Definition
Time dilation is the effect where time itself flows more slowly, depending on how an observer is moving or how strong gravity is. The closer you get to the speed of light, or the deeper you sit in the gravity well of a massive object, the slower your time runs compared to someone elsewhere.
What happens because the speed of light never changes
Imagine you're on a high-speed train and you shoot a laser beam straight up at the ceiling.
To you, inside the train, the light goes straight up and straight back down. But to a person standing on the platform watching the train zoom past, the light travels along a slanted, diagonal path in the direction the train is moving.
Here's where the universe's one unbreakable rule comes in. Light always travels at the same speed (about 186,000 miles, or 300,000 kilometers, per second), no matter who is watching. For the person on the platform, the light has to cover a longer diagonal path, but it can't speed up. So the only way it works out is if time itself stretches out to make up for the extra distance.
The result: from the platform, the clock on the fast-moving train ticks with longer gaps between ticks. Time on the train runs slower than time on the platform.
Strong gravity slows time down, too
You've probably seen what happens when you set a heavy bowling ball on a stretched rubber sheet. The sheet sinks down in a deep dip.
Einstein explained that anything with mass bends not just the space around it, but time as well. Around an extremely massive object, spacetime is like a net pulled tight and bent out of shape.
The closer you are to a massive object, the deeper in its gravity well you sit. There, not only light and matter but the flow of time itself gets dragged down and slows. In the movie *Interstellar*, the hero visits a planet near a black hole and stays only a few hours, but more than 20 years (about 23) pass back on Earth. That's time dilation caused by enormous gravity.
The same thing happens on a tiny scale here at home. Down on Earth's surface, deeper in Earth's gravity well, time ticks very slightly slower than it does on a high mountaintop or in space.
To be precise: your own time always feels normal
It's easy to assume that if you boarded a super-fast spaceship, your movements and your watch hands would look like slow motion and you'd feel stuck in molasses.
But the person on the spaceship notices nothing at all. Their heartbeat runs at its usual pace, and their watch hand still moves one tick per second. Time dilation only shows up as a relative difference when two different observers compare notes.
GPS satellites orbiting over our heads are a real example. They fly at about 8,700 miles per hour (14,000 km/h), which slows their clocks. But they're also farther from Earth, where gravity is weaker, which speeds their clocks up. Add the two effects together and a satellite clock gains about 38 microseconds per day. So engineers set the clocks to run slightly slow before launch, and keep correcting any leftover error. Without these relativity corrections, your phone's navigation would drift off by more than 6 miles (10 km) in a single day.
Common misconceptions
If you fly close to the speed of light, you'll feel your own body and watch slow down like slow motion.
In your own frame of reference, time flows perfectly normally. Time dilation only appears to another observer who is moving relative to you. Interestingly, for two observers passing each other at a steady speed (special relativity), it works both ways: an astronaut looking at a clock on Earth would see that clock running slow. But if one of them turns around and comes back (the twin paradox), or if gravity differs between them, it becomes clear which clock really ticked less.
Where you meet it
In *Interstellar*, one hour on a planet near a giant black hole equals seven years back on Earth.
To make your phone's navigation pinpoint your location, GPS satellite clocks are adjusted by exactly the time difference that relativity predicts.
In one sentence
The faster you move, or the closer you are to a massive object, the slower your time runs relative to others. This is a real law of the universe, and GPS satellites confirm it every single day.