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From: ethan@utastro.UUCP (Ethan Vishniac)
Newsgroups: net.physics
Subject: Re: "speed of light" puzzle
Message-ID: <86@utastro.UUCP>
Date: Mon, 13-May-85 14:48:43 EDT
Article-I.D.: utastro.86
Posted: Mon May 13 14:48:43 1985
Date-Received: Thu, 16-May-85 04:01:23 EDT
References: <1720@ut-ngp.UUCP>
Organization: U. Texas, Astronomy, Austin, TX
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> 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 ?
> 
Hmmm... This one is pretty old.  Nothing travels from the beam end at
one time to the beam end at another time.