The MWI is qualitatively different from the other interpretations of quantum mechanics, although that’s rarely recognized or admitted. For the interpretation speaks not just to quantum mechanics itself but to what we consider knowledge and understanding to mean in science. It asks us what sort of theory, in the end, we will demand or accept as a claim to know the world. ...That's right. Ultimately MWI says nothing that you would want from a scientific theory. There are no facts, predictions, or confirming experiments.
What the MWI really denies is the existence of facts at all. It replaces them with an experience of pseudo-facts (we think that this happened, even though that happened too). In so doing, it eliminates any coherent notion of what we can experience, or have experienced, or are experiencing right now. We might reasonably wonder if there is any value — any meaning — in what remains, and whether the sacrifice has been worth it. ...
It says that our unique experience as individuals is not simply a bit imperfect, a bit unreliable and fuzzy, but is a complete illusion. If we really pursue that idea, rather than pretending that it gives us quantum siblings, we find ourselves unable to say anything about anything that can be considered a meaningful truth. We are not just suspended in language; we have denied language any agency. The MWI — if taken seriously — is unthinkable. ...
What quantum theory seems to insist is that at the fundamental level the world cannot supply clear “yes/no” empirical answers to all the questions that seem at face value as though they should have one. The calm acceptance of that fact by the Copenhagen interpretation seems to some, and with good reason, to be far too unsatisfactory and complacent. The MWI is an exuberant attempt to rescue the “yes/no” by admitting both of them at once. But in the end, if you say everything is true, you have said nothing.
MWI just says that everything that can happen does happen in some parallel world. It allows you to think and believe whatever you want. Probabilities are meaningless. Reality and facts are meaningless.
MWI is just the same as the child's fantasy. The proponents give the impression that it is a scientific theory that gives a detailed explanation of the worlds, with Hilbert space, wave function, Schroedinger equation, atomic forces, etc. Yes, but none of them can explain how all that apparatus tells you anything beyond the simplistic child's fantasy. There are no predictions or confirming experiments.
I used to to think that string theory was the epitome of unscientific thinking. But string theory is vastly more reasonable that MWI. String theory at least had some hope of getting some theoretical explanations. MWI explains nothing, and discards almost everything we know about science.
Update: LuMo writes:
Now, Ball has written a text about some conceptual and basically insurmountable problems of the "many-world interpretation" paradigm sometimes used to misinterpret quantum mechanics. Among other things, he focused on the impossibility to define what a "splitting of the Universes" is and when and how many times it takes place. This is of course one of the problems about MWI that I see and often write about – but there are others, too. ...I had not noticed that Quanta mag allows comments, because my adblocker blocks them.
However, the comment sections are frustrating. Both articles have attracted over 100 comments by now. Pretty much all the most upvoted comments attack Wolchover's and Ball's texts. You can see that none of these people actually understands quantum mechanics and all of them assume that classical physics is right throughout their comments and lives.
I would not bother criticizing MWI, except that it has such a huge following, from leading physicists on down to the general public. Here is one of the dopey comments:
The fact is that the MWI is strictly adherent to the mathematics of quantum physics. There is no extra phenomenon like "observation" (that's just entanglement) there is no extra phenomenon like "waveform collapse" the entangled particle becomes part of a more complex waveform.MWI doesn't have to define observers because it does not make any predictions.
MWI doesn't have to justify adding any additional complexity to QM because it doesn't. Copenhagen, Pilot Wave, et. al. are the interpretations that add extra complexity that don't show up in the math, so they're the ones that have to justify that complexity. What the hell is an observer? What the hell is waveform collapse?
The math of quantum physics makes predictions that are verified by experiments. MWI makes no such predictions. Therefore MWI does not adhere to the math of quantum physics.