A question at 1:04:00 asks for observable evidence for many-worlds. For example, could you prepare a Schroedinger Cat, and somehow verify that it is alive in one world and dead in another?
The correct answer is that there is no such evidence, and the whole concept of many-worlds is unscientific and unverifiable.
He dodges the question, and says that there are experiments that could disprove quantum mechanics.
Yes, of course, but textbook (aka Copenhagen) QM does not say the two cats can be observed.
His lecture is a pretty clear explanation of QM and many-worlds.
He says, at 35:40 that many-worlds is a theory, not an interpretation. I agree with that. The interpretations of QM all have the same predictions and observations. The interpretation is just a philosophical explanation for what the variables mean, but no experiment can say that one interpretation is any better than any other.
The Copenhagen interpretation is what Bohr and Heisenberg said. And maybe Schroedinger and Dirac. The textbook interpretation is the version of it found in modern textbooks.
Many-worlds is, in essence, the theory of QM with the part about observations and predictions removed. So many-worlds cannot make predictions, and cannot be tested or verified.
Carroll is a big proponent of many-worlds, but only because he believes it gives a better explanation of what is going on. But it does not explain anything, and is an unscientific theory.
In the older lecture, he admits at 37:00 that many-worlds cannot be tested. He excuses this by saying that the assumptions that go into many-worlds can be tested. Those assumptions are the same as with quantum mechanics, so every test of QM is also a test of many-worlds.
This is just a dodge. There is no test that can show a preference to many-worlds over textbook QM.
He then goes on to say that many-worlds is an unfinished theory, maybe some day someone will figure how many-worlds could make testable predictions. With the current knowledge of the theory, it deterministically predicts that all things happen in branched universes, so all predictions come true in some universe. The theory cannot be tested.
Israeli physicist Lev Vaidman has a new paper on Why the Many-Worlds Interpretation?:
A brief (subjective) description of the state of the art of the many-worlds interpretation of quantum mechanics (MWI) is presented. It is argued that the MWI is the only interpretation which removes action at a distance and randomness from quantum theory. Limitations of the MWI regarding questions of probability which can be legitimately asked are specified.... Some speculations about misconceptions, which apparently prevent the MWI to be in the consensus, are mentioned.I give you arguments for many-worlds, as otherwise you would not believe that the theory is as stupid as it is.
Note that he says that MWI removes randomness and fails to predict probability, as if that were an advantage.
The only part of our experience, which unitary evolution of the universal wave function does not explain, is the statistics of the results of quantum experiments we performed. ...Got that? It it deterministic about things we never see and fails to predict the probabilistic events we do see.
Thus, the MWI brings back determinism to scientific description . (Before the quantum revolution, determinism was considered as a virtue of scientific explanation.) We, as agents capable of experiencing only a single world, have an illusion of randomness. This illusion is explained by a deterministic theory of the universe which includes all worlds together.
The MWI provides simple answers to almost all quantum paradoxes. Schr ̈odinger’s Cat is absurd in one world, but unproblematic when it represents one world with a live cat and a multitude of worlds with the cat which died at different times of detection of the radioactive decay. ...There you go. Your rejection of the idea that you are constanting splitting into parallel universes is just a natural human conceit about your own self-importance. You are like those narrow-minded astronomers who put the Earth at the center of the universe.
The paradoxical behaviour of Bell-type experiments disappears when quantum measure- ment does not have a single outcome . ...
The reluctance of a human to accept the MWI is natural. We would like to think that we are the center of the Universe: that the Sun, together with other stars, moves around Earth, that our Galaxy is the center of the Universe, and we are unhappy to accept that there are many parallel copies of us which are apparently not less important.
This is crackpot stuff. It is anti-science. It is saying that you can get paradoxes out of a theory by removing all predictions.