A Quantum nonlocality and "spooky action at a distance"

[Lord Jestocost]

Einstein attacked QM, and Bohr somehow got the task to try to defend it ....

The following short section in Werner Heisenberg's memoir "Der Teil und das Ganze" puts Einstein’s stubbornness regarding QM in a nutshell:

„Einstein war nicht bereit, sich — wie er es empfand — den Boden unter den Füßen wegziehen zu lassen. Auch später im Leben, als die Quantentheorie längst zu einem festen Bestandteil der Physik geworden war, hat Einstein seinen Standpunkt nicht ändern können. Er wollte die Quantentheorie zwar als eine vorübergehende, aber nicht endgültige Klärung der atomaren Erscheinungen gelten lassen. „Gott würfelt nicht“, das war ein Grundsatz, der für Einstein unerschütterlich feststand, an dem er nicht rütteln lassen wollte. Bohr konnte darauf nur antworten: „Aber es kann doch nicht unsere Aufgabe sein, Gott vorzuschreiben, wie Er die Welt regieren soll.“

(I have refrained from a translation into English)

 

[physika]

experiments to test theories on collapse.

https://www.quantamagazine.org/how-...orld-be-physicists-probe-the-limits-20210818/

https://www.nature.com/articles/s42005-021-00656-7

......

 

[Fra]

This is a misunderstanding, but probably unrelated to Bohr's vagueness. I guess it is more related to Bohr being cited by many different people, who all try to explain his thoughts, and often do this by highlighting certain features relevant for their current discussion. In your case, this was probably Bell, but Fra/Frederik gives a more current example how this happens in practice. Bohr's thoughts were simpler and more practical than Fra/Frederik's goals.

I agree that the issues of unification of all forces, was not a major thing at that time.

But I think the success is perhaps that Bohr simply acknowledged the practical matters that simply appeared mandatory to define the new theory that was just born. Not more and not less. This is how practical matters such as approximate classical reality even required to collect statistics becomes principal matters.

So while others have tried to "overthink" or deny the observing context, Bohr seems to firmly insisted that it was required because the laboratory and all it's data is effectively classical. This basic simplicity is to me the key to clarity, which paradoxally stands strong in many attempts to overthink giving QM meaning without this context.

(The question of, what another theory would be like formulated without the premise, was likely not something that concerned Bohr. It was a sufficiently major task to fine the theory give the premise)

/Fredrik

 

[gentzen]

What do you mean by "Einstein's bomb"?

I read in Moore's biography of Schrödinger that he received a letter from Einstein where he described a similar experimental setup using an explosive instead of a cat, before Schrödinger had finished his famous cat paper. See for example SEP ERP 1.3 Einstein’s versions of the argument:
"In the August 8, 1935 letter to Schrödinger Einstein says that he will illustrate the problem by means of a “crude macroscopic example”.

The system is a substance in chemically unstable equilibrium, perhaps a charge of gunpowder that, by means of intrinsic forces, can spontaneously combust, and where the average life span of the whole setup is a year. In principle this can quite easily be represented quantum-mechanically. In the beginning the psi-function characterizes a reasonably well-defined macroscopic state. But, according to your equation [i.e., the Schrödinger equation], after the course of a year this is no longer the case. Rather, the psi-function then describes a sort of blend of not-yet and already-exploded systems. Through no art of interpretation can this psi-function be turned into an adequate description of a real state of affairs; in reality there is no intermediary between exploded and not-exploded. (Fine 1996, p.78)

[...] The similarity between the gunpowder and the cat is hardly accidental since Schrödinger first produced the cat example in his reply of September 19, 1935 to Einstein’s August 8 gunpowder letter. There Schrödinger says that he has himself constructed “an example very similar to your exploding powder keg”, and proceeds to outline the cat (Fine 1996, pp. 82–83). Although the“cat paradox” is usually cited in connection with the problem of quantum measurement (see the relevant section of the entry on Philosophical Issues in Quantum Theory) and treated as a paradox separate from EPR, its origin is here as an argument for incompleteness that avoids the twin assumptions of separability and locality."

