The effects of time dilation are used often in science fiction stories, dating back to at least the 1930's. 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.