Time dilation is the phenomenon where two objects moving relative to each other (or even just a different intensity of gravitational field from each other) experience different rates of time flow.

### Relative Velocity Time Dilation

The time dilation seen due to relative velocity stems from special relativity. If two observers, Janet and Jim, are moving in opposite directions and as they pass by each other they note that the other person's watch is ticking slower than their own. If Judy were running alongside Janet at the same speed in the same direction, their watches would be ticking at the same rate, while Jim, going in the opposite direction, sees both of them having slower-ticking watches. Time seems to pass slower for the person being observed than for the observer.

### Gravitational Time Dilation

Time dilation due to being at different distances from a gravitational mass is described in the general theory of relativity. The closer you are to a gravitational mass, the slower your clock seems to be ticking to an observer farther from the mass. When a spaceship nears a black hole of extreme mass, observers see time slowing to a crawl for them.

These two forms of time dilation combine for a satellite orbiting a planet. On the one hand, their relative velocity to observers on the ground slows time for the satellite. But the farther distance from the planet means time goes faster on the satellite than on the surface of the planet. These effects may cancel each other, but also can mean a lower satellite has slower-running clocks relative to the surface while higher-orbiting satellites have clocks running faster relative to the surface.

### Time Dilation Examples

The effects of time dilation are used often in science fiction stories, dating back to at least the 1930s. One of the earliest and most well-known thought experiments to feature time dilation is the famous Twin Paradox, which demonstrates the curious effects of time dilation at its most extreme.

Time dilation becomes most apparent when one of the objects is moving at nearly the speed of light, but it manifests at even slower speeds. Here are just a few ways we know time dilation actually takes place:

- Clocks in airplanes click at different rates from clocks on the ground.
- Putting a clock on a mountain (thus elevating it, but keeping it stationary relative to the ground-based clock) results in slightly different rates.
- The Global Positioning System (GPS) has to adjust for time dilation. Ground-based devices have to communicate with satellites. To work, they have to be programmed to compensate for the time differences based on their speeds and gravitational influences.
- Certain unstable particles exist for a very brief period of time before decaying, but scientists can observe them as lasting longer because they are moving so fast that time dilation means the time that the particles "experience" before decaying is different from the time experienced in the at-rest laboratory that is doing the observations.
- In 2014, a research team announced the most precise experimental confirmation of this effect yet devised, as described in a
*Scientific American*article. They used a particle accelerator to confirm that time moves slower for a moving clock than for a stationary one.