NetNews Usenet Archive 1992 #30/NN_1992_30.iso/spool/sci/physics/21357

Text File | 1992-12-17 | 2.9 KB | 59 lines

Newsgroups: sci.physics
Path: sparky!uunet!mcsun!sunic!liuida!curofix!d91andiv
From: [e-mail] (Anders Ivner)
Subject: Re: Special relativity is SOOO irritating!
In-Reply-To: [e-mail]'s message of 16 Dec 92 19: 14:00 GMT
Message-ID: <[e-mail]>
Sender: [e-mail]
Organization: CIS Dept, University of Linkoping, Sweden
References: <[e-mail]>
    <[e-mail]>
Distribution: sci
Date: Wed, 16 Dec 1992 22:59:05 GMT
Lines: 44

In article <[e-mail]> [e-mail] (Radford Neal) writes:

   In article <[e-mail]> [e-mail] (Anders Ivner) writes:

   >Consider a staff, one meter long. Now, given the speed v it 'shrinks'
   >to half a meter. We then have a wall, and an easily controllable
   >hatch placed .75 meters from the wall. The staff passes through the
   >hatch, and we close the hatch behind it (BEFORE it hits the wall!)
   >The staff hits the wall, without being deformed (somewhat unrealistic!),
   >and since its speed now changes to 0 it is now fully 1 meters long.
   >What the **** happens????

   I'll take a crack at this... No guarantees, amateur physics only here :-)

   The actual dynamics of the staff hitting the wall is presumably not of
   interest (who knows what would really happen :-). Let's assume instead
   that the staff is equiped with a rocket motor, which it uses to
   rapidly deccelerate just before it would have hit the wall. In fact, I
   presume that we also don't want to worry about how rapidly the force
   from the rocket motor propagates through the staff, so lets assume that
   the staff is equiped with *many* rocket motors, distributed along its
   length, which all fire simultaneously, in the rest frame of the the staff.
   It's then not necessary for the rod to be enourmously rigid.

   The answer to the problem then seems to me to be pretty simple. In the
   rest frame of the wall, these rocket motors *do not* fire simultaneously.
   The ones toward the tail of the staff fire first. The observer at rest
   with the wall is then hardly surprised that the staff gets stretched out
   to a full meter. In fact, if my space-time diagrams don't lie, this
   happens before the tail passes through the hatch, so we never get to
   close it at all.

There is one problem with this theory:
(First, let's say that the staff has a sensor in the front, that starts
the rocket motors when the staff is, say .05 meters from the wall).
What if we, when we notice that the staff begins to decellerate, remove the
wall? (which, in our frame of reference occurs _before_ the front of the
staff reaches .05 meters from the wall). It would then have no reason
to decellerate... (sounds like one of those time-travelling paradoxes to
me)

      Radford Neal

Anders