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If I take a simple example using SR to start and imagine two objects A and B, which are at rest wrt each other and are separated by some distance. Anyone moving relative to these two objects would measure a different distance to the proper distance. However if they needed to use their respective measurements of distance in order to work out say the time to take a rocked ship to move between A and B or to calculate the length of an object that fits exactly between A and B, then I assume they would need to do some sort of transformation in order to make accurate predictions for the rocket ship or make the right size of object to fit exactly between A and B.

So using the examples above, my questions are:

1) Is SR, how would the proper distance be measured if A was moving wrt B?

2) How are proper distances worked out in GR, as I am told that distances are coordinate dependent in GR. Although I am not sure what that even means.

Thanks for any help