Re: Equivalence Principle

["A. R. Marlow" <marlow@loyno.edu>]

On Thu, 19 Feb 1998, John Mallinckrodt wrote:
> I recall reading and being disturbed by this paper when it first 
> appeared.  Despite the author's very specific conclusions, I still 
> can't understand on physical grounds how one could possibly 
> distinguish between a uniform gravitational field and uniform 
> acceleration.

I don't have the paper in front of me at the moment, but I recall the
author anwered that in this way.  If we lived in flat spacetime and
somehow had a uniform gravitational field, you would not be able to
distinguish it from uniform acceleration.  However, in GR to produce a
uniform gravitational field you must contrive a special nonzero Riemann
curvature tensor  --  the author constructs such in the paper  --  and
that cannot be transformed away by any mere acceleration, i. e., you can
always distinguish it from acceleration.  In other words, in GR you *can*
have uniform gravitational fields (contrary to the claims made in some
texts), and they *can* be distinguished from accelerations.
Only in the context of special relativity (which, remember, Einstein was
working from at the time he was struggling to apply the "equivalence
principle") is it possible to claim indistinguishability.

  Doesn't it seem simply a matter of definition that 
> any observation that would contradict the hypothesis that one 
> moves with uniform acceleration through a flat space would equally 
> contradict the hypothesis that space is filled with a uniform 
> gravitational field?  

But how in flat space do you get a uniform gravitational field?  The paper
referred to is written completely in the context of GR where we know how
to produce gravitational fields of all types via the Riemann curvature
tensor.


A. R. Marlow                          E-MAIL:  marlow@loyno.edu
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