Are Many Worlds and the Multiverse the Same Idea?

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 recently 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.

17 thoughts on “Are Many Worlds and the Multiverse the Same Idea?

  1. ive thought a fair amount about shrodingers cat. what i find infuriating about the experiment is that the ‘timing’ of the decay of the isotope is assumed to be ‘unknown’ and i think that is just a flaw in the setup.
    radioactive isotopes have a ‘half-life’ – its a ‘time’ that is required for ‘half’ the material to decay to daughter states.
    the thing is that i strongly feel that if more was ‘known’ about the system – what level of energy an individual atom was ionized by, for example. i think a better prediction could be made about the decay ‘time’ of said atom.
    it seems likely that an atom can ionize by a _variety_ of energy inputs – resulting in ‘radioactive’ atoms at a _variety_ of energy states. that would explain the half-life idea to me more perfectly.

  2. Really appreciate the comment, although it sounds like you know more about physics than I do. I believe the Heisenberg interpretation claims that no matter how much we know we about the system we can never predict what will happen. But then again you can’t really understand quantum theory without knowing the mathematics. JGM

  3. When I followed the link to read Sean Carroll’s blog posting, the main thing that jumped out at me was the descriptive word in his URL “preposterousuniverse”. Unfortunately, I think that pretty much says it all. While Sean Carroll is a physicist at Caltech, he describes that his blog “is absolutely not a Science Blog”. There is certainly nothing wrong with that at all. But readers need to understand speculations of a scientist do not necessarily connote any scientific reality.

    The problem with all versions of the “multiverse” is that they are mostly fantasy, not science. “String theory” does not fare much better. Sabine Hossenfelder (a physicist and Research Fellow at the Frankfurt Institute for Advanced Studies), Peter Woit (a Senior Lecturer in the Mathematics department at Columbia University) and Ethan Siegel (an astrophysicist and writer for Forbes) have been notably outspoken against calling either the “multiverse” or “string theory” science. Sabine Hossenfelder does a great job at explaining her objections, for example see her blog post

    So, I think that the answer to the titled question “Are Many Worlds and the Multiverse the Same Idea?” is “Yes”. They are each just an imaginative speculation and neither has anything to do with scientific reality.

  4. Mr Conklin:
    Experiments have long established that the probabilistic behavior of subatomic particles is not the result of our ignorance of initial conditions. There is something truly weird going on that defies our macroscopic sense of cause and effect.
    Also, ionization is not the same thing as radioactive decay. Ionization is a change in the number of electrons in the outer layers of an atom, giving the atom a net electric charge (but retaining the elemental status); radioactive decay is the conversion of protons into neutrons in the nucleus of an atom which causes an imbalance that shoots neutrons out of the core and changes the elemental nature of the atom.
    I’m not trying to be pedantic … just pointing out that what you consider “obvious” does not apply to quantum mechanics. I went through such a phase of suspicion of “hidden determinism” in my own education.

  5. I disagree with what you say here Jim. When Carroll says his isn’t a science blog he is trying to carefully distinguish science from speculation based on other scientific findings. Those speculative ideas need to be further investigated. But that doesn’t mean we should conclude they are fantasy. And having objections to string theory or any other possible ideas to explain holes in our knowledge doesn’t mean they are “just imaginative speculation” having nothing “to do with scientific reality. If that were the case then atomic or relativity theory or any other theory proposed to resolve some lacuna in the current state of scientific knowledge would have been rejected as possibly being true. They too were outlandish ideas before they were confirmed. Many worlds interpretation and the multiverse are speculative ideas that may well be true, then again they may not. They are daring hypotheses about whose truth or falsity we are now in the dark. But they are not obviously false.

  6. Regarding the topic of the post:

    Using the basic keyboard on my phone, I can enter 82 unique symbols.
    Imagine a two-dimensional grid, 82 units wide but infinitely long.
    Now place a field of static (random marks) on this ribbon. At various points along this endless road are arbitrarily long stretches with only one mark per row. Most would appear to be gibberish to us, but a subset of THESE represent every comprehensible statement that could ever be typed using my keyboard, in any language that utilizes those 82 symbols.
    Now suppose that each valid sentence represents a conscious, internal experience.
    “This ten-word sentence will end with the word “…
    In my analogy, every valid completion of that sentence is a unique universe which shares a common history: a many-worlds multiverse.
    And on an infinite tape, every valid statement will reappear infinite times. A higher-order multiverse.

  7. John, you are right that “fantasy” is not the appropriate word; “speculation” would have been better. Also, I agree that there is nothing wrong with scientific speculation. As you point out, that is how some breakthroughs in knowledge happen: first by speculation and then confirmation through predictive experimentation. So, maybe we agree more than is obvious.

    Perhaps my overreaction to the essay came more from its construction under the category “Science and Philosophy”. First, the scientific credentials of Sean Carroll were presented (establishing that he was a credible scientist that should be believed regarding science stuff), and then the essay launched into discussion of many speculative ideas. “String theory” is thrown into the discussion as if it is established scientific knowledge, when in reality it has been an utter failure at supplanting the Standard Model. Had there been some sort of up-front notice that the subject matter of the essay was speculative or if the category was “Scientific Speculations and Philosophy”, then I would have better understood the objective of the essay.

