We laymen know that high speed travel has an effect on the local time of the traveller. What exactly is it about the speed that does it though? I heard that it is not the actual high speed but the acceleration and deceleration that has the effect. That would mean that it is the change in speed rather than the speed its self. Is this true. On a related note. I have a question about 'movement' its self. Is there such a thing as movement? How can there be when the position of every thing is relative? All objects in space are on the move and there is no absolute still thing. Even the calculated center of the universe might be on the move. We don't know if the universe its self is still or moving (or even if that means anything)j. For example. If i fly counter rotation around the earth at the speed of the earths rotation it could be said that I am not moving. then if i flew around the sun like wise i would be still from the point of view of the solar system. Then if I flew counter around the galaxy, the same. Then i'd have to fly counter around the cluster of galaxys then also inward towards the center of the universe at the exact speed the universe is expanding. I know all this is actually impossible but what i'm trying to get at is, if i did all those perfectly could it be said that i'm perfectly still? But is there such a think as absolute movment or stillness seeing as everything is on the move? If there is no such absolute then how can anything be said to be moving and are we not only changing position relative to another object. And if this is so then how can there be an absolute effect of time on us at certain speeds. For example. If there were triplets and two of them flew away from the earth at half the speed of light for ten years then turned around and came back but one flew in the outward direction the universe is expanding and the other flew inward then when they arrived back on earth (i know it would have moved its self by then) then would the time difference between the two travellers and the brother they left on earth be identicle or different?