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## Main Question or Discussion Point

For the purposes of distance measurement, time measurement of signals, and so on, can we approximate the distance across the Milky Way as an extended inertial reference frame? Or is that way out of bounds? (I mean obviously there are massive bodies that already prohibit that, I suspect, but say I wanted to use a light signal to measure the distance between me and a star across the galaxy, would I be justified in just assuming SR?)

If not, about how far away does the approximation of an inertial reference frame begin to wane, or is it just the mass/energy/pressure that's the problem, screwing up the metric? That is, if mass were negligible would an infinite inertial reference frame be a reasonable approximation?

Thanks.

If not, about how far away does the approximation of an inertial reference frame begin to wane, or is it just the mass/energy/pressure that's the problem, screwing up the metric? That is, if mass were negligible would an infinite inertial reference frame be a reasonable approximation?

Thanks.