[ExI] Judging radical possibilities

[scerir]

Anders:
>> Let's (for this thread) drop the question
>> about what we actually think of FTL or relativity, and instead look at the
>> quite interesting meta-question "How do we judge the likeliehood of radical
>> changes of physics?"

Stefano:
> Much as I am fond of QM, I never liked much SR from an aesthetical POV, let
> alone the severe limitations it seems to impose as far as the realm of
> possibilities is concerned.

As for QM there are really strange situations. So strange that nobody
is even able to think of some "radical" conceptual "change". Apparently
this was well known to Bohr who wrote: "[......] with regard to the dynamical
behaviour of the individual units obviously necessitates a complete
renunciation of following their course in space and time."

One of these strangest situations is usually called the "quantum liar"
paradox.

It is well known that two quantum systems can be entangled
"ex ante" (from the past to the future) by means of a SPDC source,
or some other method. Here there is, so to speak, a "causal" vertex
(or perhaps a common cause) in the past.

It is also well known that two quantum systems (i.e. two atoms)
can be entangled "ex post" (from the future to the past). So, here
there is a "causal" vertex in the future, where is located the
detector which registers the photon emitted by one or by the other
of the two distant quantum systems (atoms) [1].

[1] Creation of entangled states of distant atoms by interference.
http://arxiv.org/abs/quant-ph/9810013
C. Cabrillo, J.I. Cirac, P. Garcia-Fernandez, P. Zoller
Abstract: We propose a scheme to create distant entangled atomic states. It is based on
driving two (or more) atoms with a weak laser pulse, so that the probability that two
atoms are excited is negligible. If the subsequent spontaneous emission is detected, the
entangled state is created. We have developed a model to analyze the fidelity of the
resulting state as a function of the dimensions and location of the detector, and the
motional properties of the atoms.

It is less known that we can entangle two quantum systems having no "causal" vertex
in the past and no "causal" vertex in the future. This is called the "quantum liar" 
paradox.

It is possible to read something about it here (chapter 7)
http://a-c-elitzur.co.il/uploads/articlesdocs/Elitzur-Dolev13.pdf
or here (chapter 10)
http://a-c-elitzur.co.il/uploads/articlesdocs/MultipleIFM.pdf

There are many speculations here, trying to explain the paradox
in terms of block-world
http://arxiv.org/abs/quant-ph/0510090
or in terms of "higher spaces of possibilities"
http://arxiv.org/abs/0906.1626
or in terms of quantum effects coming from outside space-time
http://arxiv.org/abs/1011.3440

So, there are new and huge conceptual problems here. And nobody
can even imagine what kind of "radical change" is needed.