I almost added this warning myself, though it would have been with a different emphasis:
Such debates about MWI as I have had here, in the past, have often not been a clean discussion of the merits of MWI versus some other interpretation, because I won’t shut up about these other issues, which are far more interesting and important. There are severe problems awaiting anyone who wants to explain consciousness in terms of interactions between distributed, coarse-grained physical states; there is an interesting possibility that it could instead be explained in terms of a single, microphysically exact entangled state; that is my preoccupation. The debate over MWI is just a sideshow.
MWI looks bad from my ontological perspective, because I say we should take the apparent ontology of the self more seriously, as its actual ontology, whereas MWI extends the dismissal of conscious appearances further. But MWI also looks bad from a pure physics perspective, which just wants an exact mathematical description of the world that works, and cares nothing about its relationship to the “subjective world” of “lived experience”. The most shocking feature of MWI, once I really understood it, is that it cannot by itself make any correct predictions at all, because the entire predictive content of QM comes from the Born rule (or projection postulate), and no derivation of the Born rule within MWI exists. You often hear people saying “all the interpretations of QM make the same predictions”, but this is not true for MWI. You could say it makes no predictions (since it has no substitute for the Born rule), or that it makes wrong predictions (if you just count the worlds naively), but the only version of MWI which makes the same predictions as QM is the, so far imaginary, version which contains a derivation of the Born probabilities.
It’s almost comical, how new problems for MWI keep appearing, the more I discuss it with people. For example, the standard lay understanding of MWI is that there are well-defined worlds, and they split into more worlds when there are quantum events. But among the informed defenders of MWI, it’s usually considered desirable to reject the idea of a “preferred basis”, such as would be implied by a canonical division of the wavefunction into a unique set of worlds. Instead, it’s considered a feature, not a bug, that you can express a wavefunction as a superposition of basis functions in many complementary ways. But what I’ve realized is that you can extend this perspective into the interior of a person. You can consider the density matrix of my left brain hemisphere, and the density matrix of my right brain hemisphere, and you have the same freedom to choose basis functions for each of them. So, in an MWI without a preferred basis, you can’t even say that there is a specific set of copies of you in definite states, spread across the multiverse. We can describe your left hemisphere in the position basis, and the right hemisphere in the position basis, and that will produce one set of copies; or we could describe your left hemisphere in the position basis, and your right hemisphere in the momentum basis, and that will produce a differently defined set of copies.
This isn’t even a physics debate, in the sense of making calculations and comparing their results with each other and with reality. MWI exists mostly as a verbal construction by means of which people try to make sense of QM. But if you go and chase down the implications of what is being said, you end up with nonsense. Of course, the more advanced MWI advocates like Robin Hanson, David Deutsch, etc, do have something quantitative to say; though often the key to debunking them still revolves around seeing past the equations, to the plain meaning of what they are arguing or asserting. But the debate on LW isn’t at that level.
ETA: It might seem that the Gell-Mann–Hartle formalism of decoherent histories offers a derivation of the Born rule. I would argue that the procedure whereby an absolute prior for a set of decoherent histories is obtained, is an adaptation of the Born rule to the GM–H formalism, and that it faces the same problem of motivation or interpretation, as does any attempt to just add the Born rule to MWI: why do some worlds count for more than others? GM–H provides a slightly novel way to get the right probabilities, but it still hinges on attaching unequal weights to the worlds, and what this could mean, in a multiverse context where all the worlds exist equally, is left unexplained.
Yikes! Thanks for the warning.
I almost added this warning myself, though it would have been with a different emphasis:
Such debates about MWI as I have had here, in the past, have often not been a clean discussion of the merits of MWI versus some other interpretation, because I won’t shut up about these other issues, which are far more interesting and important. There are severe problems awaiting anyone who wants to explain consciousness in terms of interactions between distributed, coarse-grained physical states; there is an interesting possibility that it could instead be explained in terms of a single, microphysically exact entangled state; that is my preoccupation. The debate over MWI is just a sideshow.
MWI looks bad from my ontological perspective, because I say we should take the apparent ontology of the self more seriously, as its actual ontology, whereas MWI extends the dismissal of conscious appearances further. But MWI also looks bad from a pure physics perspective, which just wants an exact mathematical description of the world that works, and cares nothing about its relationship to the “subjective world” of “lived experience”. The most shocking feature of MWI, once I really understood it, is that it cannot by itself make any correct predictions at all, because the entire predictive content of QM comes from the Born rule (or projection postulate), and no derivation of the Born rule within MWI exists. You often hear people saying “all the interpretations of QM make the same predictions”, but this is not true for MWI. You could say it makes no predictions (since it has no substitute for the Born rule), or that it makes wrong predictions (if you just count the worlds naively), but the only version of MWI which makes the same predictions as QM is the, so far imaginary, version which contains a derivation of the Born probabilities.
It’s almost comical, how new problems for MWI keep appearing, the more I discuss it with people. For example, the standard lay understanding of MWI is that there are well-defined worlds, and they split into more worlds when there are quantum events. But among the informed defenders of MWI, it’s usually considered desirable to reject the idea of a “preferred basis”, such as would be implied by a canonical division of the wavefunction into a unique set of worlds. Instead, it’s considered a feature, not a bug, that you can express a wavefunction as a superposition of basis functions in many complementary ways. But what I’ve realized is that you can extend this perspective into the interior of a person. You can consider the density matrix of my left brain hemisphere, and the density matrix of my right brain hemisphere, and you have the same freedom to choose basis functions for each of them. So, in an MWI without a preferred basis, you can’t even say that there is a specific set of copies of you in definite states, spread across the multiverse. We can describe your left hemisphere in the position basis, and the right hemisphere in the position basis, and that will produce one set of copies; or we could describe your left hemisphere in the position basis, and your right hemisphere in the momentum basis, and that will produce a differently defined set of copies.
This isn’t even a physics debate, in the sense of making calculations and comparing their results with each other and with reality. MWI exists mostly as a verbal construction by means of which people try to make sense of QM. But if you go and chase down the implications of what is being said, you end up with nonsense. Of course, the more advanced MWI advocates like Robin Hanson, David Deutsch, etc, do have something quantitative to say; though often the key to debunking them still revolves around seeing past the equations, to the plain meaning of what they are arguing or asserting. But the debate on LW isn’t at that level.
ETA: It might seem that the Gell-Mann–Hartle formalism of decoherent histories offers a derivation of the Born rule. I would argue that the procedure whereby an absolute prior for a set of decoherent histories is obtained, is an adaptation of the Born rule to the GM–H formalism, and that it faces the same problem of motivation or interpretation, as does any attempt to just add the Born rule to MWI: why do some worlds count for more than others? GM–H provides a slightly novel way to get the right probabilities, but it still hinges on attaching unequal weights to the worlds, and what this could mean, in a multiverse context where all the worlds exist equally, is left unexplained.