Einstein showed that, to an observer in a particular frame of reference, time appears to slow down for objects traveling near the speed of light, relative to the frame of reference. The effect is known as time dilation.

Einstein also showed that, to an observer in a particular frame of reference, time appears to slow down for objects in high gravitational fields, relative to the gravitational field of the reference frame. This is another example of time dilation.

In 1971, two very accurate caesium atomic clocks were flown east and west around the Earth in commercial airliners, to compare the elapsed time against that of a clock that remained at the US Naval Observatory. Two opposite effects came into play. The flying clocks were expected to age more quickly (show a larger elapsed time) than the reference clock, since they were in a higher (weaker) gravitational potential for most of the trip. But also, contrastingly, the moving clocks were expected to age more slowly because of the speed of their travel. The gravitational effect was the larger, and the clocks suffered a net gain in elapsed time. To within experimental error, the net gain was consistent with the difference between the predicted gravitational gain and the predicted velocity time loss.