2a] Alice measured particle A as spin-up and particle A is spin-up and particle B is spin-down +
[2b] Alice measured particle A as spin-down and particle A is spin-down and particle B is spin-up — Andrew M

So - two 'Alices'?

So - two 'Alices'? — Wayfarer

Yes, that's the result of the unitary evolution of the quantum state per the Schrodinger equation.

Yes. — Andrew M

Thank you. Perhaps one of them has indeed gone through the looking glass.

In Quantum Field Theory, as far as I know, a field is itself regarded as a real physical thing (which can be visualized as a mattress with springs). — Andrew M

Any physicist who regards a field as a real thing, has got a very strange ontology. It cannot be visualized as a mattress with springs, because numerous fields can occupy the same place, and mattresses can't do that.

As I said before, to think of it as something real requires that it is the property of a thing, and the only thing which it can be a property of is space-time. But this is to reify space and time. How is space-time a substance?

So - two 'Alices'? — Wayfarer

Yes, that's the result of the unitary evolution of the quantum state according to the Schrodinger equation. — Andrew M

Thank you. Perhaps one of them has indeed gone through the looking glass. — Wayfarer

You didn't understand MW enough to know that Alice is in both of them? Argument from incredulity? One of the main points of MW is to do away with action at a distance.

You didn't understand MW enough to know that Alice is in both of them? Argument from incredulity? One of the main points of MW is to do away with action at a distance. — noAxioms

Yes, this it's how far scientists are willing to go to preserve locality. They'll even present a never ending, every growing universe of multi-verses. How many of me does there exist? As many choices I've made in my life so far, moment by moment and combined with the choices being made by others?? It boggles the mind to imagine such a multi-universe. It totally destroys the notion of realism whatever ones notion of realism must be. Alice has indeed come to life.

In the meantime, non-locality as predicted by the Bohm model has been experimently verified as interpreted by those scientists who are working on the problem.

It totally destroys the notion of realism whatever ones notion of realism must be. — Rich

It does at least make a hash of ones dualistic notions of personal identity.
I mist-stated the position. There is an Alice in both of them. I initially said that Alice is in both of them. The reference to an objective identity doesn't work.

How many of me does there exist? — Rich

One still.

I'm not asserting the position here, just being amused at the boggling. It solves the problem of effects happening well before their causes, something that is not mind boggling, but still a violation of a lot of principles of physics. So I ask you to say what the difference in experience would be if MW turned out to be how things are objectively? Surely there must be a difference if it is so implausible.

I'm not asserting the position here, just being amused at the boggling. It solves the problem of effects happening well before their causes, something that is not mind boggling, but still a violation of a lot of principles of physics. So I ask you to say what the difference in experience would be if MW turned out to be how things are objectively? Surely there must be a difference if it is so implausible. — noAxioms

The difference lies in our understanding of the nature of the universe and what that might entail spiritually, metaphysically and physical practicality. No one could have predicted the effects that the introduction of quantum physics would have on our lives, in all of these dimensions, when we moved from the mechanical universe of Newton to the quantum universe. Entanglement at a distance, if it is a real phenomenon, would probably have a profound impact on all of our lives.

You didn't understand MW enough to know that Alice is in both of them? — noAxioms

Explaining irony is pointless, isn't it. I was trying to drive home the point of the inherent implausibility of the so-called 'Everett Interpretation', but I guess if you're willing to entertain the notion of [ x ] Alice's, then it's never going to work.

They'll even present a never ending, every growing universe of multi-verses. — Rich

The extent of my knowledge of such matters is bounded by New Scientist and other such sources, but one thing I do know is that 'multiverses' and 'parallel universes' are two different beasts. The 'multiverse conjecture' is associated with string theory, and consists of the idea that 'our universe' is but one instance of an unthinkably large number of 'universes' which are theoretically predictable on the basis of string theory (the number being larger than all the atoms in 'our universe'). But the rub is, 'other universes' are forever outside our 'event horizon', i.e. even if they exist, there's no way to know. (This is the subject of a large and ongoing controversy between George Ellis (against) and Sean Carroll (for)).

Parallel worlds, on the other hand, does fall out of the Everett conjecture. That is the idea that every possible outcome of a measurement is real in some parallel universe. It implies that everything that could happen, does happen, in one or another of these universes.

Personally, I am glad not to be a scientist in these times - if I was I would steer well clear of these subjects.

I've read journals where they have referred to Everett's Many Worlds as Multi-verses, but it is off no matter, because it is impossible to describe or imagine such a reality whether it be I've or a million words.

Even though it's obviously an abstruse topic, 'many worlds' and 'multiverses' are actually different ideas, that's what I'm saying.

