The typical and the exceptional in physics

[Simon Phoenix]

Yes, that seems fair, but I would have said that realist epistemology is doomed from the start, even in classical physics

I'm not really convinced of that. I think classical physics is essentially ontological in nature (and becomes more epistemic when we consider complex systems that require a statistical approach).

I mean if I give a golf ball a really nice satisfying thwock it's going to travel from A to B on a parabolic path. If I manage to exactly reproduce the same swing with another ball - it's going to follow exactly the same parabolic path (OK I have to neglect air resistance but I hope you see the point). Classical physics doesn't fart about trying to get all philosophical about whether we're describing 'reality' by writing down a quadratic equation for the trajectory - it would actually say that the golf ball IS traveling on a parabolic path. I don't see anything at all epistemic about that and it would be absurd (classically) to try to describe the equation as merely a representation of our 'knowledge' rather than descriptive of something that's actually happening.

It's really only QM that messed things up and lead us down the philosophical rabbit hole.

 

[ddd123]

It's really only QM that messed things up and lead us down the philosophical rabbit hole.

For Ken G, I'd guess that doesn't prove anything, or maybe it could even prove that we had to get to a certain point to have our illusion shattered - which QM did - and make us wake up to the intrinsically epistemological nature of science.

But anyway, Ken G, I still suggest you look at Feyerabend. For him, Popper wasn't too harsh with ontological presuppositions, he was too gentle, in fact Feyerabend showed that Popper's is yet another form of positivism, albeit a more refined one (here positivism is meant in the proper sense, that one recognizes only that which can be empirically verified in the sense that reality is positively characterized by empirical inquiry). I don't want to get into this but you seem to be very interested in philosophy so I'll point to that which IMHO is important for understanding the situation, even if you don't agree with that.

 

[A. Neumaier]

I wouldn't call it "very special circumstances" when those are the only circumstances we ever test!

I don't know whom you subsume under ''we'', but most physicists never make a quantum textbook measurement.

Already real measurements of position have an uncertainty of fuzzy width that cannot be described at all by collapse to an eigenstate (which doesn't exist), and only imperfectly by POVMs, the most advanced ''simple'' measurement recipe discussed in the foundations.

Measuring photons destroys them, rather puts them into an eigenstate of anything.

Just about any real measurement doesn't fit the straightjacket of a textbook description. The Stern-Gerlach experiment is almost the only exception.

 

[A. Neumaier]

does this old post of yours describe an aspect of your thermal interpretation, a consequence of it, or is it an addition?

Extended causality is most likely a consequence of relativistic quantum field theory, together with the thermal interpretation (which assigns reality to extended objects only).

 

[atyy]

I think I understand the position of @vanhees71 , as well as your position, so I believe I can explain his position in a way you can understand.

There is a consistent way to protest against collapse without caring about ontology. To do this, the most important thing is to define words one is using. So let us give the definitions:

Definition 1:
Wave-function is a mental tool used by people who understand QM.

Comment: A 100 years ago wave functions did not exist.

Definition 2:
Wave-function update is a mental act by a person who understands QM. In this act, an old wave function is replaced by a new wave function, with intention to better represent the new knowledge acquired by new measurement results.

Definition 3:
Wave-function collapse is any sudden change of wave function which cannot be described by a Schrodinger-like equation and cannot be classified as a wave-function update.

Comment: From those definitions it follows that collapse and update are mutually exclusive.

Now we need the rules for using the wave function (according to the minimal ensemble interpretation):
Rule 1: $|\psi|^2$ is probability density.
Rule 2: If no results of measurements are known, $\psi$ should be considered as changing with time according to a Schrodinger-like equation.
Rule 3: When results of measurement are known, the wave function should be updated.

Comment: The rules are not the axioms. The purpose of the rules is to provide a practical user manual.

Observation 1: From rules and definitions above it follows that wave function collapse should never be used.
Observation 2: Nothing of the above depends on ontology.

It doesn't work, because the update is nonlocal in the sense that a correct way of updating the wave function is to assign it to a hyperplane of simultaneity, and then updating it instantaneously by non-Schroedinger evolution across the hyperplane.

