Many-World vs. Multiverse

Posted by Chad Orzel on November 11, 2008
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In the recent discussion of Many-Worlds and making universes, Jonathan Vos Post asked what science fiction treatments of the idea I like. The answer is pretty much “none,” because most SF treatments are distractingly bad.

For example, last night I finished Neal Stephenson’s Anathem, a whopping huge brick of a book setting up an incredibly imaginative alternate Earth, with a detailed intellectual history paralleling our own. It’s got all sorts of great stuff, but it lost me when it started talking about parallel worlds, because it munges together the Many-Worlds Interpretation of quantum physics, and Multiverse Cosmology in a way that I found distracting.

These are very different theories, dealing with very different things:

The Many-Worlds Interpretation talks (in its popular formulation) about “alternate worlds” in which particular measurements had different outcomes. It’s not quite right to talk about these as separate universes in their own right (really, they’re just different parts of the same universal wavefunction), but that’s the basic idea– there is a branch of the wavefunction corresponding to each of the possible outcomes of any particular measurement, and those branches are inaccessible to one another.

Multiverse Cosmology, on the other hand, posits the existence of other “universes” in which the constants of nature have slightly different values. Depending on which flavor of it you’re dealing with, these may be completely separate parallel worlds (other Big Bangs leading to other universes) or “bubbles” within a single cosmos, stemming from the same Big Bang.

Stephenson blurs the distinction between the two in a way I found annoying. He talks about universes with different physical laws (Multiverse Cosmology) as if they were the same as the different wavefunction branches of Many-Worlds. It’s not critical to the resolution of the plot, but it was annoying to me in a way that hurt my enjoyment of the end of the book. (Which wouldn’t've been that great even if he had gotten the details right, but that’s a topic for another post…)

Of course, Stephenson uses these ideas about as well as anybody else in SF does. Which is why there isn’t much SF about these ideas that I would recommend.

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  1. #1 Ray Ingles
    November 11, 2008

    Of course, given an unlikely-enough branch of the wave function, you could get a universe where the physical laws appeared to be quite different. In a branch of the wavefunction where lead regularly turned into gold (exceedingly unlikely, but possible) then the laws of nature would seem quite different.

  2. #2 Peter Woit
    November 11, 2008

    You can’t really expect Stephenson to keep these two things separate when physicists themselves don’t. Perimeter recently had a workshop bringing these together, see

    http://www.perimeterinstitute.ca/en/Events/The_Multiverse/Schedule/

    The topic of why physicists are doing this came up here:

    http://www.math.columbia.edu/~woit/wordpress/?p=853

  3. #3 Matt Leifer
    November 11, 2008

    I agree that these are definitely separate issues, but it is exactly the sort of subtle distinction that is going to get lost in translation to popular media, so it doesn’t really surprise me. Also, it doesn’t bother me as much as when people give the impression that many-worlds is the ONLY way of understanding quantum theory, which seems to happen quite a lot these days.

    I think that the main reason why both multiverse theories are often discussed in the same place is just that many-worlds is popular amongst cosmologists (at least that small subset of cosmologists that are interested in quantum foundations) so the overlap between people who believe in both theories is quite large.

    Also, the arguments against both theories are quite similar, i.e. that the other universes are in principle unobservable and that it it difficult to argue for a natural probability measure, so maybe it does make sense to take on both theories at once, at least for the critics.

  4. #4 Jennifer Ouellette
    November 11, 2008

    Well, I’m a layperson, so to speak, and somehow I managed to suss out the basic difference between Many Worlds and the multiverse by reading popular treatments — so I don’t think this is necessarily getting “lost in translation.” I’d bet good money that Stephenson is smarter than me. So it’s entirely likely, to my mind, that he chose to conflate the two as a sort of creative license in world-building. Whether he was successful or not is probably a matter of taste.

    That said, as Peter Woit correctly points out in #2, even physicists sometimes discuss them together, adding to the likelihood of confusion to someone just entering the fray.

