Re: importance of Einstein

[Pentcho Valev <valevp@BAS.BG>]

Another textbook example known as "Seeing behind the stick":

"A ruler is fixed perpendicular to a wall. A stick of length L fies by at
constant speed. It travels in front of the ruler, so that it obscures part
of the ruler from your view. When the stick hits the wall it stops.
     In your reference frame, the stick is shorter than L. Therefore, right
before it hits the wall, you will be able to see a mark on the ruler which
is less than L units from the wall."

How will one (in the ruler's frame) see the mark? Obviously the mark must
emit light which is then captured by the eye of the observer. But then one
may consider the situation in which, apart from light, the mark emits a pawl
preventing the back end of the stick to move in the opposite (with respect
to the initial movement) direction. So, according to the observer in the
ruler's frame, the stick will remain "caught" between the wall and the pawl
as a result of its length contraction and at the same time cannot remain
"caught" since its length contraction must disappear after joining the
ruler's frame. Typical reductio ad absurdum which would be fatal for any
scientific theory. Not for relativity of course since it is a religion, not
a theory.

Pentcho Valev



On Mon, 25 Oct 2004 10:43:15 -0400, Britton <britton@NCSSM.EDU> wrote:

> I LOVE this textbook example.
> It is VERY accessible to (bright) high school students,
> (Einstein wrote a sort book using algebra in hopes that high school level
> students would/could appreciate the 'theory'.)
>
> Explaining 'paradoxes' such as yours is an excellent example of 'learning'
> about SR.
>
> You mustn't limit your analysis to just ONE weird result of the assumption
> (c = constant), but must realize that ALL the important results are likely
> to contribute to any interesting scenario.
>
> In your particular problem - the impossibility of simultaneity in two
> reference frames will easily save the day.
>
> If the platform rigs the barriers to close simultaneously, then the train
> will see the front barrier activate BEFORE the second one activates.
>
> The wreckage of the train will be contained between the two barriers.
>
> Other similar exercises have a pole vaulter running thru a (short) barn
with
> front & back doors. One door is always closed and they switch
simultaneously
> in the barn frame.
>
> My personal favorite is the 3 meter Lamborghini passing thru the tollbooth
> laser gates which are only 2 meters apart in the tollbooth frame.
>
> The front laser goes off when the quarter hits the bucket and the rear
laser
> turns on at the same instant. A fast enough driver can easily pass through
> unscathed even though the driver sees the gate to be less than 2 meters
> long.
>
>
> So far, no paradox with SR remains unresolved.
> But, as Einstein himself said - only ONE result that conflicts with the
> theory is required to refute it.
>
>
> On 10/25/04 2:20 AM, "Pentcho Valev" <valevp@BAS.BG> wrote:
>
> > We can PROVE it is false since it produces absurd couples of propositions
> > of the type (p, not-p). As you know, length contraction is a corollary of
> > the axiom so let us consider the following textbook example:
> >
> > "Two bombs lie on a train platform, a distance L apart. As a train passes
by
> > at constant speed,the bombs explode simultaneously (in the platform
frame)
> > and leave marks on the train. Due to the length contraction of the train,
> > the marks on the train will be a distance gamma*L apart when viewed in
the
> > train's frame (since this distance is what is length-contracted down to
the
> > given distance L in the platform frame)."
> >
> > Millions of professors have taught this example and yet nobody has found
it
> > suitable to introduce the following modifications. The bombs are replaced
> > with two barriers which are simultaneously (in the platform frame)
stretched
> > across the railway. The barriers are strong enough to be able to stop the
> > train. Also, the length of the train is L', a value limited by
L<L'<gamma*L.
> > Is the barrier mechanism capable of "catching" the train? The observer in
> > the platform frame will first say "yes" since, in this frame, the moving
> > train is shorter than L. Then the same observer will say "no" since the
> > train cannot remain length-contracted after joining the platform frame.
> > Finally, millions of relativists will say "never mind" since for them
> > relativity is a cult, not a theory that should be verified.
> >
> > Of course, if for some reason you don't like this textbook example, we
> > could analyse others.