The apparent shortening of time relative to time measured in a rest frame for observers moving at constant speed v in an inertial reference frame. Times measured in the rest frame are then measured as

in the moving frame, where is relativistic gamma and c is the speed of light. Time dilation is a counterintuitive consequence of special relativity and has the disturbing consequence that, for example, an astronaut traveling in a spaceship at very high speeds relative to the Earth would age less quickly than would friends and family. As a result, upon returning home, such an astronaut could find colleagues had grown old and died while he remains young.

Lorentz Contraction, Mass Increase, Special Relativity