Re: POE summary (was Re: Work/Energy theorem?)

["Paul Camp" <pjcamp@coastal.edu>]

> >    You keep talking around  my question.  If you can "do physics" from the 
> > "accelerating frame" then you must identify gravitational SOURCES which are 
> > synchronized to the engineer's actions.  DO it!  
> > (It can be made to work - it all depends on how "wierd" you are willing to 
> > get!)  Let's have your model.
>
> This again sounds, to me, like an attempt to draw a distinction between
> the gravitational field that appears on the surface of the earth and the
> one that appears on the decelerating train that simply does not exist from
> either viewpoint #1 or viewpoint #2. 
> If you ask me
> (again) to identify the source of a gravitational field (time-dependent or
> not), I'll express my opinion (again) that it is acceleration with respect
> to an inertial frame.  This, notwithstanding the fact that inertial frames
> are, in turn, determined somehow (this is where GR comes in) by the matter
> in the universe.  

There is an important point about the principle of equivalence that 
is being missed in this discussion -- it only applies *locally*. You 
don't get to make global statements or make measurements of things 
that are not located at the same place as you. This includes sources 
of gravitational fields. They are perforce not where you are and so 
you don't get to talk about them in a local context.

The equivalence principle does not say that gravitational fields and 
uniform accelerations are the same thing. It does say that *locally* 
they cannot be distinguished. *Locally* the only thing you can 
measure is your acceleration and you cannot tell what the source of 
it is.

Yes, you can formally replace the uniform acceleration of a train 
with a uniform gravitational field *locally* but that does not mean 
that it *is* a uniform gravitational field, simply that it is 
equivalent to one. If you want to look for sources, you must do a 
nonlocal experiment and then you can tell the difference between the 
two.

Paul J. Camp                   "The Beauty of the Universe
Assistant Professor of Physics  consists not only of unity
Coastal Carolina University     in variety but also of 
Conway, SC   29528              variety in unity.
pjcamp@coastal.edu                    --Umberto Eco
pjcamp@postoffice.worldnet.att.net    The Name of the Rose
(803)349-2227                         
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