# I Is the PBR Theorem valid?

1. Sep 6, 2017

### fanieh

https://en.wikipedia.org/wiki/PBR_theorem

"The theorem was first published as an arXiv preprint with Pusey as the principal author,[1] a subsequent version published in Nature Physics,[2] that states the theorem that either the quantum state corresponds to a physically real object and is not merely a statistical tool, or else all quantum states, including non-entangled ones, can communicate by action at a distance."

This is very fantastic claim. Since many believe quantum state is just statistical tool. Does it mean all quantum states, including non-entangled ones, can communicate by action at a distance? This can be easily proved or disproved. Since it seemed it couldn't, then the claim at Wikipedia appears not to be correct. Hope someone can edit it to give the right context. Thank you.

2. Sep 6, 2017

### Staff: Mentor

Its valid, but please read the entire paper - in particular note the assumptions (from the paper):
'One is that a system has a “real physical state” – not necessarily completely described by quantum theory, but objective and independent of the observer. This assumption only needs to hold for systems that are isolated, and not entangled with other systems. Nonetheless, this assumption, or some part of it, would be denied by instrumentalist approaches to quantum theory, wherein the quantum state is merely a calculational tool for making predictions concerning macroscopic measurement outcomes.'

That is the precise assumption many interpretations such as the ensemble reject or are silent on.

Thanks
Bill

3. Sep 6, 2017

### fanieh

I've been reading https://arxiv.org/pdf/1111.3328.pdf but I can't get the reason for the isolation.

Do you know why the assumption only needs to hold for systems that are isolated, and not entangled with other systems? What happens if the system is entangled with other systems. Why won't the PBR theorem no longer be valid?

4. Sep 6, 2017

### Staff: Mentor

Its been a while since I have gone into that theorem - it caused a big stir when released I think 6 years ago now and I don't recall the details too well - except what I posted because people sometimes ask - how can you maintain its not real anymore.

So I will have to wimp out and let someone with more current knowledge answer that one.

Thanks
Bill

5. Sep 7, 2017

### kith

The PBR theorem -under certain arguably weak assumptions- rules out that QM is a purely statistical theory about some underlying hidden variables which truly determine the physical state of a system.

In order to assess the significance of this, you should note that although some people vaguely hold such a view no currently popular interpretation is of this type. The Copenhagen interpretation denies that there are hidden variables and in the de-Broglie-Bohm theory (dBB), the wavefunction is not merely a statistical tool but influences the hidden variables.

The main influence of the PBR theorem on established interpretations is that it strengthens dBB against other conceivable hidden variable theories (HVTs). It proves that certain aspects of dBB are inevitable for HVTs and thus stands in the tradition of Bell's theorem (HVTs have to be non-local) and the Kochen-Specker theorem (HVTs have to be contextual).

For me, this leaves only one question about HVTs open: dBB is sometimes crticized for the fact that the wavefunction influences the hidden variables but not the other way round. I wonder if this is also inevitable for HVTs. (Maybe it is already known; I am no expert on these matters)

Last edited: Sep 7, 2017
6. Sep 7, 2017

### kith

Also check this blog post by Matt Leifer who discusses the PBR theorem by introducing the terminology of epistemic and ontic states which is often used in quantum foundations research.

7. Sep 7, 2017

### fanieh

Logic says the hidden variables (quantum potential or whatever) need to influence the wave function too.. what would it take to do this.. Demystifier? other Bohmians?

8. Sep 7, 2017

### DrChinese

Here is something Demystifier prepared a while back, pretty nice overview of PBR:

https://www.physicsforums.com/attachments/pbr-pdf.72453/

9. Sep 8, 2017

### vanhees71

What I still not get about this proof (as summarized nicely in @Demystifier 's slides as well as in the PBR paper) is that they somehow seem to prove the ontic nature of quantum states under the assumption that there's some hidden variable $\lambda$ that determines the QT probabilities, making these "ontic". Don't they just put what they claim to prove into this assumption? In standard minimally interpreted QT there's nothing like $\lambda$, and then isn't there again a loophole to the theorem in the sense that an epistemic interpretation of quantum state doesn't lead to a contradiction?

