Why Trust a Theory?

I noticed today that Cambridge University Press has recently published Why Trust a Theory?, a volume of articles based on a December 2015 conference held in Munich. The book is available online here (if your university is paying for it…), and preprint versions of many of the contributions are on the arXiv.

The conference had its origins in a piece published a year earlier in Nature by George Ellis and Joe Silk, entitled Scientific method: Defend the integrity of physics. Ellis and Silk made a forceful case that widely advertised but inherently untestable string theory and multiverse research does damage to the public understanding of science and is a threat to the credibility of science at a time it is under attack. The piece suggested:

A conference should be convened next year to take the first steps. People from both sides of the testability debate must be involved.

Looking through the proceedings volume, there’s lots of abstract discussion of philosophy of science and some diversity of points of view on the multiverse. When it comes to string theory though, the organizers interpreted “people on both sides” to mean bringing in one person willing to point out that there is a problem with string theory, and an army of string theorists to defend the theory. On the issue of the problems of string theory, the volume contains nearly 100 pages of pro-string theory hype, from Polchinski (two contributions), Silverstein, Kane and Quevedo. As usual with Kane, there’s a string theory “prediction” of the gluino mass (1.5 TeV +/- 10-15%) which has already been falsified. All I could find on the side of substantive criticism of string theory was in Carlo Rovelli’s contribution (preprint version here), and mainly in a single paragraph:

String theory is a living proof of the dangers of excessive reliance on non-empirical arguments. It raised great expectations thirty years ago, promising to compute all the parameters of the Standard Model from first principles, to derive from first principles its symmetry group SU(3)×SU(2)×U(1) and the existence of its three families of elementary particles, to predict the sign and the value of the cosmological constant, to predict novel observable physics, to understand the ultimate fate of black holes, and to offer a unique, well-founded unified theory of everything. Nothing of this has come true. String theorists, instead, have predicted a negative cosmological constant, deviations from Newton’s 1/r^2 law at sub-millimeters scale, black holes at the European Organization for Nuclear Research(CERN), low-energy super-symmetric particles, and more. All this was false. Still, Joe Polchinski, a prominent string theorist, writes [7] that he evaluates the Bayesian probability of string to be correct at 98.5% (!). This is clearly nonsense.

I won’t spend more time here discussing the conference and the articles in this volume, mainly because I’ve already written a lot about this in previous posts. For a contemporaneous discussion of the conference and Polchinski’s String Theory to the Rescue paper, see here and here. There are also interesting blog posts about the conference from Massimo Pigliucci, see here, here and here, and a Quanta piece by Natalie Wolchover here. For a discussion of Sean Carroll’s Beyond Falsifiability contribution, see here (and discussion here and here). For a discussion of Eva Silverstein’s contribution, see here.

Update: A few more links to material about the Munich conference: Jim Baggott here and here, Andrew Gelman here, Davide Castelvecchi here, and the conference website (with videos) here.

Update: Looking at the Preface, I notice that the editors claim:

Additional contributions were solicited by the editors with the aim of ensuring as full and balanced presentation as possible of the various positions in the debate.

With regards to string theory, the one additional contribution in the volume is from string theorist Eva Silverstein, so evidently the editors felt that balance required yet more on the pro-string theory side….

Update: I mischaracterized Polchinski’s calculation of the probability that string theory is correct as 98.5%. More accurately, he claims that the probability is “over 3 sigma” (i.e. over 99.73%).

Update: I finally got around to watching the videos of the panel discussions at the workshop (all videos available here). What most struck me about these discussions was the heavily dominant role of David Gross, who was on two of three panels, participating from the audience in the third. On the panels he was on, Gross was speaking far more than anyone else, and rarely if at all would anyone disagree with him. Gross’s point of view is that there is a testability problem with the multiverse, but all is well with string theory (although probably not at Polchinski’s “over 99.73% sure to be true” level). He’s a powerful intellect and a forceful speaker, so it’s not surprising that no one would take him on. But on the topic of string theory I think there are very serious problems with many of the claims he makes (for his arguments of 15 years ago, see the first substantive post of this blog), and the organizers should have found someone willing to challenge him on those.

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25 Responses to Why Trust a Theory?

