic particles, and were therefore less direct than the type of measurement with actual clocks as originally envisioned by Einstein. General relativity's prediction of the gravitational effect was confirmed in 1959 by Pound and Rebka. The kinematic effect was verified in the 1938 Ives–Stilwell experiment and in the 1940 Rossi–Hall experiment. In his original 1905 paper on special relativity, Albert Einstein suggested a possible test of the theory: "Thence we conclude that a spring-clock at the equator must go more slowly, by a very small amount, than a precisely similar clock situated at one of the poles under otherwise identical conditions." Actually, it is now known that all clocks located at sea level on the Earth's surface tick at the same rate, regardless of latitude, because kinematic and gravitational time dilation effects cancel out (assuming that Earth's surface is an equipotential one). The observed time gains and losses were different from zero to a high degree of confidence, and were in agreement with relativistic predictions to within the ~10% precision of the experiment. The published outcome of the experiment was consistent with special and general relativity. The results were published in Science in 1972: Since the aircraft flew at roughly the same altitude in both directions, this effect was approximately the same for the two planes, but nevertheless it caused a difference in comparison to the clocks on the ground. In the Hafele–Keating experiment, there was a slight increase in gravitational potential due to altitude that tended to speed the clocks back up. This effect has been confirmed in many tests of general relativity, such as the Pound–Rebka experiment and Gravity Probe A. That is, clocks at higher altitude tick faster than clocks on Earth's surface. General relativity predicts an additional effect, in which an increase in gravitational potential due to altitude speeds the clocks up. Main article: Gravitational time dilation Considering the Hafele–Keating experiment in a frame of reference at rest with respect to the center of the Earth (because this is an inertial frame ), a clock aboard the plane moving eastward, in the direction of the Earth's rotation, had a greater velocity (resulting in a relative time loss) than one that remained on the ground, while a clock aboard the plane moving westward, against the Earth's rotation, had a lower velocity than one on the ground. This effect, called time dilation, has been confirmed in many tests of special relativity, such as the Ives–Stilwell experiment and others. In a frame of reference in which the clock is not at rest, the clock runs more slowly, as expressed by the Lorentz factor. Overview Kinematic time dilation Īccording to special relativity, the rate of a clock is greatest according to an observer who is at rest with respect to the clock. When reunited, the three sets of clocks were found to disagree with one another, and their differences were consistent with the predictions of special and general relativity. They flew twice around the world, first eastward, then westward, and compared the clocks against others that remained at the United States Naval Observatory. Keating, an astronomer, took four caesium-beam atomic clocks aboard commercial airliners. ![]() The Hafele–Keating experiment was a test of the theory of relativity. Test of relativistic time dilation Hafele and Keating aboard a commercial airliner, with two of the atomic clocks One of the actual HP 5061A Caesium Beam atomic clock units used in the Hafele–Keating experiment ![]()
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