New evidence for the multiverse—and its implications

As skeptical as I am, I think the contemplation of the multiverse is an excellent opportunity to reflect on the nature of science and on the ultimate nature of existence: why we are here…. In looking at this concept, we need an open mind, though not too open. It is a delicate path to tread. Parallel universes may or may not exist; the case is unproved. We are going to have to live with that uncertainty. Nothing is wrong with scientifically based philosophical speculation, which is what multiverse proposals are. But we should name it for what it is.

— George Ellis, Scientific American, Does the Multiverse Really Exist?

Well, Ellis’s uncertainty may not be permanent. This short film, on the “skydivephil” playlist, presents what they say is evidence for a multiverse. It was sent to me by reader Phil, who I believe is the eponymous creator .

A multiverse is a collection of all multiple, parallel universes; and in the set taken together, a whole panoply of different things happen: many alternative outcomes are instantiated somewhere. The idea of a multiverse first came from Erwin Schrödinger, and for a long time physicists thought that a multiverse was possible but impossible to test, as there was no way we could detect the presence of universes other than ours. The Wikipedia link two sentences prior gives a good summary, as does the video at the bottom.

Now, according to this video, we’ve gotten some evidence for the multiverse, though our Official Website Physicist™ notes (see below) that the new evidence isn’t terribly decisive. The evidence adduced is cosmic inflation, but not just that: eternal cosmic inflation, in which space grows forever. One of the implications of eternal inflation is, according to some (but not all) physicists, the multiverse.

The Physics Man who presents the results below is George Efstathiou, a British physicist at Cambridge.

When I saw this, realizing that it was above my pay grade, I wrote to Sean Carroll, our Official Website Physicist™, asking him this:

Does eternal inflation really constitute evidence for a multiverse? I know you favor multiverses, but I want to know how strong the evidence is. If you want to give me a quote to post, I’d be delighted to do that, but the most important thing is that I understand what this is about.
Sean responded, and I quote him with permission:
Of course it depends on what you mean by “evidence.” In a Bayesian sense, yes: there is experimental evidence that favors inflation (e.g., in temperature fluctuations in the cosmic microwave background), and theory predicts that most inflationary models lead to eternal inflation and a multiverse, so in that sense there is evidence for a multiverse. But not in a direct, empirical sense, of course: everything we see in the observable universe is also completely compatible with ours being the only universe. And even the indirect evidence is quite weak; we don’t know for sure whether inflation happened, nor if it really does create a multiverse. So one’s credences for or against the multiverse shouldn’t be very close to 0% or 100%, they should be somewhere in between.
 The YouTube notes present a similar caveat:
A note of caution. In our opinion inflation is the dominant paradigm for early universe cosmology and most experts in inflationary cosmology seem to agree it leads to a multiverse. Does the mounting evidence for inflation then mean we should accept the multiverse? Well, inflation has passed every test to date but there is still one last hurdle and it may fail at this last test. It’s also possible that we haven’t understood inflation correctly. We need to wait and see if more data can give us a firmer picture of these fascinating questions. Whilst the evidence for inflation and the multiverse then may not be strong enough to call them facts, the statement that there is no evidence at all for these concepts looks dubious.
 Finally, here’s a list proponents and skeptics from Wikipedia, and there are Big Names on both sides:

Proponents of one or more of the multiverse hypotheses include Stephen Hawking, Brian Greene, Max Tegmark, Alan Guth, Andrei Linde, Michio Kaku, David Deutsch, Leonard Susskind, Alexander Vilenkin, Yasunori Nomura, Raj Pathria, Laura Mersini-Houghton, Neil deGrasse Tyson, and Sean Carroll.

Scientists who are generally skeptical of the multiverse hypothesis include: Steven Weinberg, David Gross, Paul Steinhardt, Neil Turok, Viatcheslav Mukhanov, Michael S. Turner, Roger Penrose, George Ellis, Joe Silk, Carlo Rovelli, Adam Frank, Marcelo Gleiser, Jim Baggott, and Paul Davies.

Now the multiverse has lots of implications for our views of physics, philosophy, and biology. Depending on how you conceive of a multiverse (and there are apparently several ways it could be), the anthropic principle—that the laws of physics seem “fine tuned” for our existence—is simply a result of different universes having different laws of physics, and the one with the “right” laws is the one that is ours, the one that allowed life to evolve. This, of course, blows the “fine-tuning” argument, and its supposed use as evidence for God, out the window. There are, of course, other implications for stuff like quantum entanglement, Schrödinger’s Cat, and other physical puzzles: many different outcomes would be realized in one universe or another. The cat would be dead in some universes, but alive in others.

Further, it means that the evolution of humans was inevitable somewhere. In one of those universes that permitted the evolution of life, it was inevitable that a thinking hominin would evolve. That, too, is evidence against theistic arguments—made famous by Simon Conway Morris—that the evolution of humans, which is taken as inevitable, is evidence for our position as God’s special creatures.

Finally, it may (and I’m not sure about this) constitute evidence for “you can choose” free will: that all possible decisions that could be the outcome of the laws of physics in our brain would be instantiated in some universe. [Rethinking this, I don’t think this buttresses “you can choose otherwise” free will unless it reflects quantum phenomena in the brain, which I don’t think is the case.]

Now I’m just speculating here, and these may not follow from any conception of the multiverse, but from what I’ve heard of the “many worlds” hypothesis, these things are possible.

If you want to watch the entire one-hour video from which the above is an excerpt, I’ve put it below.

94 Comments

  1. ThyroidPlanet
    Posted December 10, 2017 at 9:20 am | Permalink

    Sub

  2. BJ
    Posted December 10, 2017 at 9:21 am | Permalink

    Why is eternal expansion considered evidence for the multiverse theory? Is it because eternal expansion would be a necessary function of multiverses, as each moment that passes requires new universes to exist in order to account for the exponentially growing number of possible permutations of events? If not, what is the connection between cosmic inflation and the multiverse theory?

