Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Posting-Version: version B 2.10.1 6/24/83; site utastro.UUCP
Path: utzoo!watmath!clyde!burl!ulysses!mhuxl!houxm!houxz!vax135!floyd!cmcl2!seismo!ut-sally!utastro!nather
From: nather@utastro.UUCP (Ed Nather)
Newsgroups: net.physics,net.astro.expert
Subject: Re: pulsing quasars and the like
Message-ID: <64@utastro.UUCP>
Date: Sun, 3-Jun-84 15:39:10 EDT
Article-I.D.: utastro.64
Posted: Sun Jun  3 15:39:10 1984
Date-Received: Wed, 6-Jun-84 04:24:16 EDT
References: <2631@ecsvax.UUCP>
Organization: UTexas Astronomy Dept., Austin, Texas
Lines: 45

[]
   >For some time I have been reading about rapidly pulsing objects and how this
   >puts an upper limit on their size.  That is, since no signal can propagate
   >faster than light, the period of oscillation of a body cannot be less than
   >the light transit time.
   >
   >Or, more to the point, 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.
   >
   >So, does the period of pulsation really place an upper limit on size??
   >
   >Confusedly,
   >D Gary Grady
   >Duke University Computation Center, Durham, NC  27706

Your example is just the one usually used to prove the assertion.  Yes, it is
possible for a central disturbance to propagate outward at less than lightspeed
and arrive at a spherical surface "at the same time" but consider what the
distant astronomer sees:  certainly not a sudden brightening of the whole
object at once -- he sees the part nearest to him start to brighten (that
light gets to him first), followed by the slightly more distant parts (in a
kind of bright ring, if the disturbance was a short pulse) followed by ...

Even though the disturbance arrived at the surface of the object everywhere
"simultaneously" the observer sees that pulse convolved with the light travel
time across the object; if it had a radius of 1 light year (pretty tenuous
stuff, I guess) then it would take a year for the observer to see the bright
ring finally reach the most distant part of the object -- assuming it to be
opaque.  If it were transparent, he would then see the bright ring get
smaller and smaller, until light from the most distant part of the object
reached him -- 2 years after the beginning.  That's the argument used by
those in favor of the assertion.

If the object is approaching you at some substantial fraction of the speed
of light, however, then other factors enter which can make a very real
difference.

-- 

                                 Ed Nather
                                 {allegra,ihnp4}!{ut-sally,noao}!utastro!nather
                                 Astronomy Dept., U. of Texas, Austin