Undergrad Getting rid of nonlocality from quantum physics

[martinbn]

My question to you is: How do you interpret the idea that reality is wrong? What does it mean to you?

This is where you refuse to understand that realism in quantum interpretations is used differently than it is in philosophy. Realism means that all observables have definite values at all times. Antirealism rejects that proposition. It has nothing to do with the existence of say an electron, it is about whether the electron's position, momentum etc have definite values.

In addition, what do you mean by electron? A click in the electron detector? A wave function of the electron? A mental picture of an electron by a physicist? Something else? What?

Electron is the name for the thing that have objective existence.

 

[bhobba]

This particular definition of realism (which is not the only definition of realism used, but is perhaps more precise than other definitions) is called counterfactual definiteness.

That's one reason Bell made progress, he gave more precise definitions of things that are often quite 'fuzzy' like reality etc. We need to somehow overcome what Feynman very profoundly said in his famous lectures: “We can't define anything precisely. If we attempt to, we get into the paralysis of thought that comes to philosophers... one saying to the other: you don't know what you are talking about! The second one says: what do you mean by talking? What do you mean by you? What do you mean by know?” This is a very hard part, maybe even the hardest, of investigating such questions.

Thanks
Bill

 

[martinbn]

The right question is what they do have even when we don't measure it? Clearly they have a name (an "electron"), but what else? And if there is nothing specific which they have (except the name), then how is that different from the claim that they don't exist?

The electron has charge for example, doesn't it?

 

[Demystifier]

Realism means that all observables have definite values at all times. Antirealism rejects that proposition.

By that definition, Bohmian interpretation is an antirealist interpretation. And yet it is nonlocal. Moreover, the Bell theorem proves that all "antirealist" interpretations (in the sense in which the Bohmian interpretation is "antirealist") must be nonlocal. How about that?

Electron is the name for the thing that have objective existence.

Can we make any mathematical statement about properties of those objectively existing entities? Yes we can. The Bell theorem is one such statement, it says that objective properties must obey nonlocal laws.

 

[Demystifier]

The electron has charge for example, doesn't it?

Actually it doesn't. The charge $e$ is just a coupling "constant" describing the interaction between two electrons. It is not a property of one electron itself. In fact it is not even a constant, but depends on the distance (or momentum transfer) between the electrons.

Or perhaps by charge you meant the operator $\hat{Q}$? It is an observable like any other (energy, momentum, etc) and hence amenable to Heisenberg uncertainty relations.

 

[bhobba]

Electron is the name for the thing that have objective existence.

Yes but what is objective existence? Feynman thought the electron was a particle (as do I) because it gives a click on a particle detector and similar observations. That's certainly a reasonable way of thinking about it, but not the only one, which is a real problem in being precise. If everyone involved agrees with it then progress can be made, but getting that agreement to begin with is the trick. Feynman's view was quite successful, and hence has wide acceptance (I even believe it - besides few would argue with his famous Lectures), but is not the only way of thinking about it. It's likely the success in experimental confirmation of results that stem from a way it's viewed leads to sort of a circular reasoning on it - namely it reinforces that view. In saying that I have no issues with it being like that - but its wise to keep in mind it is like that when talking about fundamental things.

Thanks
Bill

 

[bhobba]

The electron has charge for example, doesn't it?

I gave it my like because its a very good question. In classical physics it certainly does because its invariably viewed that way and it definitely works. That's because it fits nicely with classical theory. But in QED we view it differently - as pointed out by Demystifier it's a coupling constant between electron and quantum EM fields (photon fields). Since everything is really quantum it would be an interesting question exactly when you can use one view or the other. Personally I think there is really only one view - a coupling constant to an EM field and that should be pointed out in EM texts, but experience has shown you do not get into trouble being 'slack'. In fact I think a lot of things in physics are like that - the truth is more complex than usually presented but we mostly do not get into trouble with being conventional. My god - I just supported Poincare's and Wittgenstein's conventionalism (I feel I can mention Wittgenstein because he also was a respected aeronautical engineer/scientist - not just a philosopher).

Thanks
Bill

 

[Demystifier]

The only thing, which many people seem to have still difficulties with is that it's not deterministic, but why should it be?