    Obviously, I am not a physicist 🙂

    • Posted December 10, 2017 at 9:27 am | Permalink

      Don’t ask me–ask a physicist here or Sean Carroll.

      • BJ
        Posted December 10, 2017 at 12:22 pm | Permalink

        It was a query for everyone 🙂

    • Posted December 10, 2017 at 9:49 am | Permalink

      Consider an explosion in a vacuum. Once a particle separates itself from others, it should proceed outward at a constant velocity (there is nothing to slow it down or speed it up). Studies now seem to indicate that the expansion of the universe is accelerating, that is the particles freed are going faster and faster over time. We have no way to explain this traditionally (vis Newtonian or Quantum mechanics) so the reasons, necessarily must be weird.

      Since the Big Bang claims that space and time were created at some point, then space seems to be something that can expand. If we are measuring the positions of starts and galaxies in this space, how can we know the motion is due to the objects and not due to the space they are moving through?

      I am holding fire on most of these concepts (dark energy, accelerated expansion, dark matter, etc.) as they are highly speculative, based upon data that is still far from confirmed in its interpretation. We seem to be learning new and fascinating things daily, so making interpretations as if we were in charge of the situation is misleading at its worst and probably useless at its best.

      • Posted December 10, 2017 at 10:05 am | Permalink

        Consider an explosion in a vacuum.

        It is very wrong to consider the Big Bang as akin to an explosion in pre-existing space.

        If we are measuring the positions of starts and galaxies in this space, how can we know the motion is due to the objects and not due to the space they are moving through?

        In the cosmological expansion, galaxies are *not* moving through space. The space is expanding and galaxies are carried long with it.

        • Posted December 10, 2017 at 10:56 am | Permalink

          The view that cosmic red-shift is an observation of expanding space itself has been around for a long time, but I thought it later became kind of an appendage to the multiverse model. Most everywhere, space is expanding at inflationary speeds, but here and there a bubble universe drops out of the inflationary space, with considerably slower expansion as much of the former energy is converted locally into matter and heat.
          In this view, galaxies are moving apart because they are moving apart in space, not because local space itself is expanding. Of course many galaxies are moving toward each other.

          • Posted December 10, 2017 at 11:03 am | Permalink

            but here and there a bubble universe drops out of the inflationary space, with considerably slower expansion as much of the former energy is converted locally into matter and heat.

            Yes, the inflaton field that was driving inflationary expansion evaporates into energy.

            In this view, galaxies are moving apart because they are moving apart in space, not because local space itself is expanding.

            No, it is still the latter. In the normal-state bubble, the space is still expanding, but at a much reduced rate.

            Of course many galaxies are moving toward each other.

            Not on cosmological scales, only as local perturbations about the cosmological expansion.

            • Posted December 10, 2017 at 1:32 pm | Permalink

              What evidence is there that the space of our universe is expanding? I know of none. I don’t see how red shifts are about that.

              • Gregory Kusnick
                Posted December 10, 2017 at 1:37 pm | Permalink

                That’s what dark energy is about. If the expansion we see were due just to galaxies moving through space, we would expect to see them slowing down as gravity pulls them back together.

                But that’s not what we see. We see them speeding up, because apparently there’s something (“dark energy”) pushing space itself apart faster than the gravitational deceleration.

      • eric
        Posted December 10, 2017 at 7:42 pm | Permalink

        AIUI…

        Under inflation, our universe formed from some bit of inflating pre-universe that stopped inflating. But our entire universe was just one small bit of that larger cosmos. Now if that cosmos is finite and the ‘stop inflating and form a universe’ condition is highly improbable, then maybe our bit is the only bit that stopped inflating and formed a universe. However under eternal inflation, that pre-universe would be infinite in scope. Which means no matter how improbable the “stop inflating and form a universe” condition is, there will be infinite bits of that cosmos that go through it and thus infinite universes.

      • Posted December 10, 2017 at 11:02 pm | Permalink

        The existence of dark energy, accelerated expansion, and dark matter are hardly speculative. The light from faraway objects is redshifted, and they get redder and redder the further we look, and the redness is increasing for each set distance we traverse. That means objects are moving away faster and faster and expansion is accelerating. Since models of gravitation can’t explain this, something must be causing it and we call the cause “dark energy”.

        There’s also an invisible halo of matter in galaxies. We know this from galaxy rotation speeds and galaxy collisions. We call this invisible matter “dark matter”.

        The observations and their explanations are demonstrable. What dark energy and dark matter are, however, are still speculative.

    • Posted December 10, 2017 at 9:58 am | Permalink

      Why is eternal expansion considered evidence for the multiverse theory?

      It’s not “eternal expansion”, it is “eternal inflation” that more or less requires a multiverse.

      In eternal inflation, most of what exists is in an “inflationary state”; localised “bubbles” then drop out of the inflationary state into the “normal state” that we see around us. Each normal-state “bubble” becomes a universe such as ours.

      Because the inflationary state expands exponentially, the amount of inflationary state increases much faster than bubbles drop out of it, therefore this process (inflationary expansion and bubbles dropping out of it) continues eternally. Hence the name “eternal inflation”.

      This process would automatically produce a multiverse (i.e. a vast number of bubble universes like ours).

      Indeed, in more or less any inflationary model, it is very hard to make one and only one normal-state universe. Nearly all variants of the model automatically produce a multiverse. This is one of the main reasons that a lot of people favour a multiverse.

      • Posted December 10, 2017 at 2:26 pm | Permalink

        Thanks, this explanation was very helpful, and explains Sean Carroll’s Bayesian conclusion nicely.

