GPS needs Einstein
Satellite clocks tick faster in orbit and slower from speed. Ignore relativity and your phone’s map drifts miles a day.
Gravitational time dilation is not a textbook curiosity. Clocks farther from Earth’s mass run faster; clocks in motion run slower relative to the ground. GPS satellites feel both effects at once — higher altitude speeds their clocks up; orbital velocity slows them down. Net result: satellite time advances relative to receivers on the surface.
Leave that uncorrected and position error accumulates fast. The science pages put the drift at about six miles a day — unusable for turn-by-turn navigation, hazardous for anything precision-guided. Every GPS receiver therefore lives inside a relativistic correction loop. Ground stations track satellite clock offsets; ephemeris updates bake in the Einstein terms engineers worked out before the constellation flew.
Time runs at different rates depending on speed and on distance from gravity — not a theory but a routinely measured, engineered-around fact.
Gravitational time dilation, science wiki
What time is remains a question page on the wiki — philosophy and physics still argue. What time does in orbit is settled enough to bet your commute on. Relativistic time dilation to GPS correction is the chain from physics to infrastructure: measure the offsets, broadcast the fix, pretend nothing spooky is happening while your mapping app depends on it.