How does QFT handle non-locality?

[HomogenousCow]

I've always found it weird that textbooks motivate by QFT by showing that particles have a non-zero probability of traveling outside their light cones in NRQM, but then after they're done with the quantization of the KG Lagrangian they completely forget about this issue, instead what they only show is that observables at different events commute with others outside of their light cones. I don't find it obvious that this alone preserves causality.

 

[zonde]

I don't understand the question. In which sense should a Hilbert space be "local" or "nonlocal"?

And I don't understand your's. I was asking about Fock space not single particle Hilbert space.

 

[atyy]

But a collapse is not part of the framework of QFT. See below.That is outside the scope of QFT already, it is bound to an interpretation. Busch let's an unspecified measurement process happen and assume this "collapses" the wave function. Equation 3 considers the state after this magic collapse.
Yes, because every relation to measurable predictions happens via an interpretation. Some interpretations are nonlocal. No one ever doubted that. Other interpretations are local. If QFT would be inherently nonlocal, there would be no local interpretations.

My comments are within the standard interpretation. Other interpretations that are able to be local are nonstandard and must state their assumptions.

Eg. Weinberg uses the standard interpretation when he write his collapse equation. So it is not only Busch. So do Landau and Lifshitz, Cohen-Tannoudji, Nielsen and Chuang. All have collapse.

 

[mfb]

My comments are within the standard interpretation. Other interpretations that are able to be local are nonstandard and must state their assumptions.

What is a "standard" interpretation?
If an interpretation is adding something like nonlocal effects, you should not claim that the theory is nonlocal: it is not. Your favorite interpretation of the local theory is nonlocal, that is a completely different statement.

Eg. Weinberg uses the standard interpretation when he write his collapse equation. So it is not only Busch. So do Landau and Lifshitz, Cohen-Tannoudji, Nielsen and Chuang. All have collapse.

The Copenhagen interpretation is widely used in descriptions, but that does not make it part of the theory.

 

[atyy]

What is a "standard" interpretation?
If an interpretation is adding something like nonlocal effects, you should not claim that the theory is nonlocal: it is not. Your favorite interpretation of the local theory is nonlocal, that is a completely different statement.

The Copenhagen interpretation is the standard interpretation. To avoid the Bell theorem, one needs something like MWI. I think it's pretty fair to say that MWI is nonstandard, eg. all outcomes occur.

 

[zonde]

Yes, because every relation to measurable predictions happens via an interpretation. Some interpretations are nonlocal. No one ever doubted that. Other interpretations are local. If QFT would be inherently nonlocal, there would be no local interpretations.

Considering what you say the best you can claim is that QFT is consistent with locality given there is scientifically sound local interpretation.

You see, if you use relative descriptions for distant things then the model is non-local. It might be consistent with locality if you can convert relative descriptions into absolute descriptions.

 

[Demystifier]

the cluster-decomposition principle as explained in Weinberg's Quantum Theory of Fields vol. I. I haven't found any mistake in this chapter. Could you point to precisely where you think there's something wrong there?

Please see the link in post #30.

 

[vanhees71]

I've always found it weird that textbooks motivate by QFT by showing that particles have a non-zero probability of traveling outside their light cones in NRQM, but then after they're done with the quantization of the KG Lagrangian they completely forget about this issue, instead what they only show is that observables at different events commute with others outside of their light cones. I don't find it obvious that this alone preserves causality.

Well, that's the point! We construct QFT such that it has this feature of microcausality. You are right, it's not enough to have this feature to prove causality. For that you need the Poincare covariance of the S-matrix elements, and one can show that the construction of microcausal local QFTs is sufficient for that. As far as I know, it's not clear whether it is also necessary. Pragmatically you can say that so far the paradigm of this kind of relativistic QT is very successful.

