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Agree with your first paragraph and the first half of your second paragraph. But if you listen to Feynman talk about the path integral, he emphasizes that all

by evanb 4y ago

Agree with your first paragraph and the first half of your second paragraph. But if you listen to Feynman talk about the path integral, he emphasizes that all the paths are real, are in fact critical to describing experimental results, and that it is simple to construct experiments to get those paths to coherently add to produce classically-unexpected results! I don't know about Dirac's "philosophy"; what you say of his stance may be correct. I don't think Feynman would say any path is actual or non-actual. It's not the case that ONE of them "really" happens and the others are a calculational trick. It's that the thing that happens is a sum of alternatives. The sum is real, full stop. If you leave out terms in the sum you get the wrong answer. Is that enough to mark them as "real"? THAT is philosophy. Certainly you may find another description for the same sum which does not discuss a sum over paths at all. Perhaps that means that the terms in the sum are not real? But we can all agree that the sum itself comes out right, whichever picture of the world you want.

skissane 4y ago
> But if you listen to Feynman talk about the path integral, he emphasizes that all the paths are real, are in fact critical to describing experimental results, and that it is simple to construct experiments to get those paths to coherently add to produce classically-unexpected results! Okay, but is he claiming that the paths are "real" in the exact same sense as the actual experimental results are? We (in principle) add up an infinite number of paths, and that infinite sum produces a probability amplitude, which gives us a probability, and then we expect that if we repeat the experiment enough times, and sufficiently control the sources of experimental error, the observed probability should converge to the calculated one. Even if he says that all those paths are "real", I don't think he is claiming that they are "real" in quite the same sense as the actual experimental results are "real". Whereas, a proper "multiverse theory" would be claiming something like that.
- evanb 4y ago
  OK, I think I appreciate the issue: “multiverse” is overloaded. When people talk about the many worlds of quantum mechanics, or alternate histories, they’re talking about the terms in the path integral sum. But multiverse can also mean, separately, at least two additional things. First, different bubble universes that may be a part of cosmology. Second, alternate quantum field theories (with different actions, or fundamental parameters, or different gauge groups, or whatever). The standard quantum-mechanical path integral is a sum over histories / a many-world theory in the sense that the different histories / parts of the wave function can be detected via interference experiments. Other meanings of multiverse are speculative (though maybe a cosmologist will come and tell me that bubble universes are a direct prediction of inflationary cosmology and are not optional either).
  - skissane 4y ago
    > When people talk about the many worlds of quantum mechanics, or alternate histories, they’re talking about the terms in the path integral sum. Hugh Everett's original "many worlds" theory wasn't just "talking about the terms in the path integral sum" – he saw all the "worlds" as "real" in exactly the same sense that the actual experimental results are "real". Everett believed in "quantum immortality", which only makes sense if you interpret the worlds in a much more literal sense than mere "terms in the path integral sum" suggests. By contrast, even though Feynman calls the paths "real", it isn't clear if he means them to be "as real" as the actual experimental results they are used to predict. People who reinterpret many-worlds as just "terms in the path integral sum" are cutting the theory down to be something far less metaphysically grandiose than its originator intended. To quote Max Tegmark: > Atheist or not, Everett firmly believed that his many-worlds theory guaranteed him immortality: His consciousness, he argued, is bound at each branching to follow whatever path does not lead to death —and so on ad infinitum. (Sadly, Everett's daughter Liz, in her later suicide note, said she was going to a parallel universe to be with her father. [149a]) https://space.mit.edu/home/tegmark/everett/everett.html https://space.mit.edu/home/tegmark/everett/everett.html
    - peteradio 4y ago
      Sadly? You presume she is not actually living in a parallel universe!
      - skissane 4y ago
        I realise you are being somewhat flippant, but it is still sad, because (among other reasons) it appears that Liz Everett misunderstood her father's theory. If the theory of quantum immortality is true, then while from our perspective, Hugh Everett died from a heart attack in 1982, from his own perspective, he's living on in some other branch of the multiverse in which he somehow survived that heart attack. Similarly, while from our perspective, his daughter Liz died from suicide in 1996, from her own perspective, she's living on in some other branch of the multiverse in which she somehow survived that suicide attempt. Indeed, if the theory is true, then no matter how many times 1996 Liz tries to commit suicide, from her own viewpoint she will never succeed – but she never gets to see her father again either – he's living on in a different branch of the multiverse from her, one which diverged from hers all the way back in 1982. And even that other Liz, whose father didn't die in 1982, is doomed to eventually watch her father die, and her father to watch her die, even as both go on living forever, trapped in different branches. Quantum immortality promises us a somewhat bleak afterlife in which we all live forever, but none of us ever get to see each other again. Eternal loneliness is our universal doom. The only way in which she actually could see her father again, would be if quantum immortality is false, but some other afterlife theory is true instead.

