> news:4ce57f48-4e4d-4dcd-9b82-4f4bc810f025@k17g2000yqb.googlegroups.com...

> >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,

> "Simultaneously" ?

> What does that mean, exactly?

> You should learn a little about SR. There is no concept of absolute
> simultaniety. Events which appear simultaneous in one frame don't appear
> simultaneous in a different reference frame (and indeed may not even appear
> simultaneous in the same reference frame, depending upon how you choose to
> interpret the word "simultaneous").

> If you do learn about SR, you will eventually be able to explain what
> happens in this thought experiment from both relevant inertial frames - the
> barn's and the pole's reference frames.

> Just to whet your appetite to learn SR, an observer in the inertial frame of
> the barn could "see" both doors open simultaneously, but from the
> perspective of the pole they are not simultaneous and the first door opens
> and closes before the second door.

> Its really quite simple when you understand the maths, you might like to
> think about buying a beginners guide to Relativity. There are many
> available, including a very good one by Einstein himself.

> If there is anything you don't undertsand in whatever book you buy, feel
> free to ask questions and I am sure people will want to help.

> HTH, a good luck learning physics!

> Peter Webb