B Where does a quantum experiment *begin*?

[ddd123]

Can you give reference to example of QED model for polarization entangled photon measurements at different measurement angles?

I asked about this earlier. I'd like to know as well.

 

[zonde]

You keep repeating your personal theories that aren't in accordance with accepted science. If PF were serious about its rules, you should have been banned by now.

There is "Report" link under every post. Simply push it and write what rule this post violates as you see it. All moderators will see your message and will take action if necessary.

QM is fully compatible with locality. One can embedd the QM probabilities in a local classical probabilistic model (https://arxiv.org/abs/1412.6987), so the compatibility with locality is established at full rigor.

I like this counter example to locality of QM. It does not use any probabilities.

 

[vanhees71]

Are the local interactions "physical" or "real"?

They are clearly defined in the mathematical formalism, using the equal-time (anti-)commutation relations, local realizations of the unitary irreps. of the covering group of the Poincare group with $m^2 \geq 0$ and polynomial Lagrangians of the fields and their first derivatives, which leads to a Poincare covariant S-matrix and microcausality.

 

[vanhees71]

Can you give reference to example of QED model for polarization entangled photon measurements at different measurement angles?

Take any textbook on quantum optics, e.g., Scully&Zubairy for the foundations. The classical paper on the parametric-downconversion process is

C. K. Hong and L. Mandel, Phys. Rev. A 31, 2409 (1985)
http://dx.doi.org/10.1103/PhysRevA.31.2409

 

[rubi]

I like this counter example to locality of QM. It does not use any probabilities.

There is of course no counterexample to a proven theorem. Using frequencies instead of probabilities doesn't change that. The existence of a local probabilistic model that predicts the QM probabilities proves beyond doubt that these probabilities are compatible with locality. What's wrong with your counterexample? Frequency proofs of Bell's theorem make the same assumptions, they are just less obvious, because nobody is used to the frequency formulation. The choice of subsequences in the frequency formulation of probability is dual to the choice of a conditional probabilities in the measure formulation.

 

[ShayanJ]

One can embedd the QM probabilities in a local classical probabilistic model (https://arxiv.org/abs/1412.6987), so the compatibility with locality is established at full rigor.

That seems to be an important paper. Can we call it an interpretation of QM?

 

[David Lewis]

...is it true that we can have a situation of two "equal probability paths" without having a situation that is modeled by a superposition of states?

Yes. If we send particles through one at a time, and place detectors that tell us which path was taken then there can be no superposition. The presence of the detectors force the particle to either take one path or the other.

 

[atyy]

You are the one who claims that collapse is compatible with relativity, so you are obliged to prove it. What are your answers to the questions I askes in my previous post? How can collapse be compatible with relativity despite non-commutativity of projections?

Your paper addresses some paradoxes, but it doesn't prove the compatibility. Compatibility with relativity means that full Poincare symmetry is somehow implemented. If this can be done, then you should be able to answer the two questions that I askes in my previous post. Moreover, I'd like to see the mathematical implementation of the Poincare group.

My claim is at the physics level of rigour - in the same way that QED is said to be compatible with relativity. In fact, my claim is found in the standard textbooks.

You keep repeating your personal theories that aren't in accordance with accepted science. If PF were serious about its rules, you should have been banned by now. QM is fully compatible with locality. One can embedd the QM probabilities in a local classical probabilistic model (https://arxiv.org/abs/1412.6987), so the compatibility with locality is established at full rigor.

It's you that it is not accepting standard science.

 

[atyy]

They are clearly defined in the mathematical formalism, using the equal-time (anti-)commutation relations, local realizations of the unitary irreps. of the covering group of the Poincare group with $m^2 \geq 0$ and polynomial Lagrangians of the fields and their first derivatives, which leads to a Poincare covariant S-matrix and microcausality.

Sure, but if you use that language, then conjugate position and momentum can also exist simultaneously. In the Heisenberg picture, after a preparation, the time evolution of position and momentum observables are governed by the Hamiltonian, and they remain canonically conjugate.

 

[vanhees71]

There's no state where position and momentum are determined simultaneously. If a position operator exists (i.e., for all kinds of massive particles or for massless particles with spin $\leq 1/2$), then is obeys the Heisenberg algebra with momentum, which implies that for any state the uncertainty relation $\Delta x \Delta p \geq \hbar/2$ holds. Of course, the Heisenberg algebra of the operators in the Heisenberg picture is invariant under time evolution since time evolution is unitary.

