Hi, I'm reading a book explaining relativity. I previously understood the subject in layman's terms but am now confused. The author has introduced relative motion in an invariant universe with the concept that motion through space is only relative to the observer. Given this, I'm now struggling with the traditional 'man travels from Earth on space ship and time slows for him relative to the observer on Earth' effect. If motion is relative then why isn't the observer on earth equally traveling away from the ship (ie ship is stationery relative to earth in it's reference)? Why doesn't the man on the ship observe time passing more slowly on earth? I realise that this is a paradox, but I can't figure it out. Confused.