Tom, Nick, MWI does not make predictions! Well, there is a version of MWI that does, but it is not the one being advocated here.
What makes predictions is a calculational procedure, like sum-over-histories. That procedure has an interpretation in a collapse theory: the theory explains why the procedure works. The version of MWI that Eliezer has expounded cannot do that. He has said so himself, repeatedly—that the recuperation of the Born probabilities is a hope, not an existing achievement.
Is that clear? I feel like I had better say it again. The bare minimum that all quantum physicists share is an algorithm for making predictions. An objective collapse theory offers an explanation as to why that algorithm works. It is a theory about what is actually there, and how it actually behaves, from which that algorithm can be derived.
Many worlds is also a theory (or a class of theories) about what is actually there. But when you count the worlds, the numbers come out wrong, badly wrong. So something has to change. Robin Hanson has suggested a different approach to the problem; but as I have objected, it remains vague on the crucial detail of exactly when the transition between one world and many worlds takes place. In any case, this brand of many worlds simply cannot yet offer an exact justification of the predictive algorithm in the way that a collapse theory can. It’s not true that MWI and collapse make the same predictions; rather, the hope is that MWI will predict what collapse already predicts, once we understand it properly.
Tom, Nick, MWI does not make predictions! Well, there is a version of MWI that does, but it is not the one being advocated here.
What makes predictions is a calculational procedure, like sum-over-histories. That procedure has an interpretation in a collapse theory: the theory explains why the procedure works. The version of MWI that Eliezer has expounded cannot do that. He has said so himself, repeatedly—that the recuperation of the Born probabilities is a hope, not an existing achievement.
Is that clear? I feel like I had better say it again. The bare minimum that all quantum physicists share is an algorithm for making predictions. An objective collapse theory offers an explanation as to why that algorithm works. It is a theory about what is actually there, and how it actually behaves, from which that algorithm can be derived.
Many worlds is also a theory (or a class of theories) about what is actually there. But when you count the worlds, the numbers come out wrong, badly wrong. So something has to change. Robin Hanson has suggested a different approach to the problem; but as I have objected, it remains vague on the crucial detail of exactly when the transition between one world and many worlds takes place. In any case, this brand of many worlds simply cannot yet offer an exact justification of the predictive algorithm in the way that a collapse theory can. It’s not true that MWI and collapse make the same predictions; rather, the hope is that MWI will predict what collapse already predicts, once we understand it properly.