I’m not sure why you say this. It’s not actually known whether the MWI mandates an “infinite” number of universes.

From literally the first paragraph of the Wikipedia page: “In layman’s terms, the hypothesis states there is a very large—**perhaps infinite**[2]—number of universes, and everything that could possibly have happened in our past, but did not, has occurred in the past of some other universe or universes.” Emphasis added.

But if it there *were* an infinite number of worlds, however defined, the case of SIMPLICITY you think you’re making still applies to regular physics processes. Does the universe extend infinitely beyond the cosmologically horizon, that is, infinitely beyond the observable universe? Is a universe of infinite **space** and **time** really the “simplest idea science can come up with about the nature of reality?” These aren’t two different questions. *They are the same question.* Physical space is just one kind of mathematical space in physics. The potential “space” on an MWI Universal Wavefunction is just another kind of potentially infinite mathematical space. (Everett, who came up with the original idea, did not refer to it as the MWI but as the theory of the Universal Wavefunction.)

If you’re wary of one infinity, then you should be equally wary of the other.

I’m personally uncomfortable with both: my own intuition is that physical space is unbounded yet finite, and that the MWI “worlds”, coherently defined, would end up as a finite number. Pleeeeeenty of people disagree with this, of course. My point here is not to argue “rawr, this must absolute be true!!!1!” but rather, “This is a sensible conclusion, derived directly from a mathematically rigorous definition of simplicity.”

You still haven’t come to grips with what SIMPLICITY means.

The consequences of literally every quantum interpretation are “far from simple”. The consequences of literally every advanced physics theory are “far from simple”. The three-body problem from Newtonian mechanics has consequences that are “far from simple”.

**Complexity arises from simplicity.**

Every sufficiently advanced collection of possible laws of physics will result in “consequences” that are as “far from simple” as it is possible to be. There is no closed-form solution to physics problems like the three-body problem. If you’re looking at a sun, a planet, and a moon, and you want to know the position of those three bodies a billion years from now, you’re going to have to deal with “consequences” that are extremely far from simple. You’re gonna have to computationally chug, chug, chug out the “consequences” of the problem, because there is no simple formula that exists that can jump you straight to the final solution. *Complexity arises from simplicity.* That’s not just true of QM. It’s true of practically everything that’s worth studying.

Does the MWI have “consequences” that are complex? Yes. Because *every physics theory* has consequences that are complex, most especially including *every other possible interpretation* of QM.

The MWI is simple where it counts: in the rule that describes what is posited to be actually happening. That simple rule has startlingly complex consequences, but then again, so does *every* other quantum rule we could possibly come up with.

Nobody is necessarily even positing an “infinite” number of universes, let alone a “*more* infinite” number of universes, whatever that is supposed to mean.

This is just meaningless. It has nothing to do with the MWI.

Rephrasing this question into something that could be sensibly interpreted: does positing the MWI make more sense than the existence of a “single” quantum world whose profound mechanisms escape our scrutiny at the moment?

Yes.

Obviously.

The entire problem with positing a “single” quantum world, where our own particular world is somehow privileged in some way, is that it is a *very complex idea*, in exactly the same sense that it’s an extremely complex idea to assume that the universe just ceases to exist outside of our own observed universe. Why would we ever posit that the earth is the “center” of the universe in that fashion, and that any light that disappears beyond our own personal provincial particle horizon just ceases to exist? Why? What do we *gain* in our understanding by assuming that?

Exactly the same argument you’re making to privilege our own quantum branch could be used to privilege our own particular place within a cosmological horizon. It makes no sense in both ways.

You can see this immediately in the “pilot-wave” interpretation that was previously brought up. The “pilot-wave” interpretation **does not get rid of the information of all those other many-worlds**. All of those worlds are still there! What the pilot-wave interpretation says is that there is a little flag on our particular quantum branch – and no other branch! – that says “You live in really-real Reality! Congratulations! All those other branches don’t matter!”

Okay… so WHY don’t they matter? The information for them is *still there* in the quantum waves. Do they not matter merely because we personally *like* the idea that we have a special physics flag planted where we are?

Or take a clever objective collapse theory like GRW: unlike the (frankly primitive) Copenhagen interpretation, GRW says that collapse just happens “randomly”, rather than being beholden to some ridiculously undefined idea like an “observation”. It basically says that when we’re doing experiments, quantum states get entangled with such a large number of particles that *at least* one of them will randomly undergo collapse at the right moment, giving us the experimental results we see. Okay. But in that case, why don’t we ever see spontaneous collapse? Because the probability is *low* enough that it won’t happen with a single particle, but *high* enough that it will essentially *always* happen with 10^16 entangled particles. Extremely clever. I like this idea a lot. But it also gives a whiff of arbitrary probability choices. Why were *those* bounds on the probabilities chosen? Where did those bounds come from? Well, because those are the only bounds that work. The parameters of the theory have to be toyed with to get the result we like… and that’s also a complex idea that has to be snuck into the rules to make it all work. If only we could do 10^16 single particle experiments…

The MWI is not complex in these ways. It just says, hey, there’s a Universal Wavefunction. It evolves according to the Schroedinger equation. BOOM. We’re done. That’s the rule.

That doesn’t have to be right. There are serious problems with it that need to be taken seriously. (Look at the previously cited Scott Aaronson blog post to look at some of them.) The MWI could easily be wrong. At the same time, it pretty clearly leads the rest of the horses in the race by at least a few feet, at least according to a sensibly rigorous definition of simplicity.

The problem with your own objections are continually that you either 1) misrepresent what the MWI actually implies, or 2) offer knee-jerk objections that, if accepted, would strike down almost all of physics. Neither of these is going to work. You need to work with the MWI as it’s actually defined, without the misconceptions, and you need to offer up objections that aren’t so clumsy and blunt that accepting your objections meant we had to give up on the entire last two centuries of physics.