I Entanglement and FTL signaling in professional scientific literature

[Fra]

Sure you can, but then one needs to note which observer has the expectations and which one makes the observations. Then it might be nether bob nor alice but a third observer which is also informes about the original prepararion.

My only point is that while i did not follow morbert logic it looks like what ia common elsewher one one makes strange combinations of observers and arrives at unnevessary weirdness.

/Fredrik

 

[vanhees71]

I don't understand, which problem you are discussing. You just have two detectors at far-distant places which store their setup and measurement results with accurate timestamps. Then you use the collected data of both detectors to investigate correlations between these measurements. There's no problem whatsoever with this.

 

[Morbert]

Yes, but that's because you picked the ordering of what to specify to make that result come out. Changing the ordering would change the result. Your ordering is equivalent to claiming that Bob's particle has a definite spin before Alice chooses whether or not to make her measurement. But what justifies that claim?

So long as both sets of histories are consistent, a consistent histories interpretation of quantum theory won't privilege one set as more valid than another, and so long as a set is indeed consistent, microscopic properties of particles can be considered, independent from whether or not they are measured.

We might say my ordering, because it considers microscopic properties of Bob's particle prior to choice of measurement, is a "sharper" set (Consistent quantum counterfactuals) (arxiv) i.e. a more fine-grained set and so we would expect more extended inferences to be possible. Though there is an interesting exchange between Griffiths and Stapp about counterfactuals concerning a slightly more complicated iteration of the EPR experiment, where a counterfactual is derived or not depending on a choice from two equally fine-grained sets (Quantum counterfactuals and locality) (arxiv)

 

[Fra]

I don't understand, which problem you are discussing. You just have two detectors at far-distant places which store their setup and measurement results with accurate timestamps. Then you use the collected data of both detectors to investigate correlations between these measurements. There's no problem whatsoever with this.

Well I am not sure I understood the problem discussed in post#266 either or if i misinterpreted it, but I it was what my comment had in mind.

An observer is of course free to gather data from past measured and recorded results from the classical domain, even those mixing different times and recordings from distributed locatations, and compute any correlation measures, but I question to what deductions you can make from such "scores" regarding causality and locality in particular becuase it tends to blur the proper sequence of inference IMO. I think it is easier if you pay close attentation to the "original observers", and how the information between different observers are "communicated". In a way the whole classical environment is often considered part of the observer, as its where the presumed irreversible pointes are store. This is where I think one enters gray area of the foundations, and it's easy to loose track of the arguments here when considers a "fact" definitive, as soon as it is recorded in the classical domain, because it makes the "observer" context delocalized in the first place. (This is one motivator why I think the QM foundations are problematic, and this is probably what allows for "interpretations" where once chooses stranges stances)

Then Morbert posted a link in #273 to Griffith's paper, and he questions the validity of various conclusions on causality or locality based on the counterfactual arguments, which relates to my point.

Quantum Counterfactuals and Locality​

"Henry Stapp [1] has challenged an argument by the author [2, 3] which claims to demonstrate that
quantum mechanics is a local theory, in the sense that it contains no long-range dynamical influences. Stapp
asserts that the validity of a certain counterfactual statement
...
Our major disagreement is over the conclusions which can be drawn from these analyses. Stapp believes
that because he has identified a framework which properly corresponds to his earlier argument for nonlocal
influences
...
is my opinion, he has only demonstrated the hazards involved in trying to use this mode of counterfactual
reasoning to reach sound conclusions about the quantum world."
-- https://arxiv.org/abs/0908.2914

From a quick skimming I side with Griffiths here. The main point beeing that the conterfactual arguments are questionable. Thich was essentially my point as well.

(I don't disagree with any of your last few post though)

/Fredrik

 

[PeterDonis]

So long as both sets of histories are consistent, a consistent histories interpretation of quantum theory won't privilege one set as more valid than another

Which means that consistent histories is useless for answering counterfactual questions like the one we've been discussing, since it does not give a unique answer but multiple answers.

 

[Morbert]

Which means that consistent histories is useless for answering counterfactual questions like the one we've been discussing, since it does not give a unique answer but multiple answers.

Consistent histories is useless for answering counterfactual questions like the ones we've been discussing only if you adopt the position that counterfactual questions, made sufficiently precise, always have unique answers. Instead, the usefulness consistent histories offers is in the way it let's us reason consistently about counterfactuals from whatever propositions we are willing to include.

Both the original family I presented and the reordering you propose are consistent sets of histories, defining domains for which classical reasoning is valid from their respective premises. If we want to premise our reasoning on orthodox traditions in quantum mechanics where microscopic properties are only asserted in the context of their imprinting on classical apparatus, then we will use sets where microscopic properties are considered only after their measurement. If we want to premise our reasoning on the idea that classical apparatus reveal pre-existing microscopic properties, as Robert Griffiths does, at least in his early presentations of quantum theory, then we can use the set I posted.

"None of this should be taken to imply that the study of counterfactual reasoning in the quantum domain
is impossible or uninteresting or must always lead to ambiguous results. Instead, it is a tool that needs to
be used carefully, with full recognition of its ambiguities" -- Robert Griffiths

 

[PeterDonis]

Consistent histories is useless for answering counterfactual questions like the ones we've been discussing only if you adopt the position that counterfactual questions, made sufficiently precise, always have unique answers.

If they don't have unique answers, then it seems pointless to ask them in the first place. In any case, this is getting into interpretation territory, which is off limits in this forum; it should be discussed, if further discussion is desired, in a separate thread in the interpretations subforum.