I may be misunderstanding what you mean. But the reference frame I'm using is time. Say an event happens in time A in body Z, and body Y only exists inside body Z at time B. If we were to move body Y into time A, which goes backwards in time? Z or Y? Time doesn't rewind for body Y, it only does so for body Z. The age of Y isn't changing, only the age of Z.... therefore Z is the thing traveling in time. A - Past B - Present Z - Universe Y - Time Traveler In my eyes, your example is about velocity. Can't have a negative velocity (negative being backwards in a sense).