15 Years of Multiverse Mania

Today is the 15th anniversary of the event that kicked off the Multiverse Mania that continues to this day, recently taking the form of a concerted attack on conventional notions of science. 2018 has seen an acceleration of this attack, with the latest example appearing this weekend.

On January 29, 2003, Kachru, Kallosh, Linde and Trivedi submitted a paper to the arXiv that outlined a construction of a supposed model of a metastable string theory state that had all moduli fixed. Ever since the first explosion of interest in string theory unification in 1984-5, it had been clear that a big problem with using string theory to get anything that looks like known physics was the so-called “moduli problem”. If you try and use 10d superstring theory to describe our universe, you need to somehow hide six of the dimensions, and the best way to do that seemed to be to argue that superstring theory implied one could do this by compactifying on an unobservably small approximately Calabi-Yau manifold. Such manifolds however come in families labeled by “moduli” parameters, which can be thought of as describing the size and shape of the Calabi-Yau. These moduli will show up as zero mass fields generating new long-range forces unless some dynamical mechanism could be found to fix their values. It was this that KKLT claimed to have found. I won’t even try to describe the complex KKLT proposal, which was aptly described by Lenny Susskind as a “Rube Goldberg mechanism”.

What string theorists had been hoping for was a moduli stabilization mechanism that would pick out specific moduli field values, getting rid of the unwanted dozens of new long-range forces and providing a way to make physical predictions. While the KKLT mechanism got rid of the unwanted forces, it had been observed three years earlier by Bousso and Polchinski, working with just parts of the Rube Goldberg mechanism, that this sort of thing led to not one specific value of the moduli fields, but an exponentially large number of possibilities. They had noted that this could allow an anthropic solution to the cosmological constant problem, and the KKLT fixing of all the moduli provided a model that accomplished this (without the long range forces).

KKLT did not mention anthropics and the multiverse, but less than a month later Lenny Susskind published The Anthropic Landscape of String Theory, a call to arms for anthropics and a founding document of Multiverse Mania. He immediately went to work on writing a book-length version of string theory multiverse propaganda aimed at the public, The Cosmic Landscape, which was published in 2005. Less than a month after Susskind’s manifesto, Michael Douglas published a statistical analysis of supposed string/M-theory vacua, and at some point the estimated number $10^{500}$ of vacua started appearing based on this sort of calculation.

I didn’t notice KKLT when it appeared, but did notice the Susskind arXiv article. I had just finished writing the first version of my book, and remember that my reaction to the Susskind article was roughly “Wow, if people like Susskind are arguing in effect that you can’t predict anything with string theory, that’s going to pull the plug on the subject.” The book took a while to find a publisher, and by the time it was published I had tacked on a chapter about the multiverse problem. I started this blog in March 2004, and recently looked back at some of the earliest postings, noticing that a huge amount of time was spent arguing with people about KKLT and its implications for the predictivity of string theory. It seemed clear to me from looking at the calculations people were doing that this kind of thing could not ever lead to a prediction of anything. I won’t go over those arguments, but claim that my point of view has held up well (no prediction of anything has ever emerged from such calculations, for reasons that are obvious if you start looking at them).

Back in 2003-4 I never would have believed that the subject would end up in the state it finds itself in now. With the LHC results removing the last remaining hope for observational evidence relevant to string theory unification, what we’ve been seeing the last few years has been a concerted campaign to avoid admitting failure by the destructive tactic of trying to change the usual conception of testable science. Two examples of this from last week were discussed here, and today there’s a third effort along the same lines, Quantum Multiverses, by Hartle. Unlike the others, this one includes material on the interpretation of quantum mechanics one may or may not agree with, but of no relevance to the fundamental problem of not having a predictive theory that can be tested.

I’m wasting far too much time discussing the obvious problems with articles like this, to no perceptible effect. Hartle like the others completely ignores the actual arguments against his position (he lists some references. describing them as “known to the author (but not necessarily read carefully by the author)”). In a section on “A FAQ for discussion” we find arguments that include

  • The cosmological multiverse is falsifiable, because maybe you’ll falsify quantum mechanics.
  • The cosmological multiverse is testable: “by experiment if a very large spacetime volume could be prepared with an initial quantum state from which galaxies, stars, life etc would emerge over billions of years.” Not surprisingly, no indication is given of how we will produce such a state or any theory that would describe what would happen if we did.
  • The theory of evolution is just like the theory of the cosmological multiverse.

