Time dilation explained

Time does not pass at the same rate for everyone. A clock that moves fast runs slower, and a clock higher above the Earth, where gravity is weaker, runs faster. The effects are tiny in daily life but real and measured: GPS satellites would drift about 38 microseconds a day without a correction.

Two kinds of time dilation

Speed (special relativity). Einstein showed in 1905 that the speed of light is the same for everyone. The cost is that a moving clock, seen from where you stand, ticks slower. The slowdown is set by the Lorentz factor, γ = 1 / √(1 − v²/c²). At everyday speeds γ is almost exactly 1; at 87% of light speed it is 2, so one hour on the ship is two hours at home.

Gravity (general relativity). In 1915 Einstein added that clocks deeper in a gravity well run slower. A clock on a mountain gains on a clock at sea level, and a clock on a satellite gains more. Labs have measured the difference with optical atomic clocks over a height change of less than a meter.

Proof: clocks on planes and satellites

• Airliners, 1971. Joseph Hafele and Richard Keating flew caesium atomic clocks around the world on ordinary flights. Compared with clocks left on the ground, the eastbound clocks lost about 59 nanoseconds and the westbound clocks gained about 273, close to what relativity predicted.
• GPS. The satellites move at about 3.9 km/s, which slows their clocks by about 7 microseconds a day, and orbit 20,200 km up, which speeds them up by about 45. The net 38 microseconds a day is corrected in the satellites, otherwise positions would drift by roughly 10 km a day. See how GPS uses time.
• Muons. These particles are made high in the atmosphere and live only about 2.2 microseconds, far too short to reach the ground at their speed. Many do, because time runs slower for them.

Does it affect people?

Barely, but yes. Astronauts on the International Space Station age about 25 microseconds less per day than people on the ground, because their speed matters more than their height. Over a year that adds up to less than a hundredth of a second. People living on a mountaintop age very slightly faster than people at sea level, by billionths of a second a year.

More on the nature of time: what is time?, how a second is defined and how NASA keeps time.

Questions people ask

What is time dilation in simple terms?

Time passes at different rates for clocks that move differently or sit at different heights. A fast-moving clock runs slow, and a clock higher above the Earth runs fast, compared with a clock on the ground.

Is time dilation real?

Yes. It has been measured with atomic clocks on airliners, satellites and in laboratories, and GPS has to correct for it every day to stay accurate.

How much does GPS time dilation add up to?

The satellite clocks run about 38 microseconds a day fast overall: about 45 microseconds faster because of weaker gravity and about 7 slower because of their speed.

Do astronauts age slower?

Slightly. On the International Space Station, astronauts age about 25 microseconds less per day than people on Earth, which adds up to less than a hundredth of a second a year.

Could you travel into the future with time dilation?

In principle, yes: at 99% of the speed of light, one year on board would be about seven years on Earth. No spacecraft can go anywhere near that fast; the fastest so far reach well under 1% of light speed.