Of course, it's not complete in several aspects: First of all it's not mathematically rigorous, i.e., it is not clear whether QFT really is a mathematical solid theory beyond the perturbative techniques or lattice gauge theory (usually applied to QCD) we use to evaluate it. Second, it's not complete concerning also the physical aspects. The Standard Model of Particle physics (updated to incorporate neutrino mass and oscillations) does not describe dark matter, and last but not least there's no consistent description of gravity yet.

 

[vanhees71]

Please see the link in post #30.

Well this doesn't refer to the linked-cluster principle as explained in Weinberg, let alone pointing out a mathematical error in his treatment. If I find the time, I'll read this chapter again carefully over the weekend.

 

[mfb]

Considering what you say the best you can claim is that QFT is consistent with locality given there is scientifically sound local interpretation.

You see, if you use relative descriptions for distant things then the model is non-local. It might be consistent with locality if you can convert relative descriptions into absolute descriptions.

If you also introduce magical fairies, you have magical fairies. Does that mean QFT has magical fairies? Do we have to say "QFT is consistent with the nonexistence of magical fairies", or can we just say "QFT does not have magical fairies"?

 

[zonde]

If you also introduce magical fairies, you have magical fairies. Does that mean QFT has magical fairies? Do we have to say "QFT is consistent with the nonexistence of magical fairies", or can we just say "QFT does not have magical fairies"?

Physical reality is a must for physics theory while magical fairies are not.
If you have mathematical model and when you establish correspondence with physical reality you attribute the same mathematical object to two distant things then it's non-local as a physics theory. Establishing correspondence with physical reality is a must for mathematical model if we view it as physics theory. Establishing correspondence with magical fairies on the other hand is not required.

 

[atyy]

Well, that's the point! We construct QFT such that it has this feature of microcausality. You are right, it's not enough to have this feature to prove causality. For that you need the Poincare covariance of the S-matrix elements, and one can show that the construction of microcausal local QFTs is sufficient for that. As far as I know, it's not clear whether it is also necessary. Pragmatically you can say that so far the paradigm of this kind of relativistic QT is very successful.

Of course, it's not complete in several aspects: First of all it's not mathematically rigorous, i.e., it is not clear whether QFT really is a mathematical solid theory beyond the perturbative techniques or lattice gauge theory (usually applied to QCD) we use to evaluate it. Second, it's not complete concerning also the physical aspects. The Standard Model of Particle physics (updated to incorporate neutrino mass and oscillations) does not describe dark matter, and last but not least there's no consistent description of gravity yet.

Microcausality is not local reality.

Weinberg's error is not mathematical, but in his English explanation of the mathematics. The correct explanation of the linked cluster principle is that no superluminal transmision of classical information is allowed (ie. spacelike observables commute), and that time evolution preserves the inability for superluminal communication (linked cluster principle).

 

[ddd123]

Isn't the Aharonov-Bohm effect nonlocal too? What does QFT say about that?

 

[mfb]

Physical reality is a must for physics theory while magical fairies are not.
If you have mathematical model and when you establish correspondence with physical reality you attribute the same mathematical object to two distant things then it's non-local as a physics theory. Establishing correspondence with physical reality is a must for mathematical model if we view it as physics theory. Establishing correspondence with magical fairies on the other hand is not required.

You are combining one specific interpretation with QFT, and you call both together "theory". The interpretations are called interpretations instead of theories for a good reason. QFT delivers amplitudes (in a broad sense) and nothing else. The calculation to get those amplitudes are local. Everything beyond that is interpretation, and there are both local and nonlocal interpretations. Yes, you need interpretations to perform experiments and to test QFT, but you do not need nonlocal interpretations.

Isn't the Aharonov-Bohm effect nonlocal too? What does QFT say about that?

It is local, and it works as local effect in all interpretations.

 

[vanhees71]

Microcausality is not local reality.

Weinberg's error is not mathematical, but in his English explanation of the mathematics. The correct explanation of the linked cluster principle is that no superluminal transmision of classical information is allowed (ie. spacelike observables commute), and that time evolution preserves the inability for superluminal communication (linked cluster principle).

