I What has changed since the Copenhagen interpretation?

[MichPod]

If to look at the foundations of QM and if to ignore various not much verifiable alternative interpretations of QM which emerged since the Copenhagen, starting with Bohm and Everett, what are the commonly accepted and recognized changes to the original views of the QM creators?

There are planty of discoveries of which I may mention Bell inequalities and their verifications and various no-go theorems. All these may be, arguably, considered as the extension of the original theory, but are there developments which may be considered not only as an extension but also as a principial change, irreversible shift in the understanding of QM?

 

[Nugatory]

Perhaps the single biggest change is discovering and appreciating the importance of decoherence. Bell and other no-go theorems tell us that quantum mechanics must be at odds with our classical intuition, so ended the search (suggested by the EPR argument) for a classical-friendly hidden variable theory underlying QM. That's important, but if you've already accepted QM it's nothing new. However, decoherence goes a long ways towards clearing up objectionable properties of the various interpretations: consciousness causes collapse in Copenhagen and the preferred-basis problem in MWI.

 

[Demystifier]

Decoherence, generalized POVM measurements, weak measurement, ...

None of those made Copenhagen obsolete, but all of them are practical aspects of QM that have deep consequences on conceptual understanding.

 

[MichPod]

Thanks!

consciousness causes collapse in Copenhagen

BTW, I though it was some views of von Neumann/Wigner, but not the Copenhagen itself. I may of course be wrong...

Decoherence, generalized POVM measurements, weak measurement, ...
None of those made Copenhagen obsolete, but all of them are practical aspects of QM that have deep consequences on conceptual understanding.

Are there any parts of Copenhagen which are obsolete as for the current mainstream QM?

 

[Demystifier]

Are there any parts of Copenhagen which are obsolete as for the current mainstream QM?

The statement that macroscopic world obeys classical laws is quite obsolete, because there are many counterexamples. For instance, superconductor in a superposition of macroscopic currents in the opposite directions.

 

[DarMM]

Are there any parts of Copenhagen which are obsolete as for the current mainstream QM?

Very recently the Frauchiger-Renner theorem has cast doubt on the fact that Copenhagen-like interpretations can be considered to give an objective view of experiments, but rather are perspectival.

In certain situations, if you try to combine the conclusions of different observers using the Copenhagen interpretation, you'll get a contradiction.

 

[Demystifier]

Very recently the Frauchiger-Renner theorem has cast doubt on the fact that Copenhagen-like interpretations can be considered to give an objective view of experiments, but rather are perspectival.

In certain situations, if you try to combine the conclusions of different observers using the Copenhagen interpretation, you'll get a contradiction.

In just a couple of months, a dozen of papers appeared on arXiv that criticize the Frauchiger-Renner paper from different points of view. So I think it's fair to say that the correctness and relevance of the Frauchiger-Renner result is not settled yet.

 

[atyy]

Very recently the Frauchiger-Renner theorem has cast doubt on the fact that Copenhagen-like interpretations can be considered to give an objective view of experiments, but rather are perspectival.

In certain situations, if you try to combine the conclusions of different observers using the Copenhagen interpretation, you'll get a contradiction.

The paper is probably wrong (eg. Bub, Aaronson).

 

[DarMM]

In just a couple of months, a dozen of papers appeared on arXiv that criticize the Frauchiger-Renner paper from different points of view. So I think it's fair to say that the correctness and relevance of the Frauchiger-Renner result is not settled yet.

Of course, that is why I said "casts doubt", some think it is incorrect, others think it's not, e.g. Matt Leifer and Robert Spekkens have said it is a major advance, others disagree. Also it should be said many of the papers more explicate the theorem or clarify what it implies. For example Baumann et al here although criticising how it states its case do agree it has found an important delimiter between interpretations (https://arxiv.org/abs/1611.01111).

The paper is probably wrong (eg. Bub, Aaronson).

Bub doesn't think it is wrong, he just dicusses how the Information Interpretation and other Neo-Copenhagen interpretations fit into the divisions it demands. Aaronson does think it is wrong, but he is only one individual, considering experts in quantum foundations like Leifer and Spekkens disagree, I don't think we can say it is probably wrong.

Most do seem to agree that it shows no-collapse and objective collapse differ and that unrestricted (i.e. modal multiple-user) subjective collapse is inconsistent, the latter probably being its main discovery.

 

[Demystifier]

Matt Leifer and Robert Spekkens have said it is major advance

Where can I see what Matt Leifer said about it?

