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From: brooks@lll-crg.ARPA (Eugene D. Brooks III)
Newsgroups: net.physics
Subject: Re: "speed of light" puzzle
Message-ID: <578@lll-crg.ARPA>
Date: Fri, 10-May-85 21:14:50 EDT
Article-I.D.: lll-crg.578
Posted: Fri May 10 21:14:50 1985
Date-Received: Mon, 10-Jun-85 04:21:03 EDT
References: <1720@ut-ngp.UUCP>
Organization: Lawrence Livermore Labs, CRG group
Lines: 28

> I find it sad that people interested in physics (at least enough to
> read net.physics) don't have *some* basic ideas about relativity,
> as shown by the recent discussion about speed of light. Sigh.
> 
> Anyway, here is the puzzle:
> 
> 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 ?
> 
> (I know the beam between earth and moon will be curved, but this is irrelevant)
> 
> Nic.                      {ihnp4,seismo,...}!ut-ngp!graner
> 
> *If Murphy's law can go wrong, it won't*

Actually there is a more real world example of this pseudo puzzle.  Consider
the movement of a spot on a high speed oscilloscope.  The answer to the puzzle
is that only the geometric intersection of the beam and the moon is moving
at a speed greater than c.  No mass is moving that fast and no information
is being transmitted that fast.  An article discussing this paradox appeared
in Scientific American a few years ago.