 

[atyy]

There's no state where position and momentum are determined simultaneously. If a position operator exists (i.e., for all kinds of massive particles or for massless particles with spin $\leq 1/2$), then is obeys the Heisenberg algebra with momentum, which implies that for any state the uncertainty relation $\Delta x \Delta p \geq \hbar/2$ holds. Of course, the Heisenberg algebra of the operators in the Heisenberg picture is invariant under time evolution since time evolution is unitary.

The position and momentum observables exist simultaneously.

 

[vanhees71]

What do you mean by that. Of course, they exist if they exist (a photon, e.g., has no position observable, but that's another story). What has this to do with the discussion about locality of interactions, etc.?

 

[atyy]

What do you mean by that. Of course, they exist if they exist (a photon, e.g., has no position observable, but that's another story). What has this to do with the discussion about locality of interactions, etc.?

What I mean is that the locality you are talking about is locality of the Hamiltonian - these concern operators just like position and momentum. So if those "exist" for you, then in the same language position and momentum also simultaneously exist.

So the language you use is not at all the usual language. In the usual language, canonically conjugate position and momentum do not simultaneously exist.

Also in the usual language, quantum mechanics does require nonlocal interactions, assuming reality.

The "locality" that you are talking about should be called "no faster than light signalling".

Relativistic quantum mechanics in the minimal interpretation is about no faster than light signalling. It is not at all about the locality of interactions of mathematical terms representing physical objects, In fact, Bell's theorem guarantees that no such local theory can exist.

Basically, there are two notions of locality - classical relativistic causality (which is where we usually say that interactions are local) and no faster than light signalling, and you are mixing them up.

 

[vanhees71]

I think this is totally confusing. What do you mean by "exist" here? Of course, the observables exist, because you can measure them. In standard relativistic QFT the "no faster than light signalling" is realized by using local interactions. Bell's inequality is violated by QT and also by standard relativistic QFT although it's built with local interactions. So something is mathematically wrong in your argument. I only talk about the mathematical construction of the theory. We should this get clear first, before we enter the unsharp terrain of philosophy again!

 

[atyy]

I think this is totally confusing. What do you mean by "exist" here? Of course, the observables exist, because you can measure them. In standard relativistic QFT the "no faster than light signalling" is realized by using local interactions. Bell's inequality is violated by QT and also by standard relativistic QFT although it's built with local interactions. So something is mathematically wrong in your argument. I only talk about the mathematical construction of the theory. We should this get clear first, before we enter the unsharp terrain of philosophy again!

But what is the interaction between? The fields? If you mean observables exist, then the field do not necessarily exist, since the fields may not be Hermitian - eg. fermion fields.

 

[vanhees71]

Fermion fields are usually not representing observables. Observables are defined via Noether's theorem, which provides you with energy, momentum, and angular momentum (densities) and with various conserved current densities of global symmetries. Everything is expressed in terms of the field operators, of course. For the details, see a good textbook on QFT (e.g., Schwartz or for the finer details and more generality Weinberg).

 

[atyy]

Fermion fields are usually not representing observables. Observables are defined via Noether's theorem, which provides you with energy, momentum, and angular momentum (densities) and with various conserved current densities of global symmetries. Everything is expressed in terms of the field operators, of course. For the details, see a good textbook on QFT (e.g., Schwartz or for the finer details and more generality Weinberg).

So in your language fermion fields do or do not exist?

 

[Demystifier]

@atyy and @vanhees71 you will never come to an agreement because you are talking at two very different levels. While atyy discusses the ontology, vanhees71 talks at the epistemic level. Since it is impossible to make vanhees71 talk about ontology, perhaps you could both agree to talk only about the epistemic aspects? But then you would come to an agreement very soon, and there would be nothing to talk about.

 

[atyy]

@atyy and @vanhees71 you will never come to an agreement because you are talking at two very different levels. While atyy discusses the ontology, vanhees71 talks at the epistemic level. Since it is impossible to make vanhees71 talk about ontology, perhaps you could both agree to talk only about the epistemic aspects? But then you would come to an agreement very soon, and there would be nothing to talk about.

