The Multiverse in a Nutshell

The Guardian has a podcast up today featuring Robert Trotta and David Wallace called The Multiverse in a Nutshell. It’s largely more of the usual uncritical multiverse hype that has been flooding the public expositions of fundamental physics for years now. Trotta gives the usual promotion of the cosmological multiverse, with no indication there is any problem with it. He assures us that this is being tested (by looking for “bruises” in CMB collisions). As far as I can tell, the Planck results released today, like all CMB data, show no evidence for anything like this. It appears that the Planck people don’t even think this is worth mentioning. The public channels used for this hype will never report the fact that there’s nothing there, instead they will just endlessly talk about this as something “scientists are looking for.”

Wallace talks about something completely different, many-worlds, with nobody telling listeners that this has nothing at all to do with the cosmological material. Instead we’re told that it’s all related, because “most theories” “tell us there must be a multiverse”. When challenged about the splitting universe business in QM, Wallace admits that at the fundamental level there is no splitting, there’s just one theory and one universe, that “many worlds” is just a way of talking about the emergent behavior of the classical approximation. His book about this, The Emergent Multiverse, is quite good and makes clear what the “Multiverse” there really is. It’s a real shame that he chooses to involve himself in this kind of attempt to muddy the waters and promote pseudo-science to the public.

Thankfully at least the physics community has one physicist trying to do something about this nonsense: Paul Steinhardt. In an interview with John Horgan, here’s his “Multiverse in a Nutshell”:

Unfortunately, what has happened since is that all attempts to resolve the multiverse problem have failed and, in the process, it has become clear that the problem is much stickier than originally imagined. In fact, at this point, some proponents of inflation have suggested that there can be no solution. We should cease bothering to look for one. Instead, we should simply take inflation and the multiverse as fact and accept the notion that the features of the observable universe are accidental: consequences of living in this particular region of the multiverse rather than another.

To me, the accidental universe idea is scientifically meaningless because it explains nothing and predicts nothing. Also, it misses the most salient fact we have learned about large-scale structure of the universe: its extraordinary simplicity when averaged over large scales. In order to explain the one simple universe we can see, the inflationary multiverse and accidental universe hypotheses posit an infinite variety of universes with arbitrary amounts of complexity that we cannot see. Variations on the accidental universe, such as those employing the anthropic principle, do nothing to help the situation.

Scientific ideas should be simple, explanatory, predictive. The inflationary multiverse as currently understood appears to have none of those properties.

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29 Responses to The Multiverse in a Nutshell

  1. Travis says:

    I don’t understand this type of objection to the multiverse. It’s entirely plausible that many features of the universe are in fact accidental, in exactly the same way that many features of earth and our solar system are accidental. Imagine for a moment that our solar system were completely surrounded by dust which made it completely impossible to see any other stars (for the purpose of this thought experiment, the dust is some kind of super dust which is opaque to all wavelengths). Would we be warranted, based on our knowledge of physics, in concluding that it is extremely likely that there are other stars out there, with planets arranged around them in different orbits from those in our solar system? Or would we say that such a model of the universe “explains nothing and predicts nothing” and keep searching for some deep reason why the orbits of the planets in our solar system are exactly the way that they are? The reason we would go with the “many stars” model of the universe is that theories which makes testable predictions, like the theories of gravity and nuclear reactions, also predict that similar processes should happen elsewhere in the universe. Analogously, inflationary theory which makes testable predictions like B modes in the CMB also predicts that similar processes should happen elsewhere in the larger universe. Even if inflationary theory in particular turns out to be wrong, the idea that many features of our universe are accidental is no more unscientific than the idea that the exact positions of the planetary orbits are accidental.

  2. Peter Woit says:

    Sure, it’s plausible that some features of what we think of as fundamental physics are historical accidents. It’s also plausible that these are not accidents but choices of an all-powerful deity. Neither conjecture is science though until you come up with a conventional scientific test of the idea. Until then, no matter how much hype proponents put out, Steinhardt is right that the “idea is scientifically meaningless because it explains nothing and predicts nothing”.