Both the absurdity and the danger of this last argument are all too clear.

By the way, for a while earlier this year the arXiv started allowing trackbacks again to this blog, but then this stopped again. The origin of the ban seems to have been in the story described here and my early criticism of the string theory multiverse. I have no idea what their current justification for the ban is.

Update: A good place to look for information about the current state of string landscape calculations is at the website for this workshop. The idea that the problems of this subject can be solved by “modern techniques in data science” seems to me absurd, but for a different point of view, look at the slides of Michael Douglas. For something more sensible, try the talk by Frederik Denef, which describes some of the fundamental intractable problems:

  • You don’t have a complete theory, with only some non-perturbative corrections known, no systematic understanding of these.
  • Dine-Seiberg Problem: When corrections can be computed, they are not important, and when they are important, they cannot be computed.
  • Measure Problem: Whenever a landscape measure is strongly predictive, it is wrong, and when it’s not, we don’t know if it’s right.
  • Tractability Problem: Whenever a low energy property is selective enough to single out a few vacua, finding these vacua is intractable.

Denef does make some very interesting comments about where modern techniques in data science might actually be useful: dealing not with the landscape of string vacua, but with the huge landscape of string theory papers (e.g. the 15,000 papers that refer to the Maldacena paper). He argues:

For obvious reasons, besides time constraints, incentives to write papers are much stronger for research scientists than to read them. So printed stacks pile up unread, PDFs remain ambitiously open until we reboot our laptops, recursive reference-backtracking gets sidetracked by the deluge of micro-distractions puncturing our days. This, plus the sheer volume of disorganized pages of important results, leads to loss of access to crucial knowledge, to repeated duplication of efforts, and to many other
inefficiencies. Worst of all, it becomes increasingly harder for young brilliant minds to stand on the shoulders of giants, and thus to make revolutionary new discoveries. It seems inevitable that we will have to outsource the tedious task of parsing the literature, in search for relevant results, insights, questions and inspiration, to the Machines.

Update: There’s an interesting article at Quanta magazine about “Big Bounce” models of the Big Bang, competitors to inflationary models. Paul Steinhardt gives his take on the multiverse: “hogwash.”

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29 Responses to 15 Years of Multiverse Mania

  1. ilovecats says:

    > By the way, for a while earlier this year the arXiv started allowing trackbacks again to this blog, but then this stopped again. The origin of the ban seems to have been in the story described here and my early criticism of the string theory multiverse. I have no idea what their current justification for the ban is.

    Hi Peter,

    I’m just a simple undergrad, but I just wanted to mention that I appreciate your blog very much. I know a few others who’d agree with me when I say that it’s because of you that:

    * we learned of folks like Sabine Hossenfelder (who also maintains an awesome blog);

    * sighed in relief when we realized that we’re not the only ones who found it difficult to read popsci books like the “The Universe in a Nutshell”;

    * or found awesome books through your reviews (e.g. “Summing It Up”!).

    Reading about your spat with Polchinski makes me appreciate your efforts more, because I don’t think I quite imagined that you’d be suffering personal attacks because of your commentary (even after you apologize for mistakes on your end!)…

    You’re a good teacher. Keep writing!

  2. Pascal says:

    A question on the 10^500 vacuaa and the resulting lack of predictive power: why don’t people just focus on the (presumably smaller?) corner of this landscape that is consistent with the standard model? Has such a “corner” even been found?

  3. Peter Woit says:

    Seiberg’s defense of string theory at least does not include the multiverse nonsense. I do wish though that sensible theorists like him would publicly address this issue. This attack on science is flourishing because of their silence.

