sci.physics.research

jacques wrote:
> This famous paradox is about the distance between two identicaly
> accelerating rockets starting from rest from an inertial lab frame. It
> is described in:
> /home/baez/physics/Relativity/SR/
> /wiki/Bell's_spaceship_paradox
> It illustrates the problem of defining a "physical"distance (something
> we would call"proper distance") in non inertial frames due to the
> breakdown of simultaneity.
>
> There is not only one definition and they do not give always the same
> result:(which one is correct?).

Which is correct depends on what you mean by "correct". That is, what
are you trying to do? Or more directly: what are you MEASURING?

There is no "correct" in the abstract here, there is only a set of
different possible measurements which obtain various different results
for "proper distance" in non-inertial coordinates. Because, as you
mentioned above, there is no definitive simultaneity in such coordinates.

> I thought that, in SR, the Lorentz "contraction" between two inertial
> systems was not physical and would not involve the string to break.

Yes. Length contraction is purely observational, and the fact that some
other observer moving past your rocket sees it as shorter than you do
does not affect the rocket at all. Just like looking at a building from
different points of view changes how you see it but does not affect the
building itself.

The difference between that and the Bell paradox is that in the latter a
PHYSICAL SITUATION was constructed (well, imagined) that breaks the
string. It is not some other observer measuring the string, it is two
rockets PULLING on it.

> Notice also that this solution does not describe the situation when
> the 2 rockets are accelerating, but the result of such situation when
> freezed..

One can imagine the two rockets stopping (briefly) in successive
inertial frames. Thus one sees that the string breaks as they are
accelerating, and there is no need to stop in any inertial frame for it
to break.

Tom Roberts