Many Worlds vs. The Multiverse

Sean Carroll 2017.jpg
Sean Carroll is a cosmologist and physicist specializing in dark energy and general relativity. He is a research professor in the Department of Physics at the California Institute of TechnologyA post on his blog caught my attention: “Are Many Worlds and the Multiverse the Same Idea?

When talking about “parallel worlds” Carroll distinguishes between: a)the “multiverse” of inflationary cosmology; b) the “many worlds” or “branches of the wave function” of quantum mechanics; and c) “parallel branes” of string theory.” While branes represent a distinct idea, Carroll thinks that the multiverse and many worlds ideas might capture the same basic idea. Here’s how he explains the differences between those two ideas:

When cosmologists talk about “the multiverse,” it’s a slightly poetic term. We really just mean different regions of spacetime, far away so that we can’t observe them, but nevertheless still part of what one might reasonably want to call “the universe.” In inflationary cosmology, however, these different regions can be relatively self-contained — “pocket universes,” as Alan Guth calls them. When you combine this with string theory, the emergent local laws of physics in the different pocket universes can be very different; they can have different particles, different forces, even different numbers of dimensions. So there is a good reason to think about them as separate universes, even if they’re all part of the same underlying spacetime.

The situation in quantum mechanics is superficially entirely different. Think of Schrödinger’s Cat. Quantum mechanics describes reality in terms of wave functions, which assign numbers (amplitudes) to all the various possibilities of what we can see when we make an observation. The cat is neither alive nor dead; it is in a superposition of alive + dead. At least, until we observe it. In the simplistic Copenhagen interpretation, at the moment of observation the wave function “collapses” onto one actual possibility. We see either an alive cat or a dead cat; the other possibility has simply ceased to exist. In the Many Worlds or Everett interpretation, both possibilities continue to exist, but “we” (the macroscopic observers) are split into two, one that observes a live cat and one that observes a dead one. There are now two of us, both equally real, never to come back into contact.

Now clearly these ideas differ. Most notably, in the multiverse, the other universes are far away whereas, in quantum mechanics, they’re right here in different possibility spaces. (technically different parts of Hilbert space.) Still, some physicists have been wondering about the connection between the two ideas. And, after reading the recent literature, Carroll has “gone from a confused skeptic to a tentative believer.”

Carroll has changed his mind because of two ideas that fit together to make this crazy-sounding proposal plausible—quantum vacuum decay and horizon complementarity. Roughly quantum vacuum decay implies that “at any point in space you are in a quantum superposition of different vacuum states.” But horizon complementarity means that “you can talk about what’s inside your cosmological horizon, but not what’s outside.” Carroll concludes:

The result is: multiverse-in-a-box. Or at least, multiverse-in-an-horizon. On the one hand, complementarity says that we shouldn’t think about what’s outside our observable universe; every question that it is sensible to ask can be answered in terms of what’s happening inside a single horizon. On the other, quantum mechanics says that a complete description of what’s actually inside our observable universe includes an amplitude for being in various possible states. So we’ve replaced the cosmological multiverse, where different states are located in widely separated regions of spacetime, with a localized multiverse, where the different states are all right here, just in different branches of the wave function.

Carroll admits not knowing if any of this is true, although he is “inclined to think that it has a good chance of actually being true.” As for the implications for physics and for us, I’m in the dark. I’m simply don’t know what to do with the idea of a multiverse and parallel universes. Try as I may, our mysterious reality confounds me.

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Author’s Note –  I hope no one mistakenly believes that because there is disagreement on the edges of physics then physicists just don’t know what they’re talking about.  Physicists know what they’re talking about or we couldn’t launch the JamesWebb telescope to an exact place in space and make it work or do a million other things!

Furthermore, because I can’t understand differential equations or relativity theory or quantum mechanics or other complicated science says something about my ignorance not that of those who understand such things. I don’t really know how fish extract oxygen from water but that doesn’t mean that those who do understand such things don’t know what they’re talking about!

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9 thoughts on “Many Worlds vs. The Multiverse

  1. I have a master’s in physics and I have to say that I don’t really grasp this concept. I’ve never felt at home with quantum mechanics. Indeed, I’m not sure that anything in quantum mechanics can be truly “grasped” because it’s all so remote from our experiences.

    I do have an excellent way of excusing myself from this issue. I doubt that this line of thinking — or ANY of the interpretations of QM — has observable manifestations. QM is a powerful computational structure that allows us to calculate almost everything about the behavior of things at the subatomic level. Trying to make sense of it, I suspect, is a fool’s errand. Our comprehension of it is analogous to that of a child’s grasp of a smartphone. The child has no idea of what’s going on inside — the child knows only that it works. The difference is that the child could eventually figure it out, whereas I don’t think that is possible with QM. Thought of this way, QM offers a powerful argument for the existence of God, albeit a God with a truly wicked sense of humor.

  2. I have no master’s in physics and I’ve read many explanations for dummies of QM (if we know the position we don’t know the velocity and the reverse) but of course, I don’t really understand it. If Einstein, Bohr, Heisenberg, etc. disagreed on the interpretation of QM then I don’t have much chance of understanding.

  3. Everyone lives in the description of reality that is in their mind, some people have greater imaginations than others so they can promulgate different descriptions of the same reality and challenge others to understand what they are describing, always with the ‘unstated’ assumption that if you can’t understand their description, then they are more intelligent than you!
    As Dr. John says; If Einstein, Bohr, Heisenberg, etc, can’t agree on what it is, perhaps the person promoting it doesn’t really know what he is talking about either.
    The danger of placing ‘Science’ and ‘Scientists’ in the position of being the sole arbiters of reality is that they sometimes speak in a language that it seems, they themselves don’t understand!

  4. I DID NOT SAY that physicists can’t agree on what QM is. QM is true beyond any reasonable doubt and without it, global positioning and a thousand other technologies wouldn’t be possible. Physicists know what they’re talking about or we couldn’t launch the JamesWebb telescope to an exact place in space and make it work or do a million other things!!!!!! The disagreement is on the philosophical implications of the beyond-any-doubt truth of QM.

    Furthermore, because I can’t understand differential equations or relativity theory says something about my ignorance not that of those who understand such things. I don’t really know how fish extract oxygen from water but that doesn’t mean that those who do understand such things don’t know what they’re talking about!!!

  5. Let me start by saying I’m in Vienna right now and just finished reading David Edmonds book “The Murder of Professor Schlick.” Your last post was about Schlick (which I will read soon), and in his Vienna Circle meetings another member, Otto Neurath, apparently used to shout “metaphysics!” every time the group talked about stuff outside of what logical positivism could consider. Now, logical positivism hasn’t held up, but still, in regards to this post, I say….metaphysics! : )

  6. that’s funny Ed. But you’re right; there are deep similarities between the edges of science and metaphysical speculation.

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