I haven't seen this questions asked here before, I used the search function so if it has been asked, then it didn't show up.

I am asking in reference to this paper that was posted to the arXiv in 2016 which claims to show than single world interpretations of quantum mechanics are not consistent.

I was wondering if any of you here have gone over this paper, or would like to give you thoughts on it, and it's usage by MWI proponents of this paper proving that Many Worlds is the only correct interpretation.


closed as off-topic by ACuriousMind May 21 '18 at 10:12

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  • $\begingroup$ I'm voting to close this question as off-topic because questions effectively asking for peer-review are off-topic $\endgroup$ – ACuriousMind May 21 '18 at 10:12
  • $\begingroup$ @ACuriousMind This question seems to have the same form as many others, though, i.e. "does no-go theorem X rule out Y?" The only difference is that the theorem is a little more recent. $\endgroup$ – knzhou May 21 '18 at 12:05
  • $\begingroup$ @knzhou The question is explicitly about the paper, asks whether people have gone through it, and asks for their thoughts on it. There may be an on-topic version of this question, but it would have to ask explicitly about the theorem itself, and be answerable without actually reading this one specific paper. A good start would be to incorporate the essential statement from the theorem into the question itself, since after reading both the question and your answer, I still don't really know what this is about without clicking on the link and reading the paper. $\endgroup$ – ACuriousMind May 21 '18 at 12:11
  • $\begingroup$ @ACuriousMind Okay, that's fair, I did have to read a paper to answer the question, so I guess it counts. $\endgroup$ – knzhou May 21 '18 at 12:12
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    $\begingroup$ "Published" is a misleading term for an arXiv preprint that has yet to appear (two years later) on any peer-reviewed journal. $\endgroup$ – Emilio Pisanty May 21 '18 at 14:52

I spent a few minutes skimming the paper, and it doesn't say much new. By definition, you cannot falsify a quantum interpretation. That is why they are philosophy, not physics.

First let me start off with what is falsified. The most naive possible version of the Copenhagen interpretation is that, whenever a system interacts with another, wavefunction collapse occurs; the larger system has "measured" the smaller one. This is not a legitimate interpretation, but a placeholder for one that's good enough to get you through a first course in quantum mechanics. It's clearly not true and I've never seen any text suggest otherwise.

We know naive Copenhagen isn't true because it directly conflicts with observation. For example, you can pass a photon through a beam splitter, or steer a particle with a magnetic field, without collapsing superpositions. And we entangle the states of two systems all the time, which would be impossible if interaction were automatically a measurement.

The real Copenhagen interpretation is much more agnostic about what counts as a measurement. There's no criterion like "interacting with a system with more than X degrees of freedom causes collapse", but it is agreed that interaction with a macroscopic, thermodynamic system does count as a measurement. The crucial difference between this and naive Copenhagen is that, like all proper interpretations of quantum mechanics, it isn't falsifiable. In principle one could tell between many worlds and Copenhagen by, say, running Wigner's friend backwards in time, but this is extravagantly impossible by the Second Law of Thermodynamics. Undoing the interactions of a trillion trillion molecules is more impossible than anything ever dreamt of in science fiction.

It seems like this paper is another variation on this old story. The point is, if the "experimenters" in the paper's thought experiment are microscopic, then all legitimate interpretations (i.e. everything but naive Copenhagen) completely agree on what happens: unitary evolution by the Schrodinger equation. If the experimenters are macroscopic, then you can get technically get logical contradictions from Copenhagen collapse, and it's not hard to do this, but by thermodynamics you can never observe them. So the paper tells us nothing about physics.

The paper does say something, but it's purely philosophical. It tells us that if you want to take the stance of a philosopher, and not worry about what is physically observable but just ponder the logical aesthetics of the postulates, then from this God's eye view everything besides Many Worlds has a sharp, ugly feature. But once you get to debating this kind of thing, you've lost all hope of objectivity: diehard proponents of every interpretation think all the others are aesthetically unacceptable. (Copenhagen people hate the ontological extravagance of Many Worlds, pilot wave people really really want their particles to have definite positions, etc.) So in either case the paper does not make a knock-down argument against anything.

