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I think one problem here - physics and the philosophy of physics are two different things, but a lot of people (even professional physicists) fail to treat them

I think one problem here - physics and the philosophy of physics are two different things, but a lot of people (even professional physicists) fail to treat them as distinct in practice. So long as we are talking about math, observation and experiment, that’s physics. But many of these “multiverse theories” are moving beyond that into philosophy, because they are not about the math, but rather how to interpret it; not only are existing observations or experiments are insufficient to determine their truth or falsehood, but we often have no idea of any practically feasible way to test them. There’s nothing wrong with doing philosophy-but let’s be honest when we are doing it, rather than pretending it is still just physics. Furthermore, your average professional physicist has indubitable expertise in physics, their expertise in philosophy (even the philosophy of physics) is much more variable.

evanb 4y ago
If you have a quantum-mechanical system that you describe by specifying a Hamiltonian, I can always write down an exactly equivalent path integral. A failure of equivalence between these two descriptions would be extremely exciting, as it'd be evidence of some paradigm beyond quantum mechanics.
- ImHereToVote 4y ago
  Can you do that for two systems that interfere? I think that is where the multiverse stuff comes from.
  - evanb 4y ago
    Well, it depends on what you mean by "two systems". If they are described by one Hamiltonian the result is one path integral. In a relativistic quantum field theory, for example, one may describe systems of any number of particles. The path integral captures all the interference effects.
- skissane 4y ago
  The mathematical equivalence of Hamiltonians and path integrals, by itself, doesn't tell us anything about a "multiverse", however. Since the mid-20th century, many philosophers have come to analyse everyday talk about possibility in terms of "possible worlds" or "possible universes". However, those who invented that language (such as Carnap and Kripke) weren't claiming that all those possible worlds were just as actual as this one – that claim only arose later (David Lewis). Similarly, the originators of the path integral formulation of QM (Dirac and Feynman) weren't claiming that all those paths were "equally real". By contrast, the majority of proponents of "multiverse" theories, are claiming those other "universes" are ultimately just as real as this one, as as opposed to merely being some theoretical construct, a mathematical metaphor. You can understand these "paths" in two ways – either as just a useful mathematical tool for analysing past observations and experimental results, and predicting future ones, but passing no judgement on their ultimate ontological status; or, as a claim that in some sense, all these paths "really exist". And, if you adopt the later reading, you can then argue whether they are all "equally real", or maybe one of them is "actual" and the others "non-actual", or even just maybe that some are "more actual" than others. Nothing wrong with those debates – but when we start engaging in them, we are no longer doing physics proper, we are doing the philosophy of physics.
  - 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.

evanb 4y ago

If you have a quantum-mechanical system that you describe by specifying a Hamiltonian, I can always write down an exactly equivalent path integral. A failure of equivalence between these two descriptions would be extremely exciting, as it'd be evidence of some paradigm beyond quantum mechanics.

ImHereToVote 4y ago
Can you do that for two systems that interfere? I think that is where the multiverse stuff comes from.
- evanb 4y ago
  Well, it depends on what you mean by "two systems". If they are described by one Hamiltonian the result is one path integral. In a relativistic quantum field theory, for example, one may describe systems of any number of particles. The path integral captures all the interference effects.
skissane 4y ago
The mathematical equivalence of Hamiltonians and path integrals, by itself, doesn't tell us anything about a "multiverse", however. Since the mid-20th century, many philosophers have come to analyse everyday talk about possibility in terms of "possible worlds" or "possible universes". However, those who invented that language (such as Carnap and Kripke) weren't claiming that all those possible worlds were just as actual as this one – that claim only arose later (David Lewis). Similarly, the originators of the path integral formulation of QM (Dirac and Feynman) weren't claiming that all those paths were "equally real". By contrast, the majority of proponents of "multiverse" theories, are claiming those other "universes" are ultimately just as real as this one, as as opposed to merely being some theoretical construct, a mathematical metaphor. You can understand these "paths" in two ways – either as just a useful mathematical tool for analysing past observations and experimental results, and predicting future ones, but passing no judgement on their ultimate ontological status; or, as a claim that in some sense, all these paths "really exist". And, if you adopt the later reading, you can then argue whether they are all "equally real", or maybe one of them is "actual" and the others "non-actual", or even just maybe that some are "more actual" than others. Nothing wrong with those debates – but when we start engaging in them, we are no longer doing physics proper, we are doing the philosophy of physics.
- 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.