Incidentally, in my universe, Everett holds no sway. X-)

OK. Thanks.

Of course, one can buy into an infinite number of universes to avoid non-locality. But then, what is being observed in all of these experiments? Bohm would say the quantum potential acting at a distance. — Rich

What is being observed is the correlation due to the initial preparation of the entangled pair of particles. If they have been prepared in a state where they have opposite spins, then that's the way they stay regardless of the distance they travel apart.

It's a philosophical choice. Local action or spooky action.

Thank you. Perhaps one of them has indeed gone through the looking glass. — Wayfarer

"Who in the world am I? Ah, that's the great puzzle!" - Alice

Any physicist who regards a field as a real thing, has got a very strange ontology. It cannot be visualized as a mattress with springs, because numerous fields can occupy the same place, and mattresses can't do that. — Metaphysician Undercover

Fair enough. See https://xkcd.com/895/

It's a philosophical choice. Local action or spooky action. — Andrew M

I would put it a little differently. The choice is:

A) Experimentally verified entanglement at a distance (non-locality)

or

B) An a never ending,, growing number of unverifiable, unknowable universes.

I think this is a much better way to describe what is actually being presented.

B) An a never ending,, growing number of unverifiable, unknowable universes. — Rich

If true, then the same would apply to the Bohmian pilot wave.

To summarize:

[1] The Everett model - realism plus quantum mechanics (the relative states of the wave function equally exist, also called "worlds" or "branches").

[2] The Bohm model - realism plus a classical modification of quantum mechanics (non-local, incompatible with special relativity and quantum field theory).

[3] The Copenhagen model - quantum mechanics with a wave function collapse postulate (inherently random, wave-particle duality, paradoxes including EPR, Schrodinger's cat).

If true, then the same would apply to the Bohmian pilot wave. — Andrew M

Absolutely. All in one universe so we can actually explore the phenomenon.

https://www.newscientist.com/article/2078251-quantum-weirdness-may-hide-an-orderly-reality-after-all/

[3] The Copenhagen model - quantum mechanics with a wave function collapse postulate (inherently random, wave-particle duality, paradoxes including EPR, Schrodinger's cat). — Andrew M

There is no 'Copenhagen model'. It is not a model or even a scientific hypothesis. The 'Copenhagen interpretation' refers merely to the kinds of things that Bohr, Heisenberg, and Pauli said about the philosophical implications of quantum physics during the 1920's and 30's. The term 'copenhagen interpretation' wasn't even coined until the 1950's.

I think it can be stated that the phenomenon of quantum entanglement undermines scientific realism, but again that's a philosophical observation. The science of the issue is simply that this is something that happens as indicated by a set of measurements.

On the whole the book contains nothing that is not to be found in previous publications, particularly in the investigations of Bohr. The purpose of the book seems to me to be fulfilled if it contributes somewhat to the diffusion of that 'Kopenhagener Geist der Quantentheorie' [i.e., Copenhagen spirit of quantum theory] if I may so express myself, which has directed the entire development of modern atomic physics. — Werner Heisenberg

Foreword to The Physical Principles of the Quantum Theory, published in 1930.

The wikipedia entry goes on:

The term 'Copenhagen interpretation' suggests something more than just a spirit, such as some definite set of rules for interpreting the mathematical formalism of quantum mechanics, presumably dating back to the 1920s.However, no such text exists, apart from some informal popular lectures by Bohr and Heisenberg, which contradict each other on several important issues. It appears that the particular term, with its more definite sense, was coined by Heisenberg in the 1950s,[3] while criticizing alternate "interpretations" (e.g., David Bohm's[4]) that had been developed.[5]

Emphasis added.

Absolutely. All in one universe so we can actually explore the phenomenon. — Rich

Agreed. For example, see the interaction-free measurements such as the Elitzur–Vaidman bomb tester.

I think it can be stated that the phenomenon of quantum entanglement undermines scientific realism, but again that's a philosophical observation. — Wayfarer

I think that's true for classical scientific realism. But, of course, quantum entanglement has a straightforward realist explanation on the Everett model as outlined earlier in this thread.

I mist-stated the position. There is an Alice in both of them. I initially said that Alice is in both of them. The reference to an objective identity doesn't work. — noAxioms

It's interesting to see what the math says here. A quantum state can itself be in a linear superposition of quantum states which, for a two-state system, can be represented as psi = psi_1 + psi_2.

So, in the double-slit experiment, we can refer to the (absolute) quantum system that is the particle that is emitted and arrives at the back screen at a location predicted by the interference pattern. We can also refer to the two (relative) quantum systems that are the two distinct particles that go through each slit. So there are three quantum systems, each with its own quantum state, and each with distinct and real identities.

