Just when I thought I was done for now with the “falsifiability” business, in our local book store I found a new book, The Scientific Attitude: Defending Science from Denial, Fraud and Pseudoscience, by Lee McIntyre. This won’t be a review of the whole book, much of which is concerned with what to do about the serious problem of the role of science in our increasingly post-truth society. I’ll just address the few pages of the book that deal with string theory, in which a quote from me appears in a misleading way.
The problem here is almost exactly the same as the problem with the Symmetry article discussed in the last posting. Both authors believe that string theory is a conventionally predictive theory, one with predictions that just happen to be hard to test. According to them, critics of string theory just don’t understand that there can be value in a theory which is testable in principle, even if a practical test is far away. Unlike the Symmetry piece, McIntyre at least names critics and links to their words, writing:
If one reads these kinds of criticisms closely one finds careful phrasing that string theory “makes no predictions about physical phenomena at experimentally accessible energies” and that “at the moment string theory cannot be falsified by any conceivable result.”23 But these are weasel words, born of scientists who are not used to taking seriously the distinction between saying that a theory is “currently” testable versus whether it is “in principle” testable. The practical limitations may be all but insurmountable, but philosophical distinctions like demarcation live in the difference.
The quoted words are mine, with footnote 23 referring to my 2002 article in American Scientist. Of course I was and am well aware of the distinction between testable “in principle” and “currently”. Bizarrely, the author has chosen to edit out from what I wrote the sentence that precisely addresses the issue I’m supposedly weaseling on. Here’s the full quote:
String theory not only makes no predictions about physical phenomena at experimentally accessible energies, it makes no precise predictions whatsoever. Even if someone were to figure out tomorrow how to build an accelerator capable of reaching the astronomically high energies at which particles are no longer supposed to appear as points, string theorists would be able to do no better than give qualitative guesses about what such a machine might show. At the moment string theory cannot be falsified by any conceivable experimental result.
As the deleted language make clear, by “any conceivable experimental result” I was making a claim about “in principle”, not “currently”. Furthermore, near the beginning of the article I explain the problem of principle:
First, string theory predicts that the world has ten space-time dimensions, in serious disagreement with the evidence of one’s senses. Matching string theory with reality requires that one postulate six unobserved spatial dimensions of very small size wrapped up in one way or another. All of the predictions of the theory depend on how you do this, but there are an infinite number of possible choices, and no one has any idea how to determine which is correct.
This article started out as an early 2001 arXiv posting and was published in early 2002, about a year before the now famous KKLT claim to have a string theory model with fully stabilized moduli. Back then, the problem I was pointing to was the basic one that, to have a self-consistent string theory model that you can confront, in principle, with experiment, you need to solve the problem of “moduli stabilization”. 6d compactifications come in families with a lot of parameters (the “moduli”) governing their size and shape, and the physics depends crucially on those parameters. You need to somehow give the moduli dynamics, and get a ground state with a correct fine-tuned vacuum energy.
KKLT claimed they could do this, but with an exponentially large “landscape” of solutions that removes the ability to get well-defined predictions from the theory. Their construction is so complicated, and non-perturbative string theory so poorly understood, that it remains controversial to this day whether these are really solutions to whatever the conjectural well-defined version of string theory might be. This is what the current “Swampland” argument is about.
I’ve put together a FAQ entry answering the Doesn’t string theory make predictions at very high energy? question. What causes all the confusion here is the common claim from string theorists that “string theory is testable at high energy”. If you ask them to tell you what the “test” is, they tell you about one of the characteristic features of the perturbative superstring (Veneziano amplitude, Regge trajectories, 10 space-time dimensions). What they are really saying is “if we did experiments at a high enough energy scale and saw one of these characteristic phenomena, we would have a successful test of string theory”, which is true enough, but not a specific, falsifiable prediction. What they are not telling you is that they are ignoring the compactification problem as well as that of not having a well-defined non-perturbative theory, and that many “string theory” models wouldn’t exhibit these characteristically perturbative features.
The main point of the new book seems to be to argue that a better way to characterize science is by whether those supposedly engaging in it are exhibiting the “scientific attitude”, which
can be summed up in a commitment to two principles:
(1) We care about empirical evidence.
(2) We are willing to change our theories in light of new evidence.
It seems to me there are lots of problems with this formulation. Sticking to the string theory question, undoubtedly string theorists “care about empirical evidence” and would like to have some. The problem though is they don’t have any, and don’t have any significant prospects for getting any. As for being willing to change one’s theories in light of new evidence, if there’s no new evidence, your willingness to change your theory won’t ever get tested.
My impression is that most people, this author included, are just fundamentally unwilling to believe that, given the high scientific profile of “string theory”, it could really have a serious problem of being inherently untestable. The technical issues involved are so formidable that non-experts don’t have any hope of understanding them. But there really is a serious problem here, and those who worry about the string theory fiasco damaging the credibility of science in a dangerously post-truth world are right to be worried.
Last Updated on