No actually, I prefer to talk at the epistemic level (ie. minimal interpretation). But at the epistemic level, collapse is compatible with relativity.

So I suspect that vanhees71 is objecting to collapse because he is working at the ontological level.

In fact, my standard position is epistemic or operational - it is vanhees71 who brings in ontology by objecting to collapse.

(You can see that this in my post #2, where I use collapse in the standard shut up and calculate way. It is vanhees71 who is somehow objecting to shut up and calculate.)

 

[stevendaryl]

I think this discussion has drifted off from the question asked in the thread title (the answer to that question is: a physics experiment begins with a grant application )

I like to think of the collapse hypothesis as having two parts:

1. When you measure an observable, you get an eigenvalue (with probabilities given by the Born rule).
2. Afterward, the system is in the state obtained by projecting the state onto the subspace corresponding to that eigenvalue.

(There is a sense in which rule #2 is only relevant for entangled systems. Typical measurements are destructive; when you measure a photon, the photon is gone afterward. So rule #2 comes into play when you have entangled subsystems: measuring a property of one subsystem can cause the other subsystem to "collapse" into a particular state.)

Rule #2 is definitely true, empirically, in the sense that it correctly predicts subsequent measurement results. But it's possible that it isn't necessary as an additional assumption, because you can always recast a sequence of measurements as a single, compound measurement, and so rule #1 would be sufficient. Rule #2 is more of a practical rule of thumb, because without collapse, you can't calculate probabilities for a sequence of measurements without describing the measuring devices quantum mechanically, which is infeasible. Collapse allows us to treat macroscopic measurement devices as if they were classical, having definite states at all times, and reserve QM for the description of microscopic systems (or extremely simple macroscopic systems).

To me, the weirdness of QM comes not from the collapse hypothesis, but from Rule #1. Why are observed values definite, when unobserved values are not? That seems to me to make "measurement" into a different class of interaction, but surely measurement should be explainable as quantum mechanical interactions between the system being measured and the measuring device?

 

[vanhees71]

I think this discussion has drifted off from the question asked in the thread title (the answer to that question is: a physics experiment begins with a grant application )

This made my day! It's very true!

 

[ddd123]

(You can see that this in my post #2, where I use collapse in the standard shut up and calculate way. It is vanhees71 who is somehow objecting to shut up and calculate.)

As I understood it, vanhees71 says that you can avoid using collapse altogether although it makes things more difficult in certain cases. So having collapse even as a calculational tool is not really "minimal", if you see the minimal interpretation not as the way to shut up and calculate but as the minimal core of quantum theory. And for him this also makes things more elegant with respect to relativity, since changing the inertial frame doesn't force you to change where collapse is supposed to happen.

 

[vanhees71]

@atyy and @vanhees71 you will never come to an agreement because you are talking at two very different levels. While atyy discusses the ontology, vanhees71 talks at the epistemic level. Since it is impossible to make vanhees71 talk about ontology, perhaps you could both agree to talk only about the epistemic aspects? But then you would come to an agreement very soon, and there would be nothing to talk about.

I don't care whether it's ontic or epistemic, I'm talking about physics, and theory has a mathematical level, which should be clarified first. The words "local interaction" and "long-range correlations" have a very clear and unambiguous meaning, and are well compatible as is proven by relativistic local QFT. A collapse implies non-local interactions and is thus not compatible with the relativistic space-time structure and not with the usual formulation of relativistic QFT which is compatible with this very space-time structure by construction.

If it comes to "ontology" it's very simple for a physicist: Things exist that can be observed reproducibly and objectively in nature.

 

[atyy]

As I understood it, vanhees71 says that you can avoid using collapse altogether although it makes things more difficult in certain cases. So having collapse even as a calculational tool is not really "minimal", if you see the minimal interpretation not as the way to shut up and calculate but as the minimal core of quantum theory. And for him this also makes things more elegant with respect to relativity, since changing the inertial frame doesn't force you to change where collapse is supposed to happen.