 

[DarMM]

See his lecture about it here:


Gets to the actual theorem around 40 minute mark.

 

[DarMM]

At a lower level, but still his opinion:
https://www.nature.com/articles/d41586-018-06749-8

 

[Demystifier]

See his lecture about it here:

Let me try to summarize and demystify all this in my own words. According to Matt Leifer, what the Frauchiger-Renner (RN) theorem rules out is one particular class of Copenhagen-like interpretations, that is the objective Copenhagen interpretation. The objective Copenhagen interpretation is an interpretation in which both of the two statements are true:

(i) The observation-induced collapse of $|\psi\rangle$ is objective.
and
(ii) The level on which this collapse happens (the level of Wigner or the level of his friend) is subjective.

The RN theorem says that (i) and (ii) are not consistent with each other, i.e. that the objective Copenhagen interpretation is inconsistent. In other words, the theorem states that it is inconsistent to treat the collapse as both objective and subjective. When put in this form, the theorem looks rather intuitive and hardly surprising. Perhaps the only surprising aspect of this is that the actual proof of this intuitive statement (that the collapse cannot be both objective and subjective) is technically quite complicated.

 

[DarMM]

When put in this form, the theorem looks rather intuitive and hardly surprising. Perhaps the only surprising aspect of this is that the actual proof of this intuitive statement (that the collapse cannot be both objective and subjective) is technically quite complicated.

Correct, that's essentially what Baumann et al say, i.e. the only surprise is that you need such an extreme scenario.

It's a shock I suppose only if you consider Quantum Mechanics to be a probability calculus with collapse as Bayesian updating in some form, as many of the Neo-Copenhagen mindset did/do. Collapse then is only epistemic or at least something like conditionalising. Hence Wigner can have no collapse and his friend can have collapse and this is fine, because one of us has conditioned in light of an observation and the other hasn't. This is subjective collapse (as you describe). Bohr and some early founders did think something along these lines.

Frauchiger-Renner shows you can't really look at things like this. Or at least if you want to have subjective collapse you need to say one can't combine any statement of Wigner with that of his friend, i.e. compose them to form objective statements for both.

 

[atyy]

But if you believed both Bohmian Mechanics and Copenhagen, wouldn't you expect that to be true? In Copenhagen, the measurement must be done by a classical observer for whom the result is irreversible. However, there is no irreversibility in Bohmian Mechanics, only unitary evolution. So it would seem possible in principle to set up in Bohmian Mechanics something that violates Copenhagen, if one was able to reverse a measurement. It is only in practice that such a setup would be impractical in Bohmian Mechanics.

 

[DarMM]

I should also say it seems to have changed the opinions of some, such as Matthew Pusey (of the PBR theorem) to move to something like QBism. You retain subjective collapse, but at the cost I mentioned above, QM is only about the expectations of a given agent and in certain scenarios you cannot combine two agent's reasoning.

 

[Demystifier]

But if you believed both Bohmian Mechanics and Copenhagen, wouldn't you expect that to be true?

No. In BM, the effective collapse happens at the level of conditional wave function, which is purely objective.

 

[atyy]

No. In BM, the effective collapse happens at the level of conditional wave function, which is purely objective.

What I meant is that wouldn't you expect to be able to set up in Bohmian Mechanics a violation of Copenhagen QM, since Bohmian Mechanics does not have true irreversibility, whereas Copenhagen QM requires a measurement to be irreversible?

 

[akvadrako]

I should also say it seems to have changed the opinions of some, such as Matthew Pusey (of the PBR theorem) to move to something like QBism. You retain subjective collapse, but at the cost I mentioned above, QM is only about the expectations of a given agent and in certain scenarios you cannot combine two agent's reasoning.

It would seem you can never combine their reasoning precisely, since every agent will have a slightly different opinion of when collapse occurs. This line of thought only leaves a few options:

1. there is only 1 agent (solipsism)
2. QM doesn't work for more than 1 agent, so needs to be modified
3. reality is inconsistent (??) or ineffable

 

[Demystifier]

whereas Copenhagen QM requires a measurement to be irreversible

The version of Copenhagen studied in the theorem is not completely irreversible. It assumes that measurement can be undone by a unitary operation.

 

[atyy]

The version of Copenhagen studied in the theorem is not completely irreversible. It assumes that measurement can be undone by a unitary operation.

So it's definitely wrong then. The Copenhagen measurement is irreversible. If you can reverse it, then it would just be a fancy version of Ballentine's disproof of Copenhagen.