But, of course, quantum entanglement has a straightforward realist explanation on the Everett model as outlined earlier in this thread. — Andrew M

And, as I responded, the fact that it implies 'two Alices' means that, as far as I am concerned, it is not realistic.

And, as I responded, the fact that it implies 'two Alices' means that, as far as I am concerned, it is not realistic. — Wayfarer

Heliocentrism was once considered unrealistic. Same with 4D spacetime. Reality doesn't seem too concerned with what we consider realistic.

Heliocentrism was once considered unrealistic. — Andrew M

You don't seem to grasp how much more radical the notion that there are many worlds, or parallel universes, is, than that of heliocentrism, which had actually already been proposed by some ancient asteronomers. But I have learned that if you can accept the idea that there are parallel universes, which myself and many others thinks is an absurd idea, then clearly there is no line of argument that can be used against it. By definition it is not a scientific concept. as it can't be falsified. There is, therefore, no point in discussing it further.

I agree with the idea that the concept of "particle", or "wave" is not the reality. There is no real wave-particle duality it's just mathematically convenient to think one way or another when that is what works.

The interpretation that comes closest to reality, for me, is Carlo Rovelli's RQM (https://en.wikipedia.org/wiki/Relational_quantum_mechanics). For him, the underlying structure is composed of quantum states. Wave-function collapse occurs when quantum states interact and human consciousness is just one of those quantum states, so the observer problem disappears. It's the most "real" interpretation I've come across.

But I have learned that if you can accept the idea that there are parallel universes, which myself and many others thinks is an absurd idea, then clearly there is no line of argument that can be used against it. — Wayfarer

The multiplicity is inherent in the mathematics of quantum mechanics - its not something that can just be ignored as if it weren't there. That's why the Everett model just is unitary quantum mechanics and is falsifiable on that basis (for example, it would be falsified if a physical collapse mechanism were discovered). And it is why the Bohm model, while also realist, is necessarily a different physical model requiring non-local action (with the multiplicity relegated to the pilot wave).

The interpretation that comes closest to reality, for me, is Carlo Rovelli's RQM — daldai

Great to hear another perspective.

The way I would characterize RQM is as quantum mechanics with a relativist premise. That is, RQM defines reality in terms of the interaction between systems. So, in the double-slit experiment, it would be real for the particle/apparatus system that the particle has gone through a particular slit. But it would also be real for an independent observer that the particle is in a superposition of going through both slits.

According to RQM, this is not a contradiction since no comparison between observables can be made until the observer and particle/apparatus system have interacted. If they do interact, then the observer will find that they agree that the particle has gone through a particular slit and so reality is then defined for the entire observer/particle/apparatus system.

Is that how you understand RQM?

The multiplicity is inherent in the mathematics of quantum mechanics - its not something that can just be ignored as if it weren't there — Andrew M

But again, what is the motivation behind the mathematics? What is the problem that the maths is trying to solve? Why go to the bother?

But again, what is the motivation behind the mathematics? What is the problem that the maths is trying to solve? Why go to the bother? — Wayfarer

The problem is to predict the behavior of particle systems which classical mechanics cannot do. What distinguishes quantum mechanics from classical mechanics is that a quantum state can be a linear superposition (i.e., multiplicity) of component quantum states. Take the multiplicity out and you're back to classical mechanics.

Much is made of the experiment’s into Bell’s inequalities and their implication for Einstein.

https://plato.stanford.edu/entries/bell-theorem/

Einstein, to speak very roughly, appeared to plump for realism - that things really did have defined properties even when they weren’t being observed or interacting with other systems.

With loopholes of the Bell equalities being closed in recent years we often paint Einstein as being wrong-headed about realism.

I suspect, however, that there are too many assumptions being smuggled in about causation when it is not clear people have thought deeply about the subject.

The implication for so-called ‘entangled’ particles seems to be that the detection of a particle here can cause the spin of a particle on the other side of the galaxy. This is NOT to say the detection of a particle here can cause you to KNOW the spin of a particle on the other side of the galaxy, which would have been a more easily explainable fact.

If all we mean by ‘causation’ is simply that the words ‘detection of a particle’ don’t give you information about the spin of a particular particle, and that the words ‘the spin of a particle’ don’t give you information about the detection of a particle, then the result IS that the detection of a particle would cause the spin of a particle.

Now that’s a particular hobby horse of mine, you may disagree that’s what causation is. Yet it is not settled in science on what causation is, so who is to to say that’s NOT what causation is?

Do your preferred understandings of causation explain the Bell experiments? Or do they simply tell us the universe is non deterministic after all?