In my opinion minimally interpreted QT strongly suggests an epistemic interpretation of the quantum state, but as I tried to explain earlier in this thread, this doesn't imply non-objectivity either.

10. Sep 8, 2017

### Demystifier

Not exactly. They assume that there is some $\lambda$, but a priori it is not obvious that $\lambda$ can determine $\psi$. As I explained at page 15, the result is actually very surprising.

You are right, this interpretation is not ruled out by the PBR theorem. What is ruled out are some Einstein-like attempts to find a hidden-variable theory in which $\psi$ is eliminated completely. Roughly speaking, the PBR theorem tells: don't even try, you cannot find such a theory.

11. Sep 8, 2017

### Demystifier

Well, Bohmian hidden variables (that is, particle positions) do not inflence the wave function. And this is fully analogous to classical HJ theory where particle trajectories do not influence the S-function. So your logic is faulty.

12. Sep 8, 2017

### fanieh

In message #5, Kith last sentence was "For me, this leaves only one question about HVTs open: dBB is sometimes crticized for the fact that the wavefunction influences the hidden variables but not the other way round. I wonder if this is also inevitable for HVTs. (Maybe it is already known; I am no expert on these matters)"

If it was criticized.. this means some want the hidden variables to affect the wave function.. but in classical HJ theory, particle trajectories do not influence the S-function. So to avoid dBB being criticized, what should be the ontology such that the wave function can be influenced.. or if it can't be fixed within dBB.. can you please give example of another interpretation where the wave function can be influenced by a hidden variable such that the PBR theorem can apply? But then Kith mentioned "no currently popular interpretation is of this type".. so how do you cook up an interpretation which can support PBR theorem per Kith context? Many thanks.

13. Sep 8, 2017

### Demystifier

Yes, some people criticize the fact that dBB trajectories do not influence the wave function. All I can do about it to accuse them for hypocrisy, for why then they don't criticize the fact that classical trajectories do not influence the HJ S-function?

14. Sep 8, 2017

### vanhees71

Ok, then again a stupid question. What are dBB trajectories good for? AFAIK they are not (!) observable according to dBB.

15. Sep 8, 2017

### martinbn

But in the classical case they don't say that S-function influences the trajectories. And in dBB they say that.

16. Sep 8, 2017

### Demystifier

Of course they do (even if by using different words). Take a look at an analytical mechanics book again.

17. Sep 8, 2017

### Demystifier

That's the wrong question. The correct one is: Who are dBB trajectories good for? The answer is: They are good for (some of) those who think that there is a measurement problem in standard QM. Which, of course, excludes you.

This is analogous to atoms in 19th century, which were not observable at that time, but were still good for Boltzmann.

18. Sep 8, 2017

### martinbn

To me the wording is important. What words do they use and why don't the Bohmists use similar phrasing instead of influence? The thing is that for me the wave influences the trajectory sounds like the electric field influences the trajectory of a charged particle, which involves interaction between the particle and the field. And I have seen the pilot wave being compared to the electromagnetic field. If there is an interaction, the particle will influence the field as well, at least in principle.

19. Sep 8, 2017

### Demystifier

Actually, Bohmians rarely say "influence", except as a response to someone else who used the same word. Personally, I usually say "determine", in both the HJ theory and Bohmin mechanics.

20. Sep 8, 2017

### martinbn

Which is perfectly clear and unambiguous.

21. Sep 8, 2017

### Demystifier

Yes, some people use this analogy. I guess it's addressed to mathematically less sophisticated readers who are familiar with electromagnetic theory but not with HJ theory. For more sophisticated readers, the HJ analogy is much better.