Wasn't it a famous physicist who said "If you think you understand Quantum Mechanics then you don't understand Quantum Mechanics"?

I'm not defending this article, but I was under the impression that the proper interpretation of quantum mechanics / QFT is far from a settled matter among physicists.

No. That's an apocryphal quote. Richard Feynman said in 1965, " I think I can safely say that nobody understands quantum mechanics." It turns out it wasn't true even when Feynman said it, and it's certainly not true today.

Feynman's problem is that he was committed to the idea that particles are real. They aren't [1].

There are a lot of arguments over the proper interpretation of QM but it turns out that when you get to the bottom of things they are all saying the same things using different terminology.

[1] https://aapt.scitation.org/doi/10.1119/1.4789885 (also https://arxiv.org/pdf/1204.4616.pdf)

In practice, things tend to go with the "Shut up and calculate"[1] philosophy.

[1] https://en.wikipedia.org/wiki/Interpretations_of_quantum_mec...

Numbers by themselves don't mean anything (for humans at least). The meaning is found in how the numbers relate to the concepts that we make use of.

(For the record, the phrase "Shut up and calculate" was coined by David Mermin as a pejorative characterization of the Copenhagen interpretation, which actually says simply that we cannot know anything more than what the math says, and that's just the way it is, and any attempt at further inquiry is doomed to fail and therefore should not be attempted. It's a deeply unscientific attitude.)

http://www.flownet.com/ron/QM.pdf

or, if you prefer a video, http://www.flownet.com/ron/QM.pdf

Like what? "Multiple worlds" does not require a commitment to the reality of multiple worlds (the misleading rhetoric notwithstanding). (Everett never called it "multiple worlds", he called it "relative state", which is IMHO much less misleading.)

Also note that Everett's dissertation was titled The Theory of the Universal Wave Function, which puts a different emphasis on the thing as well.

He did believe in quantum immortality, though, and assumed the alternate 'worlds' were real. That's the standard interpretation of MWI: While there's just a single wave function, it factors into 'branches' entangling measurement results and states of consciousness, with all the different versions of yourself coexisting in a realist sense. However, as these branches tend to no longer interact, that's an unfalsifiable claim.

The key word there is "tend". The branches can interact (via interference) it's just the the technological challenges of demonstrating this become more pronounced as you add degrees of freedom. The limits of falsifiability here are technological, not limits in principle. To be considered unfalsifiable (in the Popperian sense), a theory has to be unfalsifiable in principle, not just in practice.

In fact, interference has already been experimentally demonstrated on some very large systems, and work on quantum computing is progressing rapidly enough that post-quantum cryptography is a serious concern. The falsifiability of the universal wave function is at a similar stage as the falsifiability of gravity waves was a few decades ago.

It is true that branches may in principle join again - however, that cannot be detected by nature of the process.

And how exactly does that happen?

The answer is: it doesn't. The orthogonality of different branches is an approximation. It's a very good approximation because decoherence is a phenomenally efficient process, but it's an approximation nonetheless. The universal wave function always resides in the same Hilbert space because, as you yourself pointed out, Schrödinger time evolution is linear and unitary.