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. Massimo Post author


    The difference between speculative science and metaphysics in the case of string theory and multiverse may be very clear to you. It most definitely is not to me.


  2. saphsin


    I’ll have to disagree with you there. We can speculate about abiogenesis or even far murkier things which we’ll even more likely have never have proof for, but there’s a clear difference between that and talking about ontology. I think Labnut is saying that there is something about the nature of scientific speculation and metaphysics-talk that makes them descriptively separate.

    Liked by 5 people

  3. labnut

    The difference between speculative science and metaphysics in the case of string theory and multiverse may be very clear to you. It most definitely is not to me.

    In a certain sense I agree with you. We all bring metaphysical assumptions to bear on the way we evaluate schools of thought.

    For example. As a theist I believe in an infinitely creative God. That makes me receptive to the multiverse hypothesis since an infinite number of universes bubbling out of ‘nothing’ is exactly what I would expect from an infinitely creative God.

    On the other hand, staunch atheists, such as Krauss and Hawking, believe that the multiverse theory delivers a knockout punch to theism.

    The irony is that metaphysical preconceptions motivate both sides to believe in the same thing!

    While I continue to believe there is a clear difference between speculative science and metaphysics in this case, I readily concede that speculative science rests on a foundation of metaphysical preconceptions.


  4. Massimo Post author


    String theory and the multiverse are about the foundations of reality. And they are not (currently, foreseeably, possibly ever) empirically informed. If that’s not metaphysics, I don’t know what is.

    Liked by 1 person

  5. saphsin

    I think there’s a difference between describing the lower-level physical foundation of something and describing the ontology of something. Am I making additional metaphysical claims if I were to describe how cells are collections of molecules?


  6. Massimo Post author


    If you are describing cells as collections of molecules you are, quite clearly, making an ontological statement. If that statement is supported empirically it is science. If not, it is (perhaps science informed) metaphysics.

    Liked by 1 person

  7. saphsin

    If we’re talking about metaphysical implications by of physical descriptions, than we’re always making metaphysical implications for all scientific claims, but that’s not making an ontological statement in of itself. Or else I don’t know what the difference between metaphysics and science is at any level.


  8. synred

    if I were to describe how cells are collections of molecules?

    Cells are more than that, they are a process — more wave (soliton) than the stuff they’re made of. We’re back to process metaphysics previous discussion.

    Then we enter QM metaphysics which gets really subtle.

    Can bacteria be made simple enough that they interfere? I don’t think so, but you could try.
    The question whether quantum mechanics is complete and the nature of the transition between quantum mechanics and classical mechanics have intrigued physicists for decades. There have been many experimental breakthroughs in creating larger and larger quantum superposition and entangled states since Erwin Schro¨dinger proposed his famous thought experiment of putting a cat in a superposition of both alive and dead states in 1935. Remarkably, recent developments in quantum optomechanics and electromechanics may lead to the realization of quantum superposition of living microbes soon. Recent evidence also suggests that quantum coherence may play an important role in several biological processes. In this review, we first give a brief introduction to basic concepts in quantum mechanics and the Schro¨dinger’s cat thought experiment. We then review developments in creating quantum superposition and entangled states and the realization of quantum teleportation. Non-trivial quantum effects in photosynthetic light harvesting and avian magnetoreception are also discussed. At last, we review recent proposals to realize quantum superposition, entanglement and state teleportation of microorganisms, such as viruses and bacteria.

    Double slit with viruses, but not while they’re infecting anything. ‘Schrodinger’s Cat and the Law’ a fable with metaphysical overtones.


  9. wtc48

    Labnut: ” 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. ”

    This situation calls to mind the method of the medieval scholastics, juggling concepts to discover inconsistencies in an attempt to harmonize them with recognized authorities, a sort of subjunctive version of scientific method: “If there were angels (and they occupied no space), an infinite number of them could fit on the (dimensionless) point of a needle.”


  10. synred

    countless PhD studentships

    There aren’t that many…counting would be difficult … a rough number has likely been guessed by someone …

    But money (fairly modest amounts) is likely driving the argument.


  11. synred

    The problem with brane/strings being untestable is not one of scale, but the number of possible theories — if I recall ~10^500 — and no prescription for which we are — thus leading people to advocate for use of Weak Anthropic Principle (WAP) and such monstrosities —

    Even WAP might not make it make it completely untestable. The theory or theories you find work, might make predictions about the our world. I don’t think however they even have a means for apply WAP to the theories and picking out which to try, but that could conceivably change.

    You pick a theory that describes a world like ours and see what else it predicts. Test and repeat. 10^500 is a really big number, so you needs some way to trim it down.


  12. synred

    Multiple solutions is not the same as indeterminism. If you balance a prefect pencil on it’s point, it won’t fall if you don’t give it ever so tiny a push or miss the standing point even by infinitesimal amount.

    There’s plenty of indeterminate behavior in the world, but not in toys.

    PS. Norton’s specific dome seems irrelevant to me (‘a red herring’). Any unstable equilibrium point will do.


  13. synred

    ‘2+2=4’ is math

    ‘2+2=4 is true’ is meta-math

    ‘2+2=5 is true’ is meta-math that’s wrong.

    Then we can pile up the turtles or more sensibly just stop.


  14. Massimo Post author


    What would count as making an ontological statement “in and of itself”?

    And no, I don’t see a sharp boundary between physics and metaphysics, though theey are clearly two distinct peaks in the broader intellectual landscapes. But peaks grade into each other by way of the intervening valleys.

    Liked by 1 person

  15. synred

    : “If there were angels (and they occupied no space), an infinite number of them could fit on the (dimensionless) point of a needle.”

