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From: John Jones on 4 Nov 2009 19:24
Pentcho Valev wrote:
> http://math.ucr.edu/home/baez/physics/Relativity/SR/barn_pole.html
> "These are the props. You own a barn, 40m long, with automatic doors
> at either end, that can be opened and closed simultaneously by a
> switch. You also have a pole, 80m long, which of course won't fit in
> the barn. Now someone takes the pole and tries to run (at nearly the
> speed of light) through the barn with the pole horizontal. Special
> Relativity (SR) says that a moving object is contracted in the
> direction of motion: this is called the Lorentz Contraction. So, if
> the pole is set in motion lengthwise, then it will contract in the
> reference frame of a stationary observer.....So, as the pole passes
> through the barn, there is an instant when it is completely within the
> barn. At that instant, you close both doors simultaneously, with your
> switch. Of course, you open them again pretty quickly, but at least
> momentarily you had the contracted pole shut up in your barn. The
> runner emerges from the far door unscathed.....If the doors are kept
> shut the rod will obviously smash into the barn door at one end. If
> the door withstands this the leading end of the rod will come to rest
> in the frame of reference of the stationary observer. There can be no
> such thing as a rigid rod in relativity so the trailing end will not
> stop immediately and the rod will be compressed beyond the amount it
> was Lorentz contracted. If it does not explode under the strain and it
> is sufficiently elastic it will come to rest and start to spring back
> to its natural shape but since it is too big for the barn the other
> end is now going to crash into the back door and the rod will be
> trapped in a compressed state inside the barn."
>
> Note that, if the diameter of the rod is equal to the diameter of the
> barn's hole, Einsteinians will obtain a twofold decrease in the rod's
> volume as well! Essentially without spending any work! Therefore both
> the force exerted by the compressed rod on the doors and the work this
> (enormous?) force can do for Einsteinians are just free lunch. What a
> breathtaking discovery! And yet Einsteinians seem reluctant to develop
> the scenario further - what is the magnitude of the force, how much
> work can be extracted etc. Students should just imagine first the dull
> Newtonian world where the 80m long rod does not want to hide inside
> the 40m long barn, then the miraculous Einsteinian world where the rod
> would hide even inside a 4 cm long barn, and that is enough. Green
> lights appear in students' eyes and the tunes of "Divine Einstein" and
> "Yes we all believe in relativity, relativity, relativity" quickly
> fill the spacetime.
>
> Pentcho Valev
> pvalev(a)yahoo.com
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