What's the difference between Many Worlds Interpretation and Possible Worlds?

[u/Danpc11]

/r/askphilosophy/comments/181w1on/whats_the_difference_between_many_worlds/

[u/bugreporterinaction]

Many Worlds is a specific interpretation of quantum mechanics. "Possible worlds" is not. For me to address the latter, I'd have to know the context it is being used. But let me instead address the former.

Quantum mechanics, as formulated in textbooks, is explicitly not a theory of reality, but a theory of measurement outcomes.

Physicists love Einstein's formulation of relativity because it is mathematically elegant and easy to work with. However, it is not compatible with anything non-local.

Bell's theorem shows that if you try to explain the outcome of quantum-mechanical measurements with any sort of realistic model, that model must be non-local. There is a follow-up theorem, however, called the no-communication theorem, that shows that the direct outcome of what shows up on measuring devices is always local.

Hence, if you just take measuring devices to be fundamental, as an axiom, and model solely what shows up on those devices and nothing else, you get a nice, simple mathematical model that remains perfectly compatible with Einstein's formulation of relativity, but with the downside that it's literally not a theory of reality but a theory of measurements, and cannot even explain what it even is that we are measuring (this is called the measurement problem).

It has become a taboo in physics to posit that we should care about a theory of reality, because introducing one would necessarily overcomplicate the mathematics. This is where Many Worlds comes in. It justifies itself as a middle-ground position, by saying that quantum mechanics, as formulated in textbooks, already is secretly a theory of reality all along, and that we were mistaken to think it is a theory of measurements. Thus, we can reinterpret it as a theory of reality without introducing new mathematics at all.

However, it clearly is a theory of measurements, so you have to use a lot of weird, bizarre arguments to try and defend it as a theory of reality. For that reason, it is misleading to even talk about "the Many Worlds Interpretation." There is not one of them, there are dozens of attempts to formulate it in the literature, all proposing different methods of squaring this circle and reinterpreting a theory clearly formulated around measurements as if it actually had nothing to do with measurements at all this entire time.

It is called "Many Worlds" because there seems to be no physical mechanism by which the book-keeping tool used in textbook quantum theory, called the wavefunction, could possibly explain what we observe, unless you assume it "branches" based upon how you measure it (called the pointer basis), and these individual branches kind of then look like two different "worlds" each occupying a copy of ourselves observing one outcome or the other, and so you get a multiverse.

However, it runs into problems because, in this naive way I explained it, it is circular, because I am saying the branching happens on the pointer basis... but why? If we just assert it, then that is a measurement postulate by more complicated means. There is actually no mechanism in textbook quantum mechanics to guarantee that. This is known as the problem of the preferred basis and is where the many Many Worlds comes into play, because you have dozens of attempts in literature to try and find a solution to the problem, and they all give you different explanations.

Many Worlds is already kind of a fringe position, but even that fringe is broken up into various different camps of how to properly address that problem, with no single "Many Worlds" interpretation, and some, like Zurek's model, arguably even go beyond interpretation and become a new physical model, as they introduce a lot of new assumptions.

[u/reddituserperson1122]

Completely different things. One is a theory of quantum mechanics in physics, which contrary to the very strong feelings of @strutter395 is perfectly reasonable physics, and the other is a component of modal theories in philosophy, most famously David Lewis’s modal realism, which is perfectly reasonable philosophy.