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From: cpf@batcomputer.TN.CORNELL.EDU (Courtenay Footman)
Newsgroups: sci.physics
Subject: Re: A Question
Message-ID: <1346@batcomputer.TN.CORNELL.EDU>
Date: Fri, 31-Oct-86 20:24:02 EST
Article-I.D.: batcompu.1346
Posted: Fri Oct 31 20:24:02 1986
Date-Received: Mon, 3-Nov-86 19:52:18 EST
References: <230@sri-arpa.ARPA>
Reply-To: cpf@batcomputer.UUCP (Courtenay Footman)
Organization: Theory Center, Cornell University, Ithaca NY
Lines: 41

In article <230@sri-arpa.ARPA> JDM%SMVL%rca.com@CSNET-RELAY.ARPA writes:
>
>   Maybe someone out there can answer this one quickly. I hope its
>not too fundamental or repetitous.
>
>   Actually, now that I think about it, its really two questions:
>
>   o  Is there a difference between inertial mass and gravitational
>      mass. (I believe that is the same as asking,"is there a 
>      difference between the effects of gravity and the effects
>      of being in an accellerating frame of reference?" But
>      Im sure Im making a drastic generalization.)
There is none.
>   o  Given the old "accellerating-elevator-in-space" experiment,
>      how can one within the elevator tell the whether he is
>      accellerating or under the influence of gravity?
>
Locally, one cannot.  Key word there is locally.  If one can make 
observations over a finite area, then one can tell, if the gravitational
field is varies with position.  That is, if one measures tidal forces,
it is a gravitational field, not acceleration.   This in not a contradiction
of my first answer, or of the equivalence principle -- these state that 
the effects on a point object of a gravitational field and an acceleration
are identical;  however tidal forces can not be measured at a single point.
A formal statement contains incantations about "motions on a geodesic"
and "ability to locally transform the metric into Minkowski form", but
I don't think that such a statement would be particularly enlightening.
>
>    Any correct answer to these questions will end a heated argument
>that is rapidly degenerating into World War III. Can someone save
>the world?
>
>                            Thanks in Advance
>                                Joe


-- 
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Courtenay Footman		ARPA:	cpf@lnssun1.tn.cornell.edu
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