PBR theorem - that the wavefunction is physically existent

In PBR theorem, "wf is physically real" means "wf is not merely the (square root of) probability".

One of the important assumptions in the theorem is that some reality exists. Basically, the theorem states that if there is something real, then wf is also real. The theorem does not disprove the interpretations according to which reality does not exist at all.

 

But already the classical gravitational field is not physical if you use that criterion. http://arxiv.org/abs/1503.03754

 

OK, I'm not being sarcastic here, I'm just, well, mystified. If reality doesn't exist at all, then what are we discussing? (And who is doing the discussing?)

 

That, of course, is a very good question.
For a possible answer see
http://lanl.arxiv.org/abs/1112.2034
according to which observers are real but the things they observe are not.

 

Would you explain this part?

Is the latter idea (what the PBR disproves) some sort of hidden variable theory? I suppose it isn't, but that's how it sounds to me from this explanation, in the sense that I don't understand what else it could be saying. What's "human knowledge about underlying non-quantum state objects" supposed to mean? Does this view have a name? Thanks.

Edit: I guess the article itself answers me, they're called "psi-epistemic" theories which are indeed hidden variable theories. My brain didn't register that because I've never seen the PBR theorem explicitly described as ruling out yet another version of hidden variables, but instead is talked about using the confusing (for me) notion of "real wavefunction", is there a reason for this? (Assuming I haven't misunderstood twice).

 

I found a more detailed article that explains most of the obscure points for me:

http://mattleifer.info/2011/11/20/can-the-quantum-state-be-interpreted-statistically/

 

I read the article referenced in post #9 above. Would it be fair to say (oversimplifying) that most practitioners of QFT regard the field (perhaps the state of the field) as the underlying reality and that particles are only what we can observe? Or do they even worry about such things?

 

Exactly. And despite the fact the paper states this very carefully and clearly some people for some reason don't take it on board.

This paper goes deeper into the models it applies to and those it doesn't:
http://arxiv.org/abs/1203.4779

Thanks
Bill

 

You hit it in one. And of the few that do, sometimes like Penrose they reach some rather 'strange' conclusions:
https://www.maths.tcd.ie/~fionn/reality.pdf

Don't worry about it - it doesn't really affect anything.

Thanks
Bill

 

The PBR theorem in no way excludes the idea that the wave function of the universe is epistemic, describes only insufficient knowledge.

The point is that one has to look how hidden variable theories work, for example, by looking at dBB theory. In dBB theory itself, the wave function is considered to be objective, really existent, but one can interpret it in such a way that the wave function is epistemic only. The interesting point is how the result of a measurement is described, namely it is obtained from a common global wave function (system + measurement device) by [itex]\psi(., q_{dev}(t),t)[/itex], where [itex]q_{dev}(t)[/itex] is the trajectory of the measurement device. What is the point of this formula? It shows that, once we have prepared a state using a measurement, the wave function depends on something real, namely the trajectory [itex]q_{dev}(t)[/itex]. So, even if the global wave function is completely epistemic, the effective wave function of the subsystem is no longer completely epistemic, but contains ontic information, namely the information about [itex]q_{dev}(t)[/itex].

And that's why a theorem which proves that these effective wave functions have to be ontic prove nothing against interpretations which have interpret the global wave function of everything as epistemic.

 

I think you are both right and wrong. You are right that the wf of the universe is "epistemic" in the sense you have just described. However, the word "epistemic" in the PBR theorem has a different meaning. You have not demonstrated that the wf of the universe is "epistemic" in the strict PBR sense.