Epistemic separability and Everettian branches --- a critique of Sebens and CarrollGot that? Many-worlds does not make any quantitative predictions. None.
by Dawid, Richard and Friederich, Simon
We discuss the proposal by Sebens and Carroll to derive the Born rule in Everettian quantum mechanics from a principle they call ‘ESP-QM.’ We argue that the proposal fails: ESP-QM is not, as Sebens and Carroll argue, a ‘less general version’ of an independently plausible principle ‘ESP’ and can only be motivated by the empirical success of quantum mechanics, including use of the Born rule. Therefore, ESP-QM cannot have the status of a meta-theoretical principle of reasoning and provides no viable basis for deriving the Born rule. ...
Adherents of a `rationality principle' approach to Everettian QM, such as Deutsch and Wallace or Sebens and Carroll, acknowledge that Everettian QM does not allow for the deduction of objective probabilities of measurement outcomes. Moreover, they feel discontent with simply introducing the Born rule as a primitive instrumentalist principle. The proposed way out is to open up the additional playing field of requirements of rational reasoning in the given context.
It is not an alternative to Copenhagen, or to any scientific theory. There is nothing scientific about it. It is "not even wrong", to use the phrase Peter Woit popularized. It is just some meaningless fantasy.
The whole thing is so ridiculous that it is hard to take seriously anyone who believes in it. Yes, it causes me to doubt everything else Carroll says. Carroll is essentially saying: "QM appears to be a wonderful theory with many accurate predictions, but I don't like it so I want to replace it with a theory that does not make any predictions."
Dawid is a proponent of string theory, which does not make any predictions either, but the string theorists at least promise that they might get to some testable prediction in 50 years or more. The many-worlds people pretty much rule out the possibility of ever making any predictions.