I was thinking about "space time" today for short story that I am considering writing and the following question popped into my head. I am assuming that the following points are true:
1) All motion is relative - and therefore the lack of motion is also a relative concept. (In a universe with only one object motion becomes meaningless.) If two bodies are considered (only in relation to one another) it makes little sense to say that one is moving and the other is at rest. It is all a question of one's point of reference.
2) The more rapidly one body moves in relation to another the greater the temporal displacement between the two (this may not be the correct terminology). For example - If the space shuttle left earth and accelerated to near-light-speed for one week and then returned to the earth, the astronauts on the shuttle would experience the passage of one week's time while those on earth would experience a longer period of time.
Can you see where this is going?
The Question: When considering two bodies in motion, relative to each other, how can we know which object is moving more rapidly? (And thus) How can we know which object will experience the shorter duration of time?
For example: If I am on the space shuttle traveling away from the earth at near-light-speed, I could say, in the strictest relative sense that the earth is moving away from me. Why should I then experience the relatively short period of time?