 

[DarMM]

So it's definitely wrong then. The Copenhagen measurement is irreversible. If you can reverse it, then it would just be a fancy version of Ballentine's disproof of Copenhagen.

Irreversible versions of the Copenhagen interpretation are handled via rejection of the Q postulate, it's not wrong in that respect. It's a problem with versions of Copenhagen that seek to retain Q, by viewing Unitary evolution as a (somewhat) epistemic process and hence retaining quantum mechanics as a universal theory, i.e. subjective collapse.

 

[DarMM]

It would seem you can never combine their reasoning precisely, since every agent will have a slightly different opinion of when collapse occurs. This line of thought only leaves a few options:

1. there is only 1 agent (solipsism)
2. QM doesn't work for more than 1 agent, so needs to be modified
3. reality is inconsistent (??) or ineffable

I know a few people have moved to (or already were in) the ineffable camp (e.g. Richard Healey, Roland Omnès, Rudolf Haag thought similar), most of the main QBist people are of camp 2. I know they're all working on papers along these lines, so perhaps we'll see what they have to say over the coming year.

 

[atyy]

Irreversible versions of the Copenhagen interpretation are handled via rejection of the Q postulate, it's not wrong in that respect. It's a problem with versions of Copenhagen that seek to retain Q, by viewing Unitary evolution as a (somewhat) epistemic process and hence retaining quantum mechanics as a universal theory, i.e. subjective collapse.

Well, it's just so far away from textbook QM that it's hard to see why the paper is interesting at all.

 

[DarMM]

Well, it's just so far away from textbook QM that it's hard to see why the paper is interesting at all.

What is textbook QM in this context and what aspect of it is it violating/far away from? Genuine question.

EDIT: The presentation in something like Shankar or Griffiths for example is already known to be inconsistent, do you mean something else?

 

[atyy]

What is textbook QM in this context and what aspect of it is it violating/far away from? Genuine question.

EDIT: The presentation in something like Shankar or Griffiths for example is already known to be inconsistent, do you mean something else?

Well, it's true that the irreversibility is seldom mentioned in actual textbooks, but then neither is the classical-quantum cut (which is also textbook). Perhaps there is no actual textbook that mentions it, but Haag (Local Quantum Physics) is source that is almost a textbook that mentions the irreversibility requirement.

I don't see why Shankar or Griffiths is inconsistent, if one takes measurement to be irreversible.

 

[atyy]

Here is one quote from Haag, Local Quantum Physics, p304:
In Bohr's discussion the time asymmetry appears as obvious. For instance: "The irreversible amplification effects on which the registration of the existence of atomic objects depends reminds us of the essential irreversibility inherent in the very concept of observation" [Bohr 58]."

 

[DarMM]

Well, it's true that the irreversibility is seldom mentioned in actual textbooks, but then neither is the classical-quantum cut (which is also textbook). Perhaps there is no actual textbook that mentions it, but Haag (Local Quantum Physics) is source that is almost a textbook that mentions the irreversibility requirement.

I don't see why Shankar or Griffiths is inconsistent, if one takes measurement to be irreversible.

So let's say in the Wigner's friend scenario, Wigner should be using a mixed state, not the pure state:
$$\frac{1}{\sqrt{2}}\left(|\uparrow, A_{\uparrow}\rangle + |\downarrow, A_{\downarrow}\rangle\right)$$
with $A_{\uparrow}, A_{\downarrow}$ device states?

 

[atyy]

So let's say in the Wigner's friend scenario, Wigner should be using a mixed state, not the pure state:
$$\frac{1}{\sqrt{2}}\left(|\uparrow, A_{\uparrow}\rangle + |\downarrow, A_{\downarrow}\rangle\right)$$
with $A_{\uparrow}, A_{\downarrow}$ device states?

Does anyone actually make a wrong prediction of the probabilities of measurement outcomes in the Frauchiger and Renner paper?

 

[DarMM]

Does anyone actually make a wrong prediction of the probabilities of measurement outcomes in the Frauchiger and Renner paper?

I'm not sure how to answer, depending on what you relinquish you predict different probabilities, the whole point as such is the contradiction in the probabilities without rejecting the three conditions.

I'm not sure how it relates to what I asked.

 

[atyy]

I'm not sure how to answer, depending on what you relinquish you predict different probabilities, the whole point as such is the contradiction in the probabilities without rejecting the three conditions.

