Peter Woit vs Sean Carroll: string theory, the multiverse, and Popperazism

Peter Woit vs Sean Carroll

Peter Woit (left) vs Sean Carroll (right)

The string and multiverse wars are going strong in fundamental physics! And philosophy of science is very much at the center of the storm. I am no physicist, not even a philosopher of physics, in fact (my specialty is evolutionary biology), so I will not comment on the science itself. I take it that the protagonists of this diatribe are more than competent enough to know what they are talking about. But they keep bringing in Karl Popper and his ideas on the nature of science, as well as invoke — or criticize — Richard Dawid’s concept of non-empirical theory confirmation, so I feel a bit of a modest commentary as a philosopher of science is not entirely out of order.

Let me begin with two caveats: first, there are many people involved in the controversy, including Sean Carroll, Peter Woit, Sabine Hossenfelder, George Ellis, and Joe Silk (not to mention astute commentators such as Lee Smolin and Jim Baggott). Refreshingly, almost all of them have respect for philosophy of science, unlike ignorant (of philosophy) physicists like Lawrence Krauss and Stephen Hawking. So, who knows, some of them may even read the following with some interest. Second, I actually know most of these people, obviously some better than others. I like and respect them all, even though — as we shall see — in this post I will come squarely down on one side rather than the other.

And what are these sides? For this round, I’ll focus on an exchange between Sean Carroll and Peter Woit on the specific issue of multiverse theory, though the two disagree — for the same reasons — also about the status of string theory. I have published an extended commentary on the string wars at Aeon magazine, after having participated to a conference organized by Dawid, where Peter, unfortunately, had not been invited, and which Sean, equally unfortunately, couldn’t attend.

Sean has recently written a post at Preposterous Universe entitled “Beyond falsifiability,” in which he summarizes a paper of his, currently at Beyond falsifiability: normal science in a multiverse. Here is the abstract of that paper:

“Cosmological models that invoke a multiverse — a collection of unobservable regions of space where conditions are very different from the region around us — are controversial, on the grounds that unobservable phenomena shouldn’t play a crucial role in legitimate scientific theories. I argue that the way we evaluate multiverse models is precisely the same as the way we evaluate any other models, on the basis of abduction, Bayesian inference, and empirical success. There is no scientifically respectable way to do cosmology without taking into account different possibilities for what the universe might be like outside our horizon. Multiverse theories are utterly conventionally scientific, even if evaluating them can be difficult in practice.”

Not so fast, replies Peter at his blog, Not Even Wrong: “Much of the problem with the paper and blog post is that Carroll is arguing against a straw man, while ignoring the serious arguments about the problems with multiverse research. … None of those references [in the paper] contain anything like the naive argument that if we can’t observe something, it ‘simply shouldn’t matter,’ or one should not speculate about it, or it ‘shouldn’t count as science at all.’”

A good part of the discussion hinges on Sean accusing critics of both string theory and the multiverse of “Popperazism,” a neologism coined by him (as far as I can tell), which refers to the alleged misappropriation of the ideas of influential philosopher of science Karl Popper. Indeed, Sean already wrote a short piece for Edge back in 2014 in response to the question: “What scientific theory is ready for retirement?” His answer: falsificationism, the notion, proposed by Popper, that what demarcates science from non-science (and pseudoscience) is the feasibility of falsifying the tenets of a given theory or hypothesis. If a theory is in principle falsifiable, argued Popper, then it is scientific. If there is no way to subject it to the falsifiability criterion, it isn’t science.

Setting aside that falsificationism is not a scientific theory, but rather a notion in philosophy of science (after all, how would you falsify Popper’s account?), Sean admits that he hasn’t gone over the nuances of what Popper actually wrote. That’s unfortunate, because Popper was a bit more of a sophisticated philosopher than he is usually given credit for. Even though his ideas are no longer current in philosophy of science (you know, philosophy does make progress!), if one invokes him to dismiss a scientific theory (as Ellis and Silk do), or, conversely, rejects his insight in order to deflect criticism against one’s favorite theory (as Sean does), it would be good to take a look at what the men actually wrote.