  1. Allow me to mention that I declined writing a contribution because at the time I was writing my book & felt the overlap would be too large. With my contribution missing, the balance of the proceedings shifts somewhat towards string theory. Though I am more critical of Richard Dawid’s defense of string theory than of string theory itself.

  2. Peter Woit says:

    Thanks Jim and Sabine,

    I’ve added a few more links to the post, including Jim’s.

    I do still wonder though how the organizers of this conference and volume would justify the heavily one-sided nature of what they produced. Was it really impossible to find anyone other than Rovelli willing to write about the problems with string theory?

    By the way, I was looking for an excuse to mention Sabine Hossenfelder’s talks nearby next week, she’ll be giving multiple talks at Brookhaven on April 9, and
    a talk at Yale on April 10.

  3. Petite Kabylie says:

    Dear Peter
    In my opinion, the fact that Lee Smolin and you Peter, who were the first to have seriously and brilliantly raised the issue with your books, have not been invited to contribute says it all.
    I just wonder whether you Peter (and Lee Smolin) would have accepted to give a talk at the conference if they had invited you.

  4. Peter Woit says:

    Petite Kabylie,
    I believe Lee Smolin was invited to give a talk but declined. I would have participated if invited.

  5. Lee Smolin says:

    Dear PK,

    Peter is right, I was invited and was unable to go in the end. I was just looking at my correspondence with the organizers and, while I don’t want to quote email without permission, they indicated that they were looking for a single person to represent the “LQG point of view.” What I have to say about Richard Dawid’s book was contained in a review of it I wrote earlier. In short, I argued that the 3 non-empirical criteria he gives for taking a theory seriously could be applied equally as well to LQG as they could to string theory, which shows that they are ambiguous and of no help when we are faced with choosing between competing research programs.

    This is why I argue that when there are two or more competing research programs, each of which has non-trivial results that offer considerable, but not decisive evidence for their being the truth, we have to support them all, and also leave room for new hypotheses and research programs. This is argued in detail in Chapter 17 of TTWP.



  6. shantanu says:

    Peter, are the questions asked recorded in the proceedings?

  7. Peter Woit says:

    I don’t think there’s anything in the Cambridge volume concerning the panel discussions or questions asked at the conference. There are videos of these online (I just took a quick look, but they didn’t seem to be working now). The authors of the articles in the volume do an excellent job of not responding to the serious questions someone should have asked about the current state of string theory.

  8. James Gallagher says:

    Hi Peter

    Suppose you were invited:

    Would you just talk about the easy criticism of the current theories, or perhaps, suggest anything new yourself ? (not a theory (obviously) but a new approach to theory finding maybe?)

  9. Peter Woit says:

    James Gallagher,
    The topic of the workshop was not a call for new theories or new approaches. It was supposed to provide a venue for “both sides of the testability debate” to present their views. At least as far as string theory is concerned, the organizers decided not to do that, but to have the discussion of string theory (especially in the Cambridge volume) consist just of string theorists defending string theory.

    If I had been invited to the workshop or to contribute to the book, I would have tried to represent the otherwise unrepresented side of the debate.

  10. shantanu says:

    Sabine had some hard questions for David Gross after his talk. He mentioned 19 tests of falsification of string theory. I don’t know if they are mentioned after the talk.

  11. Peter Woit says:

    I just checked the video of the Gross talk, it cuts off at the end of the talk, no questions from the audience included. Not surprising that Sabine was the one willing to challenge Gross, too bad she wasn’t on the panels.

  12. Blake Stacey says:

    The “Popperazi” accusation particularly rankles, because by the time Karl Popper was about four years old, William James had already skewered the next-level vagueness we now see in multiversology:

    The more absolutistic philosophers dwell on so high a level of abstraction that they never even try to come down. The absolute mind which they offer us, the mind that makes our universe by thinking it, might, for aught they show us to the contrary, have made any one of a million other universes just as well as this. You can deduce no single actual particular from the notion of it. It is compatible with any state of things whatever being true here below.

  13. shantanu says:

    Peter I looked at that post of yours regarding David Gross’s talk. Do you know if the two string theory postdocs who attended that are still doing string theory?

  14. Peter Woit says:

    At this point I don’t remember exactly who I had in mind. Most of the theory postdocs from that era, like any recent era, didn’t end up staying in the field because of lack of jobs. This is true of both string theorists and non-string theorists.