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    $\begingroup$ By definition, you cannot falsify a quantum interpretation. This is a claim that I have been guilty of making in the past, but it doesn't hold up to careful examination, for multiple reasons. One reason is that any experiment that falsifies the standard foundations of quantum mechanics will automatically falsify the various interpretations. E.g., we could observe nonlinearity. Another reason is that the interpretations can be interpreted as approximations, in which case they are in fact false. $\endgroup$ – Ben Crowell May 21 '18 at 2:45
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    $\begingroup$ @BenCrowell. I guess what is meant by you cannot falsify a quantum interpretation is that you cannot do so within the current framework of QM. Of course if QM itself is falsified, then the status of its interpretations becomes a rather moot point. $\endgroup$ – Stéphane Rollandin May 21 '18 at 9:08
  • $\begingroup$ what happens is that when it becomes apparent that some new piece of evidence cannot be fit in the existing fundamental theory, a new theory that encompasses existing evidence and includes the new ones must be found. Since we don't have yet a fundamental definition of what constitutes a measurement as a nonlinear probabilistic operation, any refinement of it will constitute a bigger theory $\endgroup$ – lurscher May 21 '18 at 15:16
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    $\begingroup$ @BenCrowell As Stephane Rollandin said, I more precisely meant "interpretations of quantum mechanics are not falsifiable, besides in the sense that quantum mechanics itself is falsifiable". If QM makes the right predictions in a situation, then by definition all of the interpretations do. $\endgroup$ – knzhou May 21 '18 at 15:18

I have a migraine and, like @knzhou, don't want to read the paper in detail. Maybe I'll come back to it. But I want to say something about how quantum mechanics works.

In the mathematics of quantum mechanics, you have wavefunctions or quantum states, and you have "observables". In the original, sensible, self-consistent interpretation of quantum mechanics, the observables are what is real, and the wavefunctions and quantum states are a way to predict the values of observables.

The confusion entered when people started talking about wavefunctions as what is real. This was possible, first of all, because there are wavefunctions (called eigenstates) which imply that a specific observable (position, momentum, whatever) takes a specific value with 100% probability. So it became easy to say that "the quantum state is in the position eigenstate for x=x0" is the same thing as "the particle is at the position x=x0".

But then once you say the quantum state is a physical thing, then you will also be ascribing reality to something like "the superposition of the position eigenstate for x=x0, with sqrt(-1) times the position eigenstate for x=x1".

So suppose we don't go there. We stick to the original mathematical division of labor: observables are the only thing that's real, wavefunctions are just a tool of calculation. The next question might be, which observables actually take values, and when and where do they do so?

This is a question because not all possible observables can be real at the same time, as a consequence of the uncertainty principle. The Copenhagen interpretation set a minimum level of reality: at least those things that are measured, must exist in some sense. If they weren't real, they couldn't have been measured.

However, you don't actually have to define quantum mechanics in this observer-centric way. Something called "consistent histories" gives you a mathematical method for assigning quantum-mechanical probabilities to very general sequences of observables. You can posit that there is plenty of unobserved reality, just so long as the actually real observables are "decohered" with respect to each other.

For some reason this ontological option is relatively unexplored. But it definitely nullifies the argument for many worlds in this paper. I didn't get into the details, but clearly their argument involves saying that observers are made of wavefunctions or inhabit wavefunctions, and can engage in coherent interactions.

If you insist that observables are the only real things, and that they must be decohered, then you simply can't have an "observer in a quantum coherent state". If you did try to engineer such a thing, then according to an observables-only ontology, there would just be a gap in the universe's actual history for the duration of the coherence, rather than the many-worlds scenario of multiple copies of an observer somehow coexisting.

The closest they come to addressing this possibility is when they mention "collapse theories", which they classify as a modification of quantum mechanics. A collapse theory indeed resembles a "consistent history" in which wavefunctions and observables are both physical, and the observables are held to result from wavefunction collapse into an eigenstate. (This formal affinity between consistent histories and collapse theories is another fact that seems to have largely escaped notice.)

  • $\begingroup$ I observe reality and it is relevant to me (because for example it can kill me if I react badly), hence I refuse to wait for any confirmation that I've already "decohered". In other words, I don't think my own observables "must be decohered", as you propose. I don't really mind if my reactions inside this time window are labelled by others "a gap in the history", because reality doesn't go away even what you call it names. $\endgroup$ – kubanczyk May 21 '18 at 10:51
  • $\begingroup$ @kubanczyk Decoherence is not a choice in consistent histories. The framework only works for sets of possible histories that are mutually decoherent (as evaluated by the decoherence functional). So I suspect that the "extended Wigner's friend experiment" in this paper must correspond to a gap in the sequence of observables, in a consistent histories framework. $\endgroup$ – Mitchell Porter May 21 '18 at 13:24
  • $\begingroup$ Subjectively it might be like the jump from before general anesthesia to after general anesthesia, except that in this case, there really were no physical events between 'before' and 'after'. That may sound absurd, but remember that for branches of a wavefunction to recohere, they must reconverge on a physically identical state. All physical traces of different sequences of events must have been erased. This is also somewhat absurd. $\endgroup$ – Mitchell Porter May 21 '18 at 13:38
  • $\begingroup$ So one might want to consider whether these (extremely hypothetical!) experiments that involve quantum coherent manipulations of observers, including erasure or recombination of superposed personal histories, would really just involve precisely modulated control of a discontinuous quantum jump between initial and final mind states. $\endgroup$ – Mitchell Porter May 21 '18 at 13:42

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