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From: franka@mmintl.UUCP (Frank Adams)
Newsgroups: net.physics
Subject: Re: A Thought Experiment (Supernova)
Message-ID: <1144@mmintl.UUCP>
Date: Tue, 4-Feb-86 15:48:06 EST
Article-I.D.: mmintl.1144
Posted: Tue Feb  4 15:48:06 1986
Date-Received: Sun, 9-Feb-86 05:25:10 EST
References: <2026@teddy.UUCP>
Reply-To: franka@mmintl.UUCP (Frank Adams)
Distribution: net
Organization: Multimate International, E. Hartford, CT
Lines: 33

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.
>
>Star B undergoes supernova - a substantial part of its mass is converted to
>energy and radiated away.  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?
>
>In particular, will he observe an acceleration of A towards B that is
>inconsistent with the mass he measures for B?

Right.  Until there is time for light to get from B to A, and then to him,
he will see A accelerate towards B based on B's old mass.

Actually, A will accelerate towards B at B's old mass until a significant
fraction of the mass blown off of B is further away from B than A is.
The mass doesn't cease to exist when it stops being part of a star.
(This last is assuming that the mass lost by B is distributed evenly
in all directions.  If it is not, A may accelerate faster or slower, though
it will require a sizable fraction coming near it to make much difference.
A will still be completely unaffected until light from B has time to reach
it.)

Frank Adams                           ihpn4!philabs!pwa-b!mmintl!franka
Multimate International    52 Oakland Ave North    E. Hartford, CT 06108