There is no collapse, only updating, but the updating can be considered nonlocal.

Objecting to a nonlocal update does depend on ontology, since the update does not conflict with the locality of relativistic QFT.

Another way of saying it is that ontology is a tool for performing wave function updating. In that ontology, wave function updating is nonlocal. And there is nothing about that nonlocality that conflicts with QFT - unless one is mistaking the locality of QFT to be ontological.

 

[A. Neumaier]

It sounds like Dr. Neumeier is arguing that what we do on Earth is a position measurement that is highly approximate, so although the photon wavefunction did indeed extend over much of the visible universe

No. I argue that individual photons don't exist unless they are specifically prepared, and that the whole setting is about measuring on Earth (locally) a very weak electromagnetic field generated (long ago) by the distant star.

Note that under usual circumstances, photons are indistinguishable - to distinguish them is already an approximation, valid only when there is something definite to distinguish them!

 

[A. Neumaier]

I think he would say that there is no such thing as a wave function describing the whole LHC.

But there is a density operator describing the LHC. Otherwise the question arises - what is the largest subsystem of the LHC that can be described by quantum mechanics (and hence by a density operator). Clearly no proper subsystem matches the description, hence the whole is described by QM.

 

[ddd123]

Extended causality is most likely a consequence of relativistic quantum field theory, together with the thermal interpretation (which assigns reality to extended objects only).

Thanks. I would really like a more comprehensive exposition of the thermal interpretation but alas, you have already expressed your skepticism towards the usefulness of such endeavor.

 

[A. Neumaier]

I would really like a more comprehensive exposition of the thermal interpretation but alas, you have already expressed your skepticism towards the usefulness of such endeavor.

I cannot recall such a skepticism. Indeed, I am working on a paper on the thermal interpretation that can be be formally published, but with low priority.

But have you read my web pages on it? These are fairly comprehensive. If you have specific questions about it you can ask them here or by email.

 

[ddd123]

I cannot recall such a skepticism. Indeed, I am working on a paper on the thermal interpretation that can be be formally published, but with low priority.

Oh, it was here.

But have you read my web pages on it? These are fairly comprehensive. If you have specific questions about it you can ask them here or by email.

Mainly I'm interested in how to "solve" the EPR paradox, which is why I quoted that post.

 

[RockyMarciano]

Would you say you understand logic?

I didn't intend to discuss logic, I'd rather stick to physics. You seem to have a strong bend towards pure philosophy, which is great, don't get me wrong. But I don't find the usefulness of the radical distinction you make between ontology and epistemology in physics. In the end what you are describing as a purely epistemic way of thinking can be seen as an ontology where the entities are the models, but then in philosophy one is always bound to fall into those traps created by looking at problems as purely excluding antinomies(in this case epistemic vs.ontic).
I don't think it helps at all to make any progress in understanding what we observe, which is the goal of physics.

Also, given your signature describing physics as a dialog with nature, which sounds good to me I would have thought your answer should have been "understand nature" or "model nature".

 

[zonde]

I think I understand the position of @vanhees71 , as well as your position, so I believe I can explain his position in a way you can understand.

There is a consistent way to protest against collapse without caring about ontology. To do this, the most important thing is to define words one is using. So let us give the definitions:

Definition 1:
Wave-function is a mental tool used by people who understand QM.

Comment: A 100 years ago wave functions did not exist.

Definition 2:
Wave-function update is a mental act by a person who understands QM. In this act, an old wave function is replaced by a new wave function, with intention to better represent the new knowledge acquired by new measurement results.

Definition 3:
Wave-function collapse is any sudden change of wave function which cannot be described by a Schrodinger-like equation and cannot be classified as a wave-function update.

Comment: From those definitions it follows that collapse and update are mutually exclusive.

Now we need the rules for using the wave function (according to the minimal ensemble interpretation):
Rule 1: $|\psi|^2$ is probability density.
Rule 2: If no results of measurements are known, $\psi$ should be considered as changing with time according to a Schrodinger-like equation.
Rule 3: When results of measurement are known, the wave function should be updated.

Comment: The rules are not the axioms. The purpose of the rules is to provide a practical user manual.