  3. Katya says:

    You wondered if Paul Steinhardt agreed with Edward Witten about the “vast amount of evidence for inflation in the last 20 years.” Now you have your answer:

    “…none of the magnificent observations made over the last 30 years can be viewed as supporting inflation.”
    “As just explained, it is not possible to find evidence to support or refute inflation because an inflationary multiverse includes patches with cosmic gravitational waves and without them.”

    No one more credible on this topic.

  4. Justin says:


    If I’m not mistaken, don’t you consider yourself a believer in the many worlds interpretation of quantum mechanics. If you really believe that the universe splits off into undetectable many worlds when a quantum measurement takes place, how are multiverse ideas more nonsensical?

  5. Peter Woit says:

    No, I’m not a believer in many worlds. I’ve written a bit about QM interpretations, someday should write more about many worlds. As I tried to refer to in the posting, I think I mostly agree with Wallace: fundamentally there’s only one world, no splitting, and quantum mechanics is exact. Classical behavior emerges in some limits from quantum, and if you try and say what is happening in classical terms you can do this using the “many worlds” language if you want. But this is in many ways a misleading thing to do, it just completely ignores the actual problem (how is classical behavior emerging from quantum mechanics?). If you try and take it seriously you get all sorts of problems.

    So, I’m just not particularly opposed to people using “many worlds” language if they want for some purposes, but they need to make clear what they’re doing when they do this, that there really is just one universe and fundamentally no splitting. Unfortunately many worlds fans mislead people about this, and then, even worse, go on to promote this misleading version of many worlds as related to empty pseudo-science about the string landscape, etc.

  6. Justin says:

    @ Woit,

    Thanks for clarifying your viewpoint on quantum mechanics. Indeed the “language” of many worlds can get confusing. I highly recommend you take a look at the book Consistent Quantum Theory by Robert Griffiths. It’s available for free in pdf format easily found online. It’s very much in tune with your views on quantum mechanics.

    Keep up the good work on this multiverse nonsense!

  7. GM says:

    @ Travis

    That analogy is not really very good. The features of a particular planetary system may be “accidental”, but they are not completely random, they are what they are because of simple and understood laws of physics, even if the process is extremely complicated . And it’s worth remembering that historically we actually made a lot of progress towards understanding those laws by studying such accidents.

  8. amirpouyan says:

    I have two questions about many-worlds interpretation:

    1)when one uses an ensemble of a system to study it in statistical mechanics, there are two ways he can interpret his work:

    a) he can claim that every single picture in the ensemble(with millions of different scenarios) actually exists, or
    b) he can say that the different scenarios in ensemble only exists in “his mind”

    I think the question of importance is that is there any “experimental” distinction between these two views? (somehow i guess that the -b- interpretation sounds better!)

    2) doesn’t path integral formulation of QM support the idea that the different universes are somehow real? (because it seems that they interfere with each other in the integration process!)

    I also found the “many-worlds interpretation” section of this post by R. F. Streater very interesting,

  9. David Metzler says:

    I second Justin’s comment about Griffiths’ book. Not wanting to push you too hard, Peter, to explain in depth your views on interpretation of QM, but I would be curious about your take on the Consistent Histories approach. I find it intriguing.

  10. howie says:

    The problem Steinhardt has i think is that he needs to construct an alternative to inflation if his attack is going to be fruitful.
    But his alternatives are basically cyclic universes, in the case of the Ekpyrotic model embeded in a higher dimensional space. so its hard to see how they are not just multiverses in time rather than in space.
    Interestingly other string cosmologies seem like they might be simpler, have you seen them discussed in this video? some claims of potential tests as well:

  11. Mr MonoPole says:

    “Science and hypothesis”, by Henri Poincare, foreword by Larmor. Some good definitions of what science is and what is not. As one may expect – common sense and relation to the doable experiments characterize Poincare views. He does not mention universe at all.

