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

[John Mallinckrodt <ajmallinckro@CSUPomona.Edu>]

On Fri, 11 Apr 1997, Gentle Bob Sciamanda wrote:

> View #1 is the position toward which I was "goading" you, early on in
> these 
> exchanges when I tried to insist that you identify the *sources* of the 
> gravitational fields which appeared to the train passenger whenever the
> engineer throttled up or braked.  
> If the two views are indeed equivalent, why did you insist adamantly 
> on viewpoint #2 and seemingly disallow the (expected) answer: "the
> distant stars . . ."
> Perhaps, as you hint above, the proper interpretation of #1 still needs
> clarification for me(?). 

Hmm.  Maybe I misinterpreted your position early on, but it did not sound 
to me like viewpoint #1.  For instance, at one point, in the discussion 
of the "observer on the decelerating train" problem, you wrote,

> I don't think that your train rider is making "perfect sense" out of this.  
> You have merely given him an alternative calculational algorithm, but he 
> is at a loss to explain why it works, because these gravitational fields 
> appear as a magic result of the engineer's actions!  I think he can make 
> better physical (and equivalent calculational) sense by acknowledging his 
> acceleration!

I read this passage then, and I still read it now as indicating that you 
wanted somehow to single out his acceleration with respect to the surface 
of the earth as being of a different character--more of a *genuine* 
acceleration and less something that could be replaced by a gravitational 
field--than the acceleration of the earth's surface with respect to the 
local inertial frames.  And despite my (gentle?) initial attempt to 
explain why the time-dependence of that acceleration was a red herring, 
you seemed still to be focusing on it as a critical issue, as you had in 
your first message and as you continued to do in a later message when you 
wrote,

>    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.  As I tried to explain in my response
to that message, I understand that it is not customary and that it
*sounds* a little weird to replace the acceleration of the train with an
equivalent "horizontal" gravitational field.  That is probably why this
thread has lasted as long as it has.  But I took you to be implying that
one would have to actually *be* weird (in the sense of straining physical
principles) to do so.

I'm no expert in GR.  That's why I've tried to talk only about issues that
fall under the purview of the principle of equivalence which is far
simpler, less controversial, highly enlightening, and yet, I think, not
particularly well understood by a lot of physicists.  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.  

Although I recognize that it is partially an issue of semantics, I simply
think it is a misleading to skip that intermediate step and say that the
source of local gravitational fields is the matter in the universe.  This
is what Newton said.  He meant it in its most literal sense.  And he was
wrong. 

Free-fall John
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  A. John Mallinckrodt    http://www.intranet.csupomona.edu/~ajm
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