1. When they meet again on Earth, who is really younger? Read the following paragraph to answer this question.

After Bob has reached the distant galaxy, Bob is coming back, there is a change of reference frame. This change of reference frame is the key to understand the paradox. Since Tim has always stayed in same inertial frame in this first version (Tim stayed at rest), we must keep Tim in the same inertial frame in the second version too. So Tim must keep moving in the same direction with 3/5c speed. Now, in order for Bob and Tim meet again,. Bob now must change speed and catch up on Tim.He must zoom off with 15/17c speed.So on this trip, Bob traveling much faster than Tim. Bob would again age much less than Tim. As it turns out, he needs 4 years to catch up on Tim if we assume Tim aged 10 years.So Bob again aged 8 years.