Science › Space, time & gravity
Time dilation: moving clocks run slow
1905· Discovery / experiment
ProvenTested many times; part of working physics.
Clocks that move fast, or sit deeper in gravity, tick more slowly than clocks that do not.
See it on the Science map →How it works
Because light's speed is fixed for everyone, a moving clock must tick slower as seen by someone at rest. At everyday speeds the effect is tiny; near light speed it is huge. General relativity adds a second cause: clocks closer to a heavy mass run slower than clocks higher up. Nobody feels it locally; your own watch always seems normal. The difference shows up only when clocks are compared.
Why it matters
It is real, measurable and engineered around. It also means travel into the future is possible: a fast traveler ages less than those left home.
How we know
Fast muons from cosmic rays survive longer (Rossi and Hall, 1941). Airliner clocks (1971). In 2010 optical clocks detected the slowdown from a 33 cm height difference. GPS must correct for it daily.
Still a mystery
Why time has a direction at all, and what time fundamentally is, remain open.
Connections (4)
▸Special relativity predicts Time dilation: moving clocks run slowProven
The relationship
Imagine a clock that ticks by bouncing light between two mirrors. On a moving train, an outside observer sees the light travel a longer diagonal path. Since light's speed is fixed, each tick must take longer. So moving clocks run slow, and the effect grows sharply near light speed.
How the link was found
Einstein derived it in 1905. Larmor and Lorentz had hinted at it earlier in ether-based theories.
Why it's proven
Rossi and Hall's muon measurements (1941), Hafele-Keating airliner clocks (1971), and particle accelerators where fast unstable particles live much longer.
Open questions and mysteries
The "twin paradox" is resolved, but explaining it intuitively still trips up students.
▸General relativity: gravity is curved spacetime adds "gravity slows time" to Time dilation: moving clocks run slowProven
The relationship
Special relativity says moving clocks run slow. General relativity adds that clocks deeper in a gravitational field also run slow compared with clocks higher up. Light climbing out of a gravity well loses energy and shifts toward red. The two effects can add or partly cancel, as they do for satellites.
How the link was found
Einstein predicted gravitational time dilation in 1907 from the equivalence principle, eight years before the full theory.
Why it's proven
Pound and Rebka measured it up a 22.5-meter tower at Harvard in 1959. Gravity Probe A's rocket-borne clock (1976) and optical clocks 33 cm apart (2010) confirmed it.
Open questions and mysteries
Near a black hole's edge, time effectively freezes for outside observers; what this means for information falling in is unresolved.
▸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.
▸Event horizon: the point of no return freezes time (seen from outside) through Time dilation: moving clocks run slowProven
The relationship
Gravity slows clocks: the deeper in a gravity well, the slower time runs. Seen from far away, a clock falling toward the horizon ticks slower and slower, and its light grows redder and dimmer, appearing to freeze at the edge. The falling clock itself notices nothing odd and crosses the horizon in a short, finite time by its own count.
How the link was found
Einstein predicted gravitational time slowing in 1907–1915; the extreme horizon behavior follows from Schwarzschild's 1916 solution and was clarified by Oppenheimer and Snyder in 1939.
Why it's proven
Gravitational time slowing was measured by Pound and Rebka (1959), is corrected for daily in GPS, and was seen in light from the star S2 swinging close to the Milky Way's black hole (2018).
Open questions and mysteries
How the outside "frozen" view and the infaller's smooth view are reconciled quantum mechanically is part of the information debate.