Glossary

Time dilation

Moving (or gravitationally lower) clocks tick more slowly than stationary ones.

Image: Michael Schmid / DB (Wikimedia Commons). CC BY-SA 3.0, via Wikimedia Commons (Source)

Time dilation is the slowing of time for a clock moving relative to an observer, by the Lorentz factor γ = 1/√(1−v²/c²). At everyday speeds it is negligible; near the speed of light it becomes dramatic — the root of the famous twin paradox. General relativity adds a second kind: clocks deeper in a gravitational field also tick more slowly.

Both are routinely confirmed: fast-moving particles live longer, atomic clocks flown around the world in 1971 (Hafele–Keating) disagreed by the predicted amounts, and GPS has to allow for both every day — taken together, the satellite clocks run fast by about 38 microseconds a day.

Try it yourself

The light-clock Send a clock made of light on a journey and watch it tick more slowly. On the page: Special Relativity tried Your own twin journey Fly to a star at your real age – and work out how old you are when you come back. On the page: Special Relativity tried Cycling through slow light Ride through a town where light travels at 30 km/h – and watch houses shrink and clocks drift apart. On the page: slow-light tried GPS without the relativity correction What happens if you leave Einstein out of navigation? Worked out day by day. On the page: Death, Brain & Legacy tried Knowledge quiz: life & physics Questions on Einstein’s life and physics – with an explanation and a source for every answer. On the page: Learn
Source: Wikipedia: Time dilation — Read the full article on Wikipedia.

← All glossary terms