Re: FTL transmission?

From: mjg223 (
Date: Thu Jun 01 2000 - 16:07:14 MDT

> Well this is what I am getting at: in these "paradoxes" that I keep
> hearing about, and that you describe above, all that is really going
> on is that someone may have quicker knowledge (i.e. see an event happen)
> before someone else who is relying on normal light (travelling at normal
> lightspeed) to see the event. But the event already happened long ago.

One of my favorite frame-of-reference paradoxes:

Imagine there's a 20 meter tunnel with steel doors at each end and I'm
running through it at nearly the speed of light carrying a 20 meter
pole. When I'm halfway through, both doors snap shut at exactly the
same moment. Does the pole get cut or not? Surely the answer must be
the same in all reference frames.

>From my perspective, the pole is stationary. I observe its rest-length of
20 meters. The tunnel however is moving very quickly, so it looks much
shorter than 20 meters. The tunnel is shorter than my pole, and therefore
the pole must be cut.

>From the perspective of an observer stationary relative to the tunnel,
however, my pole has shrunk to something shorter than the tunnel - to
something as close to zero as you like, depending on my speed - so
when I'm halfway along both doors can close comfortably. Therefore,
the pole doesn't get cut.

(The resolution is that I can't say 'both doors close at the same time'
without specifying an observer.)


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