Clocks at the top of a tall building run faster than clocks at ground level
Einstein's general theory of relativity predicts that time passes more slowly in stronger gravitational fields. At higher altitudes, gravity is slightly weaker, so time passes slightly faster. The effect has been measured and confirmed: a clock on the 100th floor of a building is measurably ahead of a clock in the basement over time. GPS satellites must account for this effect — their onboard clocks run about 38 microseconds fast per day due to the weaker gravity at their altitude, which is corrected in their software. Without this correction, GPS would drift by 10km per day.
We experience time as absolute and universal — a clock is a clock. Discovering that your head ages slightly faster than your feet (due to being further from Earth's centre of mass) makes time feel genuinely subjective at a physical level.
“Clocks at altitude run faster than clocks at ground level — time passes more slowly in stronger gravity. GPS satellites must correct for this; without it, navigation would drift 10km per day. ⏱️ #OddlyHuman”