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From: srk@teddy.UUCP
Newsgroups: net.physics
Subject: A Thought Experiment (Supernova)
Message-ID: <2026@teddy.UUCP>
Date: Sun, 2-Feb-86 11:23:00 EST
Article-I.D.: teddy.2026
Posted: Sun Feb  2 11:23:00 1986
Date-Received: Mon, 3-Feb-86 05:25:19 EST
Distribution: net
Organization: GenRad, Inc., Concord, Mass.
Lines: 36

Here is a thought experiment which occurred to me many years ago but which I
have never heard a good explanation for so I still find it intriguing - perhaps
some net-people can shed some light on this:

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.  This is obviously not a stable
situation!

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?

In particular, will he observe an acceleration of A towards B that is
inconsistent with the mass he measures for B?

If he keeps watching, will he see A decelerate slightly for no apparent reason?

This is because he is observing light which B emitted after its explosion and
light which A emitted before it could have been affected by the explosion.

On the other hand, if the observer's measurements are all consistent, doesn't
this imply that A was affected by the supernova sooner than possible?

						-- Stephen Klein

						...!decvax!genrad!panda!srk