You are the only one here who even mentions the issue of determinism. But of course, let me remind you once again

Well, I don't think that one should take too seriously any non-mathematical statement on quantum interpretations too seriously.

 

[Demystifier]

it says that there is always this thing called "electron" (or more precisely "electron field" since QFT is the most fundamental version of QM we currently have), and the experimental phenomena are particular manifestations of this thing. QFT certainly does not say that the electron field is only there when we measure it.

What do you mean by "electron field"? Do you mean the field operator? The electron field operator is not even an observable (because it's not hermitian), and it doesn't even need to be a part of the formalism (as in path integral quantization). So I don't understand what do you mean when you say that "there is always electron field".

 

[Demystifier]

The philosophical argument that the electron itself may not exist between measurements is not (IMHO) demanded by Copenhagen; only that its dynamic properties are described by probabilities - and not well defined except by measurement.

That's problematic at two levels.

First, it says that (in the absence of measurement) probability is the only objective property of an individual object. It requires a propensity interpretation of probability, which is philosophically unappealing. But that's not such a serious problem.

Second, it postulates that something very deep and important happens by measurement, without explaining where does measurement come from. So it turns us back to where we started, namely at the measurement problem.

 

[Demystifier]

Why do you think so?!

See post #16 by @Lord Jestocost !

 

[PeroK]

That's problematic at two levels.

First, it says that (in the absence of measurement) probability is the only objective property of an individual object. It requires a propensity interpretation of probability, which is philosophically unappealing. But that's not such a serious problem.

Second, it postulates that something very deep and important happens by measurement, without explaining where does measurement come from. So it turns us back to where we started, namely at the measurement problem.

That may all be true. The simple point I made was that Copenhagen does not demand that the existence of an electron (insofar as that has meaning in physics) is dependent on measurement; only its dynamic properties are dependent on measurement.

I thought that was the point at debate.

 

[Demystifier]

The simple point I made was that Copenhagen does not demand that the existence of an electron (insofar as that has meaning in physics) is dependent on measurement; only its dynamic properties are dependent on measurement.

Fine, but this implies the existence of non-dynamic properties. Hence it can only help if one makes clear what one means by "non-dynamic properties".

And by the way, if probability is one such non-dynamic property, note that it is a nonlocal property. For instance, the joint probability $p({\bf x}_1,{\bf x}_2)$ of positions of two entangled particles is not local, in the sense that it is not defined on local positions ${\bf x}_1$ and ${\bf x}_2$. Hence it seems that introducing "non-dynamic properties" (whatever that means) cannot avoid the Bell's conclusion that objective properties must be nonlocal.

 

[PeroK]

Fine, but this implies the existence of non-dynamic properties. Hence it can only help if one makes clear what one means by "non-dynamic properties".

And by the way, if probability is one such non-dynamic property, note that it is a nonlocal property. For instance, the joint probability $p({\bf x}_1,{\bf x}_2)$ of positions of two entangled particles is not local, in the sense that it is not defined on local positions ${\bf x}_1$ and ${\bf x}_2$. Hence it seems that introducing "non-dynamic properties" (whatever that means) cannot avoid the Bell's conclusion that objective properties must be nonlocal.

Who's trying to avoid non-locality?

 

[Demystifier]

@Vaxjo where are you?

You entitled this thread as "Getting rid of nonlocality from quantum physics" but given the quote from https://arxiv.org/abs/2001.02977: "Thus, the whole story is not about physics, but about the rules for update of the given probability distribution on the basis of measurement output." it seems that it is really about getting rid of nonlocality from quantum non-physics.

 

[Demystifier]

Who's trying to avoid non-locality?

The thread starter, for instance.

More generally, since the Bell theorem, most attacks on realism are inspired by a desire to save locality.

 

[bhobba]

What agenda are you following with this Heisenberg/Bohr "bashing"? That you don't understand Heisenberg or Bohr, is your personal problem. Other physicists, who have some background in philosophy, do!

It's not 'bashing' Bohr to point out he had quite a philosophical view of QM, a number, including myself, find difficult. Everyone knows Bohr was a great physicist whose views need to be, and are respected. Einstein respected him, and I think understood him, but didn't agree with him. Ideas like complementarity, except as simply another name for the indeterminacy relations, I think obscure. Dirac had published its final mathematical form, and wrote 'The interpretation of quantum mechanics has been dealt with by many authors, and I do not want to discuss it here. I want to deal with more fundamental things.'.