The following Nautilus piece seems to know even more details of how Einstein inspired Schrödinger to invent his cat: How Einstein and Schrödinger Conspired to Kill a Cat

 

[vanhees71]

It's of course no paradox and it's exactly to the point of what Einstein wanted to say in the EPR paper (and he clearly said it only later in his Physica 1 paper of 1948). His real quibble was with the "inseparability" of far-distant parts of entangled quantum systems.

 

[martinbn]

It's of course no paradox ...

Usually in physics by "paradox" is meant an unintuitive consequence of the theory, like the twin paradox. None of them are actual paradoxes.

 

[Fra]

His real quibble was with the "inseparability" of far-distant parts of entangled quantum systems.

And I think Einsteins original view is sound, that there has to be a "hidden variable", of some kind. My view is no different. Not to explain the outcomes, but their correlations.

How it can explain the correlation and not the outcome, is exactly what we still do not understand. All we know is that kind of HV Bell envisioned does not work, because it assumes it's hidden from the ignorant exprimenter only.

I think Einstein wanted to understand, what others settled with describing. What progress is made until today?

/Fredrik

 

[vanhees71]

Nature doesn't care about our human quibbles due to wrong pictures/worldviews. She behaves as she does, and all very accurate empirical evidence confirms the probabilistic predictions of QT and no "local realistic hidden-variable model" (in the sense defined by Bell) can be right. That's an interpretation-independent empirical fact!

 

[GarberMoisha]

Yes, because even atoms are just useful models of experience.
Throw them the right way and they will interfere with themselves like the good probabilities that they are. Probability of experience.

 

[GarberMoisha]

If this doesn't make your head spin, I don't know what will.

 

[Structure seeker]

Nature doesn't care about

This idea is usually based on the philosophical belief that nature presents a harsh reality that is simply true regardless of your beliefs. I stress this point because I also believe that, but @vanhees71 keeps stating that before measurement there is no realism about the quantum state.

So I wonder, @vanhees71 , before interpreting measurement results we all start with our 'take' on things, our philosophical worldview. If you don't believe quantum wavefunctions are a reality, how do you justify that nature doesn't care about what people believe should be true about these wavefunctions? If you give no answer, your opinion that nature doesn't care is your belief about wavefunctions and we, as realists, can answer: nature (reality) doesn't care that you think our beliefs are superseded by measurement results.

 

[martinbn]

And I think Einsteins original view is sound, that there has to be a "hidden variable", of some kind. My view is no different. Not to explain the outcomes, but their correlations.

But everything we've learned since then points to no hidden variables!

 

[vanhees71]

This idea is usually based on the philosophical belief that nature presents a harsh reality that is simply true regardless of your beliefs. I stress this point because I also believe that, but @vanhees71 keeps stating that before measurement there is no realism about the quantum state.

So I wonder, @vanhees71 , before interpreting measurement results we all start with our 'take' on things, our philosophical worldview. If you don't believe quantum wavefunctions are a reality, how do you justify that nature doesn't care about what people believe should be true about these wavefunctions? If you give no answer, your opinion that nature doesn't care is your belief about wavefunctions and we, as realists, can answer: nature (reality) doesn't care that you think our beliefs are superseded by measurement results.

I usually avoid the word "reality", because its meaning is uncertain ;-)). The quantum state describes a preparation procedure for a quantum system. Having prepared the system in a given quantum state, you know the probabilities for the outcome of measurments of observables on the system, no more, no less, and that's what describes all observations with utmost precision.

 

[Fra]

But everything we've learned since then points to no hidden variables!

No, you missed a fine detail of my point. I think Vanhees did too.

There is no bell style HV, yes. I am with all this.

Which is a very specific HV with additional assumptions on how the average action is formed, which i think is the problem.

/Fredrik

 

[martinbn]

No, you missed a fine detail of my point. I think Vanhees did too.

There is no bell style HV, yes. I am with all this.

Which is a very specific HV with additional assumptions on how the average action is formed, which i think is the problem.

