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Journal Article 1 Mention
Around-the-World Atomic Clocks: Predicted Relativistic Time Gains
J. C. Hafele1972
Richard E. Keating
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563 citations · Ocean Engineering

TLDR

Flying super-accurate clocks around the world in airplanes can make them run a little faster or slower, just as Einstein's ideas about time predict.

Summary

1 Study Aim

The main goal of this paper is to predict how much time would be gained or lost by atomic clocks (very precise timekeeping devices) when flown around the world on airplanes, both eastward and westward. The authors want to test Einstein's theory of relativity using real, physical clocks instead of just mathematical calculations. Simply put: The study wants to see if flying clocks around the world changes how they keep time, as Einstein's theory suggests.

2 Study Design

The researchers used four cesium beam atomic clocks (clocks that use the vibrations of cesium atoms to keep very accurate time). They placed these clocks on commercial jet flights that circled the globe twice—once going east and once going west. The clocks' time readings were compared to reference clocks that stayed at the U.S. Naval Observatory. The authors used the actual flight paths to calculate what Einstein's relativity theory predicts should happen to the clocks' time. Simply put: The team flew super-accurate clocks around the world in both directions and compared their time to clocks that stayed at home.

3 Findings

The authors report that, according to Einstein's theory and the specific flight paths, the clocks should lose about 40 nanoseconds (billionths of a second) when flown eastward and gain about 275 nanoseconds when flown westward, compared to the clocks that stayed at the observatory. These predictions include a margin of error. The actual measured time differences are discussed in a follow-up report. The results are important because they provide a real-world test of relativity using everyday objects, not just subatomic particles or theory. Simply put: The study predicts that flying clocks east will make them run a little slower, and flying them west will make them run a little faster, just as Einstein's theory says.

Abstract

During October 1971, four cesium beam atomic clocks were flown on regularly scheduled commercial jet flights around the world twice, once eastward and once westward, to test Einstein's theory of relativity with macroscopic clocks. From the actual flight paths of each trip, the theory predicts that the flying clocks, compared with reference clocks at the U.S. Naval Observatory, should have lost 40 +/- 23 nanoseconds during the eastward trip, and should have gained 275 +/- 21 nanoseconds during the westward trip. The observed time differences are presented in the report that follows this one.

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