Suppose that one of your friends is four years older than you. She wants to travel for two years to space station, as measured by her, such that when she arrives there she will be the same age as you are back on earth. (a) What is the relevant proper time in this problem? (b) What is the relevant non-proper time? (c) At what speed, relative to earth, should your friend move during her trip? Answer: (c) v = (22/3)c