I don’t think I’m twisting your words to say that you have given up on explaining anything about observable physics, by giving up on unification and any of the open questions of the standard model. Worse than that, string theorists have constructed a scenario with no evidence for it, designed to avoid admitting failure, and are aggressively selling this in the media, to students and colleagues, ensuring that future generations won’t try and attack these problems, since it is “well-known” that they can’t be solved.

It is possible to pursue a question purely by the criterion of internal consistency, with no checks from the real world. Mathematicians do it all the time. But if you’re going to do this, the lesson mathematicians all know is that you need to be quite precise about what you are doing, very clear in your arguments, and very honest with yourself when something doesn’t work. As far as I can tell, the way quantum gravity is being pursued by “string theorists” (I think the scare quotes are important, at this point, any actual theory involving strings seems to have little to do with this) involves none of this discipline at all. It’s pretty clear where this kind of activity ends up, and its not with any reliable understanding of anything.

]]>I wouldn’t say that people have given up on the question of what M-theory is, rather the focus has moved beyond the aspects of that question easiest to answer using existing tools such as perturbation theory around simple backgrounds, and effective field theory.

One outstanding issue is what becomes of the equivalence principle in a quantum theory of gravity. Accelerated frames are related to thermal effects in the quantum theory, and related notions of density matrices and quantum entanglement; so these are likely to be some of the ingredients, and they also tie into the information paradox. A conceptual framework that ties all these notions together is currently lacking, but is certainly a major topic of current research. I could give other examples.

The first-principles exact calculation of black hole entropy, absorption/emission amplitudes, and so on, in particular controlled examples in string/M theory, was a tour de force of theoretical physics. The “circle of ideas” (oh those scare quotes again) in which those calculations took place is hardly devoid of content, or unscientific.

]]>So, it seems that your position is that string theory is unlikely to ever be testable by humans, but that showing it is an internally consistent untestable theory is the future of the subject. Since you don’t have an actual theory (what is M-theory again? everyone seems to have given up on even trying to answer that), what you’re actually talking about is just showing that a “circle of ideas” is consistent, while being completely empty of content. This is really nothing but a set of excuses for giving up on doing science. It’s not surprising that people doing this don’t clearly explain to the public the state of their subject, but instead produce such huge amounts of promotional hype. ]]>

You propose that “general relativity would have taken many more decades to come to fruition” if the appropriate mathematics weren’t already developed. Yet you seem remarkably impatient with string theory not being in final form thirty years on from its inception as a candidate for a fundamental theory, given that there is no ready-made theory of quantum geometry for us to read up on. (BTW, I would rather have said we are forty years on from the work of Scherk-Schwarz/Yoneya showing that string theory is a quantum theory of gravity.)

As for contact with experiment, indeed that may be hard to come by given the large disparity between the electroweak scale and the Planck scale. If the scale of extra dimensions and the string tension scale are close to the Planck scale, then string theory looks remarkably like a 4d theory of quantum gravity coupled to matter — ordinary particle physics up to near the Planck scale, a handful of odd resonances that we’ll never see, and then a spectrum of black hole states. But absence of evidence is not evidence of absence. String theory could easily be correct and not testable by humans — in fact that could well be true of any theory of quantum gravity given the remoteness of the Planck scale.

In such a situation, internal consistency is one of the few routes to progress. Einstein was initially motivated to resolve the apparent inconsistency between Newtonian gravity and special relativity. Today, we have the apparent inconsistency between locality, causality, and quantum unitarity manifested in the black hole information paradox. String theory has made undeniable progress in this direction, but not a complete resolution.

As for particle physics unification, while it would be nice for there to be some rigid structure and a limited possibility to arrive at anything other than the Standard Model at low energies, it has so far not emerged, and looks increasingly unlikely. And the number of clues we are likely to get from experiment seems increasingly limited. The situation is unfortunate, but again nothing here says string theory is wrong (rather than simply not useful in this particular context).

Which brings us back to the issue of multiverse speculation. It seems to me a legitimate scientific question whether the structure and parameters of the standard model of particle physics and cosmology are (1) predetermined, or (2) properties of the part of the state we have access to, and therefore environmental. I suppose (1) splits into subcases of being (a) calculable in principle, or (b) fixed metadata. Maybe we will know enough someday to say definitively one way or the other. What questions are worth pursuing differ depending on the answer. Again, this is an issue for any theory of quantum gravity, not just string theory.

At the moment, I think we are in no position to answer, and trumpeting one particular possibility in the media is not particularly helpful, and tarnishes the subject with a patina of unseriousness. We simply don’t know enough to say one way or the other. I cringe when I hear statements to the effect that a multiverse is a “prediction” of string theory. At best, it’s a possibility.

Such speculations are putting the cart way before the horse. Let’s get a quantum theory of gravity first, and learn how to calculate with it; only then might we hope to address such questions. I suppose it’s human nature to want to jump to the final answer, and fill in the details later. But what if the nature of the final answer depends crucially on the details?

