1. The problem statement, all variables and given/known data Consider two twins, Joe and Ed. Joe goes off in a straight line traveling at speed of 24/25 c for 7 years as measured on his clock, then reverses and returns at half the speed. Ed remains at home. 2. Relevant equations What is the age difference between twins after return? 3. The attempt at a solution Joe is moving relative to stationary Ed, so time passes slower for Joe Ed measures time: 7/sqrt(1-(v/c)^2) = 25, where v = 24/25 c For backward trip: Since the speed is halfed, the time it takes for Joe to return is doubled (in his frame of reference) 7*2=14 years. Now the speed is halfed, Ed is again stationary relative to Joe Ed measures time: 14/sqrt(1-(v/2c)^2) = 16 years So It takes for Joe total of 7+14=21 years and for Ed: 25+16 = 41 years. What is the problem here?