/Fredrik

I am not saying that it is 100% certain that there are no HV, only that so far all we know points in that direction. What makes you think that there could be HV (non Bell type of course)?

 

[gentzen]

No, you missed a fine detail of my point. I think Vanhees did too.

There is no bell style HV, yes. I am with all this.

Which is a very specific HV with additional assumptions on how the average action is formed, which i think is the problem.

I am not saying that it is 100% certain that there are no HV, only that so far all we know points in that direction. What makes you think that there could be HV (non Bell type of course)?

To Fra: Do you have an opinion on A. Neumaier's approach? It comes perhaps a bit more from the mathematical/logical/"statistical" side than your and Lee Smolin's more physical approach, but it also tries to overcome the limitations of a pure instrumental approach, and has a commitment to a single world. I know, it may seem strange that it seems to leave QT nearly unmodified, and still claims to have ("non-separable") HV hidden in plain sight, but ... What is your opinion? At least those are non-Bell style HV!

 

[martinbn]

To Fra: Do you have an opinion on A. Neumaier's approach? It comes perhaps a bit more from the mathematical/logical/"statistical" side than your and Lee Smolin's more physical approach, but it also tries to overcome the limitations of a pure instrumental approach, and has a commitment to a single world. I know, it may seem strange that it seems to leave QT nearly unmodified, and still claims to have ("non-separable") HV hidden in plain sight, but ... What is your opinion? At least those are non-Bell style HV!

I realize that is off topic but what is his approach? What are the hidden variables?

 

[PeterDonis]

But everything we've learned since then points to no hidden variables!

Not necessarily. The Bohmian interpretation has hidden variables and matches the predictions of standard QM. They're just nonlocal hidden variables--the particle positions are affected instantaneously by changes in the wave function anywhere in the universe.

 

[vanhees71]

That's consistent, of course, only in non-relativistic QM and at odds with relativistic QFT, which obeys by construction relativistic causality, i.e., no "action at a distance"!

 

[PeterDonis]

That's consistent, of course, only in non-relativistic QM

The original Bohmian interpretation was, yes. There are more recent relativistic versions (including, IIRC, one published by @Demystifier). Of course there is dispute in the literature about whether these versions actually work--just as there are disputes in the literature about every QM interpretation. I'm not trying to argue that the Bohmian interpretation is "correct", just giving it as an example of a hidden variable interpretation that is not ruled out by Bell's Theorem.

at odds with relativistic QFT, which obeys by construction relativistic causality, i.e., no "action at a distance"!

More precisely, no FTL signaling and spacelike separated measurements must commute. But that does not mean that a particular model cannot have unobservable nonlocal hidden variables that "change faster than light". It just means those variables cannot result in FTL signaling or spacelike separated measurements not commuting.

 

[vanhees71]

What are such unobservable nonlocal HIVs good for, particularly since it's claimed to solve some apparent philosophical quibbles about the physical meaning of the theory? It's just even more puzzling than the theory itself, which is consistent with the (at least special) relativistic spacetime description and causality, i.e., such ficticious hidden non-local variables breaking the intrinsic consistency of standard QFT do not solve apparent metaphysical problems but create even more serious new ones.

 

[PeterDonis]

What are such unobservable nonlocal HIVs good for

For constructing a model that has an actual mechanism for producing counterintuitive results of QM like violations of the Bell inequalities.

You might not think that's a useful goal, but others apparently do.

It's just even more puzzling than the theory itself

Perhaps to you, but not to everyone. It's certainly more concrete than just punting and saying, well, QM just predicts probabilities and doesn't tell you any actual mechanism for how things happen.

 

[Structure seeker]

I usually avoid the word "reality", because its meaning is uncertain ;-))

It's simply the collection of all true statements.

Having prepared the system in a given quantum state, you know the probabilities for the outcome of measurments of observables on the system, no more, no less, and that's what describes all observations with utmost precision.

Is this true or not? If it is, the behaviour of wavefunctions under measurement is part of reality as defined above. Is the statement "the system is in quantum state X at time t" true? The same applies. If it's not true, the state is not part of reality and therefore you can't use the idea that nature doesn't care what we think. All else is bullshit unless you come with an underlying consistent alternative for the belief in reality.