  5. #5 wokka
    November 11, 2008

    According to Max Tegmark, mr Multiverse himself, you can talk about different levels of multiverse (see http://space.mit.edu/home/tegmark/multiverse.html). His level three is actually the many worlds of quantum physics. This is his terminology, and I think you will have to accept that it’s picked up by sf authors who read Scientific American.

  6. #6 Daryl McCullough
    November 11, 2008

    I can’t say whether his novels are examples of technically correct science fiction, but Greg Egan is one of those rare science fiction writers who is also a top-notch physicist. He understands the physics as well as anyone.

  7. #7 Daryl McCullough
    November 11, 2008

    I can’t say whether his novels are examples of technically correct science fiction, but Greg Egan is one of those rare science fiction writers who is also a top-notch physicist. He understands the physics as well as anyone.

  8. #8 Neil B
    November 11, 2008

    Ray Ingles makes a good point about splitting-MW alternatives (indeed not to be confused with the MV worlds “actually” having other laws of physics.) I have too in the past and others I’m sure: if all statistical outcomes are realized “somewhere”, then there are going to be “worlds” where Co 60 decayed with a seeming half-life of weeks or centuries instead of the “real” value of about five years. (There would be even more of the messy worlds where decay went in fits and lulls so as to make a reasonable assessment of “half-life” impossible, to the extent one can accept “more” of one essentially infinite continuum set compared to another … look for “Hilbert’s Hotel, see above re “measure” … Does that wreck Bayesian assessment of one’s chances to be in X or Y world and what to expect next.)

    That challenges the very idea of empirical definitions of “law” where statistics are involved (like quantum choices: decay, point of absorption, etc.) versus the case for inverse square law attraction etc. (I’m not even so sure of the latter, since couldn’t virtual photons have odd statistical quirks that messed up the smooth appearance of classical lawfulness?)

    I suppose, scientists in such worlds could come up with theoretical justifications for why things “ought” to be different than what happened there, since apparently we can calculate what the lifetime of X “ought” to be from theoretical considerations. (True? How many calculations actually have been done to show what half-lives various nuclei and particles should have? Have all been accurate?) Of course there would be a huge chance that everything would go back to roughly “normal” the very next moment, but some physicists out there would remain stuck in such misleading universes for awhile.

  9. #9 Daryl McCullough
    November 11, 2008

    Neil B.,

    I think that the issue you bring up is a problem for any theory of probability, whether classical or quantum-mechanical. Just sticking to classical probabilities, if the universe is large enough then there should be worlds (in the ordinary sense of planets) in which, for example, a coin flipped a billion times will end up “heads” up every time. People living in such a world would assume that there is some deep reason that the coin always lands heads-up, although they will never discover such a reason.

  10. #10 Moshe
    November 11, 2008

    Regarding the difference between many worlds and the multiverse, two points to keep in mind:

    1. Even if they are really different, they tend to bring the same sort of issues, technical and conceptual, so it is not a bad idea to group them together in a professional setting. I’m sure the physicists at the PI conference managed to keep this elementary distinction in mind.

    2. There is at least one suggestion on the market that those two things are “complementary” description of the same thing, in the sense of quantum or black hole complementarity. Not that there can be much certainty in such a speculative subject, but such a suggestion is not completely absurd given that similar ideas work so successfully in the context of black hole physics.

  11. #11 Neil B
    November 11, 2008

    Daryl, that’s a good point, “but”. If the universe is large enough, then yes even without “many worlds” odd things will happen in some places – and a similar question comes up. The difference is, if we have many entire universes, then even if physicists compared notes from planets all around they’d still find strange apparent laws, hence my term “misleading universe.” It would seem to be alternate “laws” precisely because of seeming to be in effect everywhere and not just a place in a universe where things were happening a funny way.

  12. #12 Neil B
    November 11, 2008

    Daryl, that’s a good point, “but”. If the universe is large enough, then yes even without “many worlds” odd things will happen in some places – and a similar question comes up. The difference is, if we have many entire universes, then even if physicists compared notes from planets all around they’d still find strange apparent laws, hence my term “misleading universe.” It would seem to be alternate “laws” precisely because of seeming to be in effect everywhere and not just a place in a universe where things were happening a funny way.

