Although we don’t know whether parallel universes exist, we know something else about them with certainty: many people instinctively dislike them, and whenever a physicist writes a book about them, the Web erupts with claims that they are unscientific nonsense.Yes, I have criticized such multiverse proposals on this blog, and so has Peter Woit.

Many physicists have explored various types of parallel universes in recent books, including Sean Carroll, David Deutsch, Brian Greene, Michio Kaku, Martin Rees, Leonard Susskind and Alexander Vilenkin. Interestingly, not a single one of these books (my own included) makes any outright claims that parallel universes exist. Instead, all their arguments involve what logicians know as “modus ponens”: that if X implies Y and X is true, then Y must also be true.It is true that none of those books have any real evidence for the multiverse. It is all speculative conditionals.

As a warm-up example, let’s consider Einstein’s theory of General Relativity. It’s widely considered a scientific theory worthy of taking seriously, because it has made countless correct predictions – from the gravitational bending of light to the time dilation measured by our GPS phones. This means that we must also take seriously its prediction for what happens inside black holes, even though this is something we can never observe and report on inThat someone can simply accept General Relativity outside the event horizons. That is a perfectly good theory that matches every successful prediction.Scientific American.If someone doesn’t like these black hole predictions, they can’t simply opt out of them and dismiss them as unscientific: instead, they need to come up with a different mathematical theory that matches every single successful prediction that general relativity has made – yet doesn’t give the disagreeable black hole predictions.

I agree that this is a good warm-up example for thinking about unobservable universes. The theory pretty clearly predicts a singularity at the center of the black hole, and yet it also pretty clearly says that nothing inside the event horizon is observable. So do you believe in the singularity or not? My guess is that the big majority of cosmologists do believe in the singularity, but I think that they should admit that it is outside the scope of observable science. Tegmark is apparently one of the exceptional cosmologists, as he now says that he does not believe in infinities, so I don't see how he can believe in the black hole singularity.

Since the Level III multiverse is implied by the (collapse-free) Schrödinger equation of quantum mechanics, it can be demolished with a type-B attack: an experimental demonstration of a violation of the Schrödinger equation. For example, if the current multi-million dollar attempts to build quantum computers fail and the cause is determined to be that the Schrödinger equation is violated by some form of wavefunction collapse process, then there are no Level III parallel universes.The current multi-million dollar attempts to build quantum computers have failed, and I believe that they will continue to fail. The many worlds (MWI) fans like the collapse-free equations, but most people do quantum mechanics with collapse of the wave functions.

Tegmark goes on to argue that the math multiverse if falsifiable, and hence scientific:

The Level IV multiverse is also vulnerable to a type-B attack: we can simply reject the notion that there’s an external reality completely independent of us humans, for example in the spirit of Niels Bohr’s famous dictum, “no reality without observation”. A second type-B attack option is to falsify the mathematical universe hypothesis by demonstrating that there’s some physical phenomenon that has no mathematical description.I say that there is such a falsification -- the electron itself has no complete mathematical description.

Quantum mechanics gives an effective way of making predictions about observations on electrons. But to assume that the wave function is a true description of the electron requires a psi-ontic view that many physicists have rejected because of nonlocality paradoxes. I elaborate on this argument in my FQXi essay. My essay did not win the contest, but I have seen so serious rebuttal of it.

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