  8. Mr. Rogers:

    The Everett interpretation of quantum wave collapse is testable — from the internal perspective, and with enough patience.
    I am proposing that you will have conscious experiences into the indefinite future, no matter how preposterous the circumstances seem now.
    The longer you live, the less likely it is the result of chance. At a certain point (sometime after your 120th birthday), the likelihood that you remain conscious will be as small as the chance that CERN misinterpreted the data that led to the discovery of the Higgs boson. And this improbability will only increase with each additional second you count off.
    At which point, your apparent immortality will strongly suggest inevitable survival among multiple timelines.

  9. OK, I’ll toss in my own personal weirdness. I’ve always approached QM in terms of the Uncertainty Principle rather than wave mechanics. The former is useless for calculating the behavior of subatomic particles; the latter does that job very well. The value of the Uncertainty Principle is that it establishes two crucial points clearly:

    1. We cannot know anything with infinite precision. The amount of information we can obtain about the universe is finite.

    2. There are only two fundamental laws of physics that are inequalities: the Uncertainty Principle and the Second Law of Thermodynamics. I believe that the second law is a consequence of the Uncertainty Principle, although the proof is rather involved.

    All this suggests that we’re thinking in the wrong terms. The universe is not a collection of mass-energy embedded in space-time. The universe is an information processing system; mass-energy and space-time are the observable manifestations of that information, but they aren’t the fundamental components.

    However, I have always been stymied by some elements of this concept. A photon communicates one bit of information, but it does so in an amount of time proportional to its wavelength. All matter can be converted to photons, so we can measure the “information transfer content” of the earth from its mass-energy. But that’s not information, it’s an information transfer rate. So where are the bits? Is the problem that I have to let go of the notion of time so that the concept of a “rate” vanishes?

    Yep, I’m stymied here.

  10. I followed the discussion by Chris up until his statement “A photon communicates one bit of information, but it does so in an amount of time proportional to its wavelength”. My understanding is that an individual photon travels through space at the speed of light, which is independent of wavelength. The energy carried by an individual photon is inversely proportional to wavelength (longer wavelength, lower energy).

    Possibly Chris is referring to the latency of an electromagnetic signal, which is not an individual photon but a group of photons having wavelengths clustered around the wavelength of the carrier frequency of the signal. “Latency” is the start-up time between when a signal is first switched “on” to the point that information can be extracted from the signal. Latency is proportional to wavelength: the longer the wavelength, the longer that start up time and vice versa. Latency is one of the main reasons for the development of the new 5G wireless technology, which will operate at signals having wavelengths of 1/10th to 1/20th the 4G wavelengths. Reduced latency will allow faster access to more information. Of course, the downside to 5G technology is that cell towers will need to be spaced every 250 meters or so apart (small antennas on light poles are one concept for blanketing a city with a 5G network).

    The thing I found most interesting about the comment by Chris is that my “take” on QM is almost identical to his. That is, the Uncertainty Principle really stuck in my mind, in a similar fashion described by him. Possibly we both studied QM in the same era, back in the early to mid 1970s (the “Stone Age” to you youngsters). That makes me wonder what principles of physics are sticking in the minds of the current generation. If it wasn’t for Wikipedia (which is possibly the greatest single web site on the internet), I would have no hope of deciphering current discussions of physics.

  11. Thanks for this Jim. Also, I couldn’t agree more about Wikipedia. It is one of the few sources of truth on the web.

  12. Wikipedia is the only thing I pay for on the web. I donate $5 a month to keep civilization’s intellectual embers smoldering.

  13. Wholeheartedly agree. Like the phrasing too “intellectual embers smoldering.”

  14. Giordano Bruno (1584-1600) answered your question, “Are many worlds and the multiverse the same idea?” when he said, “The end is nowhere. The center is everywhere.” In “Cause, Principle, and Unity” (1584), he said, “There is no absolute position in space, the observer is always at the center of things.”

  15. mr. arends:
    i will cede that i havent done any experimental physics – my education in nuclear physics was about thirty years ago in the nuclear navy.
    i seem to recall doing some math involving MeV tho.
    what struck me then and still strikes me today that an equation that uses (for example) a 6.3 MeV bit of energy is a little too ‘specific’
    surely there is a ‘range’ of energies that can cause the exact same _macro_atomic reaction to occur. take the decay chain from u-238 as an example.
    surely if there was enough study on individual u-238 atoms with precise enough detection arrays, we could get a better idea of the ‘actual’ energy states of the daughter nuclides and therefore be better able to predict their decays in time, no?
    the experimental problem here could be that the 4.6 billion year half-life of the original u-238 atoms would mean a considerable amount of waiting.
    perhaps we could do an experiment on an isotope with a shorter half-life?
    do you know of any experiments that you could direct me to in that regard?

  16. on a perhaps related tangent, perhaps not – where do terrestrial radioactive isotopes originate? were they gathered on the primordial earth from meteoric collisions? ive always imagined solar system formation as a ‘specific gravity’ problem – why we have large gas planets with heavier metal planets and asteroids between them and the sun.
    instead of a binary star system, for example – where there are probably also heavier metal planetoids between the stars.

    can geophysical pressures and/or temperatures cause nuclear reactions? huh. thanks internet. learn something new every day. =]

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