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From: js2j@mhuxt.UUCP (sonntag)
Newsgroups: net.physics
Subject: Re: "speed of light" puzzle
Message-ID: <866@mhuxt.UUCP>
Date: Mon, 13-May-85 10:50:21 EDT
Article-I.D.: mhuxt.866
Posted: Mon May 13 10:50:21 1985
Date-Received: Tue, 14-May-85 20:16:55 EDT
References: <1720@ut-ngp.UUCP>
Organization: AT&T Bell Laboratories, Murray Hill
Lines: 23

> I have a laser on a turntable in such a way that the beam falls on
> the moon during each revolution. If my laser rotates at omega
> rad/s and the moon is at distance D, the spot moves on the moon
> at speed V = omega*D. Since I can make omega as big as I want,
> V can be made very big, and certainly much more than C. (e.g.
> with D ~ 300,000km and omega = 10rad/s, a very gentle speed,
> V = 3,000,000km/s = 10*C)
> 
> How can anything move at 10*C without violating relativity ?
> Nic.                      {ihnp4,seismo,...}!ut-ngp!graner

     This one's pretty old.  In a more familiar example, the spot on
the CRT of some very high-speed oscilloscopes can move faster than the
speed of light.  This doesn't violate relativity, as no actual object
is moving FTL, only the place where the electron beam is currently hitting
the phosphor.  Since 'a place' doesn't really have any mass, the position
of the place can change faster than any physical object's position is 
allowed to change.
-- 
Jeff Sonntag
ihnp4!mhuxt!js2j
     "I said, 'Doc, a world war passed through my brain.'
      He said, 'Nurse, grab your pad, the boy's insane.'"-Dylan