  12. Peter Woit says:

    I haven’t paid much attention to the “Consistent Histories” business. From the time I’ve spent thinking about this, the whole “interpretation” issue increasingly seems to me besides the point. The interesting question is that of really understanding what happens in a “measurement”, how classical emerges from quantum, and what the limitations of that approximation (classical, isolated system) are. Without engaging with those questions I don’t see how one addresses the question of what is really paradoxical (as opposed to an artifact of one’s over-simplified model for measurement, one’s insistence on trying to isolate a system from apparatus and environment), and what if anything is missing from the standard formalism of states and unitary time evolution in order to understand measurements.

  13. Neil Bates says:

    Peter, first let’s try to get MWI proponents’ point right, then I’ll say why I don’t agree with them. They say that the wave equation continues to evolve. That means, literally no reduction to exclusive outcomes. Yes they say *that* all “happens in one world”, but we can only see the events relative to our own entangled branch. In that sense, the unobserved possible outcomes are literally “other worlds”. Yet the branching structure of the continued superpositions contradicts the observed Born probabilities. The attempts to get actual correct frequentist proportions of events to fit the BPs, are basically circular or contrived arguments. IMHO it just doesn’t work. (See the many critical comments at e.g. Sean Carroll’s blog posts on the subject.)

    OTOH if we reject that, we are stuck with a special “collapse” happening to the wavefunction. I myself think that interactions can make that happen, it’s not about “observers”, but no one can really resolve that yet. The “quantum measurement paradox” is still unsolved.

  14. Unemployed says:

    @DavidMetzler, @Justin:
    I was about to follow your recommendations and download Griffiths’ book, but then I saw he’s also written stuff about QM&Theology. Unfortunately, time is finite, and I don’t want to waste it on god talk. Is his book that you recommended pure science, or is there any religious stuff hidden in it?

  15. David says:

    It’s always possible that Griffiths’ book has stuff in there that you won’t agree with (some kind of philosophy you won’t agree with, or some kind of religious stuff you won’t agree with), but just skip over it. What’s wrong with that? Are you worried that you won’t be able to tell what’s science and what’s religion or “god talk”? It’s a free pdf, so just skip over the stuff you don’t like.

  16. Tim May says:

    There’s nothing mystical or religious in the Griffiths book. The philosophy mentioned is not central and is mostly of the nature of mathematical theories versus actual physical reality. (Think of the point that planets and people don’t move by computing differential equations, yet Nature does indeed follow the results quite closely. Then extend this point to Hilbert spaces, tensor projections, and all that stuff. Griffiths does conclude that just as Newtonian mechanics appears to be much closer to reality than pre-Newtonian mechanics and the quantum mechanics is even closer to reality, from vast numbers of experiments carried out to very high precision.

    Griffiths does a good job of gathering together the arguments of many in this area.

    No religion that I could find, except for one line thanking his parents.

    BTW, very little said about Many Worlds, and what there was was uncontroversial.

    –Tim May

  17. David Metzler says:

    Peter, I agree with you on what the main problem is. However I do think that some interpretations of QM (e.g., for me, the Copenhagen interpretation) make it harder, psychologically, to think in productive ways about measurement, decoherence, etc. So I see an alternative interpretation as a psychological device that may possibly help get to the bottom of the key problem you mention, namely how classical emerges from quantum. The Consistent Histories approach seems particularly well suited for that purpose in some ways. (For me, MWI does not help at all, though likely that’s my problem.)

    Unemployed, as Tim May says, there’s no religious material in Griffiths’ book. And if you skim some of the detailed examples on first reading, you can get a sense of the CH approach pretty quickly.

  18. Curious Mayhem says:

    We should be grateful to the all-powerful deity–or whatever arranged things to be just so–for Paul Steinhardt and the few others like him, like Peter.

    If primordial gravitational waves are detected–and they could be soon–then inflationary theories move into firmly scientific territory. But the multiverse proper will remain a fever swamp of insanity.

  19. al says:

    Opening the new Smolin-Unger book, on finds it on page2 of the preface, in a nutshell:
    ” There is only one universe at a time, with the qualifications that we discuss. The most important thing about the natural world is that it is what it is and not something else. This idea contradicts the notion of a multiverse – of a plurality of simultaneously existing universes – which has sometimes been used to disguise certain explanatory failures of contemporary physics as explanatory successes.”