Thanks
Bill

 

[andresB]

In my view we have theorems that shed light on the issue of locality in QM, but really QM is silent on the matter. There is one 'exception' however, as is made clear in Ballentine, chapter 3, Schrodinger's equation etc depends of the Galilean transformations that are fundamentally non-local, but that is not what is usually meant in discussions of quantum locality.

Can you expand on what you mean here?

 

[Demystifier]

In my view we have theorems that shed light on the issue of locality in QM, but really QM is silent on the matter.

That's because QM (in its minimal form) is silent on ontology. Once you make an ontological commitment https://en.wikipedia.org/wiki/Ontological_commitment the absence of locality becomes explicit. As long as one uses soft talk like "Of course something is real but we don't know exactly what it is so we shall refrain from saying anything precise about reality", it's hard to make nonlocality explicit.

 

[Vaxjo]

The earliest presentation of the contextual (Bohr-like) viewpoint on quantum theory was presented in the debate with Johann Summhammer, see https://arxiv.org/abs/quant-ph/0111130, see also https://arxiv.org/pdf/quant-ph/0401072.pdf for formulation in the form of an interpretation, and it was summarized in the book by Springer, "Contextual approach to quantum formalism".

 

[Demystifier]

@Vaxjo I see your main agenda here is not to have a discussion, but to draw attention to your work. Well, I guess that's legitimate too.

 

[vanhees71]

The right question is what they do have even when we don't measure it? Clearly they have a name (an "electron"), but what else? And if there is nothing specific which they have (except the name), then how is that different from the claim that they don't exist?

What does an electron have? As any particle, it's classified by its intrinsic properties, i.e., its mass, spin, and its charges wrt. the interactions described in the standard model.

 

[EPR]

Correct me if i am wrong, but fields in QFT are treated as mathematical objects(ie operator fields). Thus 'particles' have no objective existence and properties except as excitations of the respective quantum field(upon the loosely defined concept of 'measurement').
The quantum fields permeate all of space and time and because the respective field is not in its lowest energy state everywhere, this give rise to 'particles'(fermions, etc.) which are being created at various points. Correct?

 

[martinbn]

Correct me if i am wrong, but fields in QFT are treated as mathematical objects(ie operator fields). Thus 'particles' have no objective existence and properties except as excitations of the respective quantum field(upon the loosely defined concept of 'measurement').
The quantum fields permeate all of space and time and because the respective field is not in its lowest energy state everywhere, this give rise to 'particles'(fermions, etc.) which are being created at various points. Correct?

This is self contradictory. The first paragraph says that quantum fields are mathematical objects. The second that they permeate space-time!

 

[martinbn]

What do you mean by "electron field"? Do you mean the field operator? The electron field operator is not even an observable (because it's not hermitian), and it doesn't even need to be a part of the formalism (as in path integral quantization). So I don't understand what do you mean when you say that "there is always electron field".

To me that is so obvious, I am not sure what confuses you. It seems to me that you constantly mix up the map and the territory.

 

[martinbn]

See post #16 by @Lord Jestocost !

It doesn't contradict anything I said!

 

[EPR]

This is self contradictory. The first paragraph says that quantum fields are mathematical objects. The second that they permeate space-time!

Indeed, there is a contradiction(in my understanding). What is the correct answer?

 

[Demystifier]

To me that is so obvious, I am not sure what confuses you. It seems to me that you constantly mix up the map and the territory.

What's obvious? What's the "territory" in the case of the electron?

 

[Demystifier]

It doesn't contradict anything I said!

But it answers the specific question that you asked.

 

[vanhees71]

Correct me if i am wrong, but fields in QFT are treated as mathematical objects(ie operator fields). Thus 'particles' have no objective existence and properties except as excitations of the respective quantum field(upon the loosely defined concept of 'measurement').
The quantum fields permeate all of space and time and because the respective field is not in its lowest energy state everywhere, this give rise to 'particles'(fermions, etc.) which are being created at various points. Correct?

Hm, that's a bit strange to me, because it's not different in classical physics. There you can say with the same right: "Particles are described as mathematical objects (i.e., trajectories in phase space). Thus 'particles' have no objective existence...".