Moving clocks run slower. That is time dilation, a prediction of special relativity. The faster an object moves, the slower its time passes relative to a stationary observer. At 87 percent of the speed of light, time slows by half. At 99.99 percent, time slows by a factor of 70. The effect is not an illusion. It is real. A clock on a fast-moving spacecraft would tick slower than a clock on Earth. An astronaut on that spacecraft would age more slowly than a twin left behind. That is the twin paradox, a famous thought experiment in relativity.

Time dilation also happens in gravitational fields. Clocks run slower near a massive object. That is gravitational time dilation, predicted by general relativity. It has been confirmed by experiments. Atomic clocks on airplanes run slightly slower than clocks on the ground. GPS satellites must correct for both special and general relativistic time dilation. Without those corrections, GPS would be off by kilometers per day. Time dilation is not just a curiosity. It is a real effect that must be accounted for in any precise timing system. It is one of the most counterintuitive predictions of relativity, and one of the most thoroughly confirmed.

Time dilation facts

• Special relativity. Moving clocks run slower.
• General relativity. Clocks run slower near mass.
• Twin paradox. Traveling twin ages less.
• GPS. Must correct for time dilation.
• Confirmed by. Atomic clocks, particle accelerators.

Time dilation is not the same as time travel. Time dilation slows time for a moving object. Time travel involves moving between different points in time.