But that's all you need to make QT consistent with relativistic causality. As I said, I'm not sure whether microcausality is necessary for the linked-cluster principle to be valid. It's, however, sufficient, and that's nicely shown in Weinberg's book. I guess, I have to read the chapter again to see what may be wrong with the wording around it.

In the same sense you can say, the assumption of a collapse is just words. The difference is that the linke-cluster principle is essential for QFT being compatible with the relativistic space-time structure (and causality) while the collapse is simply not needed for anything and makes the theory inconsistent with relativistic causality. In a sense it is a contradiction to the linked-cluster principle.

 

[atyy]

But that's all you need to make QT consistent with relativistic causality. As I said, I'm not sure whether microcausality is necessary for the linked-cluster principle to be valid. It's, however, sufficient, and that's nicely shown in Weinberg's book. I guess, I have to read the chapter again to see what may be wrong with the wording around it.

In the same sense you can say, the assumption of a collapse is just words. The difference is that the linke-cluster principle is essential for QFT being compatible with the relativistic space-time structure (and causality) while the collapse is simply not needed for anything and makes the theory inconsistent with relativistic causality. In a sense it is a contradiction to the linked-cluster principle.

Why but? Weinberg is simply wrong. That's all.

 

[ShayanJ]

Now that we're talking about Weinberg's mistakes, I want to mention something I've always wanted to mention but haven't found the opportunity.
Weinberg defines a ray as(sect. 2.1, page 49, end of the page):

A ray is a set of normalized vectors (i.e., $(\Psi,\Psi)=1$) with $\Psi$ and $\Psi'$ belonging to the same ray if $\Psi'=\xi \Psi$, where $\xi$ is an arbitrary complex number with $|\xi|=1$.

But as I understand it(which I'm pretty sure is correct), $\xi$ doesn't have to be unit-norm and it can be any complex number. Its just that even after choosing a particular vector on the ray, we still have the freedom to multiply it by a unit-norm complex number.
I just want to provide an example of simple mistakes that even Weinberg can make so maybe its not too bold to say he's wrong on something else too.

 

[zonde]

You are combining one specific interpretation with QFT, and you call both together "theory". The interpretations are called interpretations instead of theories for a good reason. QFT delivers amplitudes (in a broad sense) and nothing else.

You mean that I implied collapse? But I didn't, my statement was very general.
And you have to square amplitudes to establish minimum correspondence to physical reality (experimentally observed relative frequencies). I suppose that this operation is present in any interpretation.

The calculation to get those amplitudes are local.

Calculations to get single particle amplitudes can be local, that's clear. But how would you argue that you can get by local calculations amplitudes that give you coincidence rates of distant entangled particles?

Yes, you need interpretations to perform experiments and to test QFT, but you do not need nonlocal interpretations.

I don't understand this. You don't need interpretation to take module squared of probability amplitude. And that's enough to perform experimental tests of QFT, right?

 

[atyy]

In the same sense you can say, the assumption of a collapse is just words. The difference is that the linke-cluster principle is essential for QFT being compatible with the relativistic space-time structure (and causality) while the collapse is simply not needed for anything and makes the theory inconsistent with relativistic causality. In a sense it is a contradiction to the linked-cluster principle.

This is wrong. Collapse is not a contradiction to the linked cluster principle. The linked cluster principle and the commutation of spacelike observables means "no superluminal signalling". However, although collapse is inconsistent with the reality of relativistic spacetime causality, it is not inconsistent with "no superluminal signalling". One way to see that you are wrong is that the "no signalling" set is bigger than the "relativistic spacetime causality" or "local" sets, eg. Fig 2 of http://arxiv.org/abs/1303.2849.

 

[mfb]

@zonde: squares of local things are still local.
The coincidence is not a physical event. You can have observers note this coincidence - but only with a time- or lightlike connection. See how the local interpretations handle this: it works.

 

[zonde]

squares of local things are still local.

