Confusion with proper time from different frame of references

[Stevey_R]

I'm having trouble getting my head around the idea of 'proper time'. I've been thinking of this situation and I can't seem to understand what exactly the proper time is.

Say we have a planet 'A' and a rocket B moving towards this planet. From the perspective of A, B is moving towards A. If an observer on A timed how long it takes for B to reach A then would this be the proper time?

If it is the proper time then that would assume B measures a longer time to reach A. But then if an observer on A looks at B's clock as it arrives then they would see a higher time. Surely that's not possible if B appears to slow down relative to A?

If we assign proper time as measured from the rocket, then we arrive at the same problem if we take B's frame of reference where planet A is approaching B.

Sorry if I'm being completely ignorant here but it's been bugging me for a while. Hope my explanation will suffice :)

 

[xxxx0xxxx]

Proper time is "time" as it is experienced by each observer.

A's proper time differs from B until they stop moving relative to each other.

As to who's time is shorter, that depends on who had to do the stopping relative to the other.

i.e. if B lands on A, then B underwent acceleration which is not relative, (of course this ignores the mechanics of A's rotational frame of reference), but the point is, B had to initiate dynamic action to enter A's reference frame. A on the other hand, takes no dynamic/kinematic action.

This breaks the symmetry between A and B, and B's clock is found to have run slower.

 

[tom.stoer]

It's rather simple: the proper time of an observer (regardless how he moves, with constant velocity or accelerated; and regardless of any other reference frame) is the time which he measures using a co-moving clock.