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TI-17Time1977

Clocks disagree on purpose

GPS satellite clocks run faster than identical clocks on Earth’s surface by about 38 microseconds per day, as general and special relativity predict. The system corrects for it or it fails.

Established3 min read

A clock in orbit feels less gravity (general relativity: it runs fast) and is moving (special relativity: it runs slow). For GPS the gravitational effect wins. Uncorrected, positions would drift by kilometres per day.

The Hafele–Keating experiment (1971) flew cesium clocks around the world and measured the offset. Pound–Rebka (1959) saw gravitational redshift in a tower. Particle lifetimes in accelerators stretch exactly as special relativity says.

Time is not a universal drum. It is local, tied to motion and to mass. The engineering of navigation is a daily experimental confirmation.

Caveats

  • The 38-microsecond figure is a standard engineering summary for GPS orbits; other constellations differ slightly.
  • ‘Time dilation’ does not mean you feel time slowing. Proper time on your worldline is always ordinary to you.

Sources

  1. 1Hafele J. C., Keating R. E.. Around-the-World Atomic Clocks: Predicted Relativistic Time Gains. Science 177, 1972.
  2. 2Ashby N.. Relativity in the Global Positioning System. Living Reviews in Relativity 6, 2003.

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