Observation 1: From rules and definitions above it follows that wave function collapse should never be used.
Observation 2: Nothing of the above depends on ontology.

If this is fair representation of vanhees71 position then it is wrong at least in some cases.
1. Say we have beam of unpolarized photons that we register using detector. No information, no update.
2. We insert polarizer before detector. According to presented viewpoint we gain information about polarization but no sudden change of wave-function.
3. We insert two crossed polarizers in the way of beam. There are no photons so we get no information.
4. Now between two crossed polarizers we insert third polarizer at 45 deg. This operation according to point 2. should not cause sudden change of wave-function but it does as now we get 1/8 of photons instead of none.

 

[Ken G]

But that's a weird perspective. When it comes to the two-particle composite system, Bob and Alice know everything there is to know about this system. It's described by a pure state, which is, for quantum mechanics, the maximum amount of information you can have about a system. To say that Bob's mixed state reflects his ignorance about his particle means that he knows less about a part of a system than he knows about the whole system.

Actually, I read a paper once that described entanglement in exactly these terms. For a classical composite system, the entropy of the complete system has to be greater than the entropy of any of the components. But for quantum mechanics, this isn't always the case. For two-particle entangled system, the entropy for the composite system can be zero, because you know exactly what the state is. But the entropy of the components can be nonzero.

I think this is a clear indication that regarding the system as the sum of its parts is the problem here. If you regard the full system in a Bell state, its entropy is zero, but when you break it into pieces, its entropy rises, so as you say, you know less about the parts than about the whole system. That means the choice to regard it as made of parts is throwing away information. This is similar to what happens when you regard a two-slit experiment as if it required that the particle goes through one slit or the other but you don't know which-- that throws away information and gives more entropy for a system that you are treating like you have more information about (because you don't). Bohmian mechanics uses the pilot wave to recover the lowered entropy of the full system, which suggests that the action of a pilot wave is to contribute negative entropy to a system (which goes along with its superluminal character as another bizarre element to its ontology).

 

[Ken G]

I don't think it helps at all to make any progress in understanding what we observe, which is the goal of physics.

That's the source of our disagreement. I point to the history of science, and give examples where progress was delayed by a kind of false understanding of what we observe. Had the issue in Galileo's day simply been which model achieves agreement with observation while obtaining the maximal conceptual unification, the Copernican picture would have won immediately, hands down. But that wasn't at all the nature of the scientific debate, so the Copernican model took a long time to be accepted-- so much so that Galileo had to recant his views. Yes the religious authorities were involved, but they were informed by scientific ontology, a classic example of what happens in science if you mistake ontology as the goal of the whole process. When one frames scientific progress as a search for the correct ontology, one tells a story of spectacular failure after spectacular failure, culminating only in our own inevitable inability to see the full measure of our own current failures. But when framed as an epistemological journey, suddenly the story of science is one success right after another. So that's the progress I am talking about.

Also, given your signature describing physics as a dialog with nature, which sounds good to me I would have thought your answer should have been "understand nature" or "model nature".

The issue is, is nature an ontological entity, or an epistemological one? When I say we are having a dialog with nature, it is just another picture that is not intended to be taken seriously-- as it is a blatant anthropomorphism. When you start seeing anthropomorphisms, as we inevitably do, it is a clear sign that we are really doing epistemology-- not ontology.

 

[Ken G]

But this view is self-contradictory as your tracing out example shows. If you know everything about the whole system, it would imply that you know very little about the subsystem, while if you know everything about a subsystem but nothing about the remainder of the system, this cannot even be described in this model of knowledge.

I would argue it is this reasoning itself that is unsound. You are simply assuming that treating a whole system as though it was made of subsystems cannot lose information about the whole system, yet I see no basis for that claim. Instead, I see plenty of evidence that it simply is not true. Why can we not know everything there is to know about a full system, yet at the same time know very little about its subsystems? The concept of a subsystem is an effective model, not a fundamental truth, and we must not reason from our models, we must use reason to test our models. Is this not like the two slit experiment, where we know the full wavefunction of the particle, but lose information as soon as we assert that the particle must have gone through one slit or the other, we just don't know which?