I'm not sure how it relates to what I asked.

Sorry, I didn't answer directly. I'm just reluctant to think read carefully or think too hard about a paper that claims Copenhagen is wrong and assumes that measurements are reversible, since it seems obviously wrong.

 

[DarMM]

Sorry, I didn't answer directly. I'm just reluctant to think read carefully or think too hard about a paper that claims Copenhagen is wrong and assumes that measurements are reversible, since it seems obviously wrong.

Fair enough, although it isn't claiming Copenhagen is wrong, just that certain versions would be. Bub's information interpretation is an example that is still fine, just modified slightly. Copenhagen is a family of interpretations, it's not saying it wipes out all of them, it just reveals interesting restrictions on some, for example Richard Healey has a Copenhagen type interpretation that he is changing in light of the paper.

It doesn't really use the reversibility of measurements either, the measurements aren't reversed.

EDIT: Though of course @Demystifier is correct in that by assuming QM is universally valid it would admit the possibility that they could be reversed.

 

[DarMM]

My point with the Wigner's friend example above was that if measurement is irreversible, then Wigner should be using a mixed state, meaning collapse is a real physical process that one can fail to account for (objective collapse) and also one has nonlocal effects due to the collapse.

EDIT:
In other words you have either:

1. Objective collapse and so nonlocal effects.
2. No-collapse, as in Bohmian Mechanics and Many-Worlds, the whole wavefunction always applies. No-collapse cases are shown to require places where they deviate from QM, unless they drop having a single world.
   
3. Subjective collapse models need to be modified, the contradiction is really with them. Thinking of collapse as somewhat epistemic leads to contradictions.

 

[Auto-Didact]

In other words you have either:

1. Objective collapse and so nonlocal effects.
2. No-collapse, as in Bohmian Mechanics and Many-Worlds, the whole wavefunction always applies. No-collapse cases are shown to require places where they deviate from QM, unless they drop having a single world.
   
3. Subjective collapse models need to be modified, the contradiction is really with them. Thinking of collapse as somewhat epistemic leads to contradictions

Option 3 is immediately self-inconsistent, demonstrable through proof by contradiction:
I. If collapse exists, then $\psi$ exists (i.e. collapse implies ontology of $\psi$)
II. If collapse was somehow subjective it would be epistemic, making $\psi$ also epistemic
III. This would mean $\psi$ is both ontic and epistemic, i.e. both existing and at the same time only a matter of someone's knowledge
IV. Both cannot be true
V. Therefore collapse cannot be subjective

Therefore, it really is either 1 or 2.

 

[Auto-Didact]

Regarding the main topic of interpretations: a few weeks ago Smolin, Aaronson and some others commented on Woit's blog w.r.t. a question of Woit about interpretations. Smolin's comments seemed to be the most worthwhile ones, so I'll paste them here:

Dear Peter,

I’ve just picked up my head from doing the final corrections to my new book on realism in quantum foundations to find you asking, “where exactly does probability enter the theory?”

My understanding, after a lot of study, is that you have the following options:

1) Put the probabilities in at the beginning, as did Bohr, Heisenberg and von Neumann. This requires an operational approach which introduces measurement and probabilities as primitive concepts, ie through a “collapse” or “projection” postulate, which postulates Born’s rule and “eigenvalue realism”, or through a Hardy-style operational reconstruction. These are elegant but they do not answer your question as measurement and probability are primitive concepts.

2) You can attempt to derive probabilities from a formalism that has only unitary, Schrodinger evolution, which has no notion of probabilities to begin with. This is Everett’s MWI route.

This is by now a very long story. It took me a lot of time to sort out for the book, and I had help from Saunders and Wallace and others. At best, there is no consensus amongst experts that this can be done. (This agrees with Scott’s remark, above.) The rough outline is

i) the original version due to Everett fails, because you can show that with certainty there are branches of the wavefunction whose observers record measurements that disagree with Born’s rule. Because there is no primitive notion of probability you cannot say that these observers are improbable, in fact there are an infinite number of them, and also an infinite number whose observations agree with Born’s rule.

ii) There are recently several very sophisticated attempts to derive subjective probabilities and the Born rule. These are centred at Oxford, were initiated by David Deutsch and developed in different versions by Hillary Greaves, Wayne
Myrvold, Simon Saunders and David Wallace. These all use decoherence and also give up on recovering objective probabilities. Instead, they try, (in one version) from the axioms of decision theory, to show that it is rational for an observer to bet (ie choose subjective probabilities) as if Born’s rule were true. (Even though objectively Born’s Rule is false.)