Without going into too much detail (for an in-depth discussion and pertinent quotes see my Aeon article mentioned above), Popper realized that falsification is not a sharp blade capable of neatly cutting off science front non-science. He was also aware of, and discussed at length, the fact that legitimate scientific theories do include ad hoc explanations that are used by scientists as place holders until (and if) they figure out what is wrong with the theory they are working on. Nobody has ever rejected a scientific theory because all its statements were not immediately falsifiable, nor did Popper suggest such a crude practice in the first place.

To be fair to Sean, he says that what he is after is the naive version of Popper that he thinks others are using as a blunt instrument to dismiss string theory and the multiverse as outright unscientific. But, as Peter points out, evidence of such extreme “Popperazism” is hard to come by. Here, for instance, is the above mention George Ellis, in a response to a critique by Daniel Harlow, which Sean quotes approvingly:

“The process of science — exploring cosmology options, including the possible existence or not of a multiverse — is indeed what should happen. The scientific result is that there is no unique observable output predicted in multiverse proposals. This is because, as is often stated by proponents, anything that can happen does happen in most multiverses. Having reached this point, one has to step back and consider the scientific status of claims for their existence. The process of science must include this evaluation as well.”

Peter comments: “The problem with the multiverse is that it’s an empty idea, predicting nothing. It is functioning not as what we would like from science, a testable explanation, but as an untestable excuse for not being able to predict anything. In defense of empty multiverse theorizing, Carroll wants to downplay the role of any conventional testability criterion in our understanding of what is science and what isn’t.”

Does Sean do that? It appears so when he says: “The best reason for classifying the multiverse as a straightforwardly scientific theory is that we don’t have any choice. This is the case for any hypothesis that satisfies two criteria: (i) It might be true; (ii) Whether or not it is true affects how we understand what we observe.”

Those are exceedingly weak criteria indeed. As an extreme example, take the very fuzzy notion of God: it might be true, and whether it’s true or not this would affect how we understand the world. So what? Neither of those two observations — in itself — provides an iota of reason to believe in God. Or the multiverse.

Sean then moves to another target critics of string theory and the multiverse often aim at: Richard Dawid’s notion, mentioned above, of a new science based on what he calls “non-empirical confirmation.” As Sean acknowledges, that term was probably really bad PR on the part of Dawid:

“It sounds like Dawid is saying that we can confirm theories (in the sense of demonstrating that they are true) without using any empirical data, but he’s not saying that at all. Philosophers use ‘confirmation’ in a much weaker sense than that of ordinary language, to refer to any considerations that could increase our credence in a theory. Of course there are some non-empirical ways that our credence in a theory could change; we could suddenly realize that it explains more than we expected, for example. But we can’t simply declare a theory to be ‘correct’ on such grounds, nor was Dawid suggesting that we could.”

Hmm, as a philosopher, I don’t actually subscribe to this notion that we use “confirmation” in a weak sense at all. Still, Sean is right that we may, in the course of exploring the logical entailments of a given theory, discover that it has many more than we at first thought. Indeed, this is precisely what happened during the early history of string theory, and why it has attracted so much attention for so long. As for Dawid’s not suggesting that a theory should be declared correct on just such grounds, this is true. But it is also true that the whole point of Dawid’s Bayesian-informed approach is to make the argument that our belief in a theory should be updated, and even tilted toward provisional acceptance, on the basis of non-empirical entailments. This is controversial to say the least, both among philosophers and among scientists.

Sean continues: “Nobody is trying to change the rules of science; we are just trying to state them accurately. The multiverse is scientific in an utterly boring, conventional way: it makes definite statements about how things are, it has explanatory power for phenomena we do observe empirically, and our credence in it can go up or down on the basis of both observations and improvements in our theoretical understanding. Most importantly, it might be true, even if it might be difficult to ever decide with high confidence whether it is or not.”

There is a lot to unpack in those sentences. Beginning with the end, again, yes, multiverse theory may be true, but if we will never be able to decide that on the basis of empirical observation it simply shouldn’t count as a scientific theory. Nor should it be considered “probably” true, pace Dawid’s Bayesian approach. Indeed, at the moment, at least, the notion of a multiverse should be classed as scientifically-informed metaphysics. Too bad that so many scientists recoil from the “m-word,” though.

