Quantum wavefunction is real?

[Leureka]

TL;DR Summary

Lundeen, J., Sutherland, B., Patel, A. et al. Direct measurement of the quantum wavefunction. Nature 474, 188–191 (2011). https://doi.org/10.1038/nature10120

https://arxiv.org/abs/1112.3575

In this 2011 paper, Lundeen & colleagues used weak measurement to map both imaginary and real components of a wavefunction directly, without destroying the state.

It says: “with weak measurements, it’s possible to learn something about the wavefunction without completely destroying it”. And this: “We hope that the scientific community can now improve upon the Copenhagen Interpretation, and redefine the wavefunction so that it is no longer just a mathematical tool, but rather something that can be directly measured in the laboratory”. What they’re saying is wavefunction is real. So it can't just be a "probability wave".

I was always taught that psi is just a mathematical tool used to calculate probabilities. This paper is more than 10 years old and it's still taught that way, so either there is some serious rubbish going on in physics teaching or this paper is wrong in some way. Any not too technical comments?

 

[Nugatory]

Behind a paywall, but there’s an ArXiv preprint at https://arxiv.org/abs/1112.3575

(@Leureka it’s generally good practice to check arXiv - not everyone will have access to the paywalled copy.)

 

[Leureka]

Behind a paywall, but there’s an ArXiv preprint at https://arxiv.org/abs/1112.3575

(@Leureka it’s generally good practice to check arXiv - not everyone will have access to the paywalled copy.)

Thanks for that, I directly linked to the nature paper because I was told arxiv and vixra are not ideal sources to post here and the use of reputable journals is preferred. I guess they're fine then if I just post them both.

 

[Nugatory]

Thanks for that, I directly linked to the nature paper because I was told arxiv and vixra are not ideal sources to post here and the use of reputable journals is preferred. I guess they're fine then if I just post them both.

Vixra is categorically not allowed.
ArXiv preprints that have been subsequently published in a reliable and peer-reviewed journal are generally OK - the content has been properly vetted.
Other ArXiv stuff is evaluated case-by-case.

 

[anuttarasammyak]

Last year I read the news article https://scitechdaily.com/physicists...nary-part-of-quantum-mechanics-really-exists/ which says i is necessary for QM. You may be interested in it also.

 

[Leureka]

Last year I read the news article https://scitechdaily.com/physicists...nary-part-of-quantum-mechanics-really-exists/ which says i is necessary for QM. You may be interested in it also.

I did not understand that experiment. Looks like some kind of bell test? The analogy they used wasn't really conveying the necessity of imaginary numbers.

i is used simply to encode rotations, and it's much easier to integrate exponential functions of the form e^i@ than sine and cosine functions.

 

[DrChinese]

... What they’re saying is wavefunction is real. So it can't just be a "probability wave".

I was always taught that psi is just a mathematical tool used to calculate probabilities. This paper is more than 10 years old and it's still taught that way, so either there is some serious rubbish going on in physics teaching or this paper is wrong in some way. Any not too technical comments?

There are a series of papers that make this argument, that the "Wave Function is Ontic". Probably the most well-known and cited is the paper of Pusey, Barrett and Rudolph (called PBR) from 2011, called "On the reality of the quantum state":

https://arxiv.org/abs/1111.3328
"Quantum states are the key mathematical objects in quantum theory. It is therefore surprising that physicists have been unable to agree on what a quantum state truly represents. One possibility is that a pure quantum state corresponds directly to reality. However, there is a long history of suggestions that a quantum state (even a pure state) represents only knowledge or information about some aspect of reality. Here we show that any model in which a quantum state represents mere information about an underlying physical state of the system, and in which systems that are prepared independently have independent physical states, must make predictions which contradict those of quantum theory."

The PBR theorem is generally accepted as a no-go theorem, somewhat akin to Bell (in effect another nail in the coffin for EPR realism). However, there are those who reject one or more of its explicit or implicit assumptions, they say the wave function is "epistemic" instead. The debate goes on... PS One of the authors of the PBR theorem, Terry Rudolph, was formerly a member here!