    • improbable
      Posted December 10, 2017 at 10:01 am | Permalink

      Under eternal inflation the idea is that we live in a bubble of “true vacuum” within the faster-expanding “false vacuum”. Such bubbles are formed randomly, any patch of false vacuum could decay at any moment. One such decay was what looks like the big bang to us, inside the subsequent bubble.

      Because the outside expands faster, if there are different bubbles, they will almost never collide. In the exponentially growing volume of true vacuum between them, every patch has the same chance of forming another bubble at any time, and thus it is natural to think that there will be very many of them.

      These different bubbles each seem like a whole universe to their inhabitants. So sometimes people call this scenario a multiverse.

      The separation between them however is just ordinary spatial distance. But quite a lot of it, and growing fast — fast enough that any two bubbles are soon separated by an event horizon.

    • BJ
      Posted December 10, 2017 at 12:38 pm | Permalink

      Thanks for all the great answers, everyone. So, based on what has been posted in this thread, I have another question:

      If these bubbles are new universes, is the implication that these bubbles/universes form at a rate equal to every possible permutation of reality at every given moment, and that their formation is exponential?

      To explain what I mean, allow me to give an example (using two people on our planet instead of nearly 7.5 billion): in one universe, I win the lottery, Mr. Lopez in Brazil is cutting down a tree in the same moment, and the third star from planet X is 3 billion years old. In another universe, I win the lottery, and Mr. Lopez is eating a burrito at the same moment, and the star is 3 billion years old. In another universe, I don’t win the lottery, and Mr. Lopez is moving a mattress out of his apartment, and the star is 2.5 billion years old. And so on and so on, except with each possible variation for each person and object and condition and circumstance within our universe at every single moment, and every combination thereof.

      Does the multiverse theory require that this be the case? Or is it acceptable that not every permutation of every possibility has its own universe? Can it simply be an ever-expanding number of universes, but not necessarily enough to account for all possibilities? If there must be enough for every possibility, why is this the case?

      • BJ
        Posted December 10, 2017 at 12:41 pm | Permalink

        In reading other posts in this comment section, it seems the oft-repeated idea that the multiverse theory implies every possible alternative to our existence is not at all an agreed-upon result. I guess it is more pop culture than an actual part of the theory. If this is true, you can disregard my questions.

    • Posted December 11, 2017 at 12:21 pm | Permalink

      IIRC, I think it works the same way that Newtonian mechanics unifies (say) Galileo’s, Kepler’s, Huygens’ work etc.

  3. another fred
    Posted December 10, 2017 at 9:35 am | Permalink

    There are varieties of multiverse scenarios. Rather than “parallel” universes there is also the bubble universe type where the multiverse contains many “universes” like our own born of multiple “big bangs” in a dynamic whole.

    In this scenario our “Big Bang” did not occur in empty space.

    • Posted December 10, 2017 at 9:42 am | Permalink

      In no scenario I am aware of does the Big Bang occur in empty space, well, except in the minds of creationists trying to claim the event for their gods.

      • another fred
        Posted December 10, 2017 at 11:36 am | Permalink

        “Empty space” is a loose term, but there are models where the universe (big bang) is born of vacuum energy, which is a pretty good equivalence.

  4. Posted December 10, 2017 at 9:37 am | Permalink

    I found this video quite fascinating, addressed to lay people like myself:

  5. Posted December 10, 2017 at 9:41 am | Permalink

    I find the “anthropic principle” quite irritating. A pre-condition for the stating of this principle is the existence of intelligent life. If intelligent life did not exist, then the principle would not exist. (The principle does not, in fact, hold up in any case.)

    I am reminded of the physicist who suggested the puddle that became conscious. It found the depression in the ground in which it resided to be exactly perfect for holding its shape, etc.

    Similarly, the question of “Why am I here?” or “Why is there life?” is answered by “if you were not or there were not, there would be nobody to ask the question, so the question is moot, at best.

    Questions beginning with “how” are valid. Questions starting with “why” presuppose reasons that do not necessarily, and quite probably do not, exist.

    • Posted December 10, 2017 at 11:10 am | Permalink

      I am with you on that, but I had also felt that the whole idea has been a bit muddled with various strong and weak versions that confuse the picture.
      Another thing that bugs me (though I lack the creds to defend why) is the view that in a multiverse there are different versions of you and me. That always seemed a product of imagination rather than reasoning. But I could be just overly-sceptical on that.

    • Eduardo
      Posted December 10, 2017 at 11:33 am | Permalink

      Very nice.

    • danstarfish
      Posted December 10, 2017 at 12:17 pm | Permalink

      The puddle story is from Douglas Adams. https://www.goodreads.com/quotes/70827-this-is-rather-as-if-you-imagine-a-puddle-waking

    • Torbjörn Larsson
      Posted December 10, 2017 at 2:18 pm | Permalink

      Since the “anthropic principle” is in its well defined sense only a description of unavoidable selection bias, *I* find unsupported, conflated claims that it does not hold up irritating. How could it not,it is something we often have to account for!?

      But I have to give you props for trying to come up with a philosophic – so factually empty – framing of why (how?) it (in its well defined and oft used form) does not exist because of its own existence (selection bias). That was doubly irritating.

  6. Posted December 10, 2017 at 9:54 am | Permalink

    I’m not very convinced of the theory of the multiverse, at least not in the sense that there could be an opposite or divergent decision in any other universe for any decision here.
    Because there is not one single imaginable state that deviates from my present state, there are infinite possibilities of deviation, of difference.
    I wonder if the theory of inflation of the universe, not parallel to an inflation of the ego in the unconscious has taken place, that is: there is even now a scientific theory for the realization of narcissistic size needs or at least for the hope of immortality.

