“Confound it!” he said “A thousand times DARN!
my nifty new ladder won’t fit in my barn.
The barn’s forty feet, from front to back end
while the ladder is fifty and never will bend.”
The farmer despaired, you could tell by his look,
when out of a tree fell a skinny red book!
The book hit his head and when he stopped squirmin’,
he saw it was written by N. David Mermin.
“A gift from the Gods! This Mermin’s a sage!
It must be an omen….I’ll read every page.”
But try as he did, he could not read it through;
He couldn’t read nothin’ past page thirty-two.
“I needn’t read more!” he said feeling tall,
“Perhaps that new ladder will fit after all!
As objects speed length-wise at velocities steady…
this book claims I’ll witness some changes quite heady!
All fast moving objects, is says, will contract
by a factor specific-Dave proves this as fact.
If I take the square root of one minus (v squared
divided by c times itself, which is c squared)…
I now have the factor by which fast things shrink!”
The farmer grew quiet, and started to think.
This old man was simple, but perhaps somewhat wise:
He figured that ladder could be shrunk in size.
“Right now this here ladder’s too long by ten feet,
but should it zip by, its length would deplete!!!
“My prayers have been answered,” cried the jubilant man,
“I’ve done all the math, so this is my plan:
To speed up this ladder to a frame that’s inertial,
at three-fifths of c!!!!...I’ll need my son Herchel.”
“He’ll run with the ladder as I stand and look on,
and I should see that ladder fit right in the barn!!
While it’s fifty feet long as it lays here at rest,
if it moves three-fifths c it will just pass the test!”
For the barn will not move, so its length will STAY forty,
while the fifty- foot ladder will SHRINK down to 40.”
“When the ladder’s front reaches the barn’s farthest wall,
A super strong door will instantly fall
“When the ladder’s rear reaches the barn’s closer side
A front door will seal my new ladder inside!
He explained this to Herchel (a very smart lad)
Who instantly questioned his cup-half-full dad:
“Father, I fear you’ve not thought this scheme through.
You’ve only considered your own point of view.
From my frame of reference, at rest I shall be
And the barn will rush towards me at three-fifths of c.
I and the ladder will not shrink an inch,
While the barn will get smaller an even worse pinch!
“Father!” he sobbed, “you’ve become so elated
you’ve forgotten that you and your son are related!”
-Joshua M. Kershenbaum,
(Amherst College, 1996)
/**************************************
As a species, we are forever sticking our fingers into the electric socket of the Universe to see what'll happen next. It's a trait that'll either save us or kill us, but by god it's what makes us human beings. I'd rather be in the company of people who look at Mars than people who contemplate humanity's navel -- other worlds are better than fluff. ~~Sir Terence David John Pratchett
***************************************/
________________________________
From: William Maddox <maddox@physics.Auburn.EDU>
To: Forum for Physics Educators <phys-l@carnot.physics.buffalo.edu>
Sent: Tue, June 15, 2010 11:50:05 AM
Subject: Re: [Phys-l] how to explain relativity
From: WC Maddox
To go along with the astronaut/twin "paradox" there is a less well known
paradox involving length contraction. It is known as the rocket-rope
paradox and as Bell's spaceship paradox. If interested see this website: http://en.wikipedia.org/wiki/Bell%27s_spaceship_paradox
There are two versions. In one version the rockets have a constant
proper acceleration. In the other version the rocket engines are turned
off after some prearranged time so only SR is needed. The question is
whether a rope (or spring) connected between the rockets will physically
stretch (or break) or not. There are three interpretations: SR1 - rope
will stretch or break, SR2- rope will not stretch or break, and Physical
Effect- rope will stretch or break. If interested after reading above
reference, I would recommend reading the following references in order
followed by other references: original article by Dewan & Beran,
objection by Nawrocki, & response by Dewan.