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From: tino@hou2f.UUCP (A.TINO)
Newsgroups: net.physics
Subject: Re: Faster-than-light scissors
Message-ID: <496@hou2f.UUCP>
Date: Fri, 21-Jun-85 09:00:32 EDT
Article-I.D.: hou2f.496
Posted: Fri Jun 21 09:00:32 1985
Date-Received: Sun, 23-Jun-85 00:27:19 EDT
Organization: AT&T Bell Labs, Holmdel NJ
Lines: 57

In response to my comments on the Faster-than-light scissors:

>I am missing something.
>
>Why can't the crossing points at different locations along the scissors
>be treated as independent events that occur in the absolute elsewhere?
>The speed at which the blades of the scissors are moving does not have
>to be fast at all for the events to appear to be separated by times small
>enough that the apparent "rate of propagation" of the intersection exceeds
>the speed of light.
>
>Just drop a board or rotate a ruler.  There is no reason that a locus of
>points on the ruler cannot all cross a plane at the same time (or a series
>of times separated by a time difference as small as you like).
>
>With respect to the neck--the time it takes to slice the neck is only the
>time it takes for the intersection to propagate through it if the neck is
>infinitely thin, right?  So that argument is not valid.
>
>                                 --Bruce Failor
>                                   MFE  @  LLNL
-------
Bruce:

Maybe my original discussion of the "faster-than-light-scissors"
was muddled.  I'll try again.

When the person at the handle begins to squeeze the scissors,
she starts an impulse (shear force, whatever) that requires a finite
time to reach the tip of the blade.  In fact, the impulse travels
at the speed of sound in the blade material.  And you'll admit,
I hope, that there is no way that the blade tip can move, much less
connect with the second blade, until that impulse ( the "causal 
influence") reaches the the tip.  So I don't see any hyper-light
velocities here.

If there were hyper-light speeds something would be very wrong 
because relativity prohibits any "causal influence" from traveling
faster that light.  And someone squeezing the handle is very much 
the "cause" of the subsequent motion of the blade.

There is a very different situation that DOES lead to hyper-light
speeds that may be confusing the discussion.  If the top blade 
wasn't attached to the bottom blade but, instead, traveled past
the bottom blade at a uniform velocity while making a small 
angle with it, THEN the intersection point could very well reach
hyper-light speed.  But so what?  No "causal influence" is 
transmitted along the blade -- everything's already moving, no
shear forces needed -- so all's well.  (I think this is what you
are pointing out when you talk of "dropping a board".)

(P.S.: I don't get your point about the time it takes to slice the 
neck.  I was refering only to the time it takes for the blades to 
reach the neck from the time that the handle-sqeezing began.)
			
			Al Tino
			Bell Labs at Holmdel