If you read the literature you can only conclude, after some challenging technical arguments, that the experts disagree about whether this kind of approach succeeds or fails, and what the implications should be.

3) Invent a new physical theory which gives a complete description of individual processes from which the quantum probabilities are derived from ignorance about the initial state. This would then be a completion of QM rather than an interpretation. de Broglie-Bohm and collapse models are existence proofs that this is a possible route. There are also other approaches of this kind, such as Adler’s trace dynamics and my real ensemble formulation.

I have the impression you don’t find any of these 3 options satisfactory. The kind of answer to your question of where the probabilities come from would be one in which we start with QM without measurement, probabilities etc and derive them. But this was option 2 and a whole lot of very bright people have tried and failed to make it work (in a way that convinces all the experts).

My personal view is that option 3) is the only way forward for physics. But I wouldn’t try to do more here than argue that unless some notion of subjective probability can be made to work, as in option 2), you simply cannot get an answer to your question. You then either need to conclude with Bohr that the only kind of theory of atomic phenomena is operational, and has probabilities and measurement as primitive terms
or agree with Einstein, de Broglie, Schrodinger, Bohm, Bell ets that QM requires a completion that gives a complete description of individual experiments.

Thanks,

Lee

Lee,
Thanks for the comment. I look forward to seeing your book. In your categorization, I’m following option 2, and my question is being asked in that context.

Dear Peter,

I appreciate you are trying to follow path 2: “attempt to derive probabilities from a formalism that has only unitary, Schrodinger evolution, which has no notion of probabilities to begin with”. The point of my remark is that this is much harder than seems at first. A lot of really smart people have devoted years to trying to make this work and have not convincingly succeeded. Several arguments such as Everett’s original attempt, and related arguments of Hartle, Finkelstein, Banks, etc. turn out to be circular because they sneak in a measure related to probability and/or a special role for measurement. Then there are issues with the use of decoherence first pointed out by Abner Shimony, because the dynamics is unitary and reversible so there is a quantum Poincare time after which the state recoheres. So if you attempt to argue that decoherence defines the branches you can’t get an irreversible outcome to associate objective probabilities to.

It thus seems you also have to give up objective notions of probability so what you end up trying to show is that observers should chose their subjective probabilities as if Born’s rule is correct, when it is actually false. Would this much weaker notion of probability satisfy you?

So my query to you? What are you willing to give up in your beliefs about probability to make route 2 succeed?

Thanks,

Lee

 

[PeterDonis]

a few weeks ago Smolin, Aaronson and some others commented on Woit's blog w.r.t. a question of Woit about interpretations

Please give a link to the specific blog post you are quoting from. Quotes should be sourced.

 

[kith]

I'm just reluctant to think read carefully or think too hard about a paper that claims Copenhagen is wrong and assumes that measurements are reversible, since it seems obviously wrong.

I don't think that you need to think about the paper in order to sort out the difference between your position and FR. It seems that the ordinary gedankenexperiment of Wigner's friend or even simpler experiments are enough. Let me spell out things in detail to clarify my own thoughts.

(1) In the context of Copenhagen, measurements are associated with macroscopic irreversibility. In my view, all of the terms "measurement", "macroscopic" and "irreversible" contain a notion of subjectivity because they can't be strictly derived from the physical theory alone - it doesn't tell us when certain approximations are good enough for the practical purposes of the observer. This isn't specific to QM. It is true also in classical mechanics, so it also applies to the classical apparatus in Copenhagen.

(2) In almost all cases, the subjectivity of (1) is inter-subjevtivity, i.e. if different observers look at the irreversible mark which has been left on a photo plate during an experiment, they all observe the same thing. So we can get away without worrying about subjevtivity and the only cases which force us to distinguish between subjectivity and inter-subjectivity are speculative Wigner's friend type experiments.

It's been a while since I have discussed things with you, so I am not sure about your position. Do you agree that irreversibility is subjective in the way I outlined above?

If you disagree, I don't see how to escape the conclusion of @DarMM's post #33, i.e. that collapse has an objectively real element. Although the wavefunction may be purely epistemic we get a physical change in the (classical) measurement apparatus. This would imply that the freedom in choosing the Heisenberg cut and the deferred measurement principle are tools for calculations but physically wrong.

