Physics problem

Jack and Jill are the same age. Jack leaves earth on a spaceship at 0.5c. Jill waits one year then sends a radio message (message goes at speed of light) to Jack telling him to return. Jack upon receiving the message returns immediately to earth. How much younger is Jack than Jill?

>>5
I believe in it only if light is a real limit.  If people find a way to accelerate past the speed of light (impossible now, cause you'd need a negative mass but I dunno) then I don't think they will be travelling back in time.