Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: Special Relativity Question / Barn Paradox



In this problem, it is extremely helpful (if not necessary) to make a
spacetime diagram and examine the exact sequence of events. Then put
the spacetime coordinates of the other frame on your diagram and see
how the sequence changes.

The problem here is that two events are defined to be simultaneous in
one frame and in the other frame they must necessarily not be. The
important sequence of events is:

Front of pole at front of barn
Back of pole at front of barn (P)
Front of pole at back of barn (Q)
Back of pole at back of barn

The two observers will disagree on the sequence of these events. One
will say that P and Q are simultaneous but the other will say that Q
happens before P. One will say that the doors slam simultaneously but
the other will not.

I don't have time right now to go into much more detail on this topic
but there is an excellent discussion (in the form of a problem) in
Taylor and Wheeler's Spacetime Physics.


I need your help again. Here is a paraphrase of a problem from the 1993
IB(higher level exam) that I am struggling with.

A super fast runner has a 4m pole ( her frame) and is moving at 0.866c
relative to me. I have set out two poles in my frame that are 2m apart and I
notice that her pole fits neatly between the two as she passes. Now, she
sees the two markers to be 1m apart.
"Does she say that the pole will fit between the markers? Is there a
contradiction here? Explain."

I have run the RelLab simulation of the Barn where the doors close and open
so that in my frame they are closed simultaneously ( a key concept here I
think) when she is in the barn. In her frame the doors do not close at the
same time but the front and back doors close and shut to allow the pole
through. Does the runner say the pole fits? The runner sees the front of the
pole at the front door long before she sees the back of the pole at the back
door.

We had a great debate in class and most of us decided she ,too, thought the
pole did fit but we are not all (including me) convinced. Can anyone help me
out?

thanks

ken fox

Paul J. Camp "The Beauty of the Universe
Assistant Professor of Physics consists not only of unity
Coastal Carolina University in variety but also of
Conway, SC 29526 variety in unity.
pjcamp@csd1.coastal.edu --Umberto Eco
(803)349-2227 The Name of the Rose
fax: (803)349-2926