A stronger proof of nonlocality, or what?

[Demystifier]

TL;DR Summary

What's the meaning of the paper published in Nature Physics?

Half a year ago a group of authors published a paper in Nature Physics https://www.nature.com/articles/s41567-020-0990-x which seems to be a proof of nonlocality even stronger than Bell nonlocality. More precisely, according to a popular exposition by one of the authors https://theconversation.com/a-new-q...unBA8Lv0lVf8FchIm-tNumW2LfFs0ChlOlc48X9nut8_4
the theorem (and experiment) show that at least one of the 3 assumptions must be false:

1. When someone observes an event happening, it really happened.
2. It is possible to make free choices, or at least, statistically random choices.
3. A choice made in one place can’t instantly affect a distant event. (Physicists call this “locality”.)

In particular, I wonder how the minimal statistical interpretation would interpret it, which of the three assumptions would it deny?

 

[martinbn]

You forgot one more option he gives " ..or quantum mechanics itself must break down at some level. "

I haven't looked at the paper, but my guess is that all these option are everyday language statements that do not accurately represent the results in the paper.

 

[PeterDonis]

This appears to be the arxiv preprint of the paper:

https://arxiv.org/abs/1907.05607

 

[PeterDonis]

When someone observes an event happening, it really happened.

This assumption is obviously violated if we assume that, for example, Wigner's friend can perform arbitrarily quantum operations on Wigner. "Arbitrary quantum operations" includes reversing decoherence, and if decoherence can be reversed, then all statements of the form that anything "really happened" are now invalid; such statements rely on decoherence being irreversible.

 

[Demystifier]

This assumption is obviously violated if we assume that, for example, Wigner's friend can perform arbitrarily quantum operations on Wigner. "Arbitrary quantum operations" includes reversing decoherence, and if decoherence can be reversed, then all statements of the form that anything "really happened" are now invalid; such statements rely on decoherence being irreversible.

I don't understand how reversibility is related to reality. For analogy, many phenomena in classical physics are reversible, but it doesn't make them unreal.

 

[atyy]

the theorem (and experiment) show that at least one of the 3 assumptions must be false:

1. When someone observes an event happening, it really happened.
2. It is possible to make free choices, or at least, statistically random choices.
3. A choice made in one place can’t instantly affect a distant event. (Physicists call this “locality”.)

In particular, I wonder how the minimal statistical interpretation would interpret it, which of the three assumptions would it deny?

(1) is false in the minimal interpretation (thinking along the same lines as @PeterDonis). So I would say this isn't a stronger proof than the Bell inequalities. In the paper, there are real observers (Alice and Bob) and non-real observers (Charlie and Debbie).

 

[Morbert]

I don't understand how reversibility is related to reality. For analogy, many phenomena in classical physics are reversible, but it doesn't make them unreal.

Roughly speaking, classical physics is always decoherent, so reversing a classical system can never reverse decoherence.

As an aside: QM doesn't describe irreversible processes as giving rise to reality. Instead irreversibility of processes in reality are what let us successfully apply quantum theory to it.

 

[Demystifier]

See also a parody: https://www.physicsforums.com/threads/collection-of-science-jokes-p2.847743/post-6463166

 

[martinbn]

See also a parody: https://www.physicsforums.com/threads/collection-of-science-jokes-p2.847743/post-6463166

Now you are doing it again!

1. Planets exist even if no human observes them.

I thought you agreed to be more precise about this. What you mean is that at any given time the positions (or some other observables) of the planets have definite values.

 

[Demystifier]

Now you are doing it again!

I thought you agreed to be more precise about this. What you mean is that at any given time the positions (or some other observables) of the planets have definite values.

It's a parody.

 

[martinbn]

It's a parody.

I know! So?

 

[PeterDonis]

many phenomena in classical physics are reversible, but it doesn't make them unreal.

"Reversibility" in classical physics does not erase all memories and records of what happened. "Reversibility" in QM, i.e., reversing decoherence, the kind of operation that is assumed to be used by Wigner on his friend, does erase all memories and records of what happened. And that invalidates the way we talk about such things in ordinary language. When we say someone, like Wigner's friend, "observes" a result, we mean that an irreversible record of the result has been formed. But Wigner's friend-type scenarios involve quantum operations that reverse the making of the record; so any description of such a scenario that says the friend "observed" a result (and some such claim has to be made in order to derive any purported contradiction or paradox) is simply wrong.

 

[Demystifier]

"Reversibility" in classical physics does not erase all memories and records of what happened. "Reversibility" in QM, i.e., reversing decoherence, the kind of operation that is assumed to be used by Wigner on his friend, does erase all memories and records of what happened. And that invalidates the way we talk about such things in ordinary language. When we say someone, like Wigner's friend, "observes" a result, we mean that an irreversible record of the result has been formed. But Wigner's friend-type scenarios involve quantum operations that reverse the making of the record; so any description of such a scenario that says the friend "observed" a result (and some such claim has to be made in order to derive any purported contradiction or paradox) is simply wrong.

