What effect causes time to pass slower for a moving observer?

The effect that causes time to pass slower for a moving observer is called time dilation. In special relativity, a clock moving relative to an observer is measured to tick more slowly than a clock at rest in that observer's frame.

The effect follows from Einstein's postulate that the speed of light is the same for all inertial observers. A moving light clock traces a longer diagonal path between its mirrors, but light must still travel at the same speed. The interval between ticks therefore appears longer to the outside observer, meaning the moving clock runs slow.

Time dilation becomes significant only when relative speeds approach the speed of light. It has been measured using precise atomic clocks and must be included in satellite-navigation systems such as GPS. Fast-moving unstable particles also appear to survive longer in laboratory measurements because their internal processes are time-dilated.

Time dilation is often confused with the Doppler effect. Doppler shifts change the observed frequency of arriving waves, while time dilation concerns the elapsed time recorded by clocks. Relativity also predicts gravitational time dilation: clocks deeper in a gravitational field run more slowly, even when the clocks are not moving relative to one another.