If you agree, measurements may be reversed under very special circumstances and FR may have a point. Of course, it is very doubtful whether this is a point in practise. Their gedanken experiment seems to presuppose a step which corresponds to the quantum resurrection of dead cats which @stevendaryl outlined in the thread on FR. If this is true, Wigner's friend type experiments probably need to involve timescales which are wildly different in order to account for something like the Poincaré recurrance time.

I used to think that Wigner's friend type experiments are impossible and I still do but in the light of AI and quantum computers, I see a (slightly) growing need to justify this position. If the irreversibility of measurements just means that a mindbogglingly large time is needed to reverse it, an AI whose mind isn't boggled so easily because it works on a different timescale might make such an experiment feasible.

 

[stevendaryl]

Sorry, I didn't answer directly. I'm just reluctant to think read carefully or think too hard about a paper that claims Copenhagen is wrong and assumes that measurements are reversible, since it seems obviously wrong.

It seems obviously wrong that measurements are reversible, or it seems obviously wrong to call an interpretation with reversible measurements Copenhagen?

Measurements are certainly irreversible in a one-world ontology with objective collapse, but I'm not sure about a many-worlds ontology.

 

[Auto-Didact]

Please give a link to the specific blog post you are quoting from. Quotes should be sourced.

Sure, here.

The blogpost/comments even refers to some posts by @A. Neumaier on physicsoverflow. Arnold argued at length for his point of view on the matter here on PF in this thread which I then commented on after reading the thread, all of Woit's et al. comments and most of their references.

 

[A. Neumaier]

Sure, here.

Thanks for the pointer to this blog post by Peter Woit. I find his exposition of the question excellent. Still need to read through the many replies...

The blogpost/comments even refers to some posts by @A. Neumaier on physicsoverflow. Arnold argued at length for his point of view on the matter here on PF in this thread which I then commented on after reading the thread, all of Woit's et al. comments and most of their references.

I had missed your reply in the other thread, and continue the discussion of your contribution there in the other thread.

 

[Auto-Didact]

(1) In the context of Copenhagen, measurements are associated with macroscopic irreversibility. In my view, all of the terms "measurement", "macroscopic" and "irreversible" contain a notion of subjectivity because they can't be strictly derived from the physical theory alone - it doesn't tell us when certain approximations are good enough for the practical purposes of the observer. This isn't specific to QM. It is true also in classical mechanics, so it also applies to the classical apparatus in Copenhagen.

(2) In almost all cases, the subjectivity of (1) is inter-subjevtivity, i.e. if different observers look at the irreversible mark which has been left on a photo plate during an experiment, they all observe the same thing. So we can get away without worrying about subjevtivity and the only cases which force us to distinguish between subjectivity and inter-subjectivity are speculative Wigner's friend type experiments.

The argument made in (1) is incorrect for subtle non-trivial reasons:
I) You are taking the physical theory to essentially be the mathematical scheme of QM, i.e. unitary evolution or any of its equivalent mathematical formulations*.

II) The physical theory of QM however does not solely consist of unitary evolution, but also of the fact that it is validated by experiment; were it not validated by experiment it wouldn't be a physical theory but a mere physical hypothesis.

III) This validation of hypothesis by experiment requires an operationalization - literally, any operationalization whatsoever - of measurement. This operationalization isn't a part of the theorists physical hypothesis, it is part of the experimentalists empirical analysis; the mathematics of this empirical analysis need not be consistent with the work of the hypothesis of the theorist.

IV) The only possible way to view QM as a physical theory is therefore to include a) the theorists hypothesis and b) the experimentalists analysis. This conjoined mathematically self-inconsistent object is the actual physical theory, not just a) as many often try to argue - almost by a sleight of hand - by equivocating the work of the theorist (hypothesis) with the word 'physical theory' (hypothesis validated by experiment).

QM is in essence the only physical theory** which really suffers from this inconsistency between the theoreticians mathematics and the experimentalists analysis, but certainly not the only scientific theory: just step out of exact science and almost all scientific theories have this problem.

*: this argument is completely unchanged if you include the mathematical scheme of decoherence and other physical theories such as statistical mechanics as well

**: theories literally completely derived (not merely shown to be mathematically derivable) from QM like Koopman-von Neumann classical mechanics notwithstanding

I used to think that Wigner's friend type experiments are impossible and I still do but in the light of AI and quantum computers, I see a (slightly) growing need to justify this position. If the irreversibility of measurements just means that a mindbogglingly large time is needed to reverse it, an AI whose mind isn't boggled so easily because it works on a different timescale might make such an experiment feasible.