OK, I see what you mean and it makes sense. But I think that the two kinds of "reversibility" have not much to do with the difference between classical and quantum physics. Instead, the relevant difference is the fact that in one case only a subsystem is reversed, while in another case everything (including the memories and records) is reversed. In principle, both kinds of reversion are possible in both classical and quantum physics. The classical Liouville equation and the quantum von Neumann equation are not that different.

But more to the point, if I forgot that something happened, it does not mean that it was not real.

 

[Morbert]

But more to the point, if I forgot that something happened, it does not mean that it was not real.

Roland Omnes remarks that a device capable of making you forget would either i) have to violate relativity or ii) be too bulky to not collapse into a black hole. So the point might be moot since you cannot forget what is real.

 

[Demystifier]

Roland Omnes remarks that a device capable of making you forget would either i) have to violate relativity or ii) be too bulky to not collapse into a black hole. So the point might be moot since you cannot forget what is real.

My brain forgets information from my short term memory all the time, often due to distractions by devices such as TV and computer screen. But my TV neither violates relativity nor collapses into a black hole. Of course, this forgetting is only due to coarsegraining, at the fundamental microscopic level there may be no forgetting at all. So I guess by forgetting you mean fundamental microscopic forgetting, am I right?

 

[PeterDonis]

I think that the two kinds of "reversibility" have not much to do with the difference between classical and quantum physics.

Yes, they do, because "time reversibility" means different things in classical and quantum physics. In classical physics, "reversibility" applies directly to observables. In QM, it applies to the wave function.

Instead, the relevant difference is the fact that in one case only a subsystem is reversed, while in another case everything (including the memories and records) is reversed.

No, the relevant difference is that in QM, since measurement involves entanglement, it is impossible to just reverse a subsystem; the "memories and records" are entangled with the subsystem so you can only reverse everything, not just one thing.

 

[stevendaryl]

My brain forgets information from my short term memory all the time, often due to distractions by devices such as TV and computer screen. But my TV neither violates relativity nor collapses into a black hole. Of course, this forgetting is only due to coarsegraining, at the fundamental microscopic level there may be no forgetting at all. So I guess by forgetting you mean fundamental microscopic forgetting, am I right?

Presumably, the state of your brain after experiencing something and then forgetting it in the normal way is macroscopically different than the state of your brain before you ever experienced it.

 

[Morbert]

My brain forgets information from my short term memory all the time, often due to distractions by devices such as TV and computer screen. But my TV neither violates relativity nor collapses into a black hole. Of course, this forgetting is only due to coarsegraining, at the fundamental microscopic level there may be no forgetting at all. So I guess by forgetting you mean fundamental microscopic forgetting, am I right?

This notion of "forget" isn't the same as reversing a record. E.g. If you make a measurement and then immediately bump your head such that you can't remember the result, your brain still records the result of the measurement, in the same way the surrounding environment also records the measurement. Bumping your head just makes the record inaccessible to you on a conscious level. Like smashing your computer before reading the data.

 

[Demystifier]

Yes, they do, because "time reversibility" means different things in classical and quantum physics. In classical physics, "reversibility" applies directly to observables. In QM, it applies to the wave function.

No, the relevant difference is that in QM, since measurement involves entanglement, it is impossible to just reverse a subsystem; the "memories and records" are entangled with the subsystem so you can only reverse everything, not just one thing.

The comparison of classical and quantum physics can only make sense if we formulate both in similar formalisms. Hence, for the purpose of comparison, I propose to formulate classical and quantum mechanics in the language of Liouville and von Neumann equation, respectively. In this framework, none of the differences above sustain.

 

[PeterDonis]

The comparison of classical and quantum physics can only make sense if we formulate both in similar formalisms.

This is far too strong a requirement. The only thing we actually need to compare is predictions.

for the purpose of comparison, I propose to formulate classical and quantum mechanics in the language of Liouville and von Neumann equation, respectively

Doesn't this leave out measurements in the quantum case? If so, it's an incomplete comparison.

 

[Demystifier]

Doesn't this leave out measurements in the quantum case?

Not if you include the quantum state of the apparatus, e.g. as in the many-world interpretation.

 

[PeterDonis]

Not if you include the quantum state of the apparatus, e.g. as in the many-world interpretation.

So that means the validity of the comparison you describe is interpretation-dependent? That doesn't seem like a good choice of a method of comparison.

 

[Demystifier]

So that means the validity of the comparison you describe is interpretation-dependent?

Not necessarily. You can include the apparatus in a way that does not depend on interpretation. Many worlds are mentioned only because it is an example where the explicit quantum description of the apparatus is often used. Other examples are Bohmian interpretation and (interpretation independent!) theory of decoherence. All these approaches are based on the same theory of quantum measurements known as von Neumann theory. The only interpretation which is excluded by that approach is the Bohr's interpretation insisting that the apparatus must be described by classical physics.