 

[mattt]

I have only read the abstract of the PBR paper (I don't have spare time right now to have a closer look at it), so beware that I may be misunderstanding what they will really talk about in the actual article.

If we have a sufficiently large collection of identically prepared copies of a quantum system, there is at most one (pure) state that correctly describes the complete statistics of all possible measurement results, in all possible measurements of observables, for that collection (ensemble).

When they talk about epistemic states, do they refer to mixed states perhaps?

No bother, I'll try to read it during the weekend...

 

[Leureka]

There are a series of papers that make this argument, that the "Wave Function is Ontic". Probably the most well-known and cited is the paper of Pusey, Barrett and Rudolph (called PBR) from 2011, called "On the reality of the quantum state":

https://arxiv.org/abs/1111.3328
"Quantum states are the key mathematical objects in quantum theory. It is therefore surprising that physicists have been unable to agree on what a quantum state truly represents. One possibility is that a pure quantum state corresponds directly to reality. However, there is a long history of suggestions that a quantum state (even a pure state) represents only knowledge or information about some aspect of reality. Here we show that any model in which a quantum state represents mere information about an underlying physical state of the system, and in which systems that are prepared independently have independent physical states, must make predictions which contradict those of quantum theory."

The PBR theorem is generally accepted as a no-go theorem, somewhat akin to Bell (in effect another nail in the coffin for EPR realism). However, there are those who reject one or more of its explicit or implicit assumptions, they say the wave function is "epistemic" instead. The debate goes on... PS One of the authors of the PBR theorem, Terry Rudolph, was formerly a member here!

I warn everyone that I'm going to argue the following with the intent of showing my understanding, with no prejudice over either explanation. I might have misunderstood some points of that paper.

I'm not sure I understand the difference between a "mere" representation of information about a physical state and a real physical state. Do they mean "incomplete representation of information"? Whenever we talk about a system we do so by describing the information we have about it, like momentum, frequency and so on. The wavefunction is a representation of a real behaviour of the system, it's just that our interpretation of the representation might be misguided, because we don't have a clear picture of how that behaviour emerges.Quoting the first paragraph of the paper, "Does the wavefunction correspond directly [emphasis mine] to some kind of physical wave? If so, it is an odd kind of wave, since it is defined on an abstract configuration space"
It does not have to be a direct correspondence, and it does not even have to be indirect in the sense that there's hidden knowledge about the system under measurement (the "experimenter uncertainty"). I can describe the state of a falling tree in multiple ways, some more abstract than others. I could for example describe the motion of its shadow during the fall. That does not mean the shadow IS the tree, but its behaviour describes fully what the tree is doing, in relation with the position of the sun. I mean fully in the sense that there need not be hidden variables that we are not aware of to describe the motion of the tree, inside the shadow; its motion is completely understandable by the shadow, we are just ignoring every other property we are not looking at, like the texture of the bark or the smell of its flowers, which would give us a better idea of what the tree actually looks like.

In this example, the quantum wavefunction would be the shadow and the sun would be the measurement apparatus. This argument boils down to whether the entity described by the wavefunction has an independent existence to the measurement, which the paper I linked to suggests is the case. The shape of the wavefunction might change with different measurements (different, random orientations of the sun), but the essence of the system would not unless we burned the tree down with a laser light, which most of our experiments do. It appears to me that we haven't thought about hidden variables in our measurement apparatus, rather than in the system measured.

Finally, they start their reasoning by assuming a classical system with momentum and position completely determined, which goes against the uncertainty principle. It's obvious that with such an assumption we would end up with contradictions with experiment. The thing is, even "classical" system of any kind of wave phenomenon are subject to the uncertainty principle. It seems weird to me to invoke arguments such as the UP as evidence for the non-reality of the wavefunction (Though QFT has no issues in this department, and quantum fields are often interpreted as "real" at least in popular media).

At this point, another question I would then ask is, is there any bias in the mainstream probabilistic interpretation of the wavefunction?