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Re: non-inertial and inertial frames



On Mon, 29 Apr 1996, Paul Camp wrote:
...
This is one of the more popular misunderstandings of general
relativity, that it implies gravity is a fictional force loosely akin
to the centrifugal force (he said with trepidation). It is an easy
idea to acquire since the GR notion of gravity is so strange but it
is not quite correct. The gravitational force is perfectly real BUT
it is nonlocal. At a point, it is not measurable. Compare one point
to a point some distance away and then you can see the effects of
gravity.
...

Full agreement here. What was left, after Einstein pointed out that any
effect that could be transformed away by a mere change in kinematical
description (change in reference frame) was NOT gravitation, was true
gravitation: tidal effects = differences in force at different points.


... This also gives me a chance to clear up a slight oversimplification
that has cropped up in this thread and is complicating the discussion
-- the notion that a free fall frame is an inertial frame in
relativity. It is not. It is LOCALLY an inertial frame but it is the
mismatch between free fall frames for separated observers that
constitutes spacetime curvature and so gravity.
...

Again, full agreement. I have not emphasized LOCAL in the previous
discussion since that did not seem crucial to the discussion, but it
is of course true that, in curved spacetime, what is an inertial frame
at one point cannot serve also as an inertial frame at a distant point
of spacetime, simply because spacetime IS curved and not flat.

A. R. Marlow E-MAIL: marlow@beta.loyno.edu
Department of Physics PHONE: (504) 865 3647 (Office)
Loyola University 865 2245 (Home)
New Orleans, LA 70118 FAX: (504) 865 2453