 

[Ken G]

No. I argue that individual photons don't exist unless they are specifically prepared, and that the whole setting is about measuring on Earth (locally) a very weak electromagnetic field generated (long ago) by the distant star.

Note that under usual circumstances, photons are indistinguishable - to distinguish them is already an approximation, valid only when there is something definite to distinguish them!

Normally photons are not distinguishable, but we can imagine that they come with labels if we want, because we get the same questions about detecting them from ten billion light years away. The quantum mechanics is not different for distinguishable particles in this case, so we need an interpretation that works either way. I agree with a lot of what you are saying, by the way-- because I don't take an ontological perspective, I don't think photons exist, I think they are the way we think about what is happening. So we are updating our information and expectations, and generating useful models as we go along. So I was never objecting to the epistemological approach of the thermal interpretation, I was objecting to what I view as essentially window dressing to make it look like an ontological description. It just sounds so quintessentially epistemic, which is what I like about it, so why not just embrace that aspect of it and be done with all the ontology?

 

[secur]

Had the issue in Galileo's day simply been which model achieves agreement with observation while obtaining the maximal conceptual unification, the Copernican picture would have won immediately, hands down. But that wasn't at all the nature of the scientific debate, so the Copernican model took a long time to be accepted-- so much so that Galileo had to recant his views.

Incorrect. Galileo didn't know about elliptical orbits. He was aware of Kepler's work - at some point - but didn't pay attention to it. Copernicus, of course, also thought orbits had to be circular. In fact, the geocentric model, with all those epicycles, agreed with observations far more closely than Copernican without ellipses.

You're right that mistaken ontology - in fact, the entire habit of ontologizing - has caused problems in science but it's far from black-and-white. Church authorities allowed Galileo to say whatever he wanted, just with the caveat that he wasn't proposing an ontology, only epistemology. So actually the Church was firmly in your camp; it was Galileo who insisted on being an ontologist.

Anyway his trial was not due to either philosophy or religion. At that time the counter-reformation, the Church's attempt to deal with the Protestant schism, was underway. The Pope had lost influence in many countries - which meant loss of money and power. He was trying to deal with very destructive European wars and also worried about the Turks, who could credibly destroy "us". He personally, and even more, his advisors, understood the scientific issues well, but they were utterly trivial compared to the political problems. The last thing they needed was this little PITA, Galileo, stirring up trouble. The Lutherans would have used Galileo for propaganda purposes, if the Pope failed to punish the "heretic". Nothing to do with science, just politics. It's a long story and off-topic.

Generally, it's a common misunderstanding that everybody was an idiot until 1905. So it happens constantly that people notice some simple factoid, which was probably well understood by intelligent cavemen, and think they invented it today! Read some history, it's really fascinating when you get into it. Just pick up any book on the Middle Ages, open it anywhere, and start reading. Before you know it you'll have gone through the entire shelf in the library.

 

[secur]

@A. Neumaier, your thermal interpretation doesn't get rid of QM "weirdness". Only sweeps it under the rug, where it's harder to find.

Consider your 6 axioms of QM, http://arnold-neumaier.at/physfaq/topics/postulates. They comprise all of QM except the collapse. By postulating Hermitian operators with off-diagonal terms you have non-commutativity and interference. The spin operator gives spinors with their non-classical "too-strong" correlations. You have unitary evolution operator, expectations, probabilities, HUP, all contained in the first five axioms, representing all of normal QM - including the so-called "weird" aspects - except collapse.

Axiom 6 takes care of that: "Quantum mechanical predictions consist of predicting properties (typically expectations or conditional probabilities) of the measures defined in Axiom A5 ... Axiom A6 specifies that the formal content of quantum mechanics is covered exactly by what can be deduced from Axioms A1-A5 ..."

The collapse selects one possibility, one eigenvalue from the Hermitian matrix - very important. Axiom 6 explicitly says you're not going to deal with that, it's not part of your interpretation. You compute only probabilities for all "branches" and expectations derived therefrom. This is how decoherence enthusiasts try to dismiss the collapse. Once the off-diagonal elements are close enough to 0 we're dealing with purely classical probabilities, so (they say) we're done. But no, measurement selects one, and only one, of them to be instantiated in reality. They pretend the selection doesn't exist, and so do you - that's Axiom 6.

