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Person

Joseph Hafele

A man wearing glasses and a suit stands against a neutral background, looking directly at the camera.

An American physicist who in October 1971, with astronomer Richard Keating, carried four cesium-beam atomic clocks on commercial round-the-world flights to test the time dilation predictions of both special and general relativity. The eastbound clocks lost roughly fifty-nine nanoseconds; the westbound ones gained two hundred and seventy-three, both consistent with theory. The Hafele–Keating experiment is now a textbook demonstration of relativity outside the laboratory.

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