They Flew Atomic Clocks Around the World to Test Einstein

In October 1971, two scientists bought eight round-the-world plane tickets. Four of them were for "Mr Clocks": four caesium atomic clocks, strapped into seats on ordinary passenger jets. Flying east, the clocks came back 59 nanoseconds behind identical clocks on the ground. Flying west, they came back 273 nanoseconds ahead. Einstein predicted both. In this video we derive both effects ourselves: time dilation from Pythagoras and a light clock, the east–west difference from the Earth's spin, gravitational time dilation (gh/c²) from a rocket, and then check every number against what the clocks actually measured. Then: the clocks that can feel a 33 cm height difference, a 1 mm cloud of atoms, and why your GPS would drift about 11 km a day without relativity. Chapters: 0:00 Four atomic clocks fly around the world (1971) 0:57 How to measure a billionth of a second 1:40 Rule one: moving clocks run slow 2:19 Deriving the Lorentz factor from Pythagoras 2:51 Gamma for a jet airliner 3:16 Why east lost time and west gained it 4:36 Rule two: gravity, from a rocket 5:38 Running the numbers: +144 and +179 ns 6:22 The moment of truth: predicted vs measured 7:08 Better clocks: 1996, 2010, 2022 8:03 Relativity in your GPS 9:03 So time is not universal Sources & further reading: • J. C. Hafele & R. E. Keating (1972), "Around-the-World Atomic Clocks: Predicted Relativistic Time Gains" and "Observed Relativistic Time Gains", Science 177, 166–170 • J. C. Hafele (1971), "Performance and Results of Portable Clocks in Aircraft", PTTI report (flight routes, average altitudes 8.90 km and 9.36 km, flight times) • C. W. Chou et al. (2010), "Optical Clocks and Relativity", Science 329, 1630 (33 cm height difference) • T. Bothwell et al. (2022), "Resolving the gravitational redshift across a millimetre-scale atomic sample", Nature 602, 420 • National Physical Laboratory (1996) 25th-anniversary flight, London–Washington–London: predicted 39.8 ns, measured 39 ± 2 ns • GPS relativistic corrections: about −7 μs/day (speed), +45 μs/day (height), net +38 μs/day; clocks set to 10.22999999543 MHz The eastbound and westbound speeds in the "view from the North Pole" section are rounded, illustrative values (ground ≈ 1,400 km/h at the flights' average latitude of about 31–34°N, jets ≈ 800–900 km/h); Hafele and Keating's own calculation used every leg of the real flights. Watch next: How many light-years across is the universe? #relativity #einstein #physics #maths #timedilation