# I What has changed since the Copenhagen interpretation? (1 Viewer)

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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?

#### Nugatory

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.

#### Demystifier

2018 Award
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

2018 Award
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

2018 Award
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.

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#### Demystifier

2018 Award
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.

#### Demystifier

2018 Award
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

2018 Award
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?

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

2018 Award
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.

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