1905 – today

Does time really slow down? Einstein, relativity and time dilation

Yes. Einstein's relativity shows that moving clocks run slow (special relativity, 1905) and clocks in stronger gravity run slow (general relativity, 1915). The effects are tiny in daily life but measurable: GPS satellite clocks gain about 38 microseconds a day relative to the ground, and the system corrects for it.

Special relativity	1905, moving clocks tick slower
General relativity	1915, clocks deeper in gravity tick slower
GPS clock, speed effect	about −7 microseconds per day
GPS clock, gravity effect	about +45 microseconds per day
Net GPS correction	about +38 microseconds per day
Hafele–Keating experiment	1971, atomic clocks flown around the world on airliners
Smallest height difference measured	about 1 mm, JILA optical clock, 2022

For Isaac Newton, time was absolute: the same everywhere, flowing at one rate for everyone. Albert Einstein showed that it isn't. How fast time passes depends on how fast you move and how strong the gravity is where you are.

Special relativity: speed slows clocks

In 1905 Einstein started from two ideas: the laws of physics are the same for everyone moving at a steady speed, and light always travels at the same speed for every observer. Put those together and time can't be universal. A clock moving relative to you ticks more slowly, as seen by you, than one at rest beside you.

A simple way to see it is the "light clock": a pulse of light bouncing between two mirrors. If the clock moves sideways, the light has to travel a longer, diagonal path between ticks. Since light can't go faster to make up for it, each tick takes longer. The effect only becomes large near the speed of light, but it is real. Muons created by cosmic rays high in the atmosphere live long enough to reach the ground only because their internal clocks run slow at nearly light speed.

General relativity: gravity slows clocks

In 1915 Einstein extended the theory to gravity, describing it as the curving of space and time together, spacetime, by mass. One consequence: clocks lower down in a gravitational field run slower than clocks higher up. A clock on a mountain gains on a clock at sea level, a tiny bit each day.

Proving it

Pound–Rebka (1959): gamma rays sent up a 22.5-metre tower at Harvard showed the gravitational frequency shift.
Hafele–Keating (1971): four cesium atomic clocks flown around the world on airliners, eastward and westward, came back out of step with clocks left at the US Naval Observatory by the amounts relativity predicted.
Optical clocks (2010s–2020s): today's best clocks detect the gravitational difference between heights a few centimetres apart, and in 2022 across about a millimetre.

Why your phone's GPS depends on it

GPS satellites orbit about 20,200 km up, moving at about 3.9 km per second. Their speed makes their clocks run slow by about 7 microseconds a day, while the weaker gravity up there makes them run fast by about 45 microseconds a day. The net effect is about 38 microseconds a day fast. GPS works by timing radio signals, and light travels about 300 metres in a microsecond, so uncorrected position errors would build up by roughly 10 km a day. The satellite clocks are deliberately set to tick slightly slow before launch to cancel it out.

The twin paradox

If one twin flies to a distant star at near light speed and returns, she will be younger than the twin who stayed home. It's not a trick of perception: less time really passed for her. The asymmetry comes from her turning around, which means she wasn't in steady motion the whole time.

Questions people ask

Does time really slow down when you go fast?

Yes, as measured by anyone you're moving relative to. It's been confirmed with atomic clocks on aircraft and with fast-moving particles.

How much does GPS correct for relativity?

About 38 microseconds per day. Without that, GPS positions would drift by roughly 10 km per day.

Do astronauts age slower?

Very slightly. On the International Space Station the speed effect wins, so astronauts age about 0.005 seconds less per six months than people on Earth.

Sources

Last updated October 18, 2026. Found an error? Tell us.