Pure math is not local or non-local. Local or non-local are terms that describe physical reality not math. So you have to establish at least minimal correspondence with physical reality to speak about locality. With that on mind your statement is upside down: if squares (that represent relative frequencies of local detection events) are local you can argue that amplitudes should be considered local too. Unless of course you propose to establish direct correspondence between amplitudes and physical reality.

The coincidence is not a physical event.

Coincidence is not physical event but it is a physical observation. And my argument is based on how apparent FTL speeds of neutrinos in Opera experiment were perceived. It was considered that if FTL results of Opera experiment would be confirmed it would violate SR. And exactly for that reason it was considered so unbelievable and thoroughly investigated. Opera experiment looked at coincidences between emission and detection events and results of such analysis are considered physical as it takes physical observation to falsify physics theory (SR in this case).
But of course coincidence is not a basic physical observation. Basic physical observation is detection records with time tags. But derivations of coincidences and subsequent relative coincidence rates are external to QM so that results should be taken as physical observation by QM.

See how the local interpretations handle this: it works.

This thread is not about interpretations but about QFT instead. So please don't try to drag this discussion into discussion about interpretations.
But as you made the argument please name these interpretations. Then I could ask questions about these interpretations in another thread.

 

[vanhees71]

This is wrong. Collapse is not a contradiction to the linked cluster principle. The linked cluster principle and the commutation of spacelike observables means "no superluminal signalling". However, although collapse is inconsistent with the reality of relativistic spacetime causality, it is not inconsistent with "no superluminal signalling". One way to see that you are wrong is that the "no signalling" set is bigger than the "relativistic spacetime causality" or "local" sets, eg. Fig 2 of http://arxiv.org/abs/1303.2849.

Sure, it's inconsistent with "no superluminal signalling", because if you assume that the measurement of A's photon's polarization in the usual polarization-entangled biphoton state, leads to a collapse of the two-photon state, the polarization of B's photon is instantaneously determined, while before A's measurement it's maximally (in the sense of information theory) undetermined.

I've still to carefully read Brunner et al's RMP, but as long as quantum correlations are a subset of no-signalling correlations, everything is fine, right? But then one must abandone (at least the naive) collapse hypothesis.

 

[atyy]

Sure, it's inconsistent with "no superluminal signalling", because if you assume that the measurement of A's photon's polarization in the usual polarization-entangled biphoton state, leads to a collapse of the two-photon state, the polarization of B's photon is instantaneously determined, while before A's measurement it's maximally (in the sense of information theory) undetermined.

Let's suppose the initial state is |uu>+|dd>

When A measures u, then the state will immediately collapse to |uu>, so B will measure u with certainty. But can B tell that A made a measurement? He cannot, because if A always measures before B, A will collapse the state to |uu> half the time and to |dd> the other half of the time. But if A measures after B, then B will measure u half the time and d half the time. So although taking collapse as reality will violate relativistic causality as something real, collapse does not lead to any superluminal communication. This is why collapse is consistent with "no superluminal signalling".

I've still to carefully read Brunner et al's RMP, but as long as quantum correlations are a subset of no-signalling correlations, everything is fine, right? But then one must abandone (at least the naive) collapse hypothesis.

The quantum correlations are a subset of no-signalling, and the relativistic causality correlations are a subset of the quantum correlations. Quantum mechanics including collapse violates relativistic causality as something real, but it does not violate no signalling.

 

[vanhees71]

Let's suppose the initial state is |uu>+|dd>

When A measures u, then the state will immediately collapse to |uu>, so B will measure u with certainty. But can B tell that A made a measurement? He cannot, because if A always measures before B, A will collapse the state to |uu> half the time and to |dd> the other half of the time. But if A measures after B, then B will measure u half the time and d half the time. So although taking collapse as reality will violate relativistic causality as something real, collapse does not lead to any superluminal communication. This is why collapse is consistent with "no superluminal signalling".

