Originally Posted By: Bill S.
claim to occupy a F of R. Increasingly, the F of R looks like the "present", the closer you try to look at it, the more elusive it becomes.


True. But for many purposes you can ignore internal motion. Even tho there's thermal motion of the atoms in a brick, their overall momentum adds up to the momentum of the whole thing.

Inertial frames of reference are just convenient for simplifying things like SR does. If we were all really smart we could do away with them and treat everything and it's parts as accelerating in whatever way they are.