Issues on notation and concept of entanglement

[martinbn]

When you ask me, does it actually help?

It helps to understand your view. It does not help to see why you hold it!

 

[vanhees71]

The book (Verständliche Quantenmechanik: Drei mögliche Weltbilder der Quantenphysik) is not just on Bohmian mechanics. But it is indeed well written, easy to read, and clears up quite some confusion.

But I also share your confusion, because that book also talks about relativistic Bohm-Dirac theory, but vanhees71 seemed pretty sure that there is no such thing.

As I said, I can appreciate a textbook that explains a theory well, which I don't consider useful. I can accept Bohmian the Bohmian interpretation for non-relativistic QM, because it doesn't contradict any observations nor is it intrinsically inconsistent. Nevertheless I don't think that it adds anything to minimal interpreted QM as a physical theory. Maybe some philosophers feel less uneasy having the illusion of a deterministic interpretation, and I generally think one should be tolerant concerning other people's (quasi-)religious believes.

What is inacceptable for me is a reinterpretation of a valid physical theory which violates the fundamental physical meaning of the theory. In the case of relativistic QFT there seems not to exist a Bohmian reinterpretation respecting causality in some way. The trouble is that a Bohmian theory must be nonlocal and it seems hard to find a nonlocal but still causal relativistic theory at all. I'm not sure, whether there aren't even "no-go theorems" to find such a theory, but I think it's pretty probable that such a no-go theorem may be valid.

It's also hard to believe that a consistent point-particle interpretation of relativistic QFT. Note that even (interacting) classical point-particle theories don't work in the relativistic realm. Even the standard mixed particle-field framework (classical relativistic electron theory a la Dirac) is flawed and can be made valid only in a very limited case as an effective theory (the Landau-Lifshitz approximation to the Abraham-Lorentz-Dirac equation is the best model one has today).

Also pure field theoretical approaches like the many models Einstein and Schrödinger investigated seem not to work.

BTW: The book by Dürr is available in English too now:
https://www.springer.com/gp/book/9783030400675

 

[Demystifier]

It's the only book, where I thought to understand what's behind Bohmian mechanics.

A quote from the book (Durr and Teufel, Bohmian Mechanics):
"It is often said that the aim of Bohmian mechanics is to restore determinism in the quantum world. That is false. Determinism has nothing to do with ontology. What is “out there” could just as well be governed by stochastic laws, as is the case in GRW or dynamical reduction models with, e.g., flash ontology [12, 13]. A realistic quantum theory is a quantum theory which spells out what it is about. Bohmian mechanics is a realistic quantum theory. It happens to be deterministic, which is fine, but not an ontological necessity. The merit of Bohmian mechanics is not determinism, but the refutation of all claims that quantum mechanics cannot be reconciled with a realistic description of reality."

If this little paragraph doesn't make sense to you, then you have absolutely no idea what's behind Bohmian mechanics.

 

[Demystifier]

Maybe some philosophers feel less uneasy having the illusion of a deterministic interpretation

No, they feel less uneasy having the illusion of an ontic interpretation. But of course, since ontic doesn't mean measurable or deterministic, for you it means absolutely nothing.

 

[Demystifier]

What is inacceptable for me is a reinterpretation of a valid physical theory which violates the fundamental physical meaning of the theory.

I think I understand you now. "Reinterpretation" and "meaning" refer to philosophic or religious (rather than scientific) aspects of a theory. But since reinterpretations are unacceptable for you, it means that you are dogmatic about it, so in your case it's more a religion than a philosophy. Which is perfectly OK, many great physicists were religious.

 

[vanhees71]

A quote from the book (Durr and Teufel, Bohmian Mechanics):
"It is often said that the aim of Bohmian mechanics is to restore determinism in the quantum world. That is false. Determinism has nothing to do with ontology. What is “out there” could just as well be governed by stochastic laws, as is the case in GRW or dynamical reduction models with, e.g., flash ontology [12, 13]. A realistic quantum theory is a quantum theory which spells out what it is about. Bohmian mechanics is a realistic quantum theory. It happens to be deterministic, which is fine, but not an ontological necessity. The merit of Bohmian mechanics is not determinism, but the refutation of all claims that quantum mechanics cannot be reconciled with a realistic description of reality."