Agreed w.r.t. the need to settle these matters due to the rise of AI

 

[kith]

The argument made in (1) is incorrect for subtle non-trivial reasons [...]

You don't seem to disagree with my main point that irreversibility is somehow subjective. The purpose of my post is to understand @atyy's view and in this, I'm prefering brevity over philosophical nuance.

 

[DarMM]

Option 3 is immediately self-inconsistent, demonstrable through proof by contradiction:
I. If collapse exists, then $\psi$ exists (i.e. collapse implies ontology of $\psi$)

Option 3 is specifically the epistemic case where $\psi$ is (a generalisation of) a probability distribution. Collapse then is just (a generalisation of) Bayesian conditioning, so it doesn't require $\psi$ to be real.

Spekkens toy model has a nice example of this. QBism is a full blown interpretation that takes this view and accepts the restriction on subjective collapse interpretations (they must be single user) implied by the Frauchiger-Renner theorem.

 

[Auto-Didact]

You don't seem to disagree with my main point that irreversibility is somehow subjective. The purpose of my post is to understand @atyy's view and in this, I'm prefering brevity over philosophical nuance.

Okay.

I do disagree with irreversibility being subjective, but that disagreement is based in the philosophically nuanced argument I just made; namely that any physical theory actually capable of incorporating a) and b) in a mathematically self-consistent manner will necessarily be based on mathematics which can introduce irreversibility as a physically occurring phenomenon into the theory.

In other words, the consistent physical theory resulting from making a) and b) mathematically self-consistent, must actually be time irreversible - demonstrating that unitarity is only an approximation, not a fundamental principle - with collapse of $\psi$ occurring as a predicted objective dynamical phenomenon. This dynamical phenomenon is not predicted by a) i.e. standard QM (unitary evolution of the Schrodinger equation) because a) is only a limiting case of the correct equation.

 

[Auto-Didact]

Option 3 is specifically the epistemic case where $\psi$ is (a generalisation of) a probability distribution. Collapse then is just (a generalisation of) Bayesian conditioning, so it doesn't require $\psi$ to be real.

Spekkens toy model has a nice example of this. QBism is a full blown interpretation that takes this view and accepts the restriction on subjective collapse interpretations (they must be single user) implied by the Frauchiger-Renner theorem.

I know that, I used to be a QBist; w.r.t. probability theory I still am a staunch Bayesian. The problem in QBism is there that the word 'existence' doesn't necessarily mean existence out in the world, but also existence inside the mind; this makes the argument far more slippery than the QBist realizes. All of these problems are inherited directly from the field foundations of probability theory.

 

[atyy]

I don't think that you need to think about the paper in order to sort out the difference between your position and FR. It seems that the ordinary gedankenexperiment of Wigner's friend or even simpler experiments are enough. Let me spell out things in detail to clarify my own thoughts.

(1) In the context of Copenhagen, measurements are associated with macroscopic irreversibility. In my view, all of the terms "measurement", "macroscopic" and "irreversible" contain a notion of subjectivity because they can't be strictly derived from the physical theory alone - it doesn't tell us when certain approximations are good enough for the practical purposes of the observer. This isn't specific to QM. It is true also in classical mechanics, so it also applies to the classical apparatus in Copenhagen.

(2) In almost all cases, the subjectivity of (1) is inter-subjevtivity, i.e. if different observers look at the irreversible mark which has been left on a photo plate during an experiment, they all observe the same thing. So we can get away without worrying about subjevtivity and the only cases which force us to distinguish between subjectivity and inter-subjectivity are speculative Wigner's friend type experiments.

It's been a while since I have discussed things with you, so I am not sure about your position. Do you agree that irreversibility is subjective in the way I outlined above?

I think so. Without thinking, I would like to believe in both Copenhagen (irreversible) and Bohmian Mechanics (reversible). Since Bohmian Mechanics would be more fundamental, the irreversibility is a subjective approximation.

If you disagree, I don't see how to escape the conclusion of @DarMM's post #33, i.e. that collapse has an objectively real element. Although the wavefunction may be purely epistemic we get a physical change in the (classical) measurement apparatus. This would imply that the freedom in choosing the Heisenberg cut and the deferred measurement principle are tools for calculations but physically wrong.