  4. Peter Woit says:

    The weird thing about the Polchinski story is that it’s not really personal (we’ve never met, he knows nothing much about me). It is just an ideological campaign, with the arXiv trackback business an effort on the part of him and others to suppress disagreement with their ideology. Another aspect of this is what you see in the recent “multiverse is science” articles I’ve been writing about: refusal to even read the arguments of those who disagree with you, trying to pretend they don’t exist. At least Polchinski admits I exist (he was the only well-known string theorist to write a review of my book and Lee Smolin’s).

  5. Peter Woit says:

    Among the problems:
    1. The “vacua” being studied are only an infinitesimal fraction of possibilities (these days the count of possibilities looked at is not $10^{500}$ but $10^{272000}$ and surely there are many more).
    2. You can’t reliably calculate things for any of these “vacua”, i.e. you can’t identify which are consistent with the correct value of the electron mass because you can’t reliably calculate the electron mass in any of them.

    So, you are only looking at some irrelevant corner of the full space of possibilities, and have no way of finding the corner of that corner that corresponds to known physics.

  6. John Fredsted says:

    @Woit: Thanks for that update on the number of vacua: 10^27.000, and counting! A whopping factor of 10^26.500 greater than the previous stupendous estimate of 10^500. Could you perhaps point me to a reference where that new estimate appears?

  7. Peter Woit says:

    John Fredsted,

    I don’t have time to track down the exact reference for this calculation. It’s mentioned here
    and I’ve added some comments about that workshop, which is a good source for the current state of string landscape calculations.

  8. new says:

    Peter, can you clarify some issues on KKLT

    Do string cosmologists claim the universe originally started out with 10 flat dimensions of space and 1 dimension of time, then as a result of KKLT, 3 remain large, and 6 curled up as a result of the presence of anti-D3 branes and D-brane instantons or gaugino condensation as outlined in the KKLT paper? What is the origin of highly warped IIB compactifications with nontrivial NS and RR three-form fluxes?

    is there a naturalistic mechanism that gives rise to KKLT mechanism, and does this have observational evidence or experimental support? What are the origins of anti-D3 branes and how did they come to be wrapped around the extra 6 dimensions? How did these 6 dimensions acquire D-brane instantons or gaugino condensation?

    has science established the existence of any anti-D3 branes, D-brane instantons or gaugino condensation or highly warped IIB compactifications with nontrivial NS and RR three-form fluxes observed either in nature or in experiments as described in the KKLT paper?

  9. A says:

    Pascal, even if one could compute the string vacua, another problem is that the SM is described by about 150 measured digits. So, presumably, among 10^500 vacua, there are 10^350 vacua which look like the SM, just by chance.

  10. Jan Dybicz says:

    Peter, the link you provided has the exponent at 272,000, not 27,000.

  11. Peter Woit says:

    KKLT is purely a construction of a metastable ground state with stabilized moduli. There’s no theory of initial conditions, no evidence (or even complete theory) of the various elements that make up the KKLT Rube Goldberg machine. This is why the main activity of string landscapeologists is just counting these things.

  12. Peter Woit says:

    Jan Dybicz,
    Thanks, fixed! Easy to make that kind of mistake by a factor of $10^{271,173}$.

  13. Balazs Vagvolgyi says:

    On Michael Douglas’ slides, he claims about string theory that:

    “… it is not hard to find solutions … for which the effective 4d physics at low energies is the Standard Model coupled to gravity …”
    “… Only a minority of [the large number of solutions] lead to the Standard Model field content, and those which do lead to a range of values for the cosmological constant, the particle masses and the other fundamental constants …”

    This implies that it’s so easy to find suitable solutions, that string theorists already have a bunch of compactifications (solutions), that provide SM+GR, except for the values of the fundamental constants. So now we only have to find just the right compactification for our set of constants.

    Is this correct?

  14. Peter Woit says:

    Balazs Vagvolgyi,
    What Douglas doesn’t explain when he says these constructions “lead to a range of values for …” is that you can’t actually reliably or accurately compute any of these values. All you have is arguments that there is no known reason they can’t be the SM ones. Note that Denef is Douglas’s long-time collaborator on these calculations, and his description of two aspects of the issue is “When corrections can be computed, they are not important, and when they are important, they cannot be computed.” and “Whenever a low energy property is selective enough to single out a few vacua, finding these vacua is intractable.”