Your axioms are fine; your thermal interpretation may be fine, it appears to be accurate. But it doesn't dispense with weirdness. It incorporates most "weirdness" in the math, and simply denies the weirdest fact of all: "collapse".

 

[Ken G]

Incorrect. Galileo didn't know about elliptical orbits.

I wasn't talking about ellipses there, I was referring to Galileo's own observations, such as the phases of Venus. Those held more sway anyway, since anyone can use a telescope.

You're right that mistaken ontology - in fact, the entire habit of ontologizing - has caused problems in science but it's far from black-and-white. Church authorities allowed Galileo to say whatever he wanted, just with the caveat that he wasn't proposing an ontology, only epistemology. So actually the Church was firmly in your camp; it was Galileo who insisted on being an ontologist.

Indeed, I pointed out earlier that Galileo's supposed "Eppur si muove" was actually not good science, and is no longer considered to be an ontological truth, given relativity. The actual significance of Galileo's observations are how they supported models that treated the physics of the Earth and Sun in the same boat as the physics of planets and stars. The motion of the Earth was only the lightning rod that made that point.

Anyway his trial was not due to either philosophy or religion.

The Galilean affair is only one example, I have dozens to point to. Science is constantly tripping on its own ontologies-- how often have you heard the claim that the Big Bang model asserts that "space itself is expanding", despite the fact that the Big Bang model is based in general relativity, and general relativity would have to regard that description as highly dubious! It's ontology, all over again. We cling to the pictures now just as much as the ancient geocentrists did, seemingly never learning the lesson.

 

[secur]

@Ken G, I see you know more about Galileo's circumstances than I thought. Evidently you didn't feel like going into the details of what, after all, was just an off-hand example.

The Galilean affair is only one example, I have dozens to point to. Science is constantly tripping on its own ontologies-- how often have you heard the claim that the Big Bang model asserts that "space itself is expanding", despite the fact that the Big Bang model is based in general relativity, and general relativity would have to regard that description as highly dubious! It's ontology, all over again. We cling to the pictures now just as much as the ancient geocentrists did, seemingly never learning the lesson.

This is probably not the place to go into those dozens of examples but it would be useful and instructive. Your main point, "science is epistemological", may not directly involve history but a related, important, point does: that science's mistakes have often been due to ontologizing. I can't say whether you're right but can see that a good case could be made.

History of science is often considered unimportant for scientists but I don't agree. It's irrelevant when "shutting up and calculating" but very relevant when theorizing. Those who don't know history are doomed to repeat it!

 

[secur]

Yes - and that's really one of the main thrusts of Leifer's article - to examine whether it's possible to rigorously rule out certain ways of looking at things and to lift the debate out of the murky philosophical waters and into the crystal clarity of the light of science

Leifer's paper is good. The PBR paper itself seemed simpler (at a glance) but there's a lot of background I was completely unaware of. To understand PBR, and related issues, this looks like the best place to start.

My primary motivation is to understand "why the world is as it is", so to speak.

That question can, indeed should, be addressed without any modern knowledge. At the most fundamental level a caveman was confronted with exactly the same puzzle as we are. Turns out it's a matter of opinion and can't be definitively answered - as shown by generations of thinkers starting with Rig Veda and ancient Greeks, through Descartes, Leibniz, Berkeley and Hume, right up to Heidegger. The key fact that you might not realize: science has absolutely nothing to do with it. Recommend you check out the above-mentioned authors. They can't answer it either but at least their thoughts are relevant.

Before QM came along I suspect that very few scientists would have held that it is the job of science just to predict stuff, and not to say anything about 'reality'. Of course after QM, when it became awkward (to say the least) to ascribe some 'reality' to the state, I get the impression it was as if there was some collective decision to 'redefine' what science is about.

An excellent point. But science never was able to furnish any information about fundamental ontology of the world: a caveman could figure it out as well as we can. You think science is "copping out" by refusing to address it, and instead concentrating on what it does so well, prediction. But I think, more optimistically, scientists are finally rejecting the illusion (of the last few centuries) that science can answer the question "why the world is as it is", in its most fundamental sense. That's progress, IMHO.

