Getting old through quick travel

[Moonshane]

Hi all

I am trying to get my head around the 'time travel' aspects of close to light speed travel. As I understand it, travel for two objects is relative and if they are travelling relative to one another, this can be seen from the viewpoint of either side as traveling relative to the other one, even if in a layman's sense one of them is moving.

e.g. if I set off in a rocket at 99% of the speed of light and return to Earth, yo would normally say that I am moving at this speed relative to the Earth. However, could it also be said that I have been standing (floating) still and the Earth has been travelling relative to me? If this is wrong then read no further but could someone explain why?

Assuming the above is correct then how does spacetime 'know' who gets old?

[George Jones]

The situation isn't completely symmetrical.

To come back to the Earth, you have fire a rocket in order to turn around. For example, suppose4 you are moving away from Earth at 99% the speed of light. You want to come back, so you fire  a rocket that supplies thrust towards the Earth, Your speed drops to 90%, then 80%, ..., then 10%, and then zero. You keep firing the rocket in the same direction, and you start picking up speed towards, the Earth, first 10%, then 20%, ..., then 90%. When your speed reaches 99% towards the Earth, you stop firing the rocket, and you proceed back to Earth at 99% the speed of light.

During the time that the rocket fires, you experience "artificial gravity" due to the rocket's thrust. The Earth does not experience this artificial gravity, so it is possible to tell that it was you that turned around and came back, not the Earth. You have a different experience than the folks on the Earth.

[Zakalwe]

Assuming the above is correct then how does spacetime 'know' who gets old?

Spacetime doesn't need to "know". As you travel at significant percentages of the speed of light the flow of time in your frame of reference is slower than the flow of time in the frame of reference of someone not travelling at that speed. In order to travel you would, as George says, have to experience an acceleration which only exists in your frame of reference. So the individual frames of reference are not the same.