Yes, you can avoid collapse the same way you can avoid having the Bell inequalities violated at spacelike separation. But if we are talking at the level at which the Bell inequalities are violated at spacelike separation, then that is the same level at which standard quantum mechanics does contain collapse. Mind you, vanhees71 has explicitly said a standard text like Cohen-Tannoudji is not acceptable to him on the issue of collapse. So I am just using standard Cohen-Tannoudji, or Sakurai, or Weinberg, or Nielsen and Chuang which all have collapse. The big issue about interpretation is always brought up by vanhees71 because somehow the standard physics textbooks are not acceptable.

 

[atyy]

I don't care whether it's ontic or epistemic, I'm talking about physics, and theory has a mathematical level, which should be clarified first. The words "local interaction" and "long-range correlations" have a very clear and unambiguous meaning, and are well compatible as is proven by relativistic local QFT. A collapse implies non-local interactions and is thus not compatible with the relativistic space-time structure and not with the usual formulation of relativistic QFT which is compatible with this very space-time structure by construction.

If it comes to "ontology" it's very simple for a physicist: Things exist that can be observed reproducibly and objectively in nature.

Your mistake is in the phrase "a collapse implies non-local interactions". A collapse does not have any "interactions" in the way you use "local interactions".

 

[vanhees71]

Now it becomes totally bizzare. If the collapse is not caused by the interaction between the measured system and the measurement device, by what is it caused then?

 

[atyy]

Now it becomes totally bizzare. If the collapse is not caused by the interaction between the measured system and the measurement device, by what is it caused then?

The minimal interpretation is agnostic about "cause". "Local interactions" are properties of Hamiltonians. The collapse does not even affect the Hamiltonian, so how can the collapse be related to interactions?

 

[ddd123]

On my part, I don't know what to think. On one hand, the long-range correlations are there because of measurement, and avoiding collapse doesn't practically account for compound measurements (you have to believe it would work if you could do the practically impossible calculation of treating the whole measurement device quantum mechanically). On the other hand, collapse is frame-dependent, although the consequences are the same whatever frame you choose in the end, so it seems to beg for a deeper explanation.

 

[rubi]

That seems to be an important paper. Can we call it an interpretation of QM?

I wouldn't say it's important, since it just makes precise what was already consensus (QM is compatible with locality). However, I like it a lot, since it forces locality deniers to point out an error in the proof, which of course doesn't exist. I also wouldn't call it an interpretation. All interpretations of QM must predict the same probabilities and that papers just shows that they can arise from a classical probabilistic model that happens to be local.

My claim is at the physics level of rigour - in the same way that QED is said to be compatible with relativity. In fact, my claim is found in the standard textbooks.

I'm not asking for a mathematically rigorous presentation. Also a physicist understands compatibility with relativity to mean the implementation of the Poincare group, possibly at a non-rigorous level (as in QCD). I would already be satisfied if you could show me a non-rigorous implementation of the Poincare group that includes collapse. Show me one textbook that explicitly claims that collapse is compatible with relativity.

You are just telling me that I'm wrong, but you don't counter my arguments. Why don't you respond to the questions I posed earlier?

(By the way, I'm not even denying compatibility. I'm just saying that there is no evidence for it, but rather arguments against it.)

It's you that it is not accepting standard science.

The overwhelming majority of physicists interprets Bell's result in favour of locality and in disfavour of hidden variables. Only a die-hard minority of Bohmians advocates non-locality. Both groups agree that Bell can be interpreted in both ways. Moreover, the paper that I quoted proves unambiguously that QM is at least compatible with locality. So my view is in accordance with standard science.

 

[ddd123]

Rubi, why do you say "classical probabilistic model that happens to be local", when the abstract of the paper says "violation of Bell's inequality implies the impossibility to apply the classical probability model of Kolmogorov (1933) to quantum phenomena"?

 

[rubi]

Rubi, why do you say "classical probabilistic model that happens to be local", when the abstract of the paper says "violation of Bell's inequality implies the impossibility to apply the classical probability model of Kolmogorov (1933) to quantum phenomena"?

If you read the paper, you will see that he says that quantum probabilities can be interpreted as conditional probabilities that are embedded in a classical Kolmogorov model. They cannot be interpreted as absolute probabilities. However, the classical model, in which the quantum probabilities are embedded, is fully local. All of that is contained in section 8 of the paper. (In analogy to the situation in geometry: There exist non-Euclidean geometries, but they can be embedded in higher dimensional Euclidean spaces).