To clarify a possible misunderstanding, above comments concern the fundamental question. When it comes to details, like "why is the weather as it is" and millions more, of course science makes ancient thinkers completely irrelevant.

 

[vanhees71]

If this is fair representation of vanhees71 position then it is wrong at least in some cases.
1. Say we have beam of unpolarized photons that we register using detector. No information, no update.
2. We insert polarizer before detector. According to presented viewpoint we gain information about polarization but no sudden change of wave-function.
3. We insert two crossed polarizers in the way of beam. There are no photons so we get no information.
4. Now between two crossed polarizers we insert third polarizer at 45 deg. This operation according to point 2. should not cause sudden change of wave-function but it does as now we get 1/8 of photons instead of none.

I don't see, how my point of view (i.e., the minimal interpretation) can be wrong at any of these cases. In case 1. the photons are absorbed. So afterwards there are no more photons -> update to $|\Omega \rangle$ (zero-photon Fock state, photon vacuum). In case 2. half the photons get aborbed, the other half has the (linear) polarization corresponding to the filter orientation, call it "horizontal" -> Update to $|H \rangle$. In ase 3. all photons get absorbed -> update to $|\Omega \rangle$. In case 4. we get indeed what you describe, and the remaining 1/8 of the photons is again in state $|H \rangle$ (i.e., in the direction of the last filter). All this is no problem within the minimal interpretation of QED.

 

[A. Neumaier]

Why can we not know everything there is to know about a full system, yet at the same time know very little about its subsystems?

This extremely strange claim is like claiming to know everything about a person (the full system) except for its face (ostensibly a subsystem).

Every information about a subsystem is also an information about the whole system, at least according to general scientific practice. Indeed, the only way to study complex systems is to study their conspicuous subsystems and how they cooperate.

 

[zonde]

I don't see, how my point of view (i.e., the minimal interpretation) can be wrong at any of these cases. In case 1. the photons are absorbed. So afterwards there are no more photons -> update to $|\Omega \rangle$ (zero-photon Fock state, photon vacuum). In case 2. half the photons get aborbed, the other half has the (linear) polarization corresponding to the filter orientation, call it "horizontal" -> Update to $|H \rangle$. In ase 3. all photons get absorbed -> update to $|\Omega \rangle$. In case 4. we get indeed what you describe, and the remaining 1/8 of the photons is again in state $|H \rangle$ (i.e., in the direction of the last filter). All this is no problem within the minimal interpretation of QED.

In case 4. additional polarizer change polarization of passing photons from linear polarization $|V \rangle$ to $|45^0 \rangle$ linear polarization but information update should not cause physical change.

 

[A. Neumaier]

we can imagine that they come with labels if we want

Imagining something invalid doesn't make it valid. The labels are not present in nature. Labeling is a physical activity that changes the system under consideration, and hence introduces an uncontrolled approximation.

One can see it by looking at the double slit experiment - the predicted outcome depends on whether the incoming photons are or are not labeled, and experiments confirms that the labelled result appears only when the labeling is indeed physical. Otherwise it is an extremely bad approximation - so bad that it conflicts with experiment.

 

[A. Neumaier]

"Quantum mechanical predictions consist of predicting properties (typically expectations or conditional probabilities) of the measures defined in Axiom A5"

The collapse selects one possibility,

The collapse means taking probabilities conditioned on the known observations. Thus it is included in my axioms. But it amounts to a change of the modeling assumptions rather than to a change in the system.

This is exactly the same what people handling stocks do - they use propbabilities based on the most recently available information to make predictions, hence collapse their model probability distributions each time new information comes in. But I have never heard of a financial analyst complain about the weirdness of classical stochastic modeling.

Weirdness appears only when one mistakenly ascribes the collapse to the system rather than to the change in the model.

 

[vanhees71]

In case 4. additional polarizer change polarization of passing photons from linear polarization $|V \rangle$ to $|45^0 \rangle$ linear polarization but information update should not cause physical change.

