In article ,
Paul Blay wrote:
... (It is not meaningful to speak of whether it is "really"
accelerated or not; all such statements must be made relative to a
specific observer.)

Er, didn't Einstein have something to say on that subject?
/Velocity/ is only meaningful relative to a particular observer, but
"is this accelerating / not accelerating" is a meaningful question.

Actually, what Einstein had to say on the subject is precisely the
opposite of what you're suggesting. In Newtonian mechanics, acceleration
is absolute: you can work in an accelerated frame of reference only by
introducing "fictitious forces" (like centrifugal force) to fudge the
details. But General Relativity removes the absoluteness of acceleration,
by telling you that those fictitious forces could be real gravitational
forces instead, and there is no way to tell the difference. Relative
acceleration between yourself and the mass of the universe produces the
same measurable effects, in GR, regardless of who you say is "really"
moving.

Dealing with
non-inertial frames of reference gets messy.

Quite.

And *that* is unchanged in General Relativity. :-)
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