 

[Morbert]

Just to add the usual consistent histories insight re/unitary evolution:

If we have closed system $|\Psi\rangle$ consisting of a measured system + apparatus + observer, with possible measurement outcomes $\{\lambda_i\}$, then the unitary description of the entire system is incompatible with the description of possible measurement outcomes, since the property $|\Psi\rangle\langle\Psi|$ that evolves unitarily does not commute with the measurement outcomes $|\lambda_i\rangle\langle\lambda_i|$Neither the unitary nor measurement description is more correct than the other, but only the latter is accessible to any observer who is a part of $\Psi$

 

[PeterDonis]

You can include the apparatus in a way that does not depend on interpretation.

But you can't have the entire system + apparatus obey the Von Neumann equation, which, as I understand it, is unitary, unless the interpretation you are using agrees with that, as the MWI does, correct?

 

[Demystifier]

But you can't have the entire system + apparatus obey the Von Neumann equation, which, as I understand it, is unitary, unless the interpretation you are using agrees with that, as the MWI does, correct?

Correct. But to avoid interpretation dependence, I can compare formalisms (von Neumann equation vs Liouville equation) without even talking about interpretations. And if one insists that I compare interpretations too, I can always choose the interpretation in which the two look most similar (which happens to be the Bohmian interpretation*).

*There is also an alternative, in which one modifies the interpretation of classical mechanics to make it more similar to standard "Copenhgen" QM. See my papers
https://arxiv.org/abs/quant-ph/0505143
https://arxiv.org/abs/0707.2319

 

[PeterDonis]

to avoid interpretation dependence, I can compare formalisms (von Neumann equation vs Liouville equation) without even talking about interpretations

But in the quantum case the formalism you are using in the comparison doesn't cover measurement unless you are adopting an interpretation like the MWI. So whether or not your comparison is complete is interpretation-dependent.

 

[Demystifier]

But in the quantum case the formalism you are using in the comparison doesn't cover measurement unless you are adopting an interpretation like the MWI. So whether or not your comparison is complete is interpretation-dependent.

I agree. No analysis is perfect. The analysis that you proposed is unperfect too, because, in a sense, it compares apples with oranges (classical with quantum). Mine is unperfect for the opposite reason, because it compares fruits with fruits (probability distribution with probability distribution).

 

[Kolmo]

It's a stronger no-go theorem than Bell's because it locates quantum theory more precisely in the landscape of general probability theories.

Bell like inequalities, combined with results like Fine's theorem, sketch out the region where probability theories are fundamentally Kolmogorov probability theories.

These new bounds, called the "Local Friendliness" bounds show a inequality obeyed by many non-Kolmogorov theories and yet still broken by quantum theory.

The bounds sketch out theories where it is generally difficult to demonstrate that their outcomes are randomly generated. Quantum Theory violates these bounds (still obeyed by many non-Kolmogorov theories) and thus is more accurate than Bell type results which simply tell you that Quantum Theory is not Kolmogorov.

If one prefers to think in terms of hidden variable theories, Bell's theorem rules out local hidden variable theories where as this result rules out a large class of nonlocal hidden variable theories. It can also be combined with Legett's earlier work ruling out a separate class of nonlocal theories.

 

[PeterDonis]

If one prefers to think in terms of hidden variable theories, Bell's theorem rules out local hidden variable theories where as this result rules out a large class of nonlocal hidden variable theories. It can also be combined with Legett's earlier work ruling out a separate class of nonlocal theories.

How does Bohmian mechanics, which is a nonlocal hidden variable theory whose predictions exactly match those of QM, avoid the limitations imposed by these results?

 

[Kolmo]

How does Bohmian mechanics, which is a nonlocal hidden variable theory whose predictions exactly match those of QM

Do you have a reference for where that is proved? I know it is equivalent for position measurements, but I've never seen the fully general proof. I'd just need to see it to make sure I'm answering correctly.

 

[PeterDonis]

Do you have a reference for where that is proved?

The math of Bohmian mechanics is equivalent to that of standard QM; that has been known since the 1950s. Since the math is equivalent, it must make the same predictions.

 

[Demystifier]

Do you have a reference for where that is proved? I know it is equivalent for position measurements, but I've never seen the fully general proof. I'd just need to see it to make sure I'm answering correctly.

The trick is that Bohmian mechanics uses the idea that all measurements can be reduced to position measurements, or more precisely, to positions of macroscopic pointers. See e.g. the paper linked in my signature below.

 

[Kolmo]

The math of Bohmian mechanics is equivalent to that of standard QM; that has been known since the 1950s. Since the math is equivalent, it must make the same predictions.

But where is the proof of this is what I am asking? There used to be arguments in the literature that it didn't give the same results for measurements of quantities outside of the basis chosen by the hidden variables. In standard Bohmian Mechanics that would be the position basis, but there are other "Bohmian" Mechanics if one chooses other quantities. Also that it had issues with multi-time correlations.

The mathematical equations are quite different so I'd like to see a proof of the equivalence, I don't think it's true that "it is known" since it was debated at the time and there is a long argument it is not equivalent in R.F. Streater's book "Lost Causes in and beyond Theoretical Physics". I'm not saying Streater is right, just asking for where it was proven.

 

[Demystifier]

Also that it had issues with multi-time correlations.

Multi-time correlations predicted by Bohmian mechanics are also in agreement with standard QM. See Appendix I of F. Laloe, Do We Really Understand QM?