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 published in 2018 which claims to show that 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.
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.
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.)
