Originally posted by dehammer:
then explain why the person going on a high c fraction ship would return younger than his brother. its not an illusion. time does slow down for him, which means that from his perspective the universe speeds up. when he slows down at the end, the universe appears to slow down to normal, but it is really his time that is changing. [/QUOTE]
We have both posted. Mine is just before yours (see above).
Time only actually slows down within his reference frame when the spaceship accelerates and decelerates. In order to compare the ages of the two, the twin who left has to return to earth so that their ages can be compared relative to a single reference frame. The twin who has left had to accelerate to get away and to decelerate in order to return. Therefore, he will be the younger one.
If it were possible to leave with near light speed without acceleration, and the twins communicate by radio all the way, then both will conclude that they are ageing faster than the other twin. It is strange that this should be so; but, as mentioned above, both are experiencing an "illusion" within their respective reference frames of what is happening in their brother's reference frame. If, however, each twin corrects for the time he observes within the other twin's reference frame, as we are doing for the GPS satellites, they will find that they are actually ageing at exactly the same rate.