 

[PeterDonis]

It's simply the collection of all true statements.

This seems confused. Statements are statements about reality; a statement is not the same thing as the thing the statement is about.

Rather than try to make up your own definition, I would strongly recommend looking at the literature to see how the term "reality" is defined by the actual workers in the field. And I think you will find that there is not one single accepted definition of that term, nor is there even agreement in the literature that the term should be used at all.

Is this true or not?

Of course it is; it's basic QM. But that doesn't mean the rest of your claims are necessarily correct. See above.

 

[Fra]

What makes you think that there could be HV (non Bell type of course)?

There can be other arguments, but stay on track, what I had in mind here was that it was also Einsteins only (within reason) logical possibility explanation to quantum entanglement. As all other options seems just unacceptable.

The the total interference patterns, does NOT appear as if, the spins are predetermined but random, is not hte same thing. (which I would only guess Einstein would have agree with)

Ie. the problem isn't randomness, the problem is the quantum inseparability. So you must solve the latter, but not the first. So thinking there have to be some sort of "non-bell" HV, doesn't meen you have a theory for it, it seems like just a reasonable logical conclusion,

(Bell's theorem would in theory solve both, as if the HV was known, the outcome would be predetermined. And we know this gives wrong interference paterns!)

/Fredrik

 

[Fra]

To Fra: Do you have an opinion on A. Neumaier's approach? It comes perhaps a bit more from the mathematical/logical/"statistical" side than your and Lee Smolin's more physical approach, but it also tries to overcome the limitations of a pure instrumental approach, and has a commitment to a single world. I know, it may seem strange that it seems to leave QT nearly unmodified, and still claims to have ("non-separable") HV hidden in plain sight, but ... What is your opinion? At least those are non-Bell style HV!

I share alot of Nemaiers critique of current motivation of the statistical methods, so "something" needs to be done.

It's interesting but his solution is quite different to what I have in mind. His solution also, as far as I understand, does not aspire to provide insight into unification of forces without too much fine tuning (which is one of my motivators). I think Neumaiers treatise is also fairly "conservative", I just don't find that it solves all the problems I want to have answered, but it may solve some.

/Fredrik

 

[vanhees71]

It's simply the collection of all true statements.

Is this true or not? If it is, the behaviour of wavefunctions under measurement is part of reality as defined above. Is the statement "the system is in quantum state X at time t" true? The same applies. If it's not true, the state is not part of reality and therefore you can't use the idea that nature doesn't care what we think. All else is bullshit unless you come with an underlying consistent alternative for the belief in reality.

There is no general behavior of quantum states (wave functions are a very special case only) "under measurement". To describe its behavior you have to describe the interaction of the measured system with the measurement apparatus as you describe any interaction with the system with "its environment".

I still don't know, what you mean by "reality". For me "reality" are the objective observations we make in Nature. For physics we define quite refined quantified observations in terms of measurable observables, defined by an equivalence class of measurement processes for such a quantity/observable.

 

[Structure seeker]

I still don't know, what you mean by "reality". For me "reality" are the objective observations we make in Nature.

From propositional logic and the belief that statements are either true, false or some third undetermined value, I simply mean that reality is the statements (of all possible statements) that are true.
For me that is independent of being observed, measured or anything. They're true or they are not true. If that cannot work in our model of nature, quantum physics has left the foundation of logic and is therefore not anymore an exact science.

 

[GarberMoisha]

The problem is less about quantum theory and more with insisting on having objects in time and space, instead of events.
Reality as all the collection of events unfolding, has zero conceptual issues with QT. And if anything can be deduced from QT it is that this universe is about events(measurements and observations) Aka reality. Anything more would require a belief in a sophisticated structure of invisible hidden variables which may also resemble a conspiracy.

 

[Structure seeker]

And of course it is fine when the statement "the state is X at time t" has an unknown or third truth value, but then it is not yet acceptable to state "it is that way regardless of how we think it should be" (nature doesn't care what we think).