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 arxiv.org: 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 NPR.org.

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Categories: Metaphysics, Philosophy of Science

98 replies

  1. Can we retire this unnecessary Multiverse Hypothesis meme, and stop conflating it with String Theory? It really gives the appearance of intellectual laziness and an ideological agenda. What the multiverse really means is that we have many causally disconnected regions of spacetime. Classic Big Bang cosmology predicts a multiverse for large enough universes. All inflation has done is predict a global structure to these regions of spacetime. Sure, we’ll never be able to explore this global structure, but what are cosmologists supposed to do? Place a ‘Here be Dragons’ sign for anything outside our Hubble volume? Also, no one can know a priori if the analysis of the global structure will yield empirical results or not.

    Now, the idea that these regions may have different particle physics is speculation. However, even though this is a possible interpretation of the String Landscape, it’s actually an idea that has existed independent of String Theory. Coel has provided a good and clear synopsis of this. If we take the Standard Model seriously then the idea that the various symmetry breaking always happens the same way seems unlikely. And the idea that these separate spacetime regions may have different periodic tables is not considered terribly controversial by most particle physicists I’ve talked to. The fundamental laws will still be the same. I think Smolin is the only one I know who considers changes to actual fundamental laws.

    Liked by 1 person

  2. Speculative theories like string theory and inflationary multiverse theory perhaps could be a mix of (physically-motivated) metaphysics and physical mathematics (“physically-motivated mathematics” [1]).

    [1] https://en.wikipedia.org/wiki/Physical_mathematics

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  3. Massimo said:

    Synred,

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

    Depends on the nature of the failure.

    Per the issue at hand, it also sounds like it’s a question of accepting whether one has failed or not.

    Liked by 1 person

  4. https://goo.gl/hFe16t
    https://arxiv.org/abs/1801.06944

    Fine-Tuning, Complexity, and Life in the Multiverse
    Mario Livio (University of Nevada, Las Vegas), Martin J. Rees (Institute of Astronomy, University of Cambridge)
    (Submitted on 22 Jan 2018)
    The physical processes that determine the properties of our everyday world, and of the wider cosmos, are determined by some key numbers: the ‘constants’ of micro-physics and the parameters that describe the expanding universe in which we have emerged. We identify various steps in the emergence of stars, planets and life that are dependent on these fundamental numbers, and explore how these steps might have been changed, or completely prevented, if the numbers were different. We then outline some cosmological models where physical reality is vastly more extensive than the ‘universe’ that astronomers observe (perhaps even involving many ‘big bangs’), which could perhaps encompass domains governed by different physics. Although the concept of a multiverse is still speculative, we argue that attempts to determine whether it exists constitute a genuinely scientific endeavor. If we indeed inhabit a multiverse, then we may have to accept that there can be no explanation other than anthropic reasoning for some features our world.

    An argument from our lack of imagination? And why should higher levels like GR and field theory be so coherent driving from simple principles…when the larger universe is incoherent hodge podge.

    Liked by 1 person

  5. Bunsen,

    Why does the process always march forward?

    To raise an issue I’ve brought up before, when redshift was first discovered, the two assumptions were basic Doppler redshift and the universe was expanding, or something was interfering with the light, but since it was otherwise clear, this “tired light” hypothesis was dropped.

    Then it was discovered that redshift is directly and increasingly proportional to distance in all directions, creating the impression that we are at the center of the universe. So then it was argued that space itself must be expanding, based on the premise of spacetime. But that seems to overlook the central premise of Relativity and thus spacetime, that clocks and rulers dilate equally, so that light is always measured as a Constant. If it is taking light longer to cross, in order to be redshifted, between galaxies, it obviously is not Constant to this universal frame. No one seems to be able, even Coel, to explain why this isn’t some bad plot device, they just say that’s the way it is.

    Of course, given we do appear as the center of the universe, to any logical person, that might suggest looking for optical explanations, but that doesn’t seem to occur to anyone with any say so.

    I realize it’s bad politics to go against the march of progress, but I’m one of those people who hate bad plot devices.

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  6. Bunsen Burner

    , but what are cosmologists supposed to do? Place a ‘Here be Dragons’ sign for anything outside our Hubble volume?

    Perhaps the sign could say “we don’t know what be here”.

    Liked by 3 people

  7. As for Norton’s Dome, I would have thought the real problem would be that it is possible to define a dome shaped such that, in principle, an object given just the right initial velocity at the base would reach the apex in a finite time and have a zero velocity when it reaches the apex.

    Given that shape and time reversibility of Newtonian physics then a static object at the apex of such a dome ought to begin to slide down after an arbitrary time period in an arbitrary direction.

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  8. Bunsen,

    “Can we retire this unnecessary Multiverse Hypothesis meme, and stop conflating it with String Theory? It really gives the appearance of intellectual laziness and an ideological agenda”

    What “agenda” would that be? Intellectual honesty? Nobody conflates the two, but critics see a commonality there, in terms of dearth of ways to produce any testable empirical predictions. Also, sociologically, many people who accept one also accept the other.

    “what are cosmologists supposed to do? Place a ‘Here be Dragons’ sign for anything outside our Hubble volume?”

    No, something more like Robin’s suggestion: “we don’t know what and if anything is going on here, so we’ll politely shut up about it instead of sucking hundreds of millions in resources and student/faculty positions while pretending we know something we don’t.”

    Liked by 1 person

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