    • Gregory Kusnick
      Posted December 10, 2017 at 12:54 pm | Permalink

      If the bubble universe are of finite size, then then they can contain only a finite amount of information, and therefore there cannot be an infinite variety of them.

      But if inflation is eternal, then the number of such universes produced is infinite, so all possible instances must be produced repeatedly, and those instances necessarily include pairs that differ from each other in every possible way, no matter how trivial (as well as pairs that don’t differ at all).

      • Posted December 11, 2017 at 12:23 pm | Permalink

        I’m skeptical of the “everything must occur” claim. Measure zero sets are *really* weird to think through in this context. If the “multiverse” is not continuous, there are none to worry about, but there are some who regard that question as still open.

  7. peltonrandy
    Posted December 10, 2017 at 9:54 am | Permalink

    sub

  8. Posted December 10, 2017 at 9:55 am | Permalink

    There are two entirely different multiverse concepts. The first is the quantum many worlds interpretation, and the second comes from general relativity. When you speak of Schrodinger’s “all alternative outcomes are instantiated” and free will you probably mean the quantum concept, but inflation is about the GR concept.

    I’ll explain both concepts, and why they’re different.

    In quantum mechanics one can only calculate the probability that an experiment will lead to a particular outcome. The probability that we see outcome X is calculated as follows:

    1. You make a list of all the possible ways that the particles in the experiment could interact to give outcome X.
    2. You assign each of these possible ways a probability amplitude according to the rules of the theory, e.g. quantum electrodynamics.
    3. You sum up all the probability amplitudes, and take its length squared to find the probability that X happens.

    The probability that X happens involves contributions from all possible ways that the unobserved particle interactions could have lead. This leads to the many worlds interpretation in which we promote this calculation method to philosophy and say that all the possible ways did happen.

    If we were summing probabilities rather than probability amplitudes then we would not be lead to this interpretation, because we could simply say that one of the possibilities *did* happen, although we don’t know which one. This interpretation works for probabilities but not for probability amplitudes, because probability amplitudes can cancel out. It is possible that X can happen in two ways A and B, and these two ways have probability amplitudes a and b. The total probability amplitude is a+b. The thing with probability amplitudes is that they can be negative, so a+b can be zero even if a and b are not zero. This means that if X can happen in two different ways with opposite probability amplitudes, then the total probability for outcome X can be zero. We can thus no longer say that either A or B happened, because then we would see outcome X with some positive probability. If our universe is being simulated then it must be the case that the computer that is simulating our universe is simulating both A and B in parallel, because it must calculate the amplitudes for *both* A and B to know whether the amplitudes cancel out, and calculating the probability amplitude for both A and B is precisely what it means to simulate both A and B. In that sense, both A and B “happen”.

    Perhaps this philosophical interpretation is wrong (or not even wrong), but the calculation method (1,2,3) is definitely not wrong, because it leads to predictions that agree with experiment to many significant digits.

    The general relativity multiverse is very different. If you take off in a spaceship and fly far away from the earth it is difficult to return because the space between your spaceship and the earth is expanding. If you fly sufficiently far away then the space in between is expanding so fast that you can’t return even if you flew with the speed of light. In other words, if you took a flashlight and pointed it in the direction of the earth, the photons from the flashlight would never arrive at earth. Since nothing from the earth can have any effect on your spaceship and nothing from the spaceship can have any effect on the earth, they are for all practical purposes different universes. Now imagine our universe with its different galaxies. The stars within each galaxy are held together by gravitation well enough to counteract the effect of expanding space, so even if space expands they will stay together. But different galaxies are not bound to each other, so if the space in between expands you can end up with a universe in which no galaxy can possibly communicate with any other. Each galaxy has become its own island. If a civilisation developed in such a galaxy it would appear as if that galaxy was the entire universe. This is the kind of multiverse of inflation.

    • Posted December 10, 2017 at 10:00 am | Permalink

      p.s. to clarify that last point, the multiverse idea is that this separation into multiple causally distinct regions already happened during the big bang, if space was expanding really fast back then.

    • phoffman56
      Posted December 10, 2017 at 11:02 am | Permalink

      It’s good that you pointed out that often, including here towards the end of the article, there is some confusion between two speculative aspects of basic physics:

      (1) What the article began with and is mainly about; namely, that multiverse which is a logical consequence of many (most?) later developed versions of the astrophysical inflation hypothesis, originally from around 1980, due independently to USian Guth and Russian Starobinski.

      (2) The many-worlds, or Everett, interpretation of quantum mechanics due to Wheeler’s graduate student from the 1950’s (Everett was unable or unwilling to continue long as an academic and died awhile back). I think maybe the reference to Schrodinger is to this, as well as that ‘multiverse’ (not the usual term for it) confusingly occurring near the end of the article.

      I know there has recently been a very little supported theory that (1) and (2) are closely related, but historically they have been entirely separate, and clearly Sean Carroll’s remarks refer to (1).

      If I’m wrong on the above, it would be great to get a reference.

      Maybe a physicist will correct me, but I think your description of (1) is somewhat misleading in that it is not considered an astrophysical consequence of GR, but definitely goes beyond and depends on inflation theory. The almost certainty, that the visible universe extends quite considerably beyond what is ‘visible’, is an almost completely non-controversial aspect of GR astrophysics.

      On the other hand, it seems to my amateurish physics-mind that the objection by Penrose to inflation, which strongly involves the difficulties connected with need for a very low entropy of the early universe and some need for far better understanding of the (increase of entropy) 2nd law of thermodynamics, and also includes his cyclic universe alternative, must surely raise important questions. I need to be agnostic on much of Penrose’s ‘a professor is defined to be someone who thinks otherwise!’ tendency to be a contrarian. But it still seems that all his ideas need very serious consideration, perhaps more than many have gotten so far.

