David Deutsch was interviewed on Bloggingheads.tv this morning, plugging his new book.
He is mainly known as the leading advocate of the many-worlds interpretation of quantum mechanics, which he prefers to call the multiverse. He admits that only about 10% of physicists agree with him.
Like most physicists who promote crackpot ideas, he relies heavily on the concept of a Kuhnian paradigm shift. As Deutsch explains the concept, it is a new worldview like those brought by Copernicus, Galileo, and Darwin. He says that Galileo had a tough time convincing the Inquisition about heliocentrism because he had no demonstrable advantages to his view.
What is really important, says Deutsch, is not predicting experiments but having a properly satisfying explanation of reality. That is what Galileo and Darwin did, he says.
I think that this whole story exemplifies what is wrong with physics. Deutsch did not rely on any empirical evidence or any objectively verifiable theory. None of that is needed for a paradigm shift. As I explain in my book, Kuhnian paradigm shifts are not scientific advances at all, but merely fads among scientists.
What gets me the most is his insistence that he is describing reality, and that the multiverse is more realist than the alternatives.
Here is how many-worlds works. Suppose that you have a laser beam, and a polarized filter that lets 75% of the light thru. Quantum mechanics interprets this as making observations of photon spin, with each photon having a 75% chance of getting thru the filter. The theory has very precise agreement with experiment.
Many-worlds says that each time a photon strikes the filter, the universe splits into two, with the photon going thru the filter in one universe and not the other. We live in one of those universes, and we have no way of gaining knowledge about what happens in the other universe.
The main difference is that instead of saying that the photons have a 75% chance of passing thru the filter, as the standard interpretations say, many-worlds does away with the probabilities and has no way of explaining why 75% of the light passes thru, except to say that we happen to be in one of those universes where that happens. Plus many-worlds hypothesizes all those alternate universes that we can know nothing about.
The many-worlds theory explains nothing, and wrecks perfectly good explanations that we have for many common experiments. I see no merit to it at all. The main argument for it is that it is somehow more realist, if you choose to believe in the reality of the alternate universes. This is not realism. This is a mockery of realism.
Deutsch denied that he is religious or spiritual in any way. I don't see much philosophical difference between his belief in unseen worlds and common religious beliefs in unseen worlds. They are not scientific because they are not based on observable data.