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From: ntt@dciem.UUCP (Mark Brader)
Newsgroups: net.physics,net.astro.expert
Subject: Re: pulsing quasars and the like
Message-ID: <945@dciem.UUCP>
Date: Tue, 5-Jun-84 21:29:34 EDT
Article-I.D.: dciem.945
Posted: Tue Jun  5 21:29:34 1984
Date-Received: Tue, 5-Jun-84 22:58:41 EDT
References: <2631@ecsvax.UUCP>
Organization: NTT Systems Inc., Toronto, Canada
Lines: 19

D. Gary Grady (ecsvax!dgary) asks why a large body can't change
brightness quickly:
	... imagine a spherical body in space light years across.
	A signal travelling at less than c but consisting of high-frequency
	pulses travels from the center of the body and reaches the
	perimeter 'simultaneously' in all directions, causing said
	perimeter to pulsate in unison.

Well, suppose the body is N light years across and its center is M light
years from us.  For simplicity suppose the south pole of the object points
at us.  Now a pulse travelling symmetrically from the center of the object
causes the entire surface to brighten.  Fine.

But the light from the south pole takes only N/2+M years to reach us,
whereas the light from the equator takes M years, and the light from
intermediate south latitudes takes intermediate times.  Therefore we see
the brightening smeared out over N/2 years.  That's why.

Mark Brader