      • Gregory Kusnick
        Posted December 10, 2017 at 12:37 pm | Permalink

        Max Tegmark has a taxonomy of multiverse theories. What he calls the Level 1 multiverse is the GR multiverse described above by Jules, i.e. the existence of causally disconnected volumes within our bubble of spacetime.

        Tegmark’s Level 2 multiverse (if I’m remembering his numbering scheme right) is the inflationary multiverse described in the video: distinct bubbles of “normal” spacetime condensing out of a sea of rapidly inflating superheated vacuum. In this model, what we see retrospectively as the Big Bang corresponds to the moment our bubble condensed out, so there are universes that diverged from us before the Big Bang.

        Tegmark’s Level 3 multiverse is the “many worlds” of Everettian QM.

        He also adds a Level 4 multiverse of pure mathematics, but we needn’t get into that here.

        • phoffman56
          Posted December 10, 2017 at 5:17 pm | Permalink

          As a level 4 person, I believe that I myself am fundamentally nothing but a mathematical sub-structure of the solar system sub-structure, of the Milky way sub-structure, of some mathematical structure which deserves to be called every’thing’ which exists–universe, multiverse, whatever– except for a minor problem of whether its spatial or spatio-temporal I think I’m talking about.

          But don’t ask for either mathematical proof or empirical verification.

          Tegmark is Plato on steroids, at least he was before withdrawing to the computable structures.

          • Posted December 11, 2017 at 12:25 pm | Permalink

            Restricting to computability helps “a little”, but since there are undefined numbers of logics and set theories and so on, even in the computable context, I am not sure he gets out of the woods.

        • Posted December 11, 2017 at 12:24 pm | Permalink

          There’s also Storrs McCall’s “objective branch attrition” model, which is similar to but distinct from the MWI.

    • BJ
      Posted December 10, 2017 at 12:52 pm | Permalink

      This was very helpful. Thank you.

      So, if I understand (1), it’s not that all permutations of every possible circumstance and event (and combinations thereof) must have their own universes, but merely all possibilities that can lead to those circumstances and events must be carried out in different universes? If particle A ends up in location X, and there are five ways it could have arrived there, there would be a universe for each of the five ways it could occur, but not a universe for the particle ending up in location Y instead of X?

      • Gregory Kusnick
        Posted December 10, 2017 at 2:45 pm | Permalink

        I think perhaps you’re looking at it backwards. It’s not that the outcome X dictates the number of universes that can or must exist. Rather, the particle takes all trajectories open to it. The ones that terminate at X contribute to the probability of X, and the ones that terminate at Y contribute to the probability of Y. The way in which we calculate those probabilities suggest that all the possible trajectories are equally real, including ones that terminate at neither X nor Y.

        So there’s no cosmic accountant keeping score and checking off boxes to enforce some rule about what universes must exist. It’s just that the cumulative effect of particles exercising all their options adds up to an exhaustive exploration of possibility space.

  9. Eduardo
    Posted December 10, 2017 at 10:37 am | Permalink

    Ok.Then if the multiverse theory is correct and every possible outcome takes place, does that mean that there is one universe out there where there are NOT other universes? (My son asked me that one.) Or even worse: is there a universe where gods exist?

    • Posted December 10, 2017 at 10:42 am | Permalink

      does that mean that there is one universe out there where there are NOT other universes?

      No, because in a cosmological multiverse that is not one of the “possible” universes.

      Or even worse: is there a universe where gods exist?

      I guess that, if the cosmological multiverse really does extend to infinity, then every possible evolved being would exist somewhere.

      • Posted December 10, 2017 at 11:24 am | Permalink

        “then every possible evolved being would exist somewhere.”

        Exactly such conclusions make the theory of the multiverse for me suspect: Everything that is conceivable, what humans are able to conceive, should be possible and exist somewhere, in one of countless universes. Such theories seem to satisfy a particular need, you have nothing to mourn, there are no losses, nothing is wrong, everything can be right … we can be like gods: everywhere and at the same time, in so many manifestations, the theory of the multiverse explains us all to eternal beings, to gods, something that no religion has ever reached before.

        • Posted December 10, 2017 at 11:29 am | Permalink

          Everything that is conceivable, what humans are able to conceive, should be possible and exist somewhere, in one of countless universes.

          There’s a big difference between what is conceivable and what is possible.

          • Posted December 10, 2017 at 11:48 am | Permalink

            Your objection is correct. But still: The theoretically possible has a not inconsiderable intersection with what is desirable.

      • BJ
        Posted December 10, 2017 at 12:53 pm | Permalink

        Perhaps gods are like the Q Continuum 🙂

  10. Posted December 10, 2017 at 10:48 am | Permalink

    I like that the multiverse lets you “explain” the values of the physical constants we observe as simply a matter of selection (we just happen to inhabit a universe with these values), but I’m not sure that buys you anything above and beyond the so-called “brute facts” approach (those are the values, get used to it.)

  11. Posted December 10, 2017 at 10:53 am | Permalink

    Finally, it may (and I’m not sure about this) constitute evidence for “you can choose” free will: that all possible decisions that could be the outcome of the laws of physics in our brain would be instantiated in some universe.

    All possible outcomes would take place somewhere, but there would still be no choice involved. The substrate of reality, waves, particles, fields — however we name it — would merely lead to different computations and outcomes in each.

    A multiple “you” is also an illusion that arises out of our way to categorize. We are not equipped well to cope with the idea that a pattern can be copied or exist simultaneously, with our “inside” view as one of these patterns. We have no problem to copy and paste a file and see it exists twice. But we cannot really comprehend well what it means if we ourselves existed as copies.

