[ExI] R: Re: Cramer on impossibility of FTL communication

scerir at alice.it scerir at alice.it
Wed Sep 2 18:07:02 UTC 2015

----Messaggio originale----
Da: atymes at gmail.com
Data: 1-set-2015 2.27
A: "ExI chat list"<extropy-chat at lists.extropy.org>
Cc: <scerir at alice.it>
Ogg: Re: [ExI] Cramer on impossibility of FTL communication

In this case, the problem seems to be the limited speed of transmission of
information about the information.

If you entangle photons A and B, let B travel away, then measure A, you
know what B is...but your knowledge of it is not immediately available
where B is.  You can tell people, but that itself does not travel faster
than light.  As far as B and those around it are concerned, B remains
entangled until the light cone of your measurement of A reaches them.

Is that not the case?


Hi Adrian,

Yes and no. 

If you have a source of entangled photons and this source is much much closer 
 location A than to location B, and if A and B are close enough, an observer O 
could jump from A to B and you can imagine he could perform magic tricks. 

In other words ... in general the problem arises because obsevers are
 local, and quantum correlations are not. But if you have ... nonlocal
 observers ....

No story in space-time can describe nonlocal correlations: we have no tool
 in our story-toolbox to talk about nonlocal correlations (Gisin). Hence,
 we usually say things like "event A influences event B", or
"event A has a spooky action at a distance on event B" or
 "event A causes a collapse of the wavefunction at location B".
 But we know that this is all wrong: there is no time ordering between the
 events A and B ("acausality"). Hence no story in time is
 appropriate. Moreover, the distance between A and B is irrelevant. Hence
 the distance should not occur in our story. 

A good review paper about all that (and about many different - and very
often also smart - attempts to build quantum signalling machines) is this
one, by GianCarlo Ghirardi 
See also 
Ghirardi is the "G" of the GRW interpretation. 
For different arguments see Gisin here (Appendix B)

Scarani and Gisin here

Another interesting point is the speed of quantum information or, to say it
 better, the speed of quantum "influences" (between the two parts
 of the entangled quantum system, i.e. the two parts of a bi-photon). There
many interesting papers. 

(The experimental violation of Bell
 inequalities using spacelike separated measurements precludes the
 explanation of quantum correlations 
through causal influences propagating
 at subluminal speed. Yet, any such experimental violation could always be
 explained in principle through models based on hidden influences
 propagating at a finite speed v>c, provided v is large enough. Here, we
 show that for any finite speed v with c<v<infinity, such models
 predict correlations that can be exploited for faster-than-light
 communication. This superluminal communication does not require access to
 any hidden physical quantities, but only the manipulation of measurement
 devices at the level of our present-day description of quantum
 experiments. Hence, assuming the impossibility of using nonlocal
 correlations for superluminal communication, we exclude any possible
 explanation of quantum correlations in terms of influences propagating at
 any finite speed. Our result uncovers a new aspect of the complex
 relationship between multipartite quantum nonlocality and the
 impossibility of signalling.)

http://arxiv.org/abs/1304.0532(We discuss models that attempt to provide
 an explanation for the violation of Bell inequalities at a distance in
 terms of hidden influences. These 
models reproduce the quantum
correlations in most situations, but are restricted to produce local
 correlations in some configurations. The argument presented in Bancal et
 al., Nature Physics 8, 867, (2012), applies to all of these models, which
 can thus be proved to allow for faster-than-light communication. 
In other words, the signalling character of these models cannot remain hidden.)

http://arxiv.org/abs/1011.3440(Are There Quantum Effects Coming from
Outside Space-time? Nonlocality, free will and "no many-worlds".)

(In Bell inequality tests, the evolution of
 the wavefunction is not covariant, i.e. not invariant under velocity boost
 that change the time ordering of 
events, but the laws that govern the
 probability distribution of possible results are. In this note I
 investigate what this could mean and whether there could be some covariant
 "real quantum stuff". This clarifies the implication of the Free
 Will Theorem and of relativistic spontaneous localization models based on
 the flash ontology (GRW). Some implications for the concept of time(s) are
 spelled out.)

http://arxiv.org/abs/0808.3316(A real spooky action at a distance
 wouldrequire a faster than light influence defined in some hypothetical
 universally privileged reference frame. 
Here we put stringent experimental
 bounds on the speed of all such hypothetical influences. We performed a
 Bell test during more than 24 hours between two villages separated by 18
 km and approximately east-west oriented, with the source located precisely
 in the middle. We continuously observed 2-photon interferences well above
 the Bell inequality threshold. Taking advantage of the Earth's
 rotation, the configuration of our experiment allowed us to determine, for
 any hypothetically privileged frame, a lower bound for the speed of this
 spooky influence. For instance, if such a privileged reference frame
 exists and is such that the Earth's speed in this frame is less than
 10^-3 that of the speed of light, then the speed of this spooky influence
 would have to exceed that of light by at least 4 orders of magnitude.)

http://arxiv.org/abs/quant-ph/0212078(We study the apparent nonlocality of
 quantum mechanics as a transport problem. If space is a physical entity
 through which quantum information QI 
must be transported, then one can
 define its speed. If not, QI exists apart from space, making space in some
 sense 'nonphysical'. But we can still assign a `speed' of QI
 to such models based on their properties. In both cases, classical
 information must still travel at "c", though in the latter case
 the origin of local spacetime itself is a puzzle. We consider the
 properties of different regimes for this speed of QI, and relevant quantum
 interpretations. For example, we show that the Many Worlds Interpretation
 MWI is nonlocal because it is what we call `spatially complete'.)

http://arxiv.org/abs/quant-ph/0508016(This article identifies a series of
 properties common to all theories that do not allow for superluminal
 signaling and predict the violation of 
Bell inequalities. Intrinsic
 randomness, uncertainty due to the incompatibility of two observables,
 monogamy of correlations, impossibility of perfect cloning, privacy of
 correlations, bounds in the shareability of some states; all these
 phenomena are solely a consequence of the no-signaling principle and
 nonlocality. In particular, it is shown that for any distribution, the
 properties of (i) nonlocal, (ii) no arbitrarily shareable and (iii)
 positive secrecy content are equivalent.)

http://arxiv.org/abs/quant-ph/0503007(In relativity there is space-time
 out there. In quantum mechanics there is entanglement. Entanglement
 manifests itself by producing correlations 
between classical events (e.g.
 the firing of some detectors) at any two space-time locations. If the
 locations are time-like separated, i.e. one is in the future of the other,
 then there is no specific difficulty to understand the correlations. But
 if the two locations are space-like separated, the problem is different.
 How can the two space-time locations out there know about what happens in
 each other without any sort of communication? If space-time really exists,
 the locations must do something like communicating. Or it was all set up
 at the Beginning. But the correlations depend also on the free choice of
 the experimentalists, one in each space-time location. This allowed John
 Bell to derive his inequality and the experimentalists to violate it, thus
 refuting the assumption that it was all set up at the beginning: the
 Correlations can't be explained by common causes.)

http://arxiv.org/abs/quant-ph/0410025(Since Bell's theorem, it is
 known that quantum correlations cannot be described by local variables
 (LV) alone: if one does not want to abandon 
classical mechanisms for
 correlations, a superluminal form of communication among the particles
 must be postulated. A natural question is whether such a postulate would
 imply the possibility of superluminal signaling. Here we show that the
 assumption of finite-speed superluminal communication indeed leads to
 signaling when no LV are present, and more generally when only LV
 derivable from quantum statistics are allowed. When the most general LV
 are allowed, we
prove in a specific case that the model can be made again consistent with
 relativity, but the question remains open in general.)

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