NetNews Usenet Archive 1992 #30/NN_1992_30.iso/spool/sci/physics/21396

Internet Message Format | 1992-12-18 | 3.9 KB

Path: sparky!uunet!crdgw1!newsun!dseeman
From: [e-mail] (Daniel Seeman)
Newsgroups: sci.physics
Subject: Re: Special relativity is SOOO irritating!
Message-ID: <[e-mail]>
Date: 18 Dec 92 00:08:55 GMT
References: <[e-mail]> <[e-mail]> <[e-mail]>
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In article <[e-mail]> [e-mail] (Radford Neal) writes:
>
>> > > Anders Ivner:
>> > >
>> > > 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????
>
>> > Radford Neal:
>> >
>> > ... 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.
>
>> Anders Ivner:
>>
>> 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)
>
>
>The point of my rephrasing of the problem was to produce something that
>is physically realizable. Your suggestion is not. The rocket motors must
>fire _simultaneously_, in the rest frame of the staff. This can't be done
>if it's in response to a sensor at the front, since the signal to turn on
>the rocket motors will propagate no faster than the speed of light. I was
>assuming that the whole set up is pre-arrainged, so the firings can just
>be controlled by timers at each rocket motor.
>
>If you prefer, we can assume that the staff really does impact the wall.
>In this case, however, one can't assume that the staff is perfectly rigid,
>since that would require that the impact be propagated to the end of the
>staff instantaneously. What will happen is that the staff will be compressed
>(in the intertial frame in which it was previously at rest). We _can_ assume
>that this is temporary, and that the staff will later rebound to its origninal
>length. In this case, the answer is that the hatch can be closed, and that
>the tail end of the staff will smash into it when the staff rebounds to its
>full one metre length.
>
>    Radford Neal

Hey, isn't that pretty much what I said in my posting?  One would see sort of a
"virtual" rebound as the rod accelerates back into the rest frame of the wall
and the hatch so the stick would poke back through the hatch.  So, is this idea
correct or not?  I suppose I could go back to my books and check for myself.  If
I can find some rigourous proof, I will post it.

dks.