A phenomenon resulting from an experiment involving two identical clocks, initially together and showing the same time, one of which is carried off on a round-trip journey. Upon returning, the clock which moved will have lost time relative to the motionless clock by an amount prescribed by special relativity. Such effects have been observed using a pair of atomic clocks, one of which is flown around the world. However, a principle of relativity which claims that all observers are equal appears violated. The apparent paradox is resolved within special relativity by noticing that any clock undergoing a round-trip journey must be accelerated at some stage. The clock paradox was first discussed by Einstein (1905). It is sometimes expressed in terms of identical twin brothers, one of whom undertakes a journey and finds himself younger than his brother upon returning (the twin paradox).