Sure, but still the state change assumed by the collapse is instaneously acting over a long distance. Your argument of unobservability of the collapse is a perfect argument to just abandon the postulate of collapse.

It's clear that Alice get's the correct result about what Bob will find, assuming that after her measurement the state is $|uu \rangle$, but it doesn't apply that anything happens instantaneously to B's particle due to A's local spin measurement. So what you call a "collapse" here is just the adaption of A's description of the system after she made her measurement, it's not a statement of some physical process acting instantaneously on B's particle.

The quantum correlations are a subset of no-signalling, and the relativistic causality correlations are a subset of the quantum correlations. Quantum mechanics including collapse violates relativistic causality as something real, but it does not violate no signalling.

This I don't understand. If the collapse is taken as a real physical phenomenon then it violates relativistic causality. If it's taken as something non-real, you can just forget about it. I don't know of any example of the application of quantum theory where you need to assume the collapse as a real physical process, and that's why I don't understand, why it is still used today (or after 1935, when EPR pointed out that it's contradicting relativistic causality).

 

[Demystifier]

This I don't understand.

This only confirms my note in #33.
Now seriously, I am trying to understand what exactly you don't understand. Do you know what is signal locality and do you understand why is collapse compatible with signal locality?

 

[vanhees71]

No, obviously not.

 

[Demystifier]

No, obviously not.

Then let me explain signal locality (and some of the other types of locality) in a few short steps.

1. In the realm of quantum foundations and interpretations, there are several different notions of locality/non-locality. Signal locality/non-locality is only one of them.

2. As you know, different interpretations claim that QM is local or non-local in one way or another. But signal locality, as one specific notion of locality, has a special status. It is special because all interpretations agree that QM has the property of signal locality.

3. So what is signal locality? Unlike other notions of locality, signal locality is a very antropomorphic concept. Signal locality means that you cannot send signal faster than light. Here "signal" means information that can be manipulated, controlled and measured by humans in practice.

4. What is signal locality not? For example, if there is a wf collapse, you cannot use it to send a signal faster than light. That's because collapse is random, so you cannot choose to which final state the wf will collapse. Since you cannot choose it, you cannot manipulate and control the collapse. Thus, even though in collapse there is some kind of information transfer faster than light, in collapse there is no signal faster than light. Therefore collapse is compatible with signal locality.

5. Similarly, non-local hidden variables such as Bohmian theory are also compatible with signal locality. For a simple explanation see
https://www.physicsforums.com/threa...ctual-definiteness.847628/page-2#post-5319182

6. Is QFT local? It depends on what exactly one mans by "local". It certainly has property of signal locality. It also has some other types of locality. However, it does not necessarily has all possible types of locality. Depending on interpretation, it may or may not be non-local due to collapse or due to hidden variables. From the known facts about QFT we cannot exclude such non-local features.

7. Is QFT non-local in some interpretation-independent sense? Yes! QFT violates Bell inequalities, and violation of Bell inequalities is also one (of many) notion of non-locality. This non-controversial type of non-locality can be reduced to the fact that QFT contains not only local operators $\phi_1(x)$, $\phi_2(x)$, ... but also "non-local" (more precisely, multi-local) operators such as $O(x,y)=\phi_1(x)\phi_1(y)+\phi_2(x)\phi_2(y)$. Clearly, this fact does not depend on interpretation.

I hope it helps.

 

[atyy]

Sure, but still the state change assumed by the collapse is instaneously acting over a long distance. Your argument of unobservability of the collapse is a perfect argument to just abandon the postulate of collapse.

It's clear that Alice get's the correct result about what Bob will find, assuming that after her measurement the state is $|uu \rangle$, but it doesn't apply that anything happens instantaneously to B's particle due to A's local spin measurement. So what you call a "collapse" here is just the adaption of A's description of the system after she made her measurement, it's not a statement of some physical process acting instantaneously on B's particle.

