Paradox

A spaceship leaves the earth and moves for a period of time along a straight line with constant speed when it reverses in insignificant time its direction and returns back to earth moving long the same orbit with the same speed. Compare the time of flight for the astronaut in the spaceship (LCF \(\Sigma \) ) and for the observer on the earth (LCF \(\Sigma ^{\prime }\) ). (Tween paradox).