  20. Mark Thomas says:

    Marco Gleiser has a short commetary titled ‘A Quest for the Unattainable Unification of Knowledge” on E. O. Wilson’s new book “The Meaning of Human Existence” see link
    It is good to know that there are some who still believe in the faith of Einstein that there are Unities of knowledge and explanatory power in such unifications (in the Sciences). I hope we do not look at the likes of E. O. Wilson someday and say he was a relic of the 20th century and what he has to say was or is wishful thinking and that such is quaint. By God I hope it does not come to that.

  21. Katya says:

    Curious Mayhem-
    Re detection of primordial gravitational waves; Professor Steinhardt’s comments are beautifully simple:

    “…it is not possible to find evidence to support or refute inflation because an inflationary multiverse includes patches with cosmic gravitational waves and without them.”
    and therefore,
    “…even if we detect primordial gravitational waves, we should not rush to the conclusion that they are due to inflation. Better theories may come along that avoid the pitfalls of inflation and that nevertheless predict gravitational waves.”

  22. amirpouyan says:

    “A different way of conceiving of this unimaginably immense proliferation (Multiverse idea) is to locate it as happening not externally to the cosmos but internally to the mind/brain states of observers. Making that move is a turn from a manyworlds interpretation to a many-minds interpretation, but this scarcely serves to mitigate the prodigality of the proposal … for many of us it still remains a metaphysical steam hammer brought in to crack an admittedly tough quantum nut” John Polkinghorne, Quantum Theory: A Very Short Introduction,p:53,

    note that the book is highly praised by Chris Isham ,

  23. Andrew says:


    Steinhardt criticizes inflation, because if this theory is confirmed, then his own work on the cyclic universe will remain useless. It looks like a conflict of interest.

  24. Tom says:

    When the initial Bicep2 (now apparently faulty) results were released, I recall some interview w/ Steinhardt where he seeemed very accepting of the possibility that his cyclic universe hypothesis could have been falsified. He wasnt trying to handwave and make a torturous, convoluted defense.
    While he probably feels vindicated in light of Bicep2 non-results, had the Bicep2 results been corroborated, I didnt sense that he would go to his grave trying to defend cyclic cosmologies.

    I dont know the detailed chronological sequence, but was Steinhardt critical of inflation before he started advocating cyclic cosmology?

  25. S says:

    Correct me if I’m wrong, but Steinhardt might well stand to win a Nobel prize if inflation is verified. Before he became a major critic of it, he helped develop the theory.

    In any event, it doesn’t seem fair to me to call it a “conflict of interest” when a scientist with one theory critiques a competing theory. Presumably his criticisms might be part of what motivated him to develop another theory in the first place. Conflicts might arise in refereeing or the like, but I don’t think the accusation should be raised just for simple public criticism.

  26. Andrew says:

    No, if inflation is confirmed ( I’m pretty sure) Steinhardt did not receive the Nobel Prize. I mean, Steinhardt and Turok gloatingly respond to failure of Bicep2 team. It’s No Good. P.S. Sorry for my English.

  27. howie says:

    The Nobel is limited to three persons.
    If inflation were to be recognised , Guth is the stand out name. But you are right the theory was developed by others as well. Linde, Starobinsky, Albrecht and Steinhardt would be the next in line to be considered.
    Steinhardt would be just as entitled to share the prize as any of the others but given his vocal opposition its unlikely he would be picked , although it would be very amusing.
    However without a red tilted gravity wave spectrum i doubt inflaiton will be handed any nobels.
    Steinhardt developed the cyclic cosmology as an alternative to inflation as I understand becuase he was not happy with the implications of inflation ( i.e a multiverse). i dont think that’s a conflict of interest, scientists have to follow what they think is the most promising leads.

  28. Abraham says:

    This week’s Guardian Science has a decent counterpoint and discussion by Ungar and Smolin:

  29. Curious Mayhem says:


    The point you make about there being other explanations for cosmic gravitational waves is a good one. Their detection alone doesn’t prove inflation.

    But they are a distinctive predictive of inflation, and along with the classic problems that inflation solves, their detection would be strong evidence in favor of it.

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