    My hunch is that’s like having an ultimate twin, such identical that it would freak us out if we were to meet each other. We would be in our hair because we would do things exactly the same way. An identical twin may behave differently in subtle ways (and has to, because we cannot literally inhabit the exact same time-space), but their different behaviour does not make free will.

  12. Richard C
    Posted December 10, 2017 at 10:53 am | Permalink

    I don’t think you need a multiverse for the fine-tuning argument to be ridiculous. A cosmic designer didn’t have to decree that, in the universe, pi shall now be an irrational number approximately equal to 3.14159. (Imagine if He had picked 2.5 for pi! Our bridges would all collapse! Our circles would all implode!)

    Physical constants are the result of some underlying mechanism, and aren’t arbitrary design choices you pick from a Universal paint swatch booklet.

    • Posted December 10, 2017 at 11:19 am | Permalink

      Pi can’t have a different value in another universe because it can be derived as the sum of an infinite series, which has nothing to do with the universe.

    • phoffman56
      Posted December 10, 2017 at 11:24 am | Permalink

      But \pi is a number, not a physical constant. For the latter, Ch. 2, esp. the last section,
      of Yuri Manin: “Mathematics as Metaphor” is really good.

      I assure you there is lots of talk about basic physics in there, and about all the physical constants such as Newton’s, Planck’s and the speed of light.

      I used to tell calculus students (in some, but not all, seriousness) that the best definition of \pi is the first number after 0 where a particular function again takes the value zero, that unique function also whose 2nd derivative is the negative of itself, and which has slope 1 at the origin (i.e. it’s name is ‘sin’, but contemplating it isn’t sinful!)

      You can easily deduce all the elementary circle geometry stuff from that definition.

      • phoffman56
        Posted December 10, 2017 at 11:36 am | Permalink

        ‘its’ not “it’s” on line -4

      • Richard C
        Posted December 10, 2017 at 12:15 pm | Permalink

        True, pi isn’t the same as a physical constant. My point is more that pi is derived from basic geometry, so suggesting other values for it would be nonsensical.

        Physical constants aren’t just values someone chooses either, they’re derived from some fundamental behavior of the Universe. That means (if the multiverse theory ends up being wrong), suggesting other values for them may be just as nonsensical.

        That we don’t yet understand all those behaviors seems like an argument from ignorance to me.

        I’ll have to check out that book, thanks!

        • Michael Fisher
          Posted December 10, 2017 at 1:48 pm | Permalink

          @Richard C

          There are different geometries besides the Euclidean. In one that I can think of [if we define Pi as the ratio C/2r] the value of Pi is not fixed, but varies with the size of the circle. i.e. Pi tends to 3.142… as r tends to 0 & for the maximum value of C [in that geometry] Pi is exactly 2!

          Very small beings in that reality would set Pi as the real transcendental number we are familiar with, but if they developed the means to measure to large values of r…

      • Posted December 11, 2017 at 12:28 pm | Permalink

        I am not sure that’s not question-begging in some contexts. I’d hate to formalize it, though, since you’ve “quantified over” functions there, which is messy 😉

        (I’m a mathematical factionalist so it isn’t the existence claims that bother me, though.)

    • Ralph
      Posted December 10, 2017 at 1:49 pm | Permalink

      We’ve covered this ground before, but I think it’s misleading to suggest (as Jerry often does) that there is no fine tuning problem in cosmology.

      “Problem” here does not something with no conceivable resolution other than a creator. But although may all agree that the resolution to the problem is not “fine tuning therefore Jeebus”, it is wrong to deny that there is a real scientific problem with apparent fine tuning for which we do not yet know the correct resolution.

      Your last paragraph is precisely the reason that there IS a problem. Despite decades of theoretical work, nobody has come up with a viable cosmological model in which the parameters are constrained to those that prevail in our universe by “some underlying mechanism”. That’s the whole point – that’s why there’s a problem. All the models we have seem to allow a vast range of parameters for a universe, most of which are incompatible with life.

      Now, it may still be true that we have yet to discover such a model, that it’s a modeling problem, it’s down to our theoretical shortcomings. But if not, if it turns out that parameters are not constrained in this way, and that they can indeed take a very wide range of values with equal probability, the vast majority of which are incompatible with intelligent life, then the multiverse and the anthropic principle is an alternative possible resolution to the problem.

  13. mikeyc
    Posted December 10, 2017 at 11:53 am | Permalink

    I’m not a physicist so don’t have any idea the likelihood of the Multiverse but part of me is glad that if it’s true then somewhere my life turned out differently than it did.

    • claudia baker
      Posted December 10, 2017 at 12:35 pm | Permalink

      And in the multiverse, I could understand this stuff without my head hurting. I watched the 2 videos, and tried very hard to concentrate, but it is so above me, that I just felt depressed at the end. In another universe, I am a super-smart physicist, discussing these ideas with confidence and have the respect of my colleagues. In this universe, I am knitting a pair of warm wooly socks and planning my christmas baking. Sigh.

      • BJ
        Posted December 10, 2017 at 12:59 pm | Permalink

        Maybe in another universe, you’re the one who figured all this shit out and are celebrated the world over for all the new technologies your teachings have opened to humanity, which have allowed you to become an immortal being of pure consciousness who manipulates events by some quantum dark magic, like the Q Continuum.

        • claudia baker
          Posted December 10, 2017 at 2:42 pm | Permalink

          “an immortal being of pure consciousness” – so, I am a Buddhist in the multiverse? Awesome! 🙂

      • Posted December 10, 2017 at 7:32 pm | Permalink

        I would like to tag along with you, Claudia. I don’t have the education or ability to hang out with our compatriots, but do so appreciate the information they share.

        My hope is that life forms on other planets in other universes treat their homes and fellows better than we have. If we mess up this planet and/or other planets in “our” universe, I don’t imagine a visit by us to other beings on other planets in another universe would call forth cookies or a baked cake to greet us.