by evanb 4y ago

skissane 4y ago

> But if you listen to Feynman talk about the path integral, he emphasizes that all the paths are real, are in fact critical to describing experimental results, and that it is simple to construct experiments to get those paths to coherently add to produce classically-unexpected results! Okay, but is he claiming that the paths are "real" in the exact same sense as the actual experimental results are? We (in principle) add up an infinite number of paths, and that infinite sum produces a probability amplitude, which gives us a probability, and then we expect that if we repeat the experiment enough times, and sufficiently control the sources of experimental error, the observed probability should converge to the calculated one. Even if he says that all those paths are "real", I don't think he is claiming that they are "real" in quite the same sense as the actual experimental results are "real". Whereas, a proper "multiverse theory" would be claiming something like that.

evanb 4y ago
OK, I think I appreciate the issue: “multiverse” is overloaded. When people talk about the many worlds of quantum mechanics, or alternate histories, they’re talking about the terms in the path integral sum. But multiverse can also mean, separately, at least two additional things. First, different bubble universes that may be a part of cosmology. Second, alternate quantum field theories (with different actions, or fundamental parameters, or different gauge groups, or whatever). The standard quantum-mechanical path integral is a sum over histories / a many-world theory in the sense that the different histories / parts of the wave function can be detected via interference experiments. Other meanings of multiverse are speculative (though maybe a cosmologist will come and tell me that bubble universes are a direct prediction of inflationary cosmology and are not optional either).
- skissane 4y ago
  > When people talk about the many worlds of quantum mechanics, or alternate histories, they’re talking about the terms in the path integral sum. Hugh Everett's original "many worlds" theory wasn't just "talking about the terms in the path integral sum" – he saw all the "worlds" as "real" in exactly the same sense that the actual experimental results are "real". Everett believed in "quantum immortality", which only makes sense if you interpret the worlds in a much more literal sense than mere "terms in the path integral sum" suggests. By contrast, even though Feynman calls the paths "real", it isn't clear if he means them to be "as real" as the actual experimental results they are used to predict. People who reinterpret many-worlds as just "terms in the path integral sum" are cutting the theory down to be something far less metaphysically grandiose than its originator intended. To quote Max Tegmark: > Atheist or not, Everett firmly believed that his many-worlds theory guaranteed him immortality: His consciousness, he argued, is bound at each branching to follow whatever path does not lead to death —and so on ad infinitum. (Sadly, Everett's daughter Liz, in her later suicide note, said she was going to a parallel universe to be with her father. [149a]) https://space.mit.edu/home/tegmark/everett/everett.html https://space.mit.edu/home/tegmark/everett/everett.html
  - peteradio 4y ago
    Sadly? You presume she is not actually living in a parallel universe!
    - skissane 4y ago
      I realise you are being somewhat flippant, but it is still sad, because (among other reasons) it appears that Liz Everett misunderstood her father's theory. If the theory of quantum immortality is true, then while from our perspective, Hugh Everett died from a heart attack in 1982, from his own perspective, he's living on in some other branch of the multiverse in which he somehow survived that heart attack. Similarly, while from our perspective, his daughter Liz died from suicide in 1996, from her own perspective, she's living on in some other branch of the multiverse in which she somehow survived that suicide attempt. Indeed, if the theory is true, then no matter how many times 1996 Liz tries to commit suicide, from her own viewpoint she will never succeed – but she never gets to see her father again either – he's living on in a different branch of the multiverse from her, one which diverged from hers all the way back in 1982. And even that other Liz, whose father didn't die in 1982, is doomed to eventually watch her father die, and her father to watch her die, even as both go on living forever, trapped in different branches. Quantum immortality promises us a somewhat bleak afterlife in which we all live forever, but none of us ever get to see each other again. Eternal loneliness is our universal doom. The only way in which she actually could see her father again, would be if quantum immortality is false, but some other afterlife theory is true instead.