# What has changed since the Copenhagen interpretation?

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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?

Sandeep T S

Mentor
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.

bhobba, MichPod, martinbn and 3 others
Gold Member
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.

Peter Morgan, MichPod and DarMM
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?

Gold Member
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.

bhobba
Gold Member
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.

Gold Member
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.

bhobba, Peter Morgan and Mentz114
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).

Gold Member
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.

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Gold Member
Matt Leifer and Robert Spekkens have said it is major advance
Where can I see what Matt Leifer said about it?

Gold Member
See his lecture about it here:

Gets to the actual theorem around 40 minute mark.

Demystifier
Gold Member
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.

Auto-Didact, MichPod and DarMM
Gold Member
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.

Demystifier
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.

Gold Member
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.

Gold Member
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.

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?

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

DarMM
Gold Member
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.

bhobba and DrChinese
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.

Gold Member
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.

Gold Member
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.

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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.

Gold Member
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?

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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.

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]."

Gold Member
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?

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?

Gold Member
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.

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.

Auto-Didact
Gold Member
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.

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Demystifier
Gold Member
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:
Lee Smolin said:
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
Peter Woit said:
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.
Lee Smolin said:
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

MichPod and Demystifier