        • claudia baker
          Posted December 10, 2017 at 8:08 pm | Permalink

          No, and who could blame them?

    • Craw
      Posted December 10, 2017 at 8:28 pm | Permalink

      Like the universes where you are a rabid Trump supporter? 😉

  14. Posted December 10, 2017 at 1:35 pm | Permalink

    From what you quote here, it sounds like the evidence is very flimsy. I think it goes like this:

    – A recent observation adds support for inflation theories.

    – Inflation theories often (always?) hypothesize a multiverse.

    – Therefore multiverse has been boosted in likelihood.

    • Gregory Kusnick
      Posted December 10, 2017 at 1:44 pm | Permalink

      Your second point isn’t quite right. It’s not that inflation theories hypothesize a multiverse; it’s that they typically entail a multiverse. That is, if the basic theory is right, then there’s no way to avoid producing a multiverse.

      • Gregory Kusnick
        Posted December 10, 2017 at 1:44 pm | Permalink

        Whoops, italic fail.

        • Mark Joseph
          Posted December 10, 2017 at 7:50 pm | Permalink

          Actually, the italics work pretty well there!

      • Posted December 10, 2017 at 5:54 pm | Permalink

        As far as science is concerned, aren’t things always hypothesized? Many a scientific theory that just had to be true turns out not to be. Yes, I know it is mathematics but that kind of proves my point. Math is not observation. Until someone finds direct evidence for at least one other universe, the multiverse is merely a hypothesis.

        • Gregory Kusnick
          Posted December 10, 2017 at 6:12 pm | Permalink

          Is the common ancestor of chimps and humans “merely a hypothesis”? Theory says it must exist, but we have no direct evidence of it.

          • Posted December 11, 2017 at 12:24 pm | Permalink

            You are right. Hypotheses are generally not equal in terms of the evidence we have for them. Are you suggesting that the evidence for the multiverse hypothesis is as strong as that of the human-chimp-common-ancestor hypothesis?

            • Gregory Kusnick
              Posted December 11, 2017 at 1:30 pm | Permalink

              The point is that neither is an independent hypothesis that stands or falls on its own. Both are logical consequences of well-attested theories.

              • Posted December 11, 2017 at 2:00 pm | Permalink

                Your attempt at making the two hypotheses (monkey, multiverse) equivalent in evidence and confidence is a failure, IMHO. Let me try a different angle.

                – The definition of “indirect evidence” is simply “not direct evidence”.

                – We do not have any direct evidence of the multiverse and definitely not in the research which is the subject of this post.

                – Indirect evidence should give us less confidence in a hypothesis than direct evidence.

              • Gregory Kusnick
                Posted December 11, 2017 at 2:24 pm | Permalink

                Your second point is correct, but your third point doesn’t follow from it.

                If theory T makes predictions P and Q, and we have direct evidence for P that gives us high confidence in the correctness of T (and there are no rival theories that predict P but not Q), then we have equally high confidence in Q.

                In any case, I’m not attempting to claim that the multiverse and the human-chimp common ancestor are equally well attested. Sorry if I gave that impression. My only claim is that you don’t get to accept the evidence for inflation but reject the multiverse as a separate hypothesis for which you require additional evidence. Evidence for one is evidence for the other, because they’re both part of the same theory.

              • Posted December 11, 2017 at 5:07 pm | Permalink

                I think you have gotten to the root of our argument. I don’t agree with your claim about theory T and predictions P and Q. If there are multiple theories that all attempt to explain inflation (and there are, right?), then it weakens this claim, even if all such theories contain a multiverse prediction. The fact that there are so many theories that include inflation indicates there are probably many more yet to be discovered, some of which may not predict a multiverse.

                In short, it just seems unreasonable to conclude that evidence for part of a theory can be used as evidence for another part of a theory. The best that can be concluded is that the new evidence doesn’t hurt the multiverse prediction.

              • Gregory Kusnick
                Posted December 11, 2017 at 6:33 pm | Permalink

                I don’t want to beat this to death, so I’ll just finish by saying that to me, “T predicts Q” means that T logically entails Q, so that our confidence in Q cannot be less than our confidence in T, whether or not we have any direct evidence for Q. I don’t see that there’s any wiggle room there for disagreement, but if you think you’ve found some, I won’t dispute it further.

          • Posted December 11, 2017 at 12:29 pm | Permalink

            It isn’t *merely* a hypothesis, it is a well supported one. The way I think about that is the “imagine it is wrong” question – what else what would be affected?

  15. Torbjörn Larsson
    Posted December 10, 2017 at 2:07 pm | Permalink

    Thank you, very interesting find! I know that Efstathiou has a refreshing view on the evidence for inflation and now, apparently, on the evidence for eternal inflation and its natural consequences. Though I got a ´headache when the post mushed together inflationary multiverses with other types that are *not* equivalent, which commentators have already pointed out before me.

    I have to agree with the video site that there the claim that there is no evidence for a multiverse is dubious. But I would quibble with the Official Website Physicist™ that the evidence is bayesian. I have learned in bioinformatics that bayesian probabilities are optimistic, and that one can reasonably use bootstrap sampling of models to get maximum likelihoods (ML). Sampling eternal inflation, as we now understand it, the ML evidence would presumably be an unbounded number of pocket universes. And as always I ask from those who likes to describe evidence as ‘direct’ and ‘indirect’ – such as Carroll does here – for a testable definition that can discern the type. Preferably one that I can use within a hypothesis testing paradigm. It is not something you can find in textbooks as far as I am aware!*

    *I suspect that ‘direct evidence’ means “evidence I personally like” and ‘indirect evidence’ means “evidence I personally do not like”. But I may be wrong. Alas, without a testable definition, how would I know either way?