Of course it does. The interaction of the photons with the middle polarizer is a physical interaction of the photon with the charges in the polarizer material and thus a dynamical process changing the photons' polarization (if they get through, if they are aborbed there are no more photons but you have an excitation of the polarizer material manifesting itself as heat).

 

[zonde]

Of course it does. The interaction of the photons with the middle polarizer is a physical interaction of the photon with the charges in the polarizer material and thus a dynamical process changing the photons' polarization (if they get through, if they are aborbed there are no more photons but you have an excitation of the polarizer material manifesting itself as heat).

Yes of course. But then it contradicts Demystifier's definition.

Definition 2:
Wave-function update is a mental act by a person who understands QM. In this act, an old wave function is replaced by a new wave function, with intention to better represent the new knowledge acquired by new measurement results.

Result of inserting middle polarizer can not be viewed as acquisition of new knowledge and nothing more.

 

[Demystifier]

It doesn't work, because the update is nonlocal in the sense that a correct way of updating the wave function is to assign it to a hyperplane of simultaneity, and then updating it instantaneously by non-Schroedinger evolution across the hyperplane.

There is no collapse, only updating, but the updating can be considered nonlocal.

Objecting to a nonlocal update does depend on ontology, since the update does not conflict with the locality of relativistic QFT.

Another way of saying it is that ontology is a tool for performing wave function updating. In that ontology, wave function updating is nonlocal. And there is nothing about that nonlocality that conflicts with QFT - unless one is mistaking the locality of QFT to be ontological.

Well you may call it "non-local" and "ontological" if you want, but then you are using those words in a rather unusual way.

 

[Demystifier]

But there is a density operator describing the LHC.

If we accept the instrumental interpretation according to which there is no wave function of the universe, then, in the same interpretation, there is no density operator describing the whole LHC.

Of course, you are free to use a different interpretation, but then your interpretation is not the minimal instrumental interpretation advocated e.g. by Peres..

 

[Demystifier]

If this is fair representation of vanhees71 position then it is wrong at least in some cases.
1. Say we have beam of unpolarized photons that we register using detector. No information, no update.
2. We insert polarizer before detector. According to presented viewpoint we gain information about polarization but no sudden change of wave-function.
3. We insert two crossed polarizers in the way of beam. There are no photons so we get no information.
4. Now between two crossed polarizers we insert third polarizer at 45 deg. This operation according to point 2. should not cause sudden change of wave-function but it does as now we get 1/8 of photons instead of none.

First, you are using a too narrow meaning of the concept of "wave function". The wave function is nothing but a vector in the Hilbert space of all possible physical states. In this sense, even the vacuum |0> is a wave function (more precisely, a wave functional) in the Hilbert space of quantum electrodynamics. Therefore, your 3. is wrong; even though there are no photons, there is still wave function and associated information.

Second, the change in 4. is not sudden. It is a continuous transition (although very fast, due to fast decoherence) from a state with one number of photons to a state with another number of photons.

 

[atyy]

Well you may call it "non-local" and "ontological" if you want, but then you are using those words in a rather unusual way.

Well, I don't think it weakens my main point that it wrong to use the locality of relativistic QFT to object to any nonlocality associated with updating.

 

[Demystifier]

Result of inserting middle polarizer can not be viewed as acquisition of new knowledge and nothing more.

In the operational interpretation of QM, insertion of the middle polarizer (or any other polarizer) should be viewed as a choice of the effective time-dependent Hamiltonian. The Hamiltonian defines the corresponding Schrodinger equation, so any change induced by insertion of the polarizer should be viewed as evolution by Schrodinger equation.

 

[zonde]

Second, the change in 4. is not sudden. It is a continuous transition (although very fast, due to fast decoherence) from a state with one number of photons to a state with another number of photons.

Yes, I used poor wording. I should have said "physical change of subensemble that is described by wave-function".

 

[Demystifier]

Well, I don't think it weakens my main point that it wrong to use the locality of relativistic QFT to object to any nonlocality associated with updating.

Of course, but @vanhees71 does not have any objections against updating. He only objects against collapse. To clarify his opinion, I have defined the notions of "update" and "collapse" in a manner consistent with his own understanding of those words.