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Re: Equivalence Principle



Hi John Mallinckrodt-
In response to:
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Despite David Bowman's gracious and detailed response to my and Joel's
pleas for enlightenment concerning Desloge's demonstration (AJP, December
1989) that one can distinguish between uniform gravity and uniform
acceleration, I still hunger for a more conceptually satisfying
explanation.

What I want to know is, what specific observations could I make--possibly
in conjunction with others with whom I can communicate and in terms of
measurements made with local clocks and measuring rods--that would both 1)
rule out the possibility that I am uniformly accelerating with respect to
a gravitation free universe and 2) NOT rule out the possibility that the
universe is filled with a uniform gravitational field, or vice-versa.
Unless there is a reasonable answer to this question, I'm afraid I'll
simply have to go on conducting my life as though uniform gravity is
strictly equivalent to acceleration.
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I had a chance to read Desloge's article on Friday. I hadn't
seen it before, and I have yet to thoroughly digest it. A couple of
comments, however.
Desloge tells you how to distinguish between his "uniform gravitational
field" and uniform acceleration. It requires experiments (for fields giving
acceleration 10 m/s^2) over a distance of 1 light year.
Desloge does his analysis in terms of a particular choice of metric.
Choosing a metric in GR is like choosing a gauge in electrodynamics; there is
no physical content to the choice.
I give you the following quote from Misner, Thorne and Wheeler:

"I know how to measure the electromagnetic field using test charges;
what is the analogous procedure for measuring the gravitational
field?" This question has, at the same time, many answers and none.
(see pp 399 ff).

It is possible, in this light, that Desloge's demonstration is in fact
an elaborate pun on the words "constant gravitational field." There appear
to have been later papers, and at least one is readily available. I'll report
back if I learn anything new.
Regards,
Jack