MWI is not a good theory because it’s not testable

Sean M. Carroll is on his high horse again, defending many-worlds:

Longtime readers know that I’ve made a bit of an effort to help people understand, and perhaps even grow to respect, the Everett or Many-Worlds Interpretation of Quantum Mechanics (MWI) . I’ve even written papers about it. It’s a controversial idea and far from firmly established, but it’s a serious one, and deserves serious discussion.

Which is why I become sad when people continue to misunderstand it. And even sadder when they misunderstand it for what are — let’s face it — obviously wrong reasons. The particular objection I’m thinking of is:

MWI is not a good theory because it’s not testable.

... I suspect that almost everyone who makes this objection doesn’t understand MWI at all. ...

Now, MWI certainly does predict the existence of a huge number of unobservable worlds. But it doesn’t postulate them. It derives them, from what it does postulate. ...

You don’t hold it against a theory if it makes some predictions that can’t be tested. Every theory does that. ...

The people who object to MWI because of all those unobservable worlds aren’t really objecting to MWI at all; they just don’t like and/or understand quantum mechanics.

So his point here is that MWI is not so silly as to postulate parallel worlds, it just makes other postulates that imply those unobservable parallel worlds.

Someone replies, what's the point of those other worlds?

I am worried about the epistemological baggage: what does it buy you, declaring that all those potentialities actually “exist”? Is it more than a rhetorical move? And, why make that move here and not, for example, in the context of statistical mechanics?

Carroll does a dance:

The Everettian says, Why work that hard when the theory we already have is extremely streamlined and provides a perfect fit to the data? (Answer: because people are made uncomfortable by the existence of all those universes, which is not a good reason at all.)

Rejecting all those universes as superfluous is a good reason. They buy us nothing.

He says MWI is "extremely streamlined", but it is crazy to say that zillions of unobservable universes makes a theory streamlined.

Suppose I toss a coin, and observe either heads or tails. The Carroll and the MWI folks would say that a theory would be more streamlined if observing heads does not eliminate the possibility of tails. They would say that eliminating what does not happen is an extra postulate that they can do without. Just pretend that the universe splits into two, with the coin heads in one, and tails in the other. They call this a "perfect fit with the data".

This would be ridiculous even if there some practical benefit to computing things this way. But there is none. There is no quantum mechanics paper that uses MWI to simplify some real-world computation. The only practical benefit I've ever heard of is that David Deutsch says MWI helps understand how quantum computers might be possible. We should revisit that if anyone ever proves that they are possible.

Science writer Philip Ball writes on Aeon:

In any event, both ideas display a discomfort with arbitrariness in the universe, and both stem from the same human impulse that invents fictional fantasies about parallel worlds and that enjoys speculating about counterfactual histories.

Which is why, if I call these ideas fantasies, it is not to deride or dismiss them but to keep in view the fact that, beneath their apparel of scientific equations or symbolic logic, they are acts of imagination, of ‘just supposing’. But when taken to the extreme, they become a kind of nihilism: if you believe everything then you believe nothing.

An Intelligent Design (ID) site adds:

Although Philip Ball seems to think Many Worlds got started to solve a problem in quantum mechanics, there is reason to believe that it has an enormous philosophical appeal anyway to post-empirical types in science, who have no use for concepts like falsifiability or Occam’s razor.

Science is actually only an ornament, a trinket, in Many Worlds/multiverse reasoning. Sages sitting on a riverbank 2500 years ago could come up with the same sorts of ideas, and the same amount of evidence.

Today it could hardly matter less that there is no evidence for these Many Worlds. Evidence is just not hot any more.

This provokes a furious debate in the comments, as the leftist-atheists like Carroll despise ID more than anything. ID is hated because it suggests that scientific evidence might support a belief in God.

The subtext here is that if you can play some games with postulates to argue that science supports a belief in unobservable parallel universes, then the ID folks sound reasonable by comparison.

The ID-haters may call me a creationist for quoting an ID site. People say that it is unscientific, and that may be, but it is just a fringe opinion. I have no interest in stamping out fringe opinions. MWI is even more unscientific, and it is taught at our leading universities as solid science. ID is rarely mentioned in any universities, except as some sort of paranoid conspiracy to turn us into a theocracy, or some such nonsense.

The appeal of ID is to those who believe we can see evidence of God's work. Okay I get that. But what is the appeal of MWI? It appears that some people feel better thinking that all possible events are occurring in other universes, or they like to think that their decisions are meaningless, or they dislike the idea of randomness. I don't know. It makes no sense to me. But they must like MWI for some reason, because there is no evidence for all those parallel universes.

Here is another argument, in reply to Carroll:

The MWI has always seemed to me to be the most natural interpretation. The Copenhagen Interpretation is just logical positivism (denying that we can find a realist picture of reality – as such, CI not really scientific). Given that we accept ontological realism, *then* MWI naturally follows.

This is backwards. Logical positivism is the most scientific philosophy of science. He seems to be saying that we have to accept MWI because it is the only scientific view. He must have very strange definition of science if it requires unobservable universes.

I wrote my book to show how physics has lost its way by believing in untestable postulates instead of observable reality.

Update: The comments about atheists promoting MWI prompted me to post:

G.K. Chesterton: "When people stop believing in God, they don't believe in nothing -- they believe in anything."

Okay, he did not say it, but someone said it in a book about him. He could not have known how literally the saying is true. Believing in MWI is believing in anything.

Update: Lubos Motl piles on, and explains in detail why the above critic on Carroll's blog is right, and Carroll is wrong.

Another Carroll commenter says:

Although I agree that “it’s not testable” is not a good argument, this is a minor point in the article.
The main arguments are: it completely dissolves personhood, and assuming that everything (physically) possible exists trivializes the theory.

MWI is not testable, and that dooms it as a scientific theory. But that is not the worst thing about it. Those are two worse things. Streater has more. I have listed others, such as here, here, here, and here. I realize that there are some big-shot physicists who believe in MWI, but that just illustrates their sorry thinking.