[ExI] QT and SR

[scerir]

> "Two quick comments: 1) Entanglement is not "entirely different", in
> that it is usually brought about by an interaction in their common
> past; and 2) The authors seem to be unaware that entanglement is also
> apparently possible due to interactions in their common future, see
> <http://arxiv.org/abs/quant-ph/0205182> this paper. "

Damien,

1) Schroedinger himself wrote that "When two
systems of which we know the states by their
respective representations enter into a temporary
physical interaction due to known forces between
them and then, after a time of mutual influence,
the systems separate again, then they can no longer
be described in the same way as before viz. by endowing
each of them with a representative state vector.
I would not call that 'one' but rather 'the'
characteristics of quantum mechanics."
But if that interaction is the cause of the strange
superposition called entanglement, this does not mean
that non-local correlations of spacelike separate
events can be "explained" by a common cause.
(The reason might be the indeterminism of the quantum
world. Not sure, but I remember that in a deterministic
world events may be traced back to a common cause
even though they are spacelike separate.)

About 2), there is a double level here.
 A) It is true that there is no real time 
ordering behind the nonlocal correlations.
There is no "before" and "after". In other words
entanglement does not involve causality.
 B) Despite the apparent time-asymmetry 
associated with any measurement, the formalism 
of quantum theory is time symmetric. The famous 
EPR experiment involves two distant entangled 
particles, emitted from the same atom. By spin 
conservation, they have opposite spins. On the 
other hand, by the formalism of QM, and by experiments, 
and by Bell's theorem, these spins are undecided 
until they are measured. Hence, measuring one of 
them must instantly determine the spin of the other.
Since the formalism of QM is time symmetric
and since (if I remember well) time does not enter 
in Bell's reasoning, it is possible that two
particles become entangled not just when they are
emitted by the same source but also when they
are absorbed by the same source. But (to my
knowledge) there are no experiments here.