I understand why quantum mechanics always comes up in these discussions. It makes people think of "randomness", which seems to be an alternative to a deterministic view of the universe.
But the continual misconceptions about many-worlds are very difficult to deal with. I'm better at explaining the banking system than I am quantum interpretations. But from Hugh Everett's PhD dissertation:
He's contrasting this deterministic Process 2 with a supposedly random and discontinuous Process 1. He then explores a non-exhaustive list of various interpretations ("Alternatives") of quantum processes, and ends up defending Alternative 5:
This is the complete abandonment of the "probabilistic" and mysteriously discontinuous Process 1, in favor of the (determinsitic) Process 2 ("without any statistical concerns").
Many-worlds is just another way of describing the Theory of the Universal Wavefunction, and that idea entails -- in Everett's own words! -- the "complete abandonment" of the probabilistic Process 1 in favor of the deterministic Process 2. In this conception, the laws of the universe as a whole are presumed to follow the deterministic evolution of that universal wave function.
He literally mentions "all of physics" here, again his own words.
Yet I regularly come across the idea that the many-worlds interpretation is not truly deterministic. This is simply mistaken.
The "boxes" of an Everettian conception of quantum mechanics are states of the universal wavefunction. And the evolution of that wavefunction is by a deterministic process.
This comes straight from the original source.
Unfortunately, people often look at popular descriptions of the many-worlds interpretation with the result that they fundamentally misunderstand it. The false belief here is that World A leads to both World B and World B', since the universe "splits" -- the popular trap here is to view that as not one arrow out of box A in the flow chart but two arrows, one of which leads to B and the other which leads to B' in the flow chart.
That is an utter misconception.
To reach a somewhat more accurate description (still not perfect but much better), we can say that World A and World A' exist simultaneously right on top of each other. They seem to be identical worlds. Nothing new is "created" when the world "splits". World A and World A' always existed from the beginning. The universe continues to evolve deterministically according to its fixed rule. There is one arrow out of every box. World A and World A' exist on top of each other, looking exactly like. But World A becomes World B, and World A' becomes World B', and this is where the "split" happens where events diverge. It's not that there is suddenly more "stuff" in the universe, where one box suddenly becomes two boxes. It's just that this quantum moment is where where A and A' -- having existed like perfect twins until now -- finally depart from each other. Our problem being inside the universe is that we simply do not know whether we exist in World A, or whether we are the us who live in World A', because the two worlds are twins until the observation.
There are lots and lots of difficulties with the many-worlds interpretation. Can the deterministic evolution of a universal wave function [i]actually[/i] explain what we believe to observe? Can two different worlds legitimately decohere, so that the state of one of the worlds be totally uncorrelated with the state of another world for all future "time"? If that can legitimately happen, then how many worlds are there? Must it necessarily be infinite, or can it possibly be finite? These are all legitimate questions, big picture Great Debates type stuff, and there aren't really any great answers for any of them.
What is not a hard question is whether the MWI is deterministic. This is not a "philosophical" debate in the slightest. This is basic factual type stuff. The answer is yes, the MWI is deterministic, quite obviously and unambiguously.
Naturally, none of the above necessarily dictates that we live in an MWI-style world.
The appeal of the MWI is its simplicity (properly defined). How do we interpret the universe? With the MWI, we say the universe is the deterministic evolution of the Schroedinger equation. We don't need anything else. What's much, much more difficult is to explain the appeal of this particular kind of simplicity, and why we define simplicity in this particular way. That is very hard to do. It takes a lot of time.
6 is not any "more clearly" an even number than 8.
And in exactly the same sense, Bohmian mechanics is not any "more clearly" deterministic than MWI.
Both 6 and 8 are unambiguously even. Both the Bohmian interpretation and the MWI are unambiguously deterministic. And there are plenty of complexity problems with the Bohmian setup that does not seem to afflict many-worlds. There are very specific and very strong reasons why people who are inclined to determinism (such as me) are also inclined to the MWI over the pilot-wave interpretation. I might even venture to say that the same "instincts" that guide us to determinism are what guide us toward MWI and away from the pilot-wave theory.
"Shut up and multiply". I have heard that and other similar phrases offered up in the most startlingly unprofound, closed-minded, and intellectually incurious way imaginable. From physicists.
Of course, I must admit that my very limited personal experience is not a representative sample. Nevertheless.
It's a strange thing to experience this way of thinking from the people who are, ostensibly, in the business of trying to understand the way the universe works. But it genuinely happens. I don't get it. (The smartest person I know is a physicist, so before I met more people of the same professional, I had an unreasonably high expectation from the group as a whole. I had to meet other physicists -- intelligent, sure, but not smarter than my friend -- who had to disabuse me of the notion that all physicists were interested in the way the world actually works).
The weirdest had to be the person who distinguished between trying to understand how to calculate the Schroedinger equation (which was science) and trying to understand different interpretations of quantum mechancis (which was philosophy). I don't share those definitions, but at the time I was like, okay, whatever...
When that was subsequently followed by a statement: "Why should I care about philosophy?"