    So, the situation is that, after you specify some discrete data like the number of generations, you still have an exponentially large number of possibilities, with no way of determining which agree with the SM. There is no way of getting any sort of prediction of anything out of this. Claims made early on (and debated on my blog and others) that maybe some kind of prediction could be made have failed completely.

  15. Anonyrat says:

    a. Seems like a metaphor modernization replacing ‘needle in a haystack’ by ‘vacuum state in a landscape’ is appropriate.

    b. Mining a pile of documents (not of string theory documents) is a proposed project at where I am. IBM Watson is the suggested tool.

  16. David Levitt says:

    I have been following your forum almost from the beginning. Your statement that “…my point of view has held up well“ is understated. It is clear now that you were right and your vocal and, sometimes, vicious critics were wrong. This forum provides an incredibly valuable means for the non-specialist physicist to evaluate these HEP theories. You provide unlimited opportunities for other specialists to rebut your arguments – and when they can’t, or don’t, it is very strong evidence that you are right. I grew up in the 60’s reading Scientific American in awe of the brilliant physicists and their wonderful new discoveries. And now, you have shouted out that the current batch of HEP geniuses have no clothes. After a long career as a biophysicist I have developed a more jaded view of these researchers. Although they are obviously highly skilled technicians, this does not imply that they understand or care about the large picture implications of their research. I have two questions that are a little off subject. 1) Why do you think this is a recent HEP problem? Is it just the result of the lack of progress in the field, or has the character of the people attracted to the subject changed from the halcyon days of my youth. If the former, then it would imply that my youthful heroes had the same weaknesses as the current theorists and they were just lucky that there were some new experimental results that needed explaining. 2) Now that you have been essentially proven correct, what sort of response do you get from your colleagues at meetings, especially your former critics? Are they willing to admit their error?

  17. Nonsense says:

    ”dealing not with the landscape of string vacua, but with the huge landscape of string theory papers (e.g. the 15,000 papers that refer to the Maldacena paper)”

    Google Scholars indicates 16,188 citations for that paper.

  18. Peter Woit says:

    David Levitt,
    1) I don’t think earlier generations of theorists were any different. They had the huge advantage of continual new hints and checks on their conjectures from experiment. What we’ve seen over the last 30 years or so is what can go wrong when that is removed.
    2)Despite what you might think from blogosphere discusssion, I’ve always gotten along well with string theorists I know, and our views are more similar than you might think, with the main difference just our evaluation of exactly how bad the prospects for string unification are. Over the years many have told me they read my blog and agree with much of what I have to say. Most string theorists don’t believe the landscape stuff is worthwhile, and have voted with their feet and are working on things unrelated to string unification.

    No, I have yet to meet a theorist willing to say they were wrong (about string theory or almost anything…). The ones most unhappy with my criticisms have uniformly dealt with this by trying to ignore me as best they can, and continue to do so.

  19. F. G. says:

    So? When do you think we can expect the first papers on the ‘string publication landscape’ ?

  20. Louis Wilbur says:

    Peter, is it really the case that the KKLT 2003 paper achieved such stabilization? As I understand it, that paper is for type IIB string theory and the more recent paper 1709.03554 suggests, if I have read it correctly, that even meta-stability is not possible. I am not an expert in this area so perhaps I have misread it but I would definitely like your take on these papers since they all seem to suggest serious problems with stabilization.

    Supersymmetry Breaking by Fluxes

    No inflation in type IIA strings on rigid CY spaces

    On classical de Sitter and Minkowski solutions with intersecting branes

    The Swampland Conjecture and F-term Axion Monodromy Inflation

    Also, it seems that there are various no go theorems for positive vacuum energy in string theory. My understanding is that this has the potential to falsify string theory since dark energy is known to be positive. If it is not possible to have anything like cosmic inflation in string theory then string theorists need to move away from the inflationary paradigm to, say, string gas cosmology and then try to deal with dark energy in a completely different way. The below papers seem to say that there are serious problems with positive vacuum energy in string theory and they are definitely relevant to your blog.

