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From: matt@oddjob.UUCP (Matt Crawford)
Newsgroups: net.physics
Subject: Re: "speed of light" puzzle  (??spoiler??)
Message-ID: <716@oddjob.UUCP>
Date: Sun, 12-May-85 14:59:09 EDT
Article-I.D.: oddjob.716
Posted: Sun May 12 14:59:09 1985
Date-Received: Mon, 10-Jun-85 05:24:56 EDT
References: <1720@ut-ngp.UUCP>
Reply-To: matt@oddjob.UUCP (Matt Crawford)
Organization: U. Chicago, Astronomy & Astrophysics
Lines: 26

Pardon me while I repeat the question from graner@ut-ngp (Nicolas Graner):

>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)

There is no object or particle in this situation which is moving faster
than the speed of light, only the point defined by the intersection of
the beam with the surface of the moon is doing so.  To contradict any of
the accepted physical laws you would have to demonstrate a way of using
this system to communicate some information faster than light.  That the
beam is curved is not irrelevant but instead points up the fact that any
decision you may make about changing the motion of the beam at the source
will not reach the moon at any speed faster than c.
_____________________________________________________
Matt		University	crawford@anl-mcs.arpa
Crawford	of Chicago	ihnp4!oddjob!matt