In terms of not trying to change the rules of science, I beg to differ. Maybe Sean isn’t, but Dawid definitely is. That’s a major point of his book on the subject. The question is whether such change is warranted or not. (I don’t think so.)

Moreover, there seems to me — as a naive external observer to the debate — to be nothing “boring” or “conventional” about the multiverse. It is, rather, a radical theory that would dramatically revise our whole conception of what a “universe” is in the first place!

Here Woit again makes some sharp comments: “[What] Carroll ignores is that the evaluation problem is not just ‘hard,’ but actually impossible, and if one looks into the reason for this, one finds that it’s because his term ‘the theory’ has no fixed reference. What ‘theory’ is he talking about? One sort of ‘theory’ he discusses are eternal inflation models of a multiverse in which you will have bubble collisions. Some such models predict observable effects in the CMB [cosmic background radiation]. Those are perfectly scientific and easy to evaluate, just wrong (since we see no such thing). Other such models predict no observable effect, those are untestable. ‘Hardness’ has nothing to do with it, the fact that there is some narrow range of models where tests are in principle possible but hard to do is true but irrelevant.”

Here we get pretty close to the edge of my competence, and I am going to leave it to Sean, Peter and the rest to evaluate what actual (novel) predictions multiverse theory makes, and whether and how they might be tested. But the more time passes (and this goes for string theory as well), the more the burden of proof rests on defenders of the theory, while the skeptics are increasingly justified in their impatience regarding the current dearth of such tests.

Sean concludes his post by writing that “understanding how science progresses is an interesting and difficult question, and should not be reduced to brandishing bumper-sticker mottos to attack theoretical approaches to which we are not personally sympathetic.”

That is most certainly true, though again I see little evidence of bumper-sticker brandishing. But it is curious to me that he seems to imply that his critics attack string theory and the multiverse because they are not “personally sympathetic” to those notions — not because they honestly see intellectual problems with them. This comes close to poisoning the well, a type of elementary logical fallacy that Sean is usually too careful a thinker to indulge in. Besides, what makes him so confident that he and other defenders of strings and multiverse aren’t just as much personally invested in those notions, and hence subject to more or less unconscious biases? As Caroll Tavris and Elliot Ar0nson memorably put it, “mistakes were made, but not my me.”


Postscript: the term “Popperazzi” appears to have been used first by Leonard Susskind, at least since his 2006 “The Cosmic Landscape,” p. 192 (with thanks to various commenters on Twitter, especially Jim Baggott).

Also, entirely coincidentally, Sabine Hossenfelder has a (critical) piece on the multiverse at

98 thoughts on “Peter Woit vs Sean Carroll: string theory, the multiverse, and Popperazism

  1. Patrice Ayme

    Reblogged this on Patrice Ayme's Thoughts and commented:

    The present observable universe is at least 91 billion light years across. The universe may be one million billions light years across. We don’t know. But to consider we should make first wild guesses about other universes, is a bit like imagining unicorns, in another cave, when one does even know how big the cave one inhabits is. Why to make wild, untestable guesses, when more sedate guesses could be experimentally tested, if they were only considered?
    As Einstein proposed that a particle “in flight” was localized (he had no proof of this, and there are near-proofs that this is false), it came to be considered that all eigenstates were to receive said localized particle (albeit with varying probabilities). But the reception of one particle per eigenstate is possible only if the universe has split in as many universes as there are eigenstates. Therein the genesis of the Many-World “interpretation” of Quantum Mechanics…which many specialists consider equivalent to the Multiverse.
    So what is the scientifically minded to do? Well, make an experiment! In the lab. It’s simple: miniaturize, and find out whether the photon gravitationally spreads (as I believe it does), or whether the photon doesn’t spread (as Einstein believed it didn’t). My assumption is that Quantum spreads corresponds to a gravitational spread. In other words, is the Quantum Collapse real, or not? (If it is, Einstein was wrong in his 1905 paper.)
    No need to evoke other universes, and unicorns. One should just reach a higher level of experimental accuracy. Not a fancier level of religious prophecy.
    Interestingly, Einstein himself, in his 1935 EPR paper, predicted, or, more exactly observed, that the Quantum Theory implied “Entanglement” (so named by Schrodinger, soon afterwards). Entanglement is experimentally proven, collapse is not… Not yet. But one can’t get entanglement without collapse. Methinks top physicists would be better off thinking about these issues (which lead to lab experiments and the Quantum Computer), rather than to try to imagine other universes…

    Liked by 1 person

  2. Pentcho Valev


    “Ether was falsifiable and, in fact, was falsified by Michelson-Morley”

    This is mythology – something else was falsified. In 1887 Michelson and Morley had calculations showing that the two beams should arrive at different times, but the experiment demonstrated no time difference at all – the two beams arrived at the same time.

