Arguing in favor was Max Tegmark, a cosmologist at MIT. His MIT colleague, Frank Wilczek, (winner of the Nobel for his work on the strong force) took the opposing position. ...The reason for rejecting the multiverse is not that it is weird. It is no weirder than a lot of religious beliefs that people have.
And then there's the many-worlds interpretation, in which the wavefunction keeps working, but reality breaks, or at least fragments a bit. In this view, there are particles in every location the wavefunction predicts we'll find them. It's just that those locations end up in different universes once the measurement happens. So, as soon as someone makes a measurement, they split the Universe into multiple universes, each with the particle in a different location specified by the wavefunction. ...
Wilczek was a bit more detailed in his criticisms. He's perfectly happy to accept the wavefunction's existence, but feels that "many worlds is metaphysical baggage added on." His main issue is that all the worlds but one aren't accessible to our experience, and therefore can't be explored scientifically. "I am very worried about ascribing full credence to something other than reality," Wilczek said.
Tegmark for his part, thought that was a very narrow view. "This has less to do with QM than the way we perceive reality," he said, later adding, "it's partly 'what's real is what we can observe,' which seems like an ostrich with its head in the sand." But scientifically, Tegmark said that "If Schroedinger's equation applies to every system, no matter how large, we should have many worlds."...
If inflation is right—and most physicists think it is—then the Universe we see is only a small fraction of the whole. And, if you could somehow go past the observable universe, you'd come to a region where inflation is an ongoing process, rapidly expanding space and ultimately creating additional universes.
"If space is infinite, and the odds of being you is not zero—which it's not, since you exist—then you must exist in other places," Tegmark said. "Maybe you'll first find a Shmioneer Works, but if you keep looking, you'll find another [you]." And this weirdness is a necessary outcome of a theory that most physicists agree on. So, in Tegmark's view, you can't dismiss the many worlds interpretation just because it's weird.
The reasons for rejecting the multiverse are that (1) there is no evidence for it, and no prospects for getting any; (2) it does not solve any theoretical problems; and (3) it is methodologically incoherent, as it destroys the meaning of the Born rule.
If inflation is true, then it is reasonable to say that part of the universe is outside our observable horizon, and hence outside our possible knowledge. The scientific approach would be to acknowledge that there might be something out there, but leave it at that. To say that there are infinitely many copies of ourselves out there is strange and unscientific.
Tegmark is trying to trick you into one of the various paradoxes about infinity, but he has admitted that one of these paradoxes has driven him to conclude that no infinities occur in nature. So I don't know why he keeps bringing up infinities.
I am glad to see physicists debating the many-worlds MWI because it is a silly idea that cannot hold up under debate. It has become somewhat trendy for physicists to announce that they believe in it, but it is stupid on every level -- philosophically, mathematically, and physically.