Hafele–Keating experiment

In 1971,[1] Joseph C. Hafele, a physicist, and Richard E. Keating, an astronomer, took four cesium-beam atomic clocks aboard commercial airliners.

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.

[4] General relativity predicts an additional effect, in which an increase in gravitational potential due to altitude speeds the clocks up.

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.

[7] In his original 1905 paper on special relativity,[8] 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."

These experiments, however, used subatomic particles, and were therefore less direct than the type of measurement with actual clocks as originally envisioned by Einstein.

[11] 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.

[11] Hafele and Keating obtained $8000 in funding from the Office of Naval Research[12] for one of the most inexpensive tests ever conducted of general relativity.

[11][14][15] A more complex and precise experiment of this kind was performed by a research group at the University of Maryland between September 1975 and January 1976.

Special containers protected the clocks from external influences such as vibrations, magnetic fields, or temperature variations.

[19] Because the Hafele–Keating experiment has been reproduced by increasingly accurate methods, there has been a consensus among physicists since at least the 1970s that the relativistic predictions of gravitational and kinematic effects on time have been conclusively verified.

[citation needed] In 2005, van Baak measured the gravitational time dilation of a weekend at 5400' ASL on Mount Rainier using two ensembles of three HP 5071A cesium beam clocks.