Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Posting-Version: version B 2.10.1 6/24/83; site oddjob.UChicago.UUCP
Path: utzoo!watmath!clyde!burl!mgnetp!ihnp4!gargoyle!oddjob!matt
From: matt@oddjob.UChicago.UUCP (Matt Crawford)
Newsgroups: net.physics,net.astro.expert
Subject: Re: pulsing quasars and the like
Message-ID: <271@oddjob.UChicago.UUCP>
Date: Wed, 6-Jun-84 18:22:48 EDT
Article-I.D.: oddjob.271
Posted: Wed Jun  6 18:22:48 1984
Date-Received: Thu, 7-Jun-84 19:13:28 EDT
References: <255@sunybcs.UUCP>
Organization: U. Chicago: Astronomy & Astrophysics
Lines: 37

>> >> 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 oscil-
>> >> lation of a body cannot be less than the light transit time.
>> >>I'm confused by this...

>> The upper limit on the size is because the pulsing objects are actually 
>> rotating, they appear to pe pulsating from Earth the same way the rotating
>> light on a lighthouse appears to pulse on and off from a distance. THe size
>> limit is because no object can rotate such that any portion of it exceeds
>> the speed of light, c.  For example, if an object pulses at 1 pulse per
>> second, it has an upper limit on its size of 29602 miles radius, since
>> if it were larger, its outer edge would rotate at > 2*pi*29602 mi/sec
>> or 186000 mi/s or the speed of light, which is impossible.

>> >> 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.

>> This does not disprove the above, since the actual source is not the
>> object light years across, but the thing in its center which is pulsing.
>> The rest of the object is just responding to the pulses.

No, not all pulsating objects rotate.  In the case of rotating pulsars,
your explanation applies.  Nonrotational pulsation still limits the size of
the object because of the difference in light travels times for light
emitted from the nearest and farthest points on the object.  If this time
difference is comparable to or greater than the period of pulsation then
no pulsation will be observed.

The source of a light signal is the last thing it scattered from, which is
at or near the surface in the case of stars.
___________________________________________________________
Matt		University	ARPA: crawford@anl-mcs.arpa
Crawford	of Chicago	UUCP: ihnp4!oddjob!matt