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Path: utzoo!linus!decvax!tektronix!hplabs!hp-pcd!john
From: john@hp-pcd.UUCP (john)
Newsgroups: net.sf-lovers
Subject: Re: a Matter of Gravity...
Message-ID: <8200059@hp-pcd.UUCP>
Date: Wed, 7-Aug-85 20:57:00 EDT
Article-I.D.: hp-pcd.8200059
Posted: Wed Aug  7 20:57:00 1985
Date-Received: Mon, 12-Aug-85 20:58:20 EDT
References: <7800038@orstcs.UUCP>
Organization: Hewlett-Packard - Corvallis, OR
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Nf-ID: #R:orstcs:7800038:hp-pcd:8200059:000:1203
Nf-From: hp-pcd!john    Aug  7 16:57:00 1985

<<<
<
< Somebody mentioned that Gravity acted only on matter.  The one catch is,
< the equations which make up Einsteinian (?) physics all depend on matter
< being interchangeable with energy, where gravity is defined as a form of 
< energy.
<

  A good example is the planet Vulcan (Yes there really was one). One of
the best ways to discover a new planet was to chart the orbits of the 
known planets and see if you could spot any perturbation in their orbit that
might be caused by an unseen neighbor. Several of the outer ones were 
discovered using this method. Anyway, someone noticed that Mercury had a
wobble in its orbit that suggested another planet orbiting between it and
the Sun. This "planet" was dubbed Vulcan and astronomers tried unsuccesfully
for years to try and find it.

  It turned out that the wobble was due to the gravitational effect from the
"mass equivalent" of the suns gravitational field. The energy in the field
was equivalent to a certain amount of mass and if you considered the effect
of this mass the mercury was doing what it was supposed to do.

  A more complete disscussion can be found in Asimov's "The Planet that
wasn't".


John Eaton
!hplabs!hp-pcd!john