Scott Aaronson posted an interesting piece this week coming out about his favorite interpretation of quantum mechanics. I think the most relevant part is this snippet. (Although the full piece has a lot of nuance well worth reading.)
I don’t mean to say that the interpretations are all interchangeable, or equally good or bad. If you had to, you could call even me a “Many-Worlder,” but only in the following limited sense: that in fifteen years of teaching quantum information, my experience has consistently been that for most students, Everett’s crutch is the best currently on the market. At any rate, it’s the one that’s the most like a straightforward picture of the equations, and the least like a wobbly tower of words that might collapse if you utter the wrong ones. Unlike Bohr, Everett will never make you feel stupid for asking the questions an inquisitive child would; he’ll simply give you answers that are as clear, logical, and internally consistent as they are metaphysically extravagant. That’s a start.
In truth, this doesn’t seem like much of a revelation. Aaronson’s cautious sympathy with Everett has been evident in his writings and interviews for a long time. Even when he ostensibly was criticizing it, it was done in an obviously sympathetic manner. And his view isn’t unusual among quantum computing theorists. The reason is the dynamics are just easier to think through under an Everettian view. Of course, it is possible to give accounts of quantum computing under other interpretations, and some theorists do, but as Aaronson notes, it’s more work.
But this other snippet, I think, captures the caution in his support.
Do “other” branches of the wavefunction—ones, for example, where my life took a different course—exist in the same sense this one does? If you start with a quantum state for the early universe and then time-evolve it forward, then yes, you’ll get not only “our” branch but also a proliferation of other branches, in the overwhelming majority of which Donald Trump was never president and civilization didn’t grind to a halt because of a bat near Wuhan. But how could we possibly know whether anything “breathes fire” into the other branches and makes them real, when we have no idea what breathes fire into this branch and makes it real? This is not a dodge—it’s just that a simple “yes” or “no” would fail to do justice to the enormity of such a question, which is above the pay grade of physics as it currently exists.
This is a view I’ve noticed from a lot of Everettians, such as Sidney Coleman or Stephen Hawking. Supporting Everett isn’t necessarily a full throated support for the idea of many-worlds. (This is one reason I’ve started calling it “Everett” instead of using the acronym “MWI”.)
Yes, the many-worlds are an inescapable prediction of Everett. But it’s not a prediction that can currently be tested. The raw quantum formalism has been heavily tested for almost a century now, reportedly more so than any other theory in science, but there’s an enormous amount of “terrain” between the scale of those tests and the broader predictions. All we can currently say is that there’s no evidence for any factors that would frustrate those predictions.
But much of the history of science is discovering previously unforeseen complications. Who knows what might emerge in quantum systems as they scale from the currently tested thousands of elementary particles to something like 1036 particles? Concepts like “cat lives” and “cat dies” are put as variables in Bra-ket notation as though it’s a precise formulation. But we shouldn’t forget the huge assumptions in doing so, that there’s nothing in the cat’s actual wavefunction that might complicate the picture.
On the other hand, Sidney Coleman used to make the point that the world looks exactly as it should if quantum mechanics evolves as the raw formalism dictates. Maybe another way to express this idea is that the straight mathematical models of quantum mechanics successfully predict our observations as if there are macroscopic superpositions, effectively other worlds, even if it should turn out there aren’t.
I sometimes say that weak Copenhagen, the epistemic instrumentalist version, is true. But it’s true in such a cautious logically-positivist manner that another interpretation can also be true. I think something similar can be said for instrumentalist Everett. It’s true, even if another interpretation eventually turns out to be true in some realist sense. And it seems to get there with fewer assumptions and more logical coherence.
Unless of course I’m missing something.