 

[atyy]

Show me one textbook that explicitly claims that collapse is compatible with relativity.

Weinberg.

 

[rubi]

Weinberg.

Ok. where does he claim it?

(I note that you refuse to address my questions.)

 

[atyy]

Ok. where does he claim it?

(I note that you refuse to address my questions.)

He states the axioms for QM in his QFT book, and that includes collapse. The axioms for QM carry through to QFT.

 

[atyy]

@rubi, also you can look up the literature yourself. It is absolutely standard.

 

[atyy]

For example, Peres uses the collapse in his discussion of relativistic QM.

 

[rubi]

He states the axioms for QM in his QFT book, and that includes collapse. The axioms for QM carry through to QFT.

In his QFT book, he only states the Born rule. The collapse isn't even mentioned in the list of axioms. As stevendaryl wrote earlier: The Born rule is not the same thing as collapse. Moreover, I have asked for an explicit statement that the collapse is compatible with relativity, not just a statement of the collapse.

But please respond to the following question: How can the non-commutativity of collapse be compatible with relativity, when in relativity, two time evolutions always commute?

@rubi, also you can look up the literature yourself. It is absolutely standard.

I pretty much know all the literature and I'm not aware of a single book that explains the compatibility of collapse with the Poincare group. That's why I'm asking. Your first attempt apparently failed already.

 

[atyy]

http://arxiv.org/abs/quant-ph/9906034

 

[atyy]

In his QFT book, he only states the Born rule. The collapse isn't even mentioned in the list of axioms. As stevendaryl wrote earlier: The Born rule is not the same thing as collapse. Moreover, I have asked for an explicit statement that the collapse is compatible with relativity, not just a statement of the collapse.

But please respond to the following question: How can the non-commutativity of collapse be compatible with relativity, when in relativity, two time evolutions always commute?I pretty much know all the literature and I'm not aware of a single book that explains the compatibility of collapse with the Poincare group. That's why I'm asking. Your first attempt apparently failed already.

I did not attempt to answer your question, as I said, I don't know where that is shown. But certainly collapse is standard in QM, and in all special cases studied, it is consistent with relativity. So yes, it is absolutely standard to say that collapse is consistent with relativity at the physics level of rigour.

And no, you are wrong - the Weinberg book does state collapse.

See the post above for relevant literature.

BTW, I did once see a paper addressing the question you ask, but can't remember where it is - in the meantime, see the Peres paper above.

 

[ddd123]

For example, Peres uses the collapse in his discussion of relativistic QM.

If you mean his quantum theory book, he only uses collapse for a reductio, to criticize the idea that at each instant an EPR pair has a definite wave function.

 

[atyy]

If you mean his quantum theory book, he only uses collapse for a reductio, to criticize the idea that at each instant an EPR pair has a definite wave function.

Yes, that's right. I love Peres's book, but it is deeply flawed. However in this article, he takes a more standard position: http://arxiv.org/abs/quant-ph/9906034

 

[ddd123]

Yes, that's right. I love Peres's book, but it is deeply flawed. However in this article, he takes a more standard position: http://arxiv.org/abs/quant-ph/9906034

Do you think that reductio is flawed, and if so, why?

 

[atyy]

Do you think that reductio is flawed, and if so, why?

I can't remember that part exactly. My main problem with the book is that he seems to say that coarse graining a larger system (including the apparatus) will solve the measurement problem.

 

[rubi]

I did not attempt to answer your question, as I said, I don't know where that is shown. But certainly collapse is standard in QM, and in all special cases studied, it is consistent with relativity. So yes, it is absolutely standard to say that collapse is consistent with relativity at the physics level of rigour.

It may be compatible with relativistic causality, but saying that it is compatible with relativity is different. A relativistic theory must implement the Poincare group and time evolution is just one special Poincare transformation. I think this is a consensus definition of a relativistic theory among physicists. If collapse is a form of time evolution, then it must be a Poincare transformation. This is a deeply physical requirement and not just unnecessary rigor. Physicists do care about this. Weinberg's QFT book for example puts a lot of emphasis on the implementation of the Poincare group, even though it is not a rigorous text. However, it doesn't address the issue, whether collapse is a Poincare transformation or not. I'm not aware of any physics textbook that addresses this issue.