    The calculations were based on a false assumption – Michelson and Morley had assumed that the speed of light is independent of the speed of the emitter. So the thing the experiment falsified was this assumption. If Michelson and Morley had assumed that the speed of light varies with the speed of the emitter, as predicted by Newton’s emission theory, the experimental result would have matched the calculations:

    “Emission theory, also called emitter theory or ballistic theory of light, was a competing theory for the special theory of relativity, explaining the results of the Michelson–Morley experiment of 1887. […] The name most often associated with emission theory is Isaac Newton. In his corpuscular theory Newton visualized light “corpuscles” being thrown off from hot bodies at a nominal speed of c with respect to the emitting object, and obeying the usual laws of Newtonian mechanics, and we then expect light to be moving towards us with a speed that is offset by the speed of the distant emitter (c ± v).”

    John Norton: “The Michelson-Morley experiment is fully compatible with an emission theory of light that CONTRADICTS THE LIGHT POSTULATE.”

    Since the falsified assumption, “the speed of light is independent of the speed of the emitter”, became one of Einstein’s postulates in 1905, it is fair to say that Einstein’s relativity was experimentally refuted before it was created.


  3. synred

    The speed of light comes from Maxwell’s theory which predicts waves. Newton’s particle theory may have predicted the right answer, but it didn’t need ether either.


  4. synred

    Emission theory, also called emitter theory or ballistic theory of light

    I didn’t say Michelson-Morley falsified ’emission theory’. It falsified wave theory in a medium. None QM particle theory is presumably falsified by diffraction.

    We are not discussing what’s true, but what’s scientific.


  5. Robin Herbert

    Ether (or something like it) still seems to be bubbling away in the background as a contender in Bohemian mechanics.

    But then again, as far as I can see, you could get Bohemian mechanics by postulating yet tinier particles.

    Liked by 1 person

  6. Alan White

    Massimo et al thanks for a very careful post and comments. I have a question that may seem dumb but I can’t seem to get past it in my own mind.

    Is it correct to say that string/multiverse and many previous TOEs result from the theoretic-based incompatibility of GRT and QT? In other words since GRT and QT are well-founded empirically, then a logically consistent “supertheory” that at least retrodictively explains or is compatible with the empirical results in both these areas at least then has the status of a legitimate scientific hypothesis?

    I’m asking this seriously because some examples offered here have a pretty complex history in being accepted or rejected after some point. Lorentz used equations essentially like Einstein’s to defend the ether in the face of Michelson-Morley, but his contraction hypothesis was only rejected and seen as ad hoc after Einstein produced much richer claims (like E=MC2) on the basis of his view of spacetime. This particular transition accepting Einstein over Lorentz took the better part of about two generations of scientists. I wonder how much time should be allotted to string/multiverse to play out in a similar way?

    Liked by 1 person

  7. synred

    Well fields fill space in Quantum Field Theory, so that’s something like ether, though being relativistically invariant not much like it.

    Then there’s all the quantum foam of virtual particles bubbling away, though I persist in thinking virtual particles are calculational devices and not real things doing anything in particular at any particular time — just a metaphysical opinion!


  8. Philosopher Eric

    One thing that I would hope for this debate to demonstrate to the anti philosophy crowd, is how critical the questions which philosophers ponder, happen to be. And one thing that I would hope for this debate to demonstrate to the pro philosophy crowd, is how important it will be for humanity to develop a respectable community of specialists which is able to provide accepted principles of metaphysics, epistemology, and value. Is it not clear how each side of this war should contribute to the unfortunate situation that we have today?

