# Point of reference in relative time

## Main Question or Discussion Point

Hi everyone. I rediscovered a question I caught in High-school that I never encountered a answer to.

Given a spaceship going back and forth into space, during it's travel it accelerate close to c relative to earth. At an appropriate distance it breaks and returns in a similar fashion. My understanding of what would be experienced is that the equipment and/or passengers on the ship would have their net-sum of elapsed time greatly reduced compared to earth-time.

The issue arrives as I can't, from the general theory discern the ship moving from everything else in the system moving. In short, does the acceleration the ship experiences cause the relative shortage of time or is there some other indicator to whom is subject to the relative speed.

in other words, were in lies the difference between moving the ship and moving the system but not the ship.

Thanks for any insights

//Emil

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ghwellsjr
Gold Member
Hi everyone. I rediscovered a question I caught in High-school that I never encountered a answer to.

Given a spaceship going back and forth into space, during it's travel it accelerate close to c relative to earth. At an appropriate distance it breaks and returns in a similar fashion. My understanding of what would be experienced is that the equipment and/or passengers on the ship would have their net-sum of elapsed time greatly reduced compared to earth-time.

The issue arrives as I can't, from the general theory discern the ship moving from everything else in the system moving. In short, does the acceleration the ship experiences cause the relative shortage of time or is there some other indicator to whom is subject to the relative speed.

in other words, were in lies the difference between moving the ship and moving the system but not the ship.

Thanks for any insights

//Emil
I think you have answered your own question with the phrase I put in bold.

Then my question is poorly posed.

There is no great difference between the ships perspective. As, The earth is travelling away from me at high speed. Earths perspective, the ship is traveling away from me at high speed.

The difference I'm aware of is the experienced acceleration, ergo the force and and thus increased kinetic energy (but kinetic energy should be relative the observer, it is however positive in both negative and positive vector direction).

The question is thus, What dictates who is the stable, (relative)-non-moving observer when two objects perceive motion relative to themselves.

ghwellsjr
Gold Member
Then my question is poorly posed.

There is no great difference between the ships perspective. As, The earth is travelling away from me at high speed. Earths perspective, the ship is traveling away from me at high speed.

The difference I'm aware of is the experienced acceleration, ergo the force and and thus increased kinetic energy (but kinetic energy should be relative the observer, it is however positive in both negative and positive vector direction).

The question is thus, What dictates who is the stable, (relative)-non-moving observer when two objects perceive motion relative to themselves.
In this case, they both remain inertial so there's no preference. In your first post,
only the earth remained inertial so it has preference.

In terms of coordinates, each observer is the same. Each can set up a coordinate system where the other moves away and comes back. The difference for relativity is something empirical that each can measure - inertia or acceleration. While each can decide that it is the other which is accelerating in the observer's coordinate system, the experienced reality will be different. One (or both) may experience acceleration, but it is not possible for both to experience being at rest, experiencing no acceleration. This empirical thing called inertia goes beyond the coordinate systems we set up, and it is this which determines who experiences less time overall to have passed when the observers meet up.

Chestermiller
Mentor
It's the acceleration that makes the difference. But it doesn't do it all. The acceleration essentially enables the spaceship to travel in a new direction through 4d spacetime which is analogous to a shortcut. Of course, this statement is very qualitative. The exact math is contained in the Lorentz Transformation.

Dale
Mentor
The question is thus, What dictates who is the stable, (relative)-non-moving observer when two objects perceive motion relative to themselves.
The inertial one.

The question is thus, What dictates who is the stable, (relative)-non-moving observer when two objects perceive motion relative to themselves.
Who ever got there first. ,

nah Dalespam is right.