  13. #13 Ian Durham
    November 11, 2008

    I’m a little surprised Stephenson muddles that since he does have a background (albeit small) in physics. I’m reading Cryptonomicon right now (my first Stephenson novel) and thoroughly enjoying it. Most of the cryptography in it so far seems correct as does the bit of physics that comes into play here and there.

    Has anyone ever read The Schrödinger Cat Trilogy by Robert Anton Wilson? I read it back in high school before I knew anything about quantum mechanics but, now that I do this for a living, have thought about going back to reread it. Ostensibly, if I recall, it deals (in a bizarre and light-hearted way) with this very issue.

  14. #14 Neil B
    November 13, 2008

    Yes Ian, I read the Cat trilogy by Robert Anton Wilson. It’s very clever and entertaining, albeit the story line is not that much critically based on MW theory. I had RAW stay over at my house in 1987 to give a seminar, I still keep the empty bottle of Guinness he drank at a tavern we went to (“Mike’s Place” an Irish-themed pub of course.) He sure was interesting, sadly passed “away” last year.

    BTW one of the weirdest implications of MW is “quantum suicide” – just google it and read the Wikipedia to get the idea. It sounds incredible, but hardly avoidable IMHO if MW is true, but I think MW is false. Instead, the universe is just irreducibly weird.

  15. #15 dileffante
    November 13, 2008

    I agree that Stephenson muddles things a bit, but not that much, imho. The alternative universes where the Geometers come from are Multiverse stuff, and the “management of narratives” of the Incanters is MW stuff. So far so good. They are conflated in the attempts to explain them, but play different roles in the story. The fact that they *are* conflated may be related to Tegmark’s ideas that wokka cites above; the [conjectured] inclusion of the “Hylaen Theoric World” in the list of universes suggests the same Tegmarkian influence.

    Loved the book, anyway. My only complaint is that I thought it would be shipping along with a CD of cellular automata-cum-gregorian chant music, and I got no such thing :’-(

  16. #16 Kevin Riggle
    November 17, 2008

    Coming late to the party, linked via Tor.com —

    wokka @5: According to Max Tegmark, mr Multiverse himself, you can talk about different levels of multiverse (see http://space.mit.edu/home/tegmark/multiverse.html). His level three is actually the many worlds of quantum physics. This is his terminology, and I think you will have to accept that it’s picked up by sf authors who read Scientific American.

    It seems pretty clear from Mr. Stephenson’s extended acknowledgements for Anathem that he’s actually picked it up from reading Tegmark.

    dileffante @15: Loved the book, anyway. My only complaint is that I thought it would be shipping along with a CD of cellular automata-cum-gregorian chant music, and I got no such thing :’-(

    You can order Iolet: Music from the World of Anathem from CDBaby. (Apparently the CDs only went out for free with the Advanced Reading Copies.) Proceeds are donated to the Long Now Foundation. My copy shipped today. :-)

  17. #17 Roberto Szabo
    November 24, 2008

    Thinking only one universe or world or history is as difficult for me as thinking in non-infinite universes. Matter is finite, in a sense that exists a finite number of possible arrangements of particles, atoms, molecules, cells, thoughts etc. So, in some place or some time or some dimension the “arrangements”, the events, the thoughts will begin to repeat but in different alternatives or, in other words, in those different probabilities or histories that didn’t happen for us, for probability is a human concept. So, an event more likely to happen, will happen in most “universes”. This is not weird. Weird is the “entanglement” of the “universes” as David Deutsch wants to proof, and that I think he will get.

  18. #18 Jim
    January 20, 2012

    If people realized infinite universes led to Hitler eating his own poop while he declared war they might realize its just scientists admitting the odds for life are just so high they have know theorized something a philosopher would dismiss in 2 seconds.
    These guys have no imagination and have left their field of expertise. Accept the odds as they are and where they lead and stop trying to be priests of the universe. Your not terrible at it.