    To support science, as well as humanity in general, a new breed of philosopher may be required — one that’s able to effectively question inherited paradigms, and certainly the one which suggests that expert consensus isn’t needed in philosophy. As we’ve seen, physicists are in need of philosophical principles in order to more effectively do their work. All of science, however, should benefit from such support.

    How might my own two principles of epistemology help this particular situation? Well my EP1 would formalize that there is no true definition for “science”, but rather only more and less useful definitions from a given argument. Unfortunately today we commonly search for what “is” science, life, time, and so on.

    Then my EP2 proposes one unique process by which anything conscious, consciously figures anything out: It takes what it thinks it knows (evidence), which it uses to assess what it’s not so sure about (a model). As a model continues to remain consistent with evidence, it tends to become more and more believed.

    If a respected community of experts were to provide Sean and Peter these two epistemological principles, I believe that they would be able to more effectively do their work, as well as avoid these sorts of squabbles.

    Liked by 1 person

  9. Pentcho Valev


    “HUH? (On what Pentcho said, specifically in his last graf.)”

    Understandable reaction. Here is the fact: In 1887 the Michelson-Morley experiment was compatible with the variable speed of light posited by Newton’s emission theory of light, and incompatible with the constant (independent of the speed of the source) speed of light posited by the ether theory and later adopted by Einstein as his 1905 second postulate.

    The opposite has been taught for more than a century, and now everybody believes that the Michelson-Morley experiment has proved the constancy of the speed of light. (This faith has been diminishing in the last 10 years or so, mainly due to the philosopher John Norton telling the truth about the MM experiment.)

    Actually all confirmations of Einstein’s relativity are either fraudulent or inconclusive. Another example is the Pound-Rebka experiment. It confirmed the variation of the speed of light in a gravitational field predicted by Newton’s emission theory, which, on close inspection, means that general relativity was unequivocally refuted. Here is what the experimentalists themselves say:

    R. V. Pound and J. L. Snider, Effect of Gravity on Gamma Radiation: “It is to be noted that no strictly relativistic concepts are involved and the description of the effect as an “apparent weight” of photons is suggestive. The velocity difference predicted is identical to that which a material object would acquire in free fall for a time equal to the time of flight.”

    Yet, according to the mythology, the Pound-Rebka experiment has gloriously confirmed relativity.

    If the lie is taught for a long time it becomes absolute truth and further lying becomes pointless. Telling the original truth is then harmless:

    Albert Einstein Institute: “One of the three classical tests for general relativity is the gravitational redshift of light or other forms of electromagnetic radiation. However, in contrast to the other two tests – the gravitational deflection of light and the relativistic perihelion shift -, you do not need general relativity to derive the correct prediction for the gravitational redshift. A combination of Newtonian gravity, a particle theory of light, and the weak equivalence principle (gravitating mass equals inertial mass) suffices. […] The gravitational redshift was first measured on earth in 1960-65 by Pound, Rebka, and Snider at Harvard University…”


  10. Coel

    A commenter on my blog says that the sort of multiverse that I defended is uncontroverisal, saying: “I don’t know of anyone who finds this notion in the least bit controversial”. (Where I interpret this to mean controverisal as to whether it is “scientific”; there is obviously discussion as to whether inflationary cosmology and this sort of multiverse is true.)

    To be clear, that version of the multiverse is an “Emmental cheese” multiverse, where most regions of space are in an inflationary state (which behaves very differently from “normal state” space). Within that are myriad “bubble” universes that have dropped out of the inflationary state. Our entire observable universe would be contained within (and likely be much smaller than) one of these normal-state bubbles. In the most “vanilla-flavour” multiverse, all such bubble universes would have the same physics.

    The commenter implies that things only get controversial if we suggest that each bubble might have dropped out of the inflationary state in a somewhat different way (owing to local accidents of symmetry breaking; analagous to the fact that if you cool water to make snowflakes you get lots of different patterns) and thus that each bubble might have different “low-temperature” physics (different effective physical constants; analogous to different snow-flake patterns).

    There are people of a philosophical bent here, so I thought I’d ask: Do you think that there is anything “unscientific” from a philosophical/falsification point of view about the “vanilla” flavour multiverse outlined above, or are you happy to accept that as sufficiently “scientific”, and only start quibbling if we spice things up by throwing in different physical constants in different bubbles?