If this little paragraph doesn't make sense to you, then you have absolutely no idea what's behind Bohmian mechanics.

Ok, there is a bit too much philosophical gibberish in the book too. SCNR.

 

[vanhees71]

I think I understand you now. "Reinterpretation" and "meaning" refer to philosophic or religious (rather than scientific) aspects of a theory. But since reinterpretations are unacceptable for you, it means that you are dogmatic about it, so in your case it's more a religion than a philosophy. Which is perfectly OK, many great physicists were religious.

We are discussing all the time philosophical issues. For me there is no need at all for another interpretation than the standard minimal interpretation of relativistic QFT. Why should I introduce an interpretation for it which contradicts the fundamental concepts behind it, among them locality? I'm indeed at a loss, what you understand under "ontology". I thought, it's trying to answer the question what "really is". One answer is modern science, and in my opinion it's the only answer that makes sense, i.e., it is a method to make mental pictures (theories) about what really is based on objective observations of Nature.

If for an ontic interpretation you don't need determinism, then I don't know, why to bother about Bohmian mechanics at all, because for me the only merit is that for non-relativistic (and only for non-relativistic) QM it is a successful deterministic interpretation which does not contradict the very fondations of QM as a physical theory describing phenomena correctly within it's realm of validity. For relativistic QFT there seems not to be a version of Bohmian mechanics that is satisfactory, because it contradicts the very foundation of relativity, i.e., causality, which afaik is only realized in terms of local (quantum) fields, and at least the version you propose on your slides seems not to be consistent with this, particularly not for gauge theories, where you define the gauge fields (potentials) as observables.

This has nothing to do with religion or dogmatism but in the believe that a non-minimal interpretation is of little to no value if it contradicts the fundamental properties of the underlying conceptual framework of the minimally interpreted theory. In science minimal interpretations with the least unnecessary ballast are most useful.

 

[gentzen]

I'm indeed at a loss, what you understand under "ontology". I thought, it's trying to answer the question what "really is".

It is less about "what really is" and more about "what would be sufficient to exist". It is a (mathematical) model, and a model can need more or less "ontological commitments" for its constructions. Bohm's original proposal is quite "expensive" in this respect.

Just like for nonlocality, one can ask the question whether this is just an artifact of Bohm's original proposal, or whether this is nearly unavoidable for any similar model. And this question has indeed been asked and partially answered:
Quantum computing and hidden variables by Scott Aaronson, 2005 (https://journals.aps.org/pra/abstract/10.1103/PhysRevA.71.032325)
Exponential complexity and ontological theories of quantum mechanics by Alberto Montina, 2008 (https://journals.aps.org/pra/abstract/10.1103/PhysRevA.77.022104)

 

[Killtech]

As a follow up question I want to put forward this: A singlet state of entangled particles is notated in a superposition of product states as: |up,down⟩−|down,up⟩. It is not clear to me if this singlet state describes measurement outcomes or wavefunctions. Nevertheless, when the measurement is done, it represents both measurement outcome and wavefunction.

I think it means to represent the wave function therefore it is actually interesting to bring those things into interactions to see if they interact differently from other states.

... So assume you would have two photon pairs in a |up,down⟩−|down,up⟩ i.e. 4 photons in total. Now let's take one out of each pair and bring then into a two photon HOM interference. Now that state actually makes a difference for the interaction itself and specifically the parts we think not locally present at the interference do play a role in the HOM calculation. The question is how indistinguishable the remaining part of the states are to make the mixing term interfere destructively.

But if there is doubt about distinguishibility, one should take into account that once a two photon interference was done, they become indistinguishible either way. So even if there was double about it on the one side of the experiment, when the other side of the experiments does the same, it becomes hard to find an argument why the interference shouldn't appear. But that would make it seem like the outcome depends on what the away side of the experiment decides to do, i.e. the interaction of such states does not look very local.

 

[Demystifier]

It helps to understand your view. It does not help to see why you hold it!

Does this help?
https://www.physicsforums.com/threads/ontology.1007637/post-6548873