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From: rimey@ernie.berkeley.edu.BERKELEY.EDU (Ken &)
Newsgroups: net.physics
Subject: Re: Thought experiment
Message-ID: <11660@ucbvax.BERKELEY.EDU>
Date: Sat, 1-Feb-86 16:07:53 EST
Article-I.D.: ucbvax.11660
Posted: Sat Feb  1 16:07:53 1986
Date-Received: Mon, 3-Feb-86 06:26:38 EST
References: <703@milano.UUCP>
Sender: usenet@ucbvax.BERKELEY.EDU
Reply-To: rimey@ernie.berkeley.edu.UUCP (Ken Rimey)
Organization: University of California, Berkeley
Lines: 27
Keywords: speed of light

In article <703@milano.UUCP> wex@milano.UUCP writes:
>
>Now imagine constructing a rod in space that is 1 light-year long.  When I
>move my end of the rod (which we assume is rigid, and unaffected by gravity
>over the span of 1 light-year), your end moves too.  And the movement of the
>end conveys information.
>
>But doesn't this violate relativity (which I take to say that information
>cannot be propagated faster than the speed of light)?
>-- 
>Alan Wexelblat

Yes, it violates relativity.  Build rods out of whatever material you
like; none will be THAT rigid.  When you move one end of a 1 light-year
long rod, the other will not move the slightest bit until at least a
year later.

Being less abstract, you can picture the rod's atoms pushing each other
with electric force (an overly simple picture), and recall that
electromagnetic fields propagate at only the speed of light.  Or you
can observe that if the induced motion of the rod is small you are
really talking about the propagation of a sound wave down the rod.
Even in the stiffest metal, the speed of sound is much less than the
speed of light.  If the induced motion is larger, it will propagate
faster than sound, but the speed of light is still an upper limit.

						Ken Rimey