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From: wex@milano.UUCP
Newsgroups: net.physics
Subject: Thought experiment
Message-ID: <703@milano.UUCP>
Date: Fri, 31-Jan-86 15:04:32 EST
Article-I.D.: milano.703
Posted: Fri Jan 31 15:04:32 1986
Date-Received: Sun, 2-Feb-86 01:00:16 EST
Sender: wex@milano.UUCP
Organization: MCC, Austin, TX
Lines: 26
Keywords: speed of light

One of my coworkers has posed me the following problem, and my knowledge of
physics is too weak to answer him.  Please *MAIL* replies to me, as I am not
a regular reader of this group.  If people are interested, I will post the
concensus answer (if there is one).

Imagine that you and I have devised a communication scheme whereby we can
measure the movement of the end of a rod, and decode these movements to get
the meaning of the message.

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)?

What is the answer to this gedanken problem?  Is it simply some property of
the rod?  If the rod is truly inflexible and inelastic, then both ends ought
to move at (essentially) the same time.

-- 
Alan Wexelblat
ARPA: WEX@MCC.ARPA
UUCP: {ihnp4, seismo, harvard, gatech, pyramid}!ut-sally!im4u!milano!wex
"What a long, strange trip it's been"