Science › Space, time & gravity
GPS runs on relativity
1978· Discovery / experiment
ProvenTested many times; part of working physics.
GPS satellites' clocks must be corrected for both special and general relativity, or positions would drift by kilometers each day.
See it on the Science map →How it works
GPS works by timing signals from satellites, so its clocks must agree to billionths of a second. Satellites move at about 14,000 km per hour, so their clocks run slow by about 7 microseconds a day. They sit higher in Earth's gravity, so their clocks run fast by about 45 microseconds a day. Net: 38 microseconds fast daily. Engineers set the satellite clocks slightly slow before launch to compensate.
Why it matters
It is relativity as everyday infrastructure. Navigation, surveying, aviation, cell networks and financial timestamps rely on these corrections.
How we know
The first GPS satellite launched in 1978. Since then, the system has worked to meter-level precision only because the relativity corrections are applied; ignoring them would cause roughly 10 km of error per day.
Still a mystery
Settled engineering. It also serves as a constant check on general relativity.
Connections (2)
▸Time dilation: moving clocks run slow must be corrected for in GPS runs on relativityProven
The relationship
GPS satellites move fast, slowing their clocks, and sit high in weaker gravity, speeding them up. The net effect is 38 microseconds fast each day. Since positioning depends on precise timing, engineers must correct for it.
How the link was found
Engineers built the corrections into GPS from the first satellite in 1978.
Why it's proven
The system works to meter accuracy; without corrections, errors would grow about 10 km per day.
Open questions and mysteries
None major; GPS is a daily confirmation of relativity.
▸GPS runs on relativity tests every day General relativity: gravity is curved spacetimeProven
The relationship
Every GPS fix depends on satellite clocks running at the rate general relativity predicts for their height in Earth's gravity. If the theory were wrong, the corrections built into the system would be wrong too, and positions would drift measurably. Its steady, everyday accuracy is a continuous test of the theory.
How the link was found
The corrections were designed before GPS satellites launched in 1978. A 1977 test satellite first ran its clock uncorrected and drifted by the predicted amount.
Why it's proven
The system holds meter-level accuracy worldwide. Galileo satellites accidentally put in odd orbits in 2014 were used to confirm gravitational time dilation to high precision in 2018.
Open questions and mysteries
None major for this link. GPS is not precise enough to reveal any small deviations from general relativity.