Time dilation is a phenomenon in physics that describes the slowing down of time for an object in motion relative to an observer. It is one of the two main predictions of special relativity, the other being mass–energy equivalence. Time dilation is caused by the increase in the object's mass as it approaches the speed of light. The effect is very small for speeds that are much slower than the speed of light, but it becomes increasingly significant as the object's speed approaches the speed of light.