And no, you are wrong - the Weinberg book does state collapse.

My edition of Weinberg's book "The Quantum Theory of Fields, Vol. 1" makes no mention of the collapse postulate. It just states the Born rule and nothing more. I even interpret this omission to be deliberate.

See the post above for relevant literature.

BTW, I did once see a paper addressing the question you ask, but can't remember where it is - in the meantime, see the Peres paper above.

The paper just explains the compatibility with relativistic causality. I know that this works, but it doesn't address the issue of the compatibility with the Poincare group. This is not an instance of the mathematicians/physicists battle about the right amount of rigor in physical theories. Anyway, I would be really interested in the paper you mentioned. As I said, I don't claim that collapse is incompatible with relativity. I'm just saying your arguments don't guarantee compatibility.

 

[ShayanJ]

It may be compatible with relativistic causality, but saying that it is compatible with relativity is different. A relativistic theory must implement the Poincare group and time evolution is just one special Poincare transformation. I think this is a consensus definition of a relativistic theory among physicists. If collapse is a form of time evolution, then it must be a Poincare transformation. This is a deeply physical requirement and not just unnecessary rigor. Physicists do care about this. Weinberg's QFT book for example puts a lot of emphasis on the implementation of the Poincare group, even though it is not a rigorous text. However, it doesn't address the issue, whether collapse is a Poincare transformation or not. I'm not aware of any physics textbook that addresses this issue.My edition of Weinberg's book "The Quantum Theory of Fields, Vol. 1" makes no mention of the collapse postulate. It just states the Born rule and nothing more. I even interpret this omission to be deliberate.The paper just explains the compatibility with relativistic causality. I know that this works, but it doesn't address the issue of the compatibility with the Poincare group. This is not an instance of the mathematicians/physicists battle about the right amount of rigor in physical theories. Anyway, I would be really interested in the paper you mentioned. As I said, I don't claim that collapse is incompatible with relativity. I'm just saying your arguments don't guarantee compatibility.

There is a problem with your request and I don't know why atyy doesn't point it out!
Collapse is not supposed to be a time evolution in the same sense as the unitary evolution you're talking about. Its not supposed to be a fundamental evolution besides the unitary evolution. Its not even supposed to be in a fundamental theory!
The crucial point was clearly explained by @stevendaryl but looks like it was ignored. When considering an open quantum system, an effective evolution seems to emerge for the (open) system. We're still unable to completely derive this effective evolution from the fundamental unitary evolution of the larger closed system. That's why we just consider its input and output and treat it as a blackbox and call it by such a mysterious name as collapse. When this blackbox is explained, we all expect to retain the unitary evolution that is compatible with Poincare group. So even if there is an incompatibility with the Poincare group, its because we're considering an effective evolution and ignoring part of the system.
Now it seems to me that the only point of disagreement here, can be whether that effective evolution actually emerges or not.

 

[rubi]

There is a problem with your request and I don't know why atyy doesn't point it out!
Collapse is not supposed to be a time evolution in the same sense as the unitary evolution you're talking about. Its not supposed to be a fundamental evolution besides the unitary evolution. Its not even supposed to be in a fundamental theory!
The crucial point was clearly explained by @stevendaryl but looks like it was ignored. When considering an open quantum system, an effective evolution seems to emerge for the (open) system. We're still unable to completely derive this effective evolution from the fundamental unitary evolution of the larger closed system. That's why we just consider its input and output and treat it as a blackbox and call it by such a mysterious name as collapse. When this blackbox is explained, we all expect to retain all the unitary evolution that is compatible with Poincare group.

I agree fully with this view. Collapse is an effective process that arises from a fully Poincare invariant theory with only unitary evolution. But that only means that the collapse is not part of a fundamental relativistic theory, but rather emerges from such a theory! Think of the following analogy: The quantum harmonic oscillator $H=p^2+q^2$ is not Galilei invariant, but we can embedd it into a larger theory $H=P^2 + p^2 + (Q-q)^2$ which is fully Galilei invariant. The QHO just emerges as the center of mass part of this larger system. In the same way, a theory with collapse emerges from a larger theory without collapse, in which only Poincare transformations are allowed.

I think atyy doesn't point this out, because he doesn't agree that collapse can emerge from a larger theory.

