[ExI] QT and SR

[Lee Corbin]

On the difficulties of understanding Special Relativity,
John Bell (he of Bell's Theorem fame) used to pose
the following problem to the other physicists at CERN
while he worked there.

Suppose that 20 mighty spaceships with extremely
powerful nuclear engines are all lined up nose to stern
on a very long runway at the spaceport, except for a 
10 meter stretch between each pair. All their clocks are
synchronized, and their orders are as follows:  "At 
exactly 12 noon in our local time zone, you are to all
take off, accelerating at one gee until you reach velocity
c/2, at which point you will continue at velocity c/2 
relative to our solar system."

But someone has linked all the spaceships together with
19 pieces of string. The question is, will the pieces of string
break?
In other words, will the spaceships, upon reaching 
constant velocity, maintain their original separation?
The question is tricky because on the one hand, it
seems that the pieces of string should break because
the spaceships all become shorter as they approach
lightspeed. And the strings too!  On the other hand,
they're traveling all in unison, and that shouldn't depend
on your frame of reference.

Bell found that about half of the physicists, clearly a
very exceptional pool of very capable physicists, got
the answer wrong.

I consulted my own intuition, but decided to sit down
at a table and draw a spacetime diagram before I checked
to see what Bell's answer was. The result contradicted my
intuition!  But at least I got the right answer (though only,
I admit, by having been warned by his statement that half
of those professionals at CERN got the wrong answer).

Later I gave it to a friend of mine who has a Ph.D in physics.
He got it wrong too, (though we were on the phone, and
he didn't draw any diagram). This stuff is very tricky.

Lee