In case you doubt this, listen to Sean M. Carroll, a leading advocate of many-worlds. Here is his latest podcast:
19:02 Victor Tiffany says "After you answered my question about energy conservation in the many worlds interpretation of 19:08 quantum mechanics you added there are real worries that you can have about many worlds but energy conservation is 19:14 not a true scientific worry." So what are those real worriesIf you listen carefully, he is acknowledging that these are problems with many-worlds. Probabilities do not make sense. Maybe it will all be explained by "self-locating uncertainty" or some other concept yet to be developed.i think I've said what the real worries are at different 19:20 times but you know not everyone listens to every word so I'll I'll it's worth mentioning again. It's been a while Um I 19:25 think there are two sets of worries that are that are valid um one is the 19:30 question of deriving the born rule the probabilities in many worlds and it's 19:36 not that I don't think we can I think we can I think I know how to do it but the 19:41 origin of probability in many worlds is very very different than what it would 19:46 be in a truly stochastic theory or in a truly stochastic theory. um single world 19:52 theory you can just say well there's something that we don't know and can never know about what will happen next 19:58 in quantum mechanics. And the best we can do is assign a probability to it And we can figure out what that probability is by looking at 20:05 the frequencies of previous events and we get that the born rule fits the data and it all sort of makes sense It's at 20:10 least it not only does it make sense but it's comfortable and familiar to us Okay
20:16 Whereas in many worlds as has been pointed out many many times uh every allowed outcome happens The word allowed 20:23 is super important there Some outcomes are not allowed all outcomes that are supported by the Schroinger equation and 20:30 the current state of the quantum state of the universe are allowed which is very very different than saying everything happens but anyway many 20:37 different uh outcomes actually do happen and with 100% probability they will 20:42 happen in one of the branches of the wave function. right if I have a spin that is a superposition of spin up and 20:48 spin down I measure its spin in one branch it's definitely going to be spin up in the other branch. it's definitely 20:53 going to be spin down nothing probabilistic about it So where do the probabilities come from and again.
I 20:59 think there's an answer to this question and it has to do with self-locating uncertainty and things like that But the 21:05 kind of answer it is is deeply different than the kind of answer it is in the case of a truly stochastic theory. And 21:11 therefore I think it is okay to worry that we are cheating Uh you know we know what the answer is that we want to get. 21:18 We want to get the borne rule We want to get that the probability is proportional to the wave function squared I think we do get that in a very natural way but a 21:25 very different way than what we're used to. So we should be thinking very very deeply about whether we're just cheating 21:31 ourselves.
The other thing to worry about in Everett is that it's a very austere 21:37 theory. So the origin of structure broadly construed in the quantum state 21:42 of the universe is a big question Why do we see a world with three-dimensional space and you know matter and energy 21:49 rather than just seeing a wave function. okay again I think it's perfectly possible to answer this question but I 21:56 think it is much less answered than the probability question because you know the answer that we want is a lot more 22:02 detailed and structured as it were than just the probability question.
So there's a lot of work to be done and this is one 22:09 of the biggest things that I'm working on myself right now um how do you take that abstract quantum wave function and 22:16 divide it up using words like emergence and coarse graining and and effective theories and things like that and say 22:22 that oh look there's a classical limit with matter and particles and things like that. The reason why this is a 22:29 uniquely Everettian problem is because Everett's theory is the simplest theory 22:34 of quantum mechanics. Um it is the one that doesn't have any extra stuff It just has the quantum state and the 22:41 Schrodinger equation.
Other approaches to quantum mechanics don't need to address 22:46 the structure problem because they just put the structure in as part of the 22:51 posits of the theory. You have particles moving in space if you're bohmian. You 22:56 have certain ways that the wave function collapses if you're an objective collapse person And all of these rely on 23:02 this extra structure. So they quote unquote answer the question by just in 23:08 you know making an assumption that it's there right and maybe it is there that's perfectly valid I think that it is 23:14 better if we can derive it from something more fundamental but that doesn't mean that it's not uh uh work to 23:21 actually do that derivation
And it predicts wave functions as the "simplest theory", but no one has figured out how to relate the theory to what anyone observes.
The phrase "not even wrong" applies here. There is no theory that is scientific in any way. It is all a delusional fantasy. He complains that Pres. Trump is probably not going to renew his federal grant to philosophize about this stuff, but this is crazier than transgender mice. There is no possibility of any good coming out of many-worlds theory.
Any scientific theory has to face the fact that if you look at the range of possible events, some things happen and some do not. Many-worlds theory says you get a simpler theory by dropping that fact, and assuming that anything can happen. It refuses to even say that some outcomes are more likely than others. It assures us that future work will make it all make sense somehow.
Listen to Carroll if you think I am misrepresenting many-worlds.