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> The measurement problem is a solved problem. I think this is overselling it slightly. Yes, Everettian style interpretations have helped to shed insight into

• tim333 10y ago
  Indeed it's not that solved. A big problem is as Weinberg puts it >Several attempts following the realist approach have come close to deducing rules like the Born rule that we know work well experimentally, but I think without final success. which is kind of the heart of things.
  • lisper 10y ago
    Just because Steven Weinberg is unaware of or does not accept the solution does not mean that the problem is not in fact solved.
    • tim333 10y ago
      I don't think that's been solved either or most physicists for that matter. I've read some of the attempts and they are not terrible good. I don't think Ron Garret even touched on the Born rule, Everett of many worlds just hand waves and assumes it. The attempts I've seen are basically of the form that if the rule was different from the probability equaling the absolute amplitude squared then reality wouldn't come out as we find, so it must be so, which is not much of an explanation in my book.
      • lisper 10y ago
        FYI, I am Ron Garret. It's true that I don't talk about the Born rule or multiple worlds in the paper, but I do talk about multiple worlds in the video (at the end, during the Q&A) and in the blog posts I linked to above. As for the Born rule, what kind of an explanation are you hoping for? Some things can't be explained beyond, "That's just how it is." For example, why do the fundamental physical constants have the values that they have? Maybe this will help: https://news.ycombinator.com/item?id=13406241 https://news.ycombinator.com/item?id=13406241 http://www.scottaaronson.com/democritus/lec9.html http://www.scottaaronson.com/democritus/lec9.html
        • n4r9 10y ago
          Two of the appealing arguments for many worlds type interpretations are that the number of required assumptions is small, and that it gives a reasonable physical intuition for why measurement outcomes occur as they do. Having to bolt on the Born rule weakens both of these. In fact, I would even go so far as to say that if you are going to merely "enforce" the Born rule without justification, then you are no longer proposing an Everettian interpretation. The whole point of these interpretations is that simply "unitary quantum mechanics" describes the whole universe.
          • tim333 10y ago
            I agree that bolting on the Born rule seems a weakness of the many world view. It would be nice if you could say everything follows the Schrodinger equation and hence the probabilities naturally come out the way they do but it doesn't seem to work that way very easily. Everett himself however seems to basically "enforce" the Born rule without justification so I guess that would be Everettian. See https://www-tc.pbs.org/wgbh/nova/manyworlds/pdf/dissertation.pdf https://www-tc.pbs.org/wgbh/nova/manyworlds/pdf/dissertation... page 34 >we define a square-amplitude distribution, Pi... Probability is a funny thing to deal with. Say you have an experiment where you push a button and a red light or green comes on based on some quantum effect but the green light is 1000x more likely. In many worlds you'd end up with different worlds with an observer seeing one or another but it's tricky to see how the 1000x thing comes in.
    • tigershark 10y ago
      If you are religiously convinced that the QIT interpretation is the only true one that doesn't make it true. For me it is just one of the many interpretations and I think that it is wrong on par with all the others that I'm aware of. If this wasn't the case then the measurement problem and the unification with the relativity theory (or with a theory that supersedes the relativity) would be already solved today. I'm really not in the field, but I don't think that anyone that actively works on QM is debating on solved problems.
      • lisper 10y ago
        I am absolutely not religiously convinced that QIT is the only true explanation. In fact, I explicitly say at the end of my talk that it is not, that multiple-worlds is equally valid, and which you choose is a matter of personal preference. But if want to argue that no solution exists (and this is Weinberg's claim) then you have to consider all of the solutions on the table, and Weinberg does not. He deliberately ignores the fundamental (and IMHO ultimately clarifying) fact that entanglement and measurement are intimately related (and are in fact the same physical phenomenon). It is entirely unsurprising that anyone who ignores this fact ends up believing that there is a big mystery. But in fact there is no big mystery. There are only people who are mystified because they are ignoring (or ignorant of) this crucial fact.
        • akvadrako 10y ago
          > There are only people who are mystified because they are ignoring (or ignorant of) this crucial fact. I don't think that's the only reason people are mystified. From what Weinberg says, and I've heard from other physicists, when you get to the root of the issue they just won't accept a multiverse theory. It has profound philosophical consequences, so I guess they go looking for ways to make their understanding of QM fit their strongly held preconceptions. Some avoid this fact you just mentioned, but others fight the Born rule derivations or wrongly apply Occam's Razor or have more original objections.
          • lisper 10y ago
            > they just won't accept a multiverse theory They don't have to. But they do have to accept that entanglement and measurement are the same physical phenomenon.
  • akvadrako 10y ago
    I know of 3 modern ways to derive the Born rule for WMI and all have various reasonable (yet not airtight) assumptions. In physics you can never really prove anything, but there is no serious technical issues here. Actually probability (the Born rule) is much better defined in WMI then in any other interpretation or even classically. A related beautiful thing about WMI is that is creates genuine subjective probability inside a deterministic system (the wave function). One thing QM shows us is that we experience real random events, which cannot just come from lack of knowledge. Yet, if you are a computer expert you should know that it's impossible to create real randomness without an outside source. And the universe has no outside sources at all, by definition. Also, it's clear that the basis of Weinberg's objection to WMI is philosophical if you don't selectively quote him so much. If he was concerned about deriving the Born rule we would talk about the specific assumptions he disagrees with.
    • n4r9 10y ago
      Have you aware of Adrian Kent's critiques of (many of) these attempts? https://arxiv.org/abs/0905.0624 https://arxiv.org/abs/0905.0624
      • akvadrako 10y ago
        I have read lots of critiques about the Born rule derivations but I've missed this one. Thanks for the pointer. He is only attacking one attempt, the decision-theoretic one. I have trouble finding his point in this paper because it's so rambling but he seems to be saying: > It seems prima facie surprising to claim that mathematical analysis could show that Born-weight mean utilitarianism, or any other strategy, is the unique rational way of optimizing the welfare of one’s own, and other people’s, many future selves in a multiverse. Okay, sure - but it's also just as ridiculous to say that you can prove with no assumptions how a rational agent should act in a classical world. Actually there is no explanation of probability in the classical world that's as clear as that in a multiverse, where you have actual proportions of outcomes. All you really need is to assume that branches of equal magnitude have an equal chance of occurring and the Born rule becomes obvious. That seems like a safe assumption to me but you can't prove it beyond all doubt without some axioms of probability. EDIT: I found a critique of his paper you might also want to read: https://arxiv.org/pdf/1111.2563.pdf https://arxiv.org/pdf/1111.2563.pdf