The collapse postulate cannot be abandoned even if it one is agnostic about its reality. This because it is very difficult to argue that it is "just the adaption of A's description of the system after she made her measurement, it's not a statement of some physical process acting instantaneously on B's particle". If it were true, then that would follow from the laws of probability, but I am not aware of any successful derivation of collapse as simply an updating of knowledge without any physical process.

This I don't understand. If the collapse is taken as a real physical phenomenon then it violates relativistic causality. If it's taken as something non-real, you can just forget about it. I don't know of any example of the application of quantum theory where you need to assume the collapse as a real physical process, and that's why I don't understand, why it is still used today (or after 1935, when EPR pointed out that it's contradicting relativistic causality).

Even if you treat collapse as non-real, you cannot save relativistic causality unless you assume something like many worlds, retrocausation etc. That is the content of the Bell theorem: relativistic causality is dead or empty.

One can be agnostic about the reality of collapse. However, it is wrong to reject on the basis of superluminal communication, since collapse does not allow superluminal communication. It is also wrong to reject collapse in order to save relativistic causality, unless one adopts many worlds, retrocausation etc, since apart from those ways of avoiding the Bell theorem, quantum mechanics implies that relativistic causality is dead or empty.

 

[vanhees71]

Then let me explain signal locality (and some of the other types of locality) in a few short steps.

1. In the realm of quantum foundations and interpretations, there are several different notions of locality/non-locality. Signal locality/non-locality is only one of them.

2. As you know, different interpretations claim that QM is local or non-local in one way or another. But signal locality, as one specific notion of locality, has a special status. It is special because all interpretations agree that QM has the property of signal locality.

3. So what is signal locality? Unlike other notions of locality, signal locality is a very antropomorphic concept. Signal locality means that you cannot send signal faster than light. Here "signal" means information that can be manipulated, controlled and measured by humans in practice.

4. What is signal locality not? For example, if there is a wf collapse, you cannot use it to send a signal faster than light. That's because collapse is random, so you cannot choose to which final state the wf will collapse. Since you cannot choose it, you cannot manipulate and control the collapse. Thus, even though in collapse there is some kind of information transfer faster than light, in collapse there is no signal faster than light. Therefore collapse is compatible with signal locality.

5. Similarly, non-local hidden variables such as Bohmian theory are also compatible with signal locality. For a simple explanation see
https://www.physicsforums.com/threa...ctual-definiteness.847628/page-2#post-5319182

6. Is QFT local? It depends on what exactly one mans by "local". It certainly has property of signal locality. It also has some other types of locality. However, it does not necessarily has all possible types of locality. Depending on interpretation, it may or may not be non-local due to collapse or due to hidden variables. From the known facts about QFT we cannot exclude such non-local features.

7. Is QFT non-local in some interpretation-independent sense? Yes! QFT violates Bell inequalities, and violation of Bell inequalities is also one (of many) notion of non-locality. This non-controversial type of non-locality can be reduced to the fact that QFT contains not only local operators $\phi_1(x)$, $\phi_2(x)$, ... but also "non-local" (more precisely, multi-local) operators such as $O(x,y)=\phi_1(x)\phi_1(y)+\phi_2(x)\phi_2(y)$. Clearly, this fact does not depend on interpretation.

I hope it helps.

Yes, that helps a lot, and it underlines that the assumption of a collapse as a physical objective process is empty and unnecessary, because you can never test it against the minimal (ensemble) interpretation.

 

[vanhees71]

The collapse postulate cannot be abandoned even if it one is agnostic about its reality. This because it is very difficult to argue that it is "just the adaption of A's description of the system after she made her measurement, it's not a statement of some physical process acting instantaneously on B's particle". If it were true, then that would follow from the laws of probability, but I am not aware of any successful derivation of collapse as simply an updating of knowledge without any physical process.
Even if you treat collapse as non-real, you cannot save relativistic causality unless you assume something like many worlds, retrocausation etc. That is the content of the Bell theorem: relativistic causality is dead or empty.