  11. brodix

    Here is an interesting paper on redshift:

    Click to access 2008CChristov_WaveMotion_45_154_EvolutionWavePackets.pdf

    “5. Conclusions
    In the present work, the effect of attenuation and dissipation on propagation of waves governed by the Jeffrey equation is addressed. When packets of small but finite breadth are considered the presence of dissipation changes the central wave number of the packet. The distribution of the wave length around the central length is assumed to be Gaussian which is the most frequently encountered case in cosmology when hot stars are observed. Dispersion relation for the damped wave equation is derived and the evolution of the packet density is investigated in time (or space). It is shown that the attenuation acts merely to decrease the amplitude of the shifts packed, while the dissipation damps the higher frequencies stronger than the lower frequencies and shifts the maximal frequency of the packet to lower frequencies (longer wave lengths), i.e., the packet appears redshifted upon its arrival. For Gaussian wave packets, this kind of redshift is linearly proportional to the time passed or the distance traveled. The coefficient of proportionality contains the ratio of the dissipation coefficient and the initial width of the distribution which means that the thicker packets are redshifted more than the narrower ones for the same distance or for the same time. We call this liner relationship ‘‘Hubble Law’’ for redshifting of wave packets.
    The new approach can be used in acoustics for devising methods for estimating the bulk viscosity of air, or other slightly compressible liquids based on the relationship between the ‘‘baseshift’’ and viscosity coefficient. An application to cosmology is also possible because the spectral lines measured in the experiments are wave packets, and never a single isolated wave comprising it. Thus, one has to take special care to distinguish between the redshift of the packet (as outlined in the present work) and the redshift due to the dilation of a single wave.”


  12. Massimo Post author


    Great questions. My understanding is that yes, string theory has emerged (in part) because of the incompatibilities between GR and QM.

    How much time should we give it in order to produce testable reasults? Clearly, there is no specific cut off point. But as I indicated in the OP, the more time passes the more the impatience of its critics seems justified.

    Parallels with previous situations are difficult, I think, because a major criticism of both string theory and the multiverse is that they do not seem to be able to make novel testable in principle. But of course that’s where the experts disagree, and that’s where I retire to my perch as an external observer and enjoy the spectacle…

    Liked by 3 people

  13. SocraticGadfly

    Pentcho, per Wiki’s page, Pound-Rebka doesn’t disprove Einstein.

    “The Pound–Rebka experiment is a well known experiment to test Albert Einstein’s theory of general relativity. It was proposed by Robert Pound and his graduate student Glen A. Rebka Jr. in 1959,[1] and was the last of the classical tests of general relativity to be verified (in the same year).”

    Note the word “verified.”

    Assuming that you’re trying to promote alt-physics of some sort, I’ll move along. Please feel free to do the same yourself.

    Liked by 1 person

  14. brodix


    It’s a bit like asking if you can have both rainbow striped and candy striped unicorns. If you can prove unicorns, why not?
    Does Inflation exit in the first place, or is it adding another epicycle to make the underlying theory fit the evidence?
    Given all the patches already required to make BBT work, is there anything that can falsify it?
    I think it is safe to say the geocentric cosmology didn’t have nearly as many patches before cosmologists started looking for other ideas.
    Given we do appear to be at the center of the expansion, wouldn’t reviewing optical explanations for redshift be a good place to start?


  15. synred

    Pentcho: The point was the waves in ether was a scientific theory. I don’t think anybody disputes that a particle theory of light was scientific. It was also falsifiable, and, falsified, but not by MM.

    Norton I think is the guy with what I view as the silly idea the classical mechanics is not deterministic because he can find two solutions to the equations sometimes. This neglects Newton’s second law — no force, no acceleration. Curiously, Carroll seems to buy into Norton.


  16. synred

    Astrology and creationism are falsifiable and have been, right? Well there are always epicycles, like God creating star light in transit…


  17. davidlduffy

    Bondi in Chapter 1 of Cosmology [1959]:

    “One possible mode of progress in answering these questions was to construct mentally a number of different models of such a universe and to investigate whether the physical laws could be applied to them consistently. The first such investigation seems to have been due to Olbers (1826)…It is not surprising that such a mode of attack led to a considerable number of model universes, each of them interesting and remarkable in its own right…Which one of these possible ‘universes’ is the one actually realized then becomes a secondary question, the answer to which must at present be found observationally, since there are no theoretical grounds for preferring any one of the possible models to any of the others.

