Undergrad Some more questions about the PBR theorem

[ftr]

There are two recent threads about the subject, however I want to ask a different question.

What does the wavefunction is "real" imply or what such "clarification" actually mean to the deeper understanding of QM. The theory is purported to be revolutionary and important breakthrough, yet it does not seem to clarify anything. Ordinary interpretations seem to have much better "explanatory" potential.

 

[PeterDonis]

What does the wavefunction is "real" imply or what such "clarification" actually mean to the deeper understanding of QM.

The term used in the PBR paper is $\psi$ ontic. A model is $\psi$ ontic if either the wave function $\psi$ is the real state of the system, or can be known with 100% certainty from knowledge of the real state of the system (for example, if each quantum state $\psi$ corresponds to some set of real states of the system instead of just a single real state, and no real state is in the set corresponding to more than one quantum state $\psi$).

The reason the theorem is considered important is that it claims to rule out any interpretation of QM that is not $\psi$ ontic in the above sense. Since many physicists have argued for interpretations that are not $\psi$ ontic, and claim that interpretations that are not $\psi$ ontic avoid a number of difficult issues, ruling out such interpretations altogether would be a big deal. However, it's not entirely clear that the PBR theorem does quite what its proponents claim it does. The Matt Leifer review article that I linked to in my previous thread on this topic gives a good discussion of the issues involved.

 

[PeterDonis]

Ordinary interpretations

What do you consider "ordinary interpretations"?

Note that the PBR theorem does not introduce a new interpretation of QM. It only considers existing intepretations, divides them into two classes ($\psi$ ontic and not $\psi$ ontic), and claims to show that interpretations in the second class (not $\psi$ ontic) are ruled out.

 

[ftr]

What do you consider "ordinary interpretations"?

BM,Manyworlds ...etc.

Note that the PBR theorem does not introduce a new interpretation of QM. It only considers existing intepretations, divides them into two classes (ψψ\psi ontic and not ψψ\psi ontic), and claims to show that interpretations in the second class (not ψψ\psi ontic) are ruled out.

Yes we talked about the "technical" definition and I know it is not an interpretation per say, I am just asking what it buys us in terms of expanding our understanding of The foundation of QM and in what way.

 

[DrChinese]

What does the wavefunction is "real" imply or what such "clarification" actually mean to the deeper understanding of QM. The theory is purported to be revolutionary and important breakthrough, yet it does not seem to clarify anything. Ordinary interpretations seem to have much better "explanatory" potential.

Some QM interpretations imply (or state) that the quantum mysteries represent a lack of knowledge. This would apply more to that side of things. My personal beliefs lean more toward believing that the quantum state itself is "real", so the PBR theorem tends to support the view I already held.

 

[DrChinese]

For example: QBism.

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

Although naturally, there aren't too many people who actually drop their favored interpretation due to things like PBR. There's always an "out".

 

[ftr]

quantum state itself is "real"

So what do you think of the probability distribution is "real" mean.

 

[DrChinese]

So what do you think of the probability distribution is "real" mean.

To me, it means that the possibilities (summing to probabilities) act as if they are themselves real. Until (and unless) a measurement collapses things to a specific outcome. Don't take what I am saying literally, it's just a mental picture.

If you input a 45 degree stream of photons into a beamsplitter (PBS), the output is a 50% probability out of the H port and 50% probability out of the V port. Until measured as one or the other, each output is qualitatively different than what you would get if you had sent an input stream of 0 degree photons into the PBS. You know that because of things like MZIs which tend to demonstrate the reality of the wave function (via related interference effects).

 

[ftr]

Ok thaks to you both. I will reread all related articles, but now it is late and my brain is shutting down. Actually I found that thinking about foundation when retiring is a good way to sleep quickly.

 

[DarMM]

The reason the theorem is considered important is that it claims to rule out any interpretation of QM that is not $\psi$ ontic in the above sense.

Just to be completely accurate, it claims to rule out any realist interpretation that is not $\psi$-ontic (in the sense you said) and also does not violate:

1. Experiments have multiple objective outcomes.
2. There are no retrocausal or acausal dynamics.
3. Spacetime is not highly topologically nontrivial
4. There is no superdeterminism

For simplicity I'll call them the "ontological framework axioms", any model violating them is called "Exotic". And the PBR theorem assumes the interpretation does not violate preparation independence (systems prepared separately at space-like distances do not affect each other).

So rejecting these is a form of "out" as @DrChinese mentioned.

The even bigger "out" is to reject that $\psi$ is related to the properties of the quantum system at all (i.e. Anti-Realist/Participatory-Realist interpretations).

(I know you know this, more so for @ftr )

So you could break $\psi$-epistemic into two classes, Realist and AntiRealist. Sometimes these are called Type 1 and Type 2 $\psi$-epistemic or (my preference and what I'll use below) $\psi$-statistical and $\psi$-epistemic.

So the PBR theorem rules out non-Exotic $\psi$-statistical interpretations that obey preparation independence.

You could violate preparation independence, make the model Exotic or move to $\psi$-epistemic theories and still retain the wave-function as not real.