Two identical twins, Romulus and Remus, are the same age. Remus is abducted by space aliens and flown to a nearby star and back at almost the speed of light, while Romulus remained stationary. When the twins meet again, Romulus is significantly older. Briefly explain why we may not consider that the Earth moved and Remus remained stationary (leading to the inconsistent conclusion that Remus is older). Really have no attempt at a solution as I can't get my head round this at all.