    If they were QM angels they might not even be in an eigenstate of number…</:-)


  16. labnut

    And no, I don’t see a sharp boundary between physics and metaphysics,

    All science makes metaphysical assumptions and in that sense, if you look carefully, with a suspicious mind, you will find some metaphysics in papers about physics. But that does not mean there are no sharp distinctions. Think of it this way. My house and my guest cottage may share a common foundation (metaphysics?) but there is a very clear distinction between my main house and my guest cottage. Arguing that they share a foundation does not makes this distinction go away.


  17. Massimo Post author


    I said in a comment to Sapshin that there are distinctions, and also gradations. So we don’t seem to disagree on the substance of the matter (the relationship between metaphysics and physics), but only about whether the specific cases of string theory and the multiverse belong more properly to one “peak” or the other. Nothing crucial hinges on such disagreement.

    Liked by 1 person

  18. saphsin


    I don’t believe in a sharp distinction either, philosophy and science are human constructed categories that we use to generalize certain types of intellectual activities, and they end up meshing because even empirical science has elements of “pure thought” to it. But I think merely describing and understanding cells as made up of collection of molecules is just a matter of naming parts and processes and how they fit together rather than trying to make a point about the essence of the system of parts. Or else we’re lead to say that architects recognizing how all the parts of the building fit together are practicing ontology, and I don’t think that describes well philosophy as an activity.


  19. saphsin

    Also I have to admit I had to struggle to make any comment that might be worth contributing here because the details of String Theory are completely out of my depth, and I have done some reading before. This exchange interesting to read though.

    Liked by 1 person

  20. labnut

    they are clearly two distinct peaks in the broader intellectual landscapes. But peaks grade into each other by way of the intervening valleys.

    You like your intellectual landscape metaphor and I admit it is a powerful metaphor, so I will build on it. Your landscape may be riven by deep crevasses. You have said as much when you argued against the unity of the sciences. I will go further and argue that your intellectual landscape is an island bounded by the sea of the unknown.

    You, the meta-physicist, are free to speculate about what lies in any part of the sea while you keep your feet on dry ground.

    The physicist, on the other hand, will construct a pier, attached to the rocks on the shore, and with footings driven into the sand. The attachment to the rocks is the firm connections with good, existing science, as are the sturdy footings driven into the sand, and the planks of the pier. This is what makes his work science, the firm connection with existing science and the use of the building materials of science and this is what sharply differentiates it from metaphysics..

    The physicist will extend his pier as far out into the sea as he can find good footings. Having extended his pier as far as it will go he finds he has a better view than the armchair meta-physicist. From this vantage point he dares to imagine what the view would be like further out to sea. The armchair meta-physicist never dared to imagine this because he never left his armchair to go to the end of the pier and so cannot see what the physicist has seen. Our physicist is comfortably ensconced with his fishing rod on the end of the pier. He is more likely to get a bite than our armchair meta-physicist who is still somewhere on the shore, counting grains of sand. 🙂


  21. Robin Herbert

    The Scholastics never really did discuss how many angels could fit on the head of a needle.

    The Scholastics did discuss what would happen if a rock were dropped down a hole bored through the centre of the Earth, or what would happen if a rock were poised exactly between two worlds, would it fall to the one or the other or remain exactly between. They discussed whether we could tell from the evidence whether the Sun orbited the Earth or whether the Earth remained in place and rotated. They discussed whether you could tell from any demonstration whether you were moving or not if you could not feel or see the movement. And they pointed out that inhabitants of any star or planet would see themselves as the immobile centre of the Universe just as we do, and that an infinite universe would have no centre, except relatively. They asked if distance was constant or varied with velocity. They were also busy working out such things as the mathematical relationship between acceleration and distance travelled.

    There was plenty of Scholastic metaphysics that informed science.

    Liked by 1 person

  22. Massimo Post author


    Did you miss my caveat about dinstinctions being made on the basis of empirical evidence? It is when the latter is missing that we are practicing metaphysics. As in the case of string theory and the multiverse.


  23. Massimo Post author


    “The physicist will extend his pier as far out into the sea as he can find good footings”

    The problem is that, in the case of strings / multiverse, there is no such good footing.

    Liked by 1 person

  24. synred

    I would thing the assertions that ‘string theory is correct’ or ‘everything is made of strings’ is meta-physics.

    String theory is physics or maybe it’s ‘just’ math. It was intended to be physics, but may not work out…

    Trying to develop a theory and failing is still doing physics.

    Liked by 1 person

  25. synred

    The existence of regions beyond our reach such as inside black holes or beyond the event horizon is pretty clear cut.

    The Andre Linde vision of vista’s of bubbling multi-verses is problematic.


  26. brodix

    Is empty space physics, or metaphysics?

    It seems Physics treats it as an effect of geometry, rather than geometry a mapping of space, but if clocks and rulers are dilated in a moving frame, what are they moving relative to? Wouldn’t the frame with the fastest clock and longest ruler be closest to the equilibrium of the vacuum, through which light travels at C?

    If so, than the equilibrium of space, the balance of a non-physical state, is measurable with physical instruments.


  27. synred

    Well, yes and no. There are ‘crackpots’. You can fail empirically or it may turn out that your theory is not very predictive.

    An earlier example is Regge poles and S-matrix theory. This attempted to end the infinite regress by supposing that hadrons are made out of each other and the constraints of analyticity, unitarity and symmetry could allow you derive it’s their properties. It has a ‘process ontology’ sort of feel to it.

    The Regge part was pretty ad-hoc, and at first fairly predictive, but at some point too man ‘epicycles’ were needed and so it failed and was dropped [a],

    The analogy to what’s happening to string theory is imperfect, but it seems to me to be going down in a somewhat similar way.

    String theory math has proved useful in Quantum Field Theory, so it’s not a dead loss.

    [a] Some baby might have gotten tossed with bath water.


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