Darren New <dne### [at] sanrrcom> wrote:
> Warp wrote:
> >   Thinking about it, unless I'm completely mistaken, quantum mechanics
> > doesn't disagree with the notion that *information* cannot be transported
> > at superluminal speeds (even if quantum effects can).

> QM doesn't show that particles cannot be transported at superluminal speeds. 
> Indeed, you have to take superluminal speeds into account when you're doing 
> the infinite sums in order to make them come out.

  Unless I'm mistaken (hmm, I'm using that expression a lot), it's a bit
more complicated than that.

  Just because, according to quantum mechanics, a particle can interact
over great distances with no delay doesn't mean that *information* can
be transferred over that distance without the delay (it's one of those QM
weirdnesses that I have no hope of fully understanding). I have the
impression that quantum mechanics doesn't disagree with GR about this
subject.

  For example it was specifically noted in the relatively recent test
result of quantum teleportation that although the particles were (at least
apparently) teleported at superluminal speeds, the technique could still
not be used to transfer information from one place to another at superluminal
speeds. Thus there was no contradiction with GR.

  GR itself doesn't forbid the distance between two points in space growing
faster than c. This is actually what is currently mostly accepted as
happening in the universe, and the cause for the so-called cosmological
horizon (the farthest parts of the Universe are receding from us faster
than c, which means we can *never* get any information about those parts
by any possible means, which in turn means that there's a "horizon" which
effectively completely conceals the rest of the Universe from us). This is
accepted because it actually doesn't break GR equations.

  However, even in this circumstance *information* cannot be transferred at
superluminal speeds between two points in space. In other words, even if
we invented a machine which would create new space between two points so
fast that the two points recede from each other faster than c, this could
not be used to transport anything between the points faster than c.

> GR says information cannot be transported at superluminal speeds, which is 
> why Bell's Inequality is of interest.

  I think that the first sentence in the related wikipedia page is the
best summarization of QM I have seen:

"Bell's theorem is a theorem that shows that the predictions of
quantum mechanics (QM) are not intuitive, and touches upon several
fundamental philosophical issues related to modern physics."

-- 
                                                          - Warp

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