    It was interesting to go back and see what the situation was 15 year ago. My commentary then was
    “Gross and others seem intent on ignoring the failures of string theory, desperately hoping that superpartners will pop out of the LHC, thereby providing at least some vindication of the train of reasoning that led to string theory. What will be interesting to see will be what Gross et. al. do when this doesn’t happen. Will they drop string theory?”

    Unfortunately we now know the answer, and it is clear that belief in string theory has become completely decoupled from any possibility of experimental test.

  15. Bernhard says:

    I sometimes think this string theory/multiverse story is best left alone. Whenever I’m in a conference where real physics results are discussed this stuff tends to be ignored and I star believing this group lives already in their own parallel universe.

    But, whenever I give an outreach talk I remember again of the danger. A few months ago I gave a BSM talk for high school kids. At the end of the talk, they were a bit disappointed to know I was not going to talk about strings and the multiverse…

  16. Petite Kabylie says:

    Thank you Lee.
    I am sure it would have been a historic conference if both you and Peter were there.

  17. tulpoeid says:

    The banner today is a nice touch.

  18. AcademicLurker says:

    tulpoeid: I was actually a bit alarmed to open this page and find myself looking into the Eye of Sauron…

  19. The older I get, the more Felliniesque do discussions of the frontiers of HEP seem. Surely they have always been so, but we tend to imbue things with more seriousness when young. “Known for his distinct style that blends fantasy and baroque images with earthiness, he is recognized as one of the greatest and most influential filmmakers of all time.” He was a fan of vaudeville, of course, and I imagine were he alive today, and did he care at all about theoretical physics, he would have referred to it as HEV.

  20. Low Math, Meekly Interacting says:

    Stunning. Jean-Pierre Luminet’s 40-year wait has ended.

  21. Why trust theory?
    Because you can make consistent computations…
    This is the message I can understand from Petr Horava in a panel discussion at the recent European workshop: Quantum Spacetime ’19 Bratislava, 11-15 February 2019 (https://youtu.be/S_nvO4Kembw?t=3245)
    “I think mathematically, on paper, String Theory is that [a kind of theory of quantum gravity as least as good as QED ] because it allows you to calculate graviton-graviton scattering consistently, it has all kinds of beautiful properties that make you think this theory is in some technical sense UV complete and how much more you want to ask at a first stage?”

    “Why the standard model?” comments Ali Chamseddine from the audience (https://youtu.be/S_nvO4Kembw?t=3459) who don’t intend to attack string theory but want to emphasize the urge to learn the lessons gained from other fundamental interactions to address quantum gravity.

  22. Peter Woit says:

    cedric bardot,

    This is just the usual argument that “OK string theory doesn’t work as a unified theory, but at least it’s a consistent theory of quantum gravity”. This isn’t actually true, since there is no non-perturbative theory (what is the amplitude for a black hole in the final state when you collide two gravitons?). The secondary dodge is “OK, all we have is an effective theory at low energies, but QED is just an effective theory at low energies”. I long ago decided that trying to argue with the “string theory is exactly like QED” people is just a waste of time, no different than trying to argue with people who claim “black is white, since they are both shades of gray”.

  23. Jim Given says:

    A “testability debate” is such an unusual occurrence in the history of the natural sciences. Where is there a precedent? Perhaps mid-1990’s attempts by Freudian analysts to re-assert the reality of Freudian theory against the barrage of attacks by senior scientists and philosophers that ended the 1980’s. But the discussion, by that time, had ended, except in the minds of true believers. (Something similar may have occurred with string theory.)
    Really, “testability” like the earlier “falsifiability” seems not to be a good way to evaluate scientific theories. Rather, the test of a productive research program should be the interesting new experiments, and new discoveries in science motivated by efforts to test it. What would be recent examples in this category for string theory?

  24. Peter Woit says:

    Jim Given,
    I agree that “is it testable” is too simplistic. A better criterion may be the Lakatos productive vs. degenerative one, and string theory unification is a classic example of a degenerative research program. Put differently, instead of asking how close string theory is to giving a unified theory (these days, string theorists seem happy with “maybe centuries away”), you should ask whether it is getting closer or farther. I think there’s no question the answer is “farther”: 30 years ago string theory was almost there, now it’s much, much farther.

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