If you agree, measurements may be reversed under very special circumstances and FR may have a point. Of course, it is very doubtful whether this is a point in practise. Their gedanken experiment seems to presuppose a step which corresponds to the quantum resurrection of dead cats which @stevendaryl outlined in the thread on FR. If this is true, Wigner's friend type experiments probably need to involve timescales which are wildly different in order to account for something like the Poincaré recurrance time.

I used to think that Wigner's friend type experiments are impossible and I still do but in the light of AI and quantum computers, I see a (slightly) growing need to justify this position. If the irreversibility of measurements just means that a mindbogglingly large time is needed to reverse it, an AI whose mind isn't boggled so easily because it works on a different timescale might make such an experiment feasible.

Sure, if Bohmian Mechanics (the more fundamental theory) allows reversibility, and still produces definite outcomes at all times, I would expect it to be possible for QM to be wrong in cases where something seems irreversible to the QM observer, but it is in fact reversible in the underlying Bohmian Mechanics. I think they shouldn't sell it as Copenhagen or QM being wrong, especially if they break fundamental bits of QM in deriving their conclusions. Nor should they be surprised that Bohmian Mechanics does not reproduce QM exactly. Of course, if they have produced a specific case where QM fails but Bohmian Mechanics works, then that is an advance, since my arguments are just vague sketches.

 

[atyy]

In post #27, I quoted Haag's quote of Bohr as a source that says that measurements must be irreversible.

Here is another example from Peres's textbook:

"It must now be shown that no inconsistency arises if the measuring instrument, or the “observer” (in the above example, the center of mass position r), is considered as a genuine quantum object for which no classical description is used, but this quantized instrument is, in turn, observed by some other instrument, having both quantum and classical descriptions. For example, it should not matter whether Schrödinger’s cat is considered as the observer, or is an intrinsic part of the atom-cat dynamical system which is observed by someone else. The reason for suspecting a possible inconsistency is the following: If the observer were not a cat or some other, possibly inanimate but utterly complicated measuring apparatus, the unitary evolution leading to Eq. (12.1) would be reversible. Simple, highly idealized models of measuring apparatuses can easily be concocted, which have that property. After the measurement is achieved (i. e., the instrument is correlated with the measured system), it still is possible to undo the whole process: A superobserver, capable of fully analyzing the dynamical behavior of the measuring apparatus (e. g., capable of writing explicitly the Hamiltonian of Schrödinger’s cat) could cause the original observer (or apparatus—this makes no difference) to decorrelate itself from the measured system and to “unlearn” the result of the measurement. For example, the cat would be resurrected. And then, the superobserver, by measuring again the same system, could obtain a different result for his measurement.

If such a scenario were indeed possible, the notion of measurement would become meaningless, as no measurement would ever be conclusive. Consistency thus requires the measuring process to be irreversible."

 

[DarMM]

I think they shouldn't sell it as Copenhagen or QM being wrong, especially if they break fundamental bits of QM in deriving their conclusions.

Having read the paper I don't see where they are saying Copenhagen is wrong or what fundamental parts of QM they are breaking.

The quotes from Peres and Haag are fine, but there's nothing there that disagrees with the paper. They break assumption Q, just as Many Worlds would break S.

The main result is really about subjective collapse being inconsistent if QM applies everywhere and you combine different agents conclusions. That's it, nothing about Copenhagen being wrong, just certain forms of it.

 

[DarMM]

I know that, I used to be a QBist; w.r.t. probability theory I still am a staunch Bayesian. The problem in QBism is there that the word 'existence' doesn't necessarily mean existence out in the world, but also existence inside the mind; this makes the argument far more slippery than the QBist realizes. All of these problems are inherited directly from the field foundations of probability theory.

Yeah, I find the interpretation of probability theory quite difficult. Perhaps a better example would be the retrocausal theories which have subject collapse without $\psi$ being ontic.

Or some of the $\psi$-epistemic constructions, like those of Aaronson et al: https://arxiv.org/abs/1303.2834

 

[atyy]

Having read the paper I don't see where they are saying Copenhagen is wrong or what fundamental parts of QM they are breaking.

The quotes from Peres and Haag are fine, but there's nothing there that disagrees with the paper. They break assumption Q, just as Many Worlds would break S.

The main result is really about subjective collapse being inconsistent if QM applies everywhere and you combine different agents conclusions. That's it, nothing about Copenhagen being wrong, just certain forms of it.

I think Haag's and Peres's statements are pretty fundamental parts of QM. If Frauchiger and Renner break it, I'm not sure what they are talking about can be called QM.

Anyway, would you agree with Demystifier that collapse in BM is objective (post #17)?