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?

>>11
Not negative mass. Imaginary mass.

Yes, you read that right. Complex imaginary mass. FTL travel is non-trivial, to say the least.