Also known as the twins paradox or two-clock paradox. An apparent paradox of special and general relativity theory. The time dilation effect in the special theory of relativity is that clocks at rest in different inertial frames observe each other to run slow. The alleged consequence is that if we consider a universe of two adjacent clocks at rest with respect to each other, which are then separated and subsequently reunited, after this event each will be slow relative to the other (i.e. each will show less time to have elapsed than the other), which is a contradiction. The argument is that in a universe with no independent objects providing a frame of reference, there is no fact of the matter which clock should be considered to be the one that has moved. The flaw in the reasoning is that if a clock goes on a journey then there is no inertial frame in which it is at rest throughout the interval: it needs to accelerate outwards and accelerate back. Whichever clock does this will indeed show that less time has elapsed when reunited with another clock with no past history of acceleration, for this is a physical effect consequent upon accelerating with respect to space. However, the clock paradox does show that this real acceleration does not supervene upon the symmetrical properties of the two clocks, in the universe of which they are the only constituent masses.

Asymmetric ageing as a consequence of changes in velocity is a well-confirmed empirical consequence of general relativity theory. In general relativity theory, space involves a dynamic field created by gravitational mass, and movement relative to it is a physical state with its own consequences.