    • Torbjörn Larsson
      Posted December 10, 2017 at 2:09 pm | Permalink

      “I have to agree with the video site that there the claim that there is” – I have to agree with the video site that the claim that there is.

    • Posted December 10, 2017 at 5:59 pm | Permalink

      Certainly, what is considered “direct evidence” is somewhat subjective but surely “multiverses are one part of my theory and this observation supports another part, so all parts of my theory must be equally true” is a classic example of indirect evidence.

  16. jimroberts
    Posted December 10, 2017 at 3:57 pm | Permalink

    As far as the probability of humans evolving is concerned, I do not see that a multiverse is any better than a single infinite universe. An infinite number of infinite universes offers no more than one infinite universe, as a brief consideration of Hilbert’s hotel shows.

  17. Alan
    Posted December 10, 2017 at 4:29 pm | Permalink

    I wonder who are typically better mathematicians, physicists or population geneticists?

    • Craw
      Posted December 10, 2017 at 8:30 pm | Permalink

      In which universe?

  18. chris moffatt
    Posted December 10, 2017 at 4:54 pm | Permalink

    Well if doG wanted thinking(?) hominims surely all he had to do was make a doll out of spit and dust and breathe life into it. Simples. Then take a rib from this thing and make another one but different. Why do physicists haver to make this so complicated?

    On a serious note if Steinhardt and Turok don’t buy it neither do I.

  19. Mark Joseph
    Posted December 10, 2017 at 7:52 pm | Permalink

    “When the cosmos wakes, if ever she does, she will find herself not the single beloved of her maker, but merely a little bubble adrift on the boundless and bottomless ocean of being.” (Olaf Stapledon, Star Maker)

  20. Mark Joseph
    Posted December 10, 2017 at 7:52 pm | Permalink

    What would the collective wisdom of this web site recommend as the best recent book on this topic for an educated layman? Thanks in advance.

  21. Jimbo
    Posted December 10, 2017 at 8:27 pm | Permalink

    I really don’t grasp how contracausal free will can be smuggled in via quantum indeterminancy. While the quantum can introduce randomness “all the way down” such that prior arrangements of matter may be altered in a causal chain of deterministic events, I fail to see how the helps the free will case. My simplistic thinking is this: if humans have free will but animals don’t (overstated but hear me out) and diseased or neurologically damaged human brains lose free will (e.g. a vegetative state), nothing at the quantum level has changed. It is either operative in humans or not. And why would the brain of say a raccoon, made from the same protein machinery as our own brains, not have free will? What am I missing?

    • Gregory Kusnick
      Posted December 10, 2017 at 11:27 pm | Permalink

      No sensible person (that I know of) claims that quantum indeterminacy grants us magical, libertarian free will.

      What it does do is make it clear that anyone who wants to claim that choice is an illusion must find a better argument than “you could not have done otherwise” or “only one outcome was physically possible”.

      Amending the argument to “you could not have chosen otherwise” doesn’t help, because that simply assumes the conclusion: that choice is not a thing. (And shifting the emphasis to “you could not have chosen otherwise” smuggles in dualism by defining “you” as something separate from the chooser.)

      So now we’re left with the question of what we actually mean when we talk about choice. To me the most sensible answer is that choice is what we call the kind of calculation the brain does when evaluating alternatives. Whether the outcome is completely determined or has some irreducible element of randomness is beside the point; the question of whether calculation counts as “real” choice has to be decided on some other basis.

  22. Posted December 10, 2017 at 11:14 pm | Permalink

    I really don’t think we need a multiverse to defeat the fine-tunibg argument. Imagine that our universe had entirely different laws from the outset. If intelligent self-replicating chemical systems evolved, the universe necessarily has to be “fine-tuned” for the intelligent observers.

    It just happens that the universe had our constants and humans evolved (instead of something else).

    • Gregory Kusnick
      Posted December 10, 2017 at 11:53 pm | Permalink

      The question is, how likely is it that a random set of physical laws permit intelligent self-replicating chemical systems to evolve? If it’s very unlikely, then being such a system is like winning the lottery.

      Now we have to ask how likely it is for there to be a winner in that lottery. If only one ticket was sold, and it turns out to be the winning ticket, our suspicions are aroused that maybe the game was rigged. But if a million tickets were sold, it’s no great surprise if one of them turns out to be a winner.

      So you need the multiverse to make the fact of a winner unsurprising and remove suspicion that the game is rigged.

  23. Posted December 11, 2017 at 4:54 am | Permalink

    My brain hurts.

  24. Faze kingslayer
    Posted December 11, 2017 at 7:05 pm | Permalink

    IF there are other universes there’s a very high chance they exist ok the other sides of black holes. But humans theoretically cannot survive traveling through a black hole to find out the truth. Seemingly black holes functions are to distribute space matter from one universe to another and possibly give birth to new stars or spit matter that will collect more and more until big enough to form asteroids or stars possibly planets.

    • Michael Fisher
      Posted December 11, 2017 at 7:36 pm | Permalink

      Can you hook me up with your dealer? He moves quality product!

      There is a respectable, fairly old idea that the other ‘side’ of a black hole is a new, growing baby universe fed by the black hole, but you certainly can’t claim “there’s a very high chance they [new universes] exist on the other sides of black holes” – we simply don’t know.

    • Posted December 12, 2017 at 11:55 am | Permalink

      If another universe can be contained inside a black hole, and information is conserved such that it eventually leaks outside the hole, we can theoretically detect the (former?) existence of that other universe. Not sure what we should be looking for though.

      • Posted December 12, 2017 at 1:06 pm | Permalink

        Unless, as some astrophysicists have posited, black holes are gateways (wormholes, perhaps) or passageways to other universes, through which matter is transferred.


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