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The wave function is real [LINK]
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 represents. One possibility is that a pure quantum state corresponds directly to reality. But there is a long history of suggestions that a quantum state (even a pure state) represents only knowledge or information of some kind. Here we show that any model in which a quantum state represents mere information about an underlying physical state of the system must make predictions which contradict those of quantum theory.
So, it turns out it is more than "just" a useful mathematical tool after all. Does this confirm the universe basically runs on maths?
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Comments (19)
The idea is:
Suppose all wavefunctions encode some constant, sub-unity fraction of the information about the 'true state' - that all wavefunctions are equally specific. Then we should be able to tease out more of that information with some very basic experiments (edited to clarify: by measuring in multiple ways - not that the new state would be intrinsically better constrained). This contradicts the results of the many many instances of these experiments.
So you can escape by supposing that it's possible to prepare a wavefunction that has more or less information about the 'true state'.
It's nice to have yet another very strong constraint to put on global hidden variables, but nothing could ever be total proof.
~~~~ edited to add: But if you pull that escape route, then it's trivial to perform (by which I mean, they have already been done zillions of times) additional experiments showing that quantum states with different distributions of 'real' states (as we just allowed by the escape route) actually have identical dynamics.
The more I think about this, the less wiggle room I see.
Also note the companion article "The quantum state can be interpreted statistically," by two of the same authors. A previous title for this paper was "The quantum state cannot be interpreted statistically," by the way.
This heavily implies that what's being argued here is deeply philosophical, hinging crucially on the interpretation of "statistically" and therefore has a high probability of being incorrect.
I'm withholding my judgment, for the moment.
Don't lend too much credence to a single paper and don't make far-reaching conclusions just yet. This is a nice result, but it is not of the same level as, say, the Bell inequalities.
I have an (undergraduate) degree in physics, or will at the end of the semester, and either I don't fully understand the paper after a second read-through, or it is wrong. The claimed contradiction doesn't seem to be particularly damning: if the measured state is excluded from being in a |0>|0> initial state, that doesn't mean that the two cannot have started in a different state, and the excluded state will change in later measurements anyway, so the measurement is not telling you anything new.
(Motl's explanation is harsh and caustic, but here I agree with him in interpreting the paper.)
Edit: I agree with Motl's explanation of why the exclusion of 1 of the 4 combined states is not problematic.
Motl's explanation is completely off-base because he's trying to defend QM from attack when the paper is not trying to attack QM, but trying to distinguish QM from hidden-variables theories.
Of course P(i) = Tr( L | Z(i) >< Z(i) |) ... that's what QM says!
What they've just shown is that hidden variables theories that can't do that, because the 'real state' L has specific values, and not a quantum nature.
http://motls.blogspot.co.uk/2011/11/nature-hypes-anti-qm-crackpot-paper-by.html
Motl refuses to understand that they are not questioning the probabilistic nature of QM (map from amplitude to probability), but rather whether amplitude itself is only a reflection of some underlying physical state (map from unknown underlying state to amplitude). Basically, discount most of what Motl writes in this thread.
The paper authors say that if amplitude itself reflects our incomplete knowledge of the state of a physical system, and so is probabilistic in nature, then they can obtain a contradiction with the predictions of conventional QM. Unfortunately, unlike in the case of the celebrated Bell theorem, it doesn't look to me like they are making any new predictions, only reasoning things out from what is already known.
If someone can clarify this issue (can this theorem be tested experimentally?) or link to a relevant discussion, I would appreciate it.
Yeah, that person clearly has too much of his identity tied up in this stuff. His post reads like an atheist who has been consigned to debate creationists for all his days.
The paper's authors found that X is false, and Motl contends that in fact X is obviously false so the authors are bad people for even talking about it.
It's as though the paper proved that if the moon were made of cheese then it could not be green, and Motl took them to task for being pro- "the moon is made out of cheese".
Given Motl's record of being vocally wrong about many things in basic ways, this should be more or less the default position.
From the article:
Skimming the article I haven't found a precise definition of what having a real physical state means (which would be interesting and important in such a discussion, especially given the authors scare quotes around the term). Nevertheless, the assumption that the system has a state is quite a strong one. If one wants to deny that the wave function is real (whatever it means) one can easily deny the reality of states, and treat QM only as a tool predicting which sequences of measurements can happen and how probably.