    “The opposite point of view is reached from investigations in the borderland between physics and philosophy…[T]he fundamental laws of cosmology are immediate consequences of the a priori assumptions
    involved in any physical science. These fundamental laws play the part of axioms, and the main body of cosmology may be deduced from them in the manner of a discipline of pure mathematics. It may be found necessary, in one or two places, to supplement these basic axioms by other, less fundamental ones, in
    order to make progress, and in the choice of these supplementary axioms terrestrial physics will be a guide…
    making cosmology the truly all-embracing science, the long-sought link between philosophy and the physical world.

    “The other philosophical argument, while rather more vague, leads to the same result by suggesting that the world should be simple when viewed on a sufficiently large scale…This deep feeling that complexity is a feature of phenomena on our own scale only is one of the most powerful driving forces of science…

    “The contrast between the ‘extrapolating’ and the ‘deductive’ attitudes to cosmology is very great indeed, and most of the workers in the field adopt an intermediate position combining parts of both. However, it is clear that there is room for wide divergences of opinion..There is, however, a widespread lack of understanding and of appreciation of the differing points of view that makes the disagreements less fruitful for advancing the subject than they would otherwise be. In particular, the tendency on the part of the empirical school to regard any work of the deductive type as being ‘too speculative’ seems to be based on a complete misunderstanding…”


  18. davidlduffy

    I have snipped the subsequent paragraphs (which can be found at, which actually flag some of the complaints about the multiverse re lack of fecundity as one type of legitimate criticism of “deductive” models. If I understand correctly, the steady state model that Bondi and his mates championed was “deductive”, based on an axiom – the strong Cosmological Principle. Popper very much liked this style of hypothesis generation, with Einstein’s development of SR and GR from simple axioms as the exemplar.

    Liked by 1 person

  19. wtc48

    Massimo: “Great questions. My understanding is that yes, string theory has emerged (in part) because of the incompatibilities between GR and QM.”

    In this sense, perhaps the question of scale also plays a part, because of the difficulty of conceiving of
    objects that exist on a scale vastly different from our own, a difficulty that is in part due to the technological challenge of making, say, subatomic particles appear to be the size of basketballs. I would guess that continuing technological improvements in access might make some of these processes less mysterious. A hundred years ago, we had endless speculation about the canals, etc., on Mars, but now that we’ve been there (in a sense), we’re in a position to make some real observations.


  20. SocraticGadfly

    Maybe we need string astrology? Multiverse astrology? That would perhaps be a more up-to-date skewering than would luminiferous ether.

    “Your seventh string for the Higgs boson is currently in Venus ascending. Today’s ventures for finding the eighth string are profitable but should be approached with caution.”

    Liked by 1 person

  21. labnut

    I reiterate my claim that the multiverse ‘theory’ is a speculative hypothesis. That is not to criticise it since generating speculative hypotheses is a vital stage in the scientific process. It is at this stage that science is at its most creative and fecund as it creates many alternative scenarios for consideration. This is where Mao Zhedong’s dictum rules – let a hundred flowers bloom and a hundred schools of thought contend. The contention of ideas and their gradual refinement lies at the heart of Western thought and especially science.

    It is an unsettling and uncomfortable process for the grey beard partisans of established thought as they defend their orthodoxies against spirited challenges. But it gives encouragement to the Young Turks from whose ranks important new ideas will emerge.

    And so, instead of condemning the speculative multiverse hypothesis, we should engage in more ‘what if’ thinking, following these lines of thought as far as possible. We need a light playful touch in our thinking, one that liberates us from the strict confines of orthodoxy. De Bono would ‘colourfully’ advise us to put on as many thinking hats as possible. Eventually, of course, the profusion of ideas are culled by subjecting them to the discipline of testing against reality. The central question, in my opinion, is when this should be done. Some problems are exceedingly difficult and in that case we should not be in a hurry to cull the field of potential solutions.