    Refining the boundaries of the classical de Sitter landscape, Nogo theorems

    Revisiting constraints on uplifts to de Sitter vacua

    Constraints on Dbar Uplifts

    Loop corrections to the antibrane potential

    A no-go theorem for monodromy inflation

    Constraining de Sitter Space in String Theory

    If the very early universe did have a strong but short lived vacuum energy then it has to be the case that it is just as mysterious as dark energy. In other words, very little is known about it except that it has negative pressure, positive energy, and a few other things. But it is a big leap from that to other universes. If there are other universes then the only way to have no measure problem is for the number of such universes to be finite and each universe itself is finite in terms of all of its parameters. One problem with an infinite number of universes is well known by mathematicians since the time of Cantor: one can pair up the universes to each other in any way one wants and therefore get any measure that one wants. Questions like what percent of the universes contain life are just as meaningless as asking what percent of the positive integers are odd. The percentage can be anything one wants depending upon how the universes or positive integers are bijected with one another. Thank you for the interesting post.

  21. Mitchell Porter says:

    Peter Woit said

    “you can’t identify which are consistent with the correct value of the electron mass because you can’t reliably calculate the electron mass in any of them”

    A paper yesterday offers a glimpse of what remains to be done, in order to calculate such quantities:


    In the standard model and also in string theory, masses of elementary fermions come from yukawa couplings to the Higgs field. The introduction to this paper tells us that in string theory, there are three steps to calculating the yukawa coupling, and that the authors report progress with the second step.

  22. S says:

    Surely one difference between theorists now and fifty years ago is the speed with which they rush to the popular press? Few theorists did that with a new, half-baked theory back then (or am I wrong?). They might have had more experimental evidence, but that would have provided *more* justification for rushing out books, not less, and yet they rarely did so it seems.

  23. simplicio says:

    There’s a fair amount of pretty speculative science in old newspapers and magazine articles. I don’t think the public appetite for these sort of thing, or Science’s willingness to provide it has really changed in the last five decades. The difference is that the absence of new results means the *same* speculative ideas can basically linger for years, never being proven or disproven, making them seem less like speculative ideas and more like well supported science.

  24. Peter Woit says:

    Louis Wilber,
    There has always been debate about whether the KKLT Rube Goldberg machine really works or not. But this really doesn’t matter, the conclusion that there is no scientific theory of a multiverse is the same either way. Either the theory is radically inconsistent with observation (unstabilized moduli) or completely empty and consistent with anything (KKLT-stabilized moduli).

    Mitchell Porter,
    The technical problem (lack of explicit Calabi-Yau metric) they are discussing goes back to the beginnings of the subject, and they seem to me to make little progress in dealing with it. In any case this technical problem is not at all the main problem. Note that they just ignore the moduli stabilization problem completely. They don’t even get to the point of introducing the pieces of the KKLT machine.

    A counter-example is the story of Schrödinger in 1947. Quoting a random source about this (https://www.salon.com/2015/04/18/albert_is_an_old_fool_einstein_vs_schrodinger_in_battle_of_the_nobel_laureates/)
    “The leading announcer was the Irish Press, from which the international community learned about Schrödinger’s challenge. Schrödinger had sent them an extensive press release describing his new “theory of everything,” immodestly placing his own work in the context of the achievements of the Greek sage Democritus (the coiner of the term “atom”), the Roman poet Lucretius, the French philosopher Descartes, Spinoza, and Einstein himself. “It is not a very becoming thing for a scientist to advertise his own discoveries,” Schrödinger told them. “But since the Press wishes it, I submit to them.”

    Scientists are often misguidedly in love with their own dubious ideas, and there always have been ones willing to go to the press and get public attention. The media environment now is different and much more extensive, but I think the main difference is the lack of real progress being made, so fake claims of progress dominate because they have no competition.

  25. The Observer says:

    Bravo Peter for a bravura performance over the years. I cannot help wonder whether you would be a professor by now had you not taken your stance against the established “way forward” and stuck to it.

  26. Peter Woit says:

    The Observer,
    Thanks, but I should make it clear that I haven’t suffered professionally because of this in any way, and am very much content with my current academic position and employer.

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