PBR theorem - that the wavefunction is physically existent

[Quotidian]

Main Question or Discussion Point

I have been told on another forum I post to that there is a revolutionary theorem in physics which proves beyond doubt that the wavefunction (I presume meaning the one originally described by Schrodinger) is physically real. I have had various exchanges with the contributor who has told me this who is adamant that this is the case and equally adamant that I'm not sufficiently educated to understand it.

I'm not a physics grad, but I have, I think, a reasonable layman's understanding of some of the issues sorrounding philosophy of physics. But doing a search on the theorem doesn't reveal much. It certainly doesn't seem to be a revolutionary idea, but who am I to say? So I just thought I would mention it here and see what other folks with some background in physics might have to say.

 

[Borek]

First thing first: as him for a reference. Where and by whom was it published?

 

[Nugatory]

First thing first: as him for a reference. Where and by whom was it published?

Quotidian may be assuming that "PBR theorem" is a sufficient reference: published in Nature, arxiv preprint at http://arxiv.org/abs/1111.3328

 

[Nugatory]

proves beyond doubt that the wavefunction (I presume meaning the one originally described by Schrodinger) is physically real.

That's somewhere between an overstatement and a misstatement, I think, but...

It certainly doesn't seem to be a revolutionary idea, but who am I to say? So I just thought I would mention it here and see what other folks with some background in physics might have to say.

PBR does pretty convincingly eliminate one way of thinking about QM. That's certainly interesting and important. Whether it's revolutionary or not depends on whether you've based your worldview on that way of thinking.

 

[Quotidian]

The wikipedia entry on this theory says

The theorem was first published as an arXiv preprint with Pusey as the principal author, a subsequent version published in Nature Physics, 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.

So if the wavefunction is a physically real object, this seems to alter the definition of what 'physically real' means - which is the point at issue.

 

[Demystifier]

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.

 

[Quotidian]

But whether or in what sense it is physical is the point at issue. If it occupies no location and has no mass, then it seems to me that defining it as 'physically real' is taking liberties with the very word 'physical'.

 

[atyy]

But whether or in what sense it is physical is the point at issue. If it occupies no location and has no mass, then it seems to me that defining it as 'physically real' is taking liberties with the very word 'physical'.

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

 

[Nugatory]

But whether or in what sense it is physical is the point at issue. If it occupies no location and has no mass, then it seems to me that defining it as 'physically real' is taking liberties with the very word 'physical'.

Have you read the paper itself? That will go a long ways towards clarifying exactly what the theorem says. Pusey and crew are also fairly careful in their use of words - they don't casually sling "real" around.

You might be also be interested in this overview: http://www.scottaaronson.com/papers/getreal.pdf

 

[sandy stone]

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.

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?)

 

[Demystifier]

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.

 

[fresh_42]

Sorry for leveling down the debate. I couldn't resist:

"Vulcan Computer: What was Kiri-Kin-Tha's first law of metaphysics?
Spock: Nothing unreal exists."

 

[ddd123]

You might be also be interested in this overview: http://www.scottaaronson.com/papers/getreal.pdf

Would you explain this part?

if you adhere to the shut-up-
and-calculate philosophy or the Copenhagen
interpretation (which I think of as shut-
up-and-calculate minus the shutting-up
part) then the PBR result shouldn’t trouble
you. You don’t have an ontology: you
consider it uninteresting or unscientific to
discuss reality before measurement. For
you, ψ is indeed an encoding of human
knowledge, but it’s merely knowledge about
the probabilities of various measurement
outcomes, not about the state of the world
before someone measures.
[...]
But if you think that the rules of quantum
mechanics are fine and the wavefunction
is merely a summary of human knowledge
about underlying objects that are not
themselves quantum states — then, and only
then, the PBR theorem spells big trouble
for you.

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).

 

[Quotidian]

Thanks, I will dig into those sources a bit, although, as I said, I'm not trained in physics and generally have trouble understanding the math. But the fact that there's something to be discussed about it, i.e. that the interpretation is not unequivocal, is more or less the only point I couldn't get across to the poster on the other forum.

 

[ddd123]

Thanks, I will dig into those sources a bit, although, as I said, I'm not trained in physics and generally have trouble understanding the math. But the fact that there's something to be discussed about it, i.e. that the interpretation is not unequivocal, is more or less the only point I couldn't get across to the poster on the other forum.

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/

 

[sandy stone]

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?

 

[bhobba]

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.

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

 

[bhobba]

Or do they even worry about such things?

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

 

[Ilja]

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 $\psi(., q_{dev}(t),t)$, where $q_{dev}(t)$ 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 $q_{dev}(t)$. 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 $q_{dev}(t)$.

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.

 

[Demystifier]

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 $\psi(., q_{dev}(t),t)$, where $q_{dev}(t)$ 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 $q_{dev}(t)$. 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 $q_{dev}(t)$.

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.

 

[Ilja]

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.

First, I do not claim that PBR is somehow wrong, thus, it proves that the effective wave function is ontic - my point is that this is very compatible with the global wave function being epistemic. Because what defines the actual wave function of a subsystem - which is the chosen measurement device and its result - is ontic even if only the dBB trajectory of the measurement device and its pointer is ontic.

But this argument cannot prove, of course, that the wf of the universe is epistemic. What it shows is that PBR is irrelevant for this question. Arguments that the wave function is epistemic have to be, of course, different. See http://arxiv.org/abs/1103.3506 for some such arguments.