 

[ShayanJ]

I think atyy doesn't point this out, because he doesn't agree that collapse can emerge from a larger theory.

I'm not sure what you mean by a larger theory, but I expect that collapse emerges from the same QM we already have, when applied to an open system.

 

[rubi]

I'm not sure what you mean by a larger theory, but I expect that collapse emerges from the same QM we already have, when applied to an open system.

Yes, that's what I mean too. However, this is a controversial topic. Many physicists (like me) believe that this works out and others don't. I'd say the issue isn't settled completely.

 

[atyy]

It may be compatible with relativistic causality, but saying that it is compatible with relativity is different. A relativistic theory must implement the Poincare group and time evolution is just one special Poincare transformation. I think this is a consensus definition of a relativistic theory among physicists. If collapse is a form of time evolution, then it must be a Poincare transformation. This is a deeply physical requirement and not just unnecessary rigor. Physicists do care about this. Weinberg's QFT book for example puts a lot of emphasis on the implementation of the Poincare group, even though it is not a rigorous text. However, it doesn't address the issue, whether collapse is a Poincare transformation or not. I'm not aware of any physics textbook that addresses this issue.

In fact it must be, so I am not dismissing your question. It is true that is you read the standard texts, you may come to the erroneous view that the standard QFT Poincare group discussion is sufficient, when in fact it is not, since that only guarantees classical relativistic causality. In fact there is a gap in reasoning which you mention, that is, the standard discussion does not address how Poincare invariance fits with collapse. The usual assumption is that it does, because of the many special cases that have been worked out.

My edition of Weinberg's book "The Quantum Theory of Fields, Vol. 1" makes no mention of the collapse postulate. It just states the Born rule and nothing more. I even interpret this omission to be deliberate.

Weinberg gives a version of the Born rule from an old tradition in which collapse is stated as part of the Born rule.

Also, it is standard in Bell tests discussions throughout to assume collapse. In that case the theory is free relativistic QFT.

The paper just explains the compatibility with relativistic causality. I know that this works, but it doesn't address the issue of the compatibility with the Poincare group. This is not an instance of the mathematicians/physicists battle about the right amount of rigor in physical theories. Anyway, I would be really interested in the paper you mentioned. As I said, I don't claim that collapse is incompatible with relativity. I'm just saying your arguments don't guarantee compatibility.

In fact there is not much battle - most physicists would love to see a rigourous completion of QED, or string theory etc. The question is appropriateness. Would it be right for me to interrupt every thread about relativistic QED by saying that there is no proof that relativistic QED even exists? Here the start of the thread is just simple QM, and I used collapse in a completely standard shut up and calculate way, and there is the interruption that this is somehow not standard.

BTW, this is not the paper I was thinking of which dealt with collapse in the minimal interpretation, but here is one that deals with a physical collapse http://arxiv.org/abs/1111.1425.

 

[atyy]

There is a problem with your request and I don't know why atyy doesn't point it out!
Collapse is not supposed to be a time evolution in the same sense as the unitary evolution you're talking about. Its not supposed to be a fundamental evolution besides the unitary evolution. Its not even supposed to be in a fundamental theory!
The crucial point was clearly explained by @stevendaryl but looks like it was ignored. When considering an open quantum system, an effective evolution seems to emerge for the (open) system. We're still unable to completely derive this effective evolution from the fundamental unitary evolution of the larger closed system. That's why we just consider its input and output and treat it as a blackbox and call it by such a mysterious name as collapse. When this blackbox is explained, we all expect to retain the unitary evolution that is compatible with Poincare group. So even if there is an incompatibility with the Poincare group, its because we're considering an effective evolution and ignoring part of the system.
Now it seems to me that the only point of disagreement here, can be whether that effective evolution actually emerges or not.

I think atyy doesn't point this out, because he doesn't agree that collapse can emerge from a larger theory.

I don't discuss this because if all you have is unitary evolution, you will end up with unitary evolution of the universe with all the problems of interpretation including MWI etc.

My aim in this thread is to defend the minimal interpretation or shut up and calculate because it works. Vanhees71 claims to support shut up and calculate or the minimal interpretation, but if you notice, it is he that is always bringing up issues of interpretation by objecting to collapse.