One can be agnostic about the reality of collapse. However, it is wrong to reject on the basis of superluminal communication, since collapse does not allow superluminal communication. It is also wrong to reject collapse in order to save relativistic causality, unless one adopts many worlds, retrocausation etc, since apart from those ways of avoiding the Bell theorem, quantum mechanics implies that relativistic causality is dead or empty.

I think we discuss in circles again, but for me the very successful application of local microcausal QFT to the real world proves this statement wrong. It explains perfectly the violation of Bell's inequality in accordance with very accurate observations thereof without killing relativistic causality. To the contrary: Relatistic causality is used in the very construction of this class of QT models. As I said before, locality and microcausality is sufficient but AFAIK not necessary for relativistic causality.

 

[Demystifier]

Yes, that helps a lot, and it underlines that the assumption of a collapse as a physical objective process is empty and unnecessary, because you can never test it against the minimal (ensemble) interpretation.

I agree with you that the minimal ensemble interpretation is in many respects better than the physical collapse interpretation. Yet, I don't think that the idea of a physical collapse is completely useless, at least for some physicists. For psychological reasons, many physicists can more easily think about physics if they have a visual picture in their mind of the physical processes involved. The minimal ensemble interpretation, unfortunately, does not provide such a picture. After all, that's why it is called minimal. Therefore some physicists look for alternative interpretations which do provide some picture. And among many pictures provided by many non-minimal interpretations, the physical collapse collapse interpretation is in some sense "minimal" itself. Namely, such a picture does not require any other object except the wave function, and, at the same time, does not require any other world except the world that we see. That's why the physical collapse picture is still popular among some physicists. And if that picture helps them to make calculations, as long as the results of their calculations do not differ from results of calculations done by physicists using other pictures or using no pictures at all, I do not see a reason to judge them for using a picture that works for them.

 

[vanhees71]

Hm, but also for the minimal interpretation, I can stick with the position representation and wavemechanics as long as we restrict ourselves to non-relativistic systems of constant particle number. I don't know, what the collapse can provide in addition to the ensemble representation in the sense of heuristic pictures. State preparation in the sense of von Neumann filter measurements are much more natural than when the collapse hypothesis is applied. I just don't bother about the formalism but filter out "partial beams" from the ensemble that don't have the properties I like to prepare, e.g., a certain spin state using a Stern Gerlach apparatus. I just block the unwanted beams and get a practically well-determined spin component in direction of the magnetic field. That's it. No complicated thinking in terms of fictitious collapses needed :-).

 

[atyy]

I think we discuss in circles again, but for me the very successful application of local microcausal QFT to the real world proves this statement wrong. It explains perfectly the violation of Bell's inequality in accordance with very accurate observations thereof without killing relativistic causality. To the contrary: Relatistic causality is used in the very construction of this class of QT models. As I said before, locality and microcausality is sufficient but AFAIK not necessary for relativistic causality.

But that is simply wrong. "Microcausality" is not what you believe it to be. "Microcausality" is not relativistic causality. "Microcausality" means no superluminal signalling. QFT in the minimal interpretation is not consistent with relativistic causality - this conclusion can only be evaded by eg. many-worlds or retrocausation.

 

[vanhees71]

Well, we seem to have different language. Microcausality+locality of the interactions indeed excludes superluminal signalling. Together with the dynamics of QT that implies relativistic causality, or what else do you need to establish it?

 

[Demystifier]

I just block the unwanted beams

This sentence is very problematic in minimal ensemble interpretation (MEI). Namely, this sentence sounds as if the "beam" is a physical object existing even without our observations. On the other hand, using only MEI, I think you cannot answer whether the beam physically exists without our observations. Thus, the language you use does not seem compatible with MEI. So either
i) you really use something more than MEI (even if you fail to recognize it), or
ii) within MEI you have to answer whether the beam exists without our observations, or
iii) stay agnostic about this question and adopt your language accordingly, to prevent false impression of believing in beams existing without our observations.

So what is your choice, i), ii), or iii)?