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

[John Mallinckrodt <ajmallinckro@CSUPomona.Edu>]

On Fri, 11 Apr 1997, Bob Sciamanda wrote:

> Perhaps, as you hint above, the proper interpretation of #1 still needs
> clarification for me(?). 

Bob, 

Reading over my response of last night, it occurs to me that I might have 
more directly addressed the clarification of your "viewpoint #1" that, I 
believe shows it to be completely compatible with "viewpoint #2."  Again, 
the "viewpoints" were: 

> 1) GR abolishes the preferred position of the Newtonian class of inertial 
> frames (the "fixed stars" being one) by allowing an observer to transform 
> away his (Newtonian) acceleration by taking account of dynamic, covariant
> gravitational fields with identifiable sources (including the distant 
> stars). Here we are transforming away acceleration in favor of gravitation.
>
> 2) The only inertial frame worthy of the name is the local free fall frame. 
> The observer in that frame is neither accelerating nor gravitating.
> Departures from that frame are produced by accelerations (relative to 
> that frame).  An observer thus accelerating will see kinematical effects 
> which will be identical to the effects of a gravitational field, but 
> this field is fictitious and has no source beyond his acceleration 
> itself. Here we are transforming away gravitation in favor of acceleration.

As is true (I think) of all respectable modern theories of gravitation, 
the principle of equivalence (POE) is a fundamental guiding principle of 
GR and is built right in.  Thus, GR "abolishes ... the Newtonian ... 
inertial frames," as you say in the first sentence of #1, for precisely 
the reason stated in the first sentence of #2.  That is, it is not so 
much GR that does the abolishing as it is the POE.  And it is not *just* 
"Newtonian" acceleration (by which I assume you mean something like 
"acceleration wrt Newtonian inertial frames") that the POE (and, thus, 
GR) allows us to transform into gravitational fields; it is *all* 
accelerations.  The transformation is particularly simple when the 
acceleration is already reckoned with respect to the new (local) inertial 
frames identified by the POE rather than other frames which are 
themselves accelerating--e.g., the surface of the earth.

GR's real function is to go one step further than the POE and explain how 
the matter in the universe *determines* the local inertial frames.  Thus 
it does "connect" gravitational fields to the configuration of matter in 
the universe.  However, as I said last night, I think it is misleading to 
say that the matter is the "source" of the gravitational field even if it 
may be semantically excusable.

John
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