  22. saphsin

    I found Peter Woit’s piece really helpful, and somewhat leaving me confused at the same time. I had the idea that string theory being untestable was about being too small in scale to confirm with our technological means was the extent of the issue rather than inherent problems as a scientific theory itself, which seems sort of a new thing. Is there any other known area of science where there is a similar issue? Massimo calls this scientifically-informed metaphysics but that doesn’t sound fitting, I’d call it…..well I have no idea what to call it to be honest.


  23. labnut

    To Continue…
    What makes a speculative hypothesis, science, in the sense of being part of the scientific process?
    1) It is rigorously derived from established science;
    2) It extends established science by using scientific reasoning;

    These two requirements distinguish it from metaphysics. The multiverse hypothesis meets these two requirements, so contrary to Massimo’s statement, it is not metaphysics but is instead a scientific claim.

    There is usually a third requirement:
    3) It is in principle capable of empirical testing.

    But it can happen, given the present state of scientific knowledge, that the means of testing cannot be imagined. In that case the speculative hypothesis has provisional status as science until scientific knowledge has advanced sufficiently to decide the matter.

    But what if it eventually becomes clear that the truth of the speculative hypothesis can never be empirically demonstrated for the reason that science has reached an impenetrable boundary?

    In that case we are reduced to asking which candidate is the most plausible and credible. Then we anoint it as such. Science will reach impenetrable boundaries and it is in our nature to reason about what lies beyond those boundaries. If this reasoning is rigorously derived from established science and extends it by using scientific reasoning then we can call these answers provisional science or speculative science. It is still science of a certain, and incomplete kind, that lacks the final seal of approval conferred by empirical testing.


  24. Bunsen Burner


    ‘…what I view as the silly idea the classical mechanics is not deterministic because he can…’

    No, classical mechanics is not generally deterministic, at least not over an entire domain. Norton’s example is one such issue, triple collisions are another. In fact gravitational problems have solutions where a particle can escape to infinity in a finite time. Currently we don’t even have a complete classification of all the singular solutions possible in a general N body problem. This is the proper response to Laplace – even systems of deterministic differential equations cannot guarantee you a unique solution for any initial condition for all time. Relativity in many ways makes this even worse, as the singularity theorems of Penrose and Hawking show, any dynamic spacetime will be geodesically incomplete.


  25. Massimo Post author


    “This is where Mao Zhedong’s dictum rules – let a hundred flowers bloom and a hundred schools of thought contend.”

    The problem with that approach is that hundreds of millions in funding, lots of faculty positions, and countless PhD studentships, hang in the balance. Physics simply can’t afford to let all that blooming go to waste.


  26. Massimo Post author


    “These two requirements distinguish it from metaphysics. The multiverse hypothesis meets these two requirements, so contrary to Massimo’s statement, it is not metaphysics but is instead a scientific claim.”

    Those requirements are met also by so-called scientific, or naturalistic, metaphysics, of the kind practiced by Ladyman & Ross.


  27. labnut

    Those requirements are met also by so-called scientific, or naturalistic, metaphysics, of the kind practiced by Ladyman & Ross.

    I have a copy of Everything Must Go, Metaphysics naturalized, by Ladyman and Ross. By dint of repetition, close reading and copious side reading I can begin to make sense of the book, though I struggle mightily because of my poor background in philosophy.

    But when confronted with the dense mathematical reasoning used to describe the multiverse hypothesis and its roots in physics, I have no chance whatsoever in understanding it.

    The two are hardly comparable. Everything Must Go is readily recognised as metaphysics and understandable as such. On the other hand, the multiverse hypothesis is readily recognised(by those with suitable training) as a scientific statement strongly rooted in both theoretical and experiemental physics and abstruse mathematics. The so-called metaphysical element in its reasoning is more of an aspersion cast by those who dislike its prediction of untestable outcomes while the metaphysical element is everything in .Everything Must Go.

    What should science do when it encounters seemingly impermeable barriers? Through up its hands in despair, saying thus far and no further, leaving the field to ill equipped philosophers? Or should they struggle mightily to see through these barriers, even if only in their mathematical imagination? I suggest the second path is more likely to be fruitful because a sustained scientific assault from many directions may yield unexpected insights and push back that barrier. We will only know if we try.

    I am critical, not of the multiverse hypothesis, but of its proponents, for greatly overstating the strength of their claims.


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