🚀 go-pugleaf

In article <1992Aug10.190327.6019@bnr.ca>, bucknerb@bnr.ca (Brent Buckner) writes...
>In article <25362@dog.ee.lbl.gov> sichase@csa1.lbl.gov writes:
>>It's not a joke, but you didn't get it.  You are correct about the 
>>limiting speed of the tips of the scissors.  But Jim was referring to the
>>point of contact between the two blades, which speeds off towards the tips
>>at much faster than the speed of light if the two blades are long enough.
>>Imagine a pair of scissors with each blade two light-seconds long. They
>>are open at the far end by only 1 inch.  Now close the scissors.  The 
>>tips move 1 inch in, say, one second.  But the leading point of contact
>>travels down 2 light-seconds of blade in the same one second.
> 
>It seems to me that Richard Bond was refering to the fact that
>no point along either scissor blade will begin to move in reaction
>to the closing of the scissors until the E/M signal propagates
>from the end of the scissors (where you are closing them)
>to that point.
> 
>Consider your own example.  Close the scissors.  One second later
>an observer at the tip of the scissors knows that you did so.
>But that observer is stationed two light-seconds away from you.

Hmmm.  Clarification is clearly needed.  To fix my example, imagine
hiring a few dozen demons to sit at fixed intervals along the scissors.
We agree that in order for them to receive payment they must push 
on one arm of the scissor at precisely 1:00 PM.  Their watches
have all been synchronized.

At 1:00 PM the scissors are closed by the demons, no one of which
actually moves the blades more than a fraction of an inch.  The leading 
point of contact between the two blades moves two light-seconds in 
only one second, but no information has been transmitted.  

My point here was to attempt to illustrate the distinction between
phase velocity and group velocity.  The phase velocity can and does
often exceed the speed of light.  But the group velocity may not
do so.  Another oft-quoted example is a slightly oblique wave hitting
a beach.  The wave rolls in at a few miles per hour, but the 
point of contact between the wave and the beach can run along the 
beach at many times the speed of the wave.  For an angle of attack
arbitrarily close to 90 degrees, an arbitrary wavefront moves laterally
across the beach at arbitrarily large speed - in principle it could
be faster than the speed of light.

-Scott
--------------------
Scott I. Chase			"The question seems to be of such a character
SICHASE@CSA2.LBL.GOV		that if I should come to life after my death
				and some mathematician were to tell me that it
				had been definitely settled, I think I would
				immediately drop dead again."      - Vandiver