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From: ethan@utastro.UUCP
Newsgroups: net.physics
Subject: Re: A Thought Experiment (Supernova)
Message-ID: <335@utastro.UUCP>
Date: Tue, 4-Feb-86 17:35:52 EST
Article-I.D.: utastro.335
Posted: Tue Feb  4 17:35:52 1986
Date-Received: Thu, 6-Feb-86 21:20:24 EST
References: <2026@teddy.UUCP>
Distribution: net
Organization: U. Texas, Astronomy, Austin, TX
Lines: 44
Summary: General Relativity in the weak field limit

In article <2026@teddy.UUCP>, srk@teddy.UUCP writes:
> 
> Assume there are two stars, A and B, of equal mass.  They are close enough to
> have significant gravitational attraction, but far enough apart that it takes
> light a week or two to travel from one to the other.  They are not in mutual
> orbit but are approaching each other head on.
> 
> Star B undergoes supernova - a substantial part of its mass is converted to
> energy and radiated away (at least let's assume it's substantial).  It is a
> fast supernova, and it is all over before any light from the event reaches
> star A.
> 
> At a safe distance, equidistant from both A and B, is an observer who decides
> to measure these two stars immediately after the light from the supernova
> passes his planet (he was watching a soap opera at the time...).  He measures
> the mass and acceleration of both stars.
> 
> What will he see?
> 
The gravitational field will propagate changes in its structure at the speed
of light.  To do this problem in gory detail would require GR, but we can
simply use a modified form of Newton's law in which the field disturbances
propagate at the speed of light to get a rough idea of what goes on.

If the distribution of mass and radiation around star B remains spherically
symmetric then the perceived gravitational field due to B will be the mass
and *energy* interior to the position of the observer.  Once the radiation
(and other ejecta) start to pass the radius of the observer then she will
perceive a drop in the gravitational force due to B.  Assuming A is farther
away than she is, she will notice that A is reacting to more mass than she
is and will conclude (correctly) that there is a distribution of energy/mass
surrounding B a significant amount of which is between the radius of the
observer and the radial distance to A.  After some time the acceleration of
A will drop and she will conclude (correctly) that the ejected mass and
radiation has continued to move outward.
-- 
"These are not the opinions    Ethan Vishniac
 of the administration of      {charm,ut-sally,ut-ngp,noao}!utastro!ethan
 the University of Texas,      ethan@astro.UTEXAS.EDU
 but they are the opinions     Department of Astronomy
 of your favorite deity, who   University of Texas
 is in daily communication 
 with me on this (and every 
 other) topic.