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From: hadeishi@husc4.harvard.edu (mitsuharu hadeishi)
Newsgroups: sci.physics
Subject: Re: A Question
Message-ID: <523@husc6.HARVARD.EDU>
Date: Fri, 24-Oct-86 12:20:28 EDT
Article-I.D.: husc6.523
Posted: Fri Oct 24 12:20:28 1986
Date-Received: Sat, 25-Oct-86 05:45:50 EDT
References: <230@sri-arpa.ARPA>
Sender: news@husc6.HARVARD.EDU
Reply-To: hadeishi@husc4.UUCP (mitsuharu hadeishi)
Organization: Harvard Science Center
Lines: 52

In article <230@sri-arpa.ARPA> Joe 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.)

	The basic question is, is the ratio between inertial mass
and gravitational mass the same for everything?  If not, you would be
able to detect this by observing different rates of acceleration of
objects in a gravitational field.  This would be very problematic
because states of free fall would be impossible to define; your lab
might be free falling at a different acceleration than your lab
equipment, so you would be able to measure forces in your free falling
lab that operated on different materials to different degrees.

	If so, however, you can equate the two for convenience and
just talk about "mass".  This is in fact the case.  From the point of
view of general relativity, objects simply follow geodesics; no
matter what the composition of the object (no matter what its mass)
it will follow the same geodesics through space-time.  (This is not
perfectly true for macroscopic objects, since they have extent they
are acted upon by tidal forces which may change their trajectory
somewhat.  However, in the limit as the objects become small
in extent the assertion is true, within the framework of general
relativity.)  This is assuming free fall in vacuum, of course.

	Recently some physicists were going over the data of
old experiments trying to establish the equivalence of inertial
and gravitational mass and claimed to have found slight
differences between atoms of different atomic number.  They attributed
this to an effect related to a quantity they called "hypercharge."
Apparently their result was flawed by a sign error, however,
so it is very dubious at the moment.

>   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?

	You could determine this by trying to measure tidal
forces (i.e., the gravitational field is not uniform, but varies
slightly through the elevator; this causes tidal forces).  Again,
however, in the limit as the elevator becomes small the two systems
are identical from the point of view of the people inside.

				-Mitsu