I suppose you’ll dismiss this as some dodge that it’s premature to judge string theory (again the scare quotes). Your mind was made up long ago and I’m sure I won’t change it. I will simply conclude by stating that I still think the subject is making interesting progress on important questions; they’re just not the questions we were initially asking 30 (or 40) years ago about unification of particles and forces, and instead have more to do with the nature of quantum gravity.

]]>While the total DOE high-energy physics budget hasn’t gone down by much percentage-wise, the cuts haven’t been distributed equally. Some large collaborations at certain universities received much larger percentage cuts than this average (I don’t know of any place that received anywhere near a 50% cut, but possibly Strassler does.)

]]>One thing that strikes me is that you decided to comment here in response to an item I wrote, completely ignoring what that item was about. I’m curious: do you really think it’s all right for physicists to go on PBS and tell the public that the multiverse is “Einstein’s legacy” and that there are “very specific equations” describing it? If it isn’t, who should be saying something about this? If not me and my “little diatribes” (that’s what this one was about, not string theory), who is or should be taking on this job?

The argument for string theory that, while we don’t know what it is (other than a “circle of ideas”) various different kinds of mathematics have turned out to be useful to analyze some of the complicated structures that have turned up, isn’t a very convincing one. Yes, this circle of ideas that grew out of string theory has led people to wander around in some mathematically rich areas, raising questions and finding interesting things that would not otherwise have turned up. That’s great, and a perfectly good argument for pursuing this circle of ideas if you can’t think of anything else to do. It’s not an argument though for the 10d superstring (or related circle of ideas) as having anything to do with fundamental physics.

Back in 1985, at the beginning of your (and my, we were educated in exactly the same era) career, the idea that something like the heterotic string could unify physics was a reasonable one to try, but it’s now thirty years later. The idea that you could predict something, anything, this way has collapsed, giving us multiverse pseudo-science. The idea that higher energy colliders would find SUSY, giving experimental clues pointing towards this scenario, has also pretty much collapsed, with the final part of that story to happen over the next year or two. Given this, I think there’s a very strong argument that you and others need to face facts as scientists, admit failure, and move on. You can dismiss this as a “little diatribe”, dismiss any evaluation of the current state of string unification efforts as “premature” (with the right time to consider admitting failure only when you’re no longer around) and try and prop up a failed idea by hyped connections to mathematics, but I don’t think you’re doing the subject any favors this way.

]]>I wouldn’t say that string theory (removing the scare quotes) is an ill-defined term; rather that it is a somewhat inapt designation for a circle of ideas about fundamental physics, given that the primacy of strings is now understood as an artifact of perturbation theory. And the fact that all sorts of very different things (from finite simple groups to K-theory, to name but two examples) are related to string theory is not so much a ‘tedious ideological debating point’ as it is an indication that this circle of ideas has the depth and richness that one would hope for in a fundamental theory.

If your notion of deep and difficult ideas about physics is restricted to a narrow set of issues in particle physics, then no, mathematicians are not who one should run to for help. But fundamental physics is so much more than that. Einstein taught us the deep interconnection between geometry and gravitation, a notion which has been extended by the central role of gauge theory in the other fundamental forces. A useful role for mathematicians in this enterprise has been and remains to help develop the geometrical/topological/algebraic ideas that might be of use in constructing a physical theory. Sometimes those ideas are already at hand when the time is ripe for an advance on the physics side (e.g. Riemannian geometry was available to Einstein), sometimes mathematics is the beneficiary of developments in physics (e.g. quantum mechanics leading to developments in algebra and representation theory). String theory has been both beneficiary and benefactor of mathematics, for example K-theory was available to help understand brane dynamics, while properties of string worldsheet dynamics have led to developments in algebraic geometry such as mirror symmetry and quantum cohomology.

As for the so-called landscape, I regard the whole discussion as premature. A certain segment of the community is piling speculation upon speculation, and not really getting anywhere. It is useful to recognize that there is always going to be an issue with quantum cosmology — that the universe we inhabit had substantial quantum fluctuations early on, and we are now in one decohered branch of the wavefunction; and so what to make of all the other branches out there, and the question of how many are there. To make progress on a problem, one needs the tools to do so. Most of the analysis of the landscape is based on an analysis of effective field theory and the seeming existence of a plethora of solutions to the effective field equations. People spun their wheels for twenty years trying to solve the Hawking paradox using the same methods, without success.

We now understand that black holes are consistent with quantum mechanics in specific examples in string theory, yet we cannot point to the dynamical process which supersedes Hawking’s analysis of horizon dynamics in effective field theory. I suspect that making progress on cosmology (i.e. global dynamics) will have to await a better understanding of such local dynamical questions. One of the least understood aspects of string theory is dynamics.

As for your desire for physicists to “admit that string theory unification has been a failure and abandon empty, dead ideas”, good luck with that. People actually working in the field will pursue the directions they deem most promising, regardless of your little diatribes here.

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