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MWI makes the very strong claim that the Schrödinger equation is sufficient to explain observations that do not naturally fit into its framework, such as wavefu

MWI makes the very strong claim that the Schrödinger equation is sufficient to explain observations that do not naturally fit into its framework, such as wavefunction collapse and the Born rule. You can simplify any theory by throwing out half its postulates. But now your theory has less explanatory power. The critiques of MWI are that it cannot explain observations. For example, consider the recent article on generating random bits through quantum fluctuations with a laser. MWI, like the Schrödinger equation, is fully deterministic; there is no randomness. The bits are not random in MWI. How to reconcile these?

jiggawatts 6y ago
> MWI, like the Schrödinger equation, is fully deterministic; there is no randomness. The bits are not random in MWI. How to reconcile these? There is a key concept in MWI that people (even Nobel laureate level physicists) are just unable to wrap their heads around: You're also a quantum object, not just the experiment. In MWI, the fundamental concept is that quantum objects expand into multiple dimensions. Hence by the rule above, so do you. This is the source of "randomness". From your in-universe perspective, you can't know apriori which "path" into the multiverse you take. You take all of them, but after some time, any observation you make is from the perspective of one of your copies, and each copy thinks of themselves as the unique, singular copy. To summarise: You can talk meaningfully about the statistics that your future selves will experience, but you can't know ahead which one's perspective you'll have, because your current self will become all of them. So from "God's point of view", outside the multiverse, the whole thing looks deterministic. From inside the universe it seems inherently random. If you're a computer scientist by background, imagine this: You create a human-equivalent AI that you can converse with. Inside its memory you set up a 1-byte cell that it can observe. You then make 256 copies of the AI, one for each possible combination of bits set in the memory cell. If you ask one of the randomly selected copies what value it sees, it'll see whatever you've set, which will more than likely look some arbitrary -- random seeming -- combination of bits. If you repeat the procedure, each time with a randomly chosen AI, it'll report that the cell is changing "randomly". However, the overall system of all copies is entirely deterministic! The randomness is in your external selection. From the outside, you can remove the randomness by asking every AI the same question to reveal the deterministic pattern. From in-universe, this is not an option. The AIs can't ask the other AIs what they see!
- goldenkey 6y ago
  Right, in the universal view, it's just a huge wavefunction following the Schrodinger eq at all times. God can peek at it, and not have to collapse anything. If he decides to sample it, he's choosing 1 universe from all possibilities. It's a huge space, but it's linear from his dimension in which collapse doesn't appear. [1] https://en.wikipedia.org/wiki/Universal_wavefunction https://en.wikipedia.org/wiki/Universal_wavefunction
  - millstone 6y ago
    Ok, how does one derive the Born rule from that idea?
    - ben_w 6y ago
      Could you ELI5 why the Born rule is incompatible with MWI? (It being a problem with MWI is new to me, I am absolutely a physics amateur).
      - millstone 6y ago
        Say there's a quantum bowl with two identical red balls, and one blue ball. You take a ball with your eyes closed. Probably your ball is red, maybe it's blue. You open your eyes, see a color - what happened! Here's three choices, none of them good: 1. Local Hidden Variables: When you open your eyes, you see which color your ball had all along! This is so obvious, but it's wrong for tricky reasons (Bell's Theorem) which I can't fit into the analogy; basically the ball cannot have "had a color all along." 2. Copenhagen: When you open your eyes, the ball just decides what color it is. The other balls will mysteriously agree, so there's not two blue or three red or whatever. How did they do that? Is that a real physical change, or just learning something new? (This is wavefunction collapse and it's very uncomfortable for obvious reasons.) 3. MWI: When you open your eyes, one version of you sees a red ball, and one version of you sees blue. Ok, but if there's red you and blue you, how can the red ball be "more probable?" Are there two separate "red ball" yous? Or maybe the red ball you is "more real?" (This is the relationship between the measure of the wavefunction and experimental probability, aka Born's Rule; MWI struggles to explain it, Copenhagen just postulates it).
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jiggawatts 6y ago

> MWI, like the Schrödinger equation, is fully deterministic; there is no randomness. The bits are not random in MWI. How to reconcile these? There is a key concept in MWI that people (even Nobel laureate level physicists) are just unable to wrap their heads around: You're also a quantum object, not just the experiment. In MWI, the fundamental concept is that quantum objects expand into multiple dimensions. Hence by the rule above, so do you. This is the source of "randomness". From your in-universe perspective, you can't know apriori which "path" into the multiverse you take. You take all of them, but after some time, any observation you make is from the perspective of one of your copies, and each copy thinks of themselves as the unique, singular copy. To summarise: You can talk meaningfully about the statistics that your future selves will experience, but you can't know ahead which one's perspective you'll have, because your current self will become all of them. So from "God's point of view", outside the multiverse, the whole thing looks deterministic. From inside the universe it seems inherently random. If you're a computer scientist by background, imagine this: You create a human-equivalent AI that you can converse with. Inside its memory you set up a 1-byte cell that it can observe. You then make 256 copies of the AI, one for each possible combination of bits set in the memory cell. If you ask one of the randomly selected copies what value it sees, it'll see whatever you've set, which will more than likely look some arbitrary -- random seeming -- combination of bits. If you repeat the procedure, each time with a randomly chosen AI, it'll report that the cell is changing "randomly". However, the overall system of all copies is entirely deterministic! The randomness is in your external selection. From the outside, you can remove the randomness by asking every AI the same question to reveal the deterministic pattern. From in-universe, this is not an option. The AIs can't ask the other AIs what they see!

goldenkey 6y ago
Right, in the universal view, it's just a huge wavefunction following the Schrodinger eq at all times. God can peek at it, and not have to collapse anything. If he decides to sample it, he's choosing 1 universe from all possibilities. It's a huge space, but it's linear from his dimension in which collapse doesn't appear. [1] https://en.wikipedia.org/wiki/Universal_wavefunction https://en.wikipedia.org/wiki/Universal_wavefunction
- millstone 6y ago
  Ok, how does one derive the Born rule from that idea?
  - ben_w 6y ago
    Could you ELI5 why the Born rule is incompatible with MWI? (It being a problem with MWI is new to me, I am absolutely a physics amateur).
    - millstone 6y ago
      Say there's a quantum bowl with two identical red balls, and one blue ball. You take a ball with your eyes closed. Probably your ball is red, maybe it's blue. You open your eyes, see a color - what happened! Here's three choices, none of them good: 1. Local Hidden Variables: When you open your eyes, you see which color your ball had all along! This is so obvious, but it's wrong for tricky reasons (Bell's Theorem) which I can't fit into the analogy; basically the ball cannot have "had a color all along." 2. Copenhagen: When you open your eyes, the ball just decides what color it is. The other balls will mysteriously agree, so there's not two blue or three red or whatever. How did they do that? Is that a real physical change, or just learning something new? (This is wavefunction collapse and it's very uncomfortable for obvious reasons.) 3. MWI: When you open your eyes, one version of you sees a red ball, and one version of you sees blue. Ok, but if there's red you and blue you, how can the red ball be "more probable?" Are there two separate "red ball" yous? Or maybe the red ball you is "more real?" (This is the relationship between the measure of the wavefunction and experimental probability, aka Born's Rule; MWI struggles to explain it, Copenhagen just postulates it).

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