I’m busy with other things, so no possible way I can keep up with the claims about string theory flooding the media for some reason these days. It’s hard enough to find the time to read all of this, much less write something thoughtful about it… One obvious point to make though is that none of it acknowledges the obvious: the widely promoted idea that we can get a unified theory and explain the Standard Model by using a theory of strings has turned out to be an empty one. The result of tens of thousands of papers and more than 30 years of work is that all the evidence is that if you can get something this way that looks at all like the Standard Model, you can get anything. Normally when that happens you simply acknowledge the problem and give up, but for some reason that hasn’t happened. Instead of a description of this straightforward situation, the public gets the following:
- Frank Close describes the situation as
In recent years, however, many physicists have developed theories of great mathematical elegance, but which are beyond the reach of empirical falsification, even in principle. The uncomfortable question that arises is whether they can still be regarded as science. Some scientists are proposing that the definition of what is “scientific” be loosened, while others fear that to do so could open the door for pseudo-scientists or charlatans to mislead the public and claim equal space for their views…
Is physics moving towards an era in which elegance will suffice and into the domain of theories that are beyond the reach of experimental proof? Or will empirical evidence remain the arbiter of science?
Close correctly identifies the problematic nature of multiverse pseudo-science, but misses the basic facts about the string theory landscape. This is not a theory of “great mathematical elegance”, quite the opposite, and there is no such thing developed “in recent years”. If you go back 30 years, there were then claims of “elegant” string theory models, but those never worked out. KKLT is the opposite of “elegant”.
- Clifford Johnson takes the Close piece as a starting point to explain his own view. He lacks interest in string unification, thinks that string theory should be thought of as a “method” for solving problems. He doesn’t really explain though why it is a “method” that deserves so much more attention than any number of other methods used in physics. He also doesn’t acknowledge that, besides the huge amount of TOE hype (which he and other string theorists often appear on TV to promote), the hype problem for the string theory “method” may be just as bad. For example, he was quite proud of his efforts to promote string theory as the method to understand heavy-ion physics (see here and here), but that’s something that really hasn’t worked out very well.
- Over at Starts With a Bang, by Sabine Hossenfelder, there’s Will the LHC be able to test String Theory: Definitely maybe. The article actually does a very good job of explaining why the answer is “no”, so I have no idea why the misleading headline. The fact that the AdS/CFT heavy ion predictions haven’t worked out is explained, with the comment that
The LHC, thus, has already tested string theory!
and that this failed, which might be a more accurate headline.
- There’s a long interview with Sean Carroll at the Edge website. He’s quite defensive about the multiverse, claims it’s a prediction of our best theories, and gives his usual characterization of multiverse critics as zealots unable to understand the idea of an indirect test
But certain zealous colleagues of mine are saying that because you can’t see the other universes in the multiverse or because you can’t see the little super strings moving around, these theories are not falsifiable and, therefore, should not count as science.
He’s also defensive about string theory, there the argument is
Either we will bring it down to earth and connect it to the world we see or people will lose interest. People cannot maintain this optimistic idea that we’re going to get the right theory of quantum gravity, the theory of everything, if it’s literally decades and decades of people writing down equations and never predicting the experimental outcome of anything. But we’re not there yet. It would be a terrible shame if we gave up on string theory when maybe next year someone will figure out how to bring it in connection to observations, or maybe ten years from now it will happen. This is how science works, and this is it at work.
The problem here is that it is “literally decades and decades of people writing down equations and never predicting the experimental outcome of anything”, and no matter how many decades of this go on, someone can always argue that “maybe next year”. He’s avoiding the very real issue at the center of things: why hasn’t string theory been held to account for its failures the way any normal speculative scientific idea is supposed to?
As far as his current interests go, from what he says, he seems to be losing interest in the multiverse, which soon may no longer be a hot topic, and moving into research into complexity and consciousness.
Dr. Woit – Thank you for continuing to write such interesting, informative articles. Always look forward to the latest showing up in my Twitter feed.
Peter,
surely you need to be following more closely what top young post docs have to say about String Theory, rather than focusing on the opinions of theoretical physicists much older and possibly ignorant in comparison?
For example, this is what David Simmons-Duffin has to say:
http://www.quora.com/Is-it-true-that-many-physicists-are-abandoning-String-Theory
“Here’s my personal favorite at the moment (Warning: technical jargon — feel free to skip this paragraph). Physicists have recently been able to compute operator dimensions and scattering amplitudes in planar N=4 Super Yang Mills theory nonperturbatively at all values of the ‘t Hooft coupling. These quantities interpolate between 5-loop Feynman diagram calculations at weak coupling (note, these are calculations in QFT, a theory no physicist — even Woit or Smolin — disputes the correctness of) and 10-dimensional supergravity calculations at strong coupling, with full String Theory calculations in between. This is spectacular evidence that N=4 SYM (a close cousin of the theory of quarks) is both a Quantum Field Theory and a String Theory (as conjectured by Maldacena), in a way that can be made quantitatively precise.
I don’t know what high energy theorist isn’t impressed with 5-loop Feynman diagrams. And if you’re impressed with that, you’d better be impressed with all-loop results that agree with 5-loops when Taylor expanded to 5th order.
That kind of result is so impressive that I am convinced without a shadow of a doubt about the merit of studying String Theory. It also shows why String Theory is an inevitable and inextricable part of physics. Gauge theory is real and correct: it describes electrons and photons and W-bosons and quarks and has been verified innumerable times by a variety of experiments. The above evidence shows that some gauge theories literally are String Theories. Given that many scientists would like to understand gauge theory better, it would be totally crazy to abandon String Theory!”
and moving into research into complexity and consciousness
This does not bode well as there lies a fertile savannah where the tenure buffaloos of crackpottery congregate, free of the restraints of realistic physics and the theory of computation. At night, the ghostly moonlight of fantasist attributes ascribed to the human mind illuminates their ruminations of ethereal qualia.
John McVirgo,
Young post-docs are not a very good opinion gauge, since they are typically still bedazzled by the theories they study (I’m also struggling with that, so…). The first signal of becoming a mature scientist is when one recognizes all the *problems* that the theory has, and starts paying attention to those rather than saying “Oh wow, this can be evaluated to five loops! Wow!”.
The serious scientist focuses on what is wrong with their own pet theory, and emphasizes that in public and among peers, so that others may also attack the problems. Putting one’s head in the sand doesn’t make the problems go away, but only adds to confusion. Senior scientists are aware of these problems (at least they should be), while junior scientists somewhat lack the experience to recognize them as such. Btw, that is why mentoring is so important in academia.
As for the concrete example you quoted, the “planar N=4 Super Yang Mills theory” has a very big symmetry, and nobody should be surprised that something like that can be integrated in some cases. But this is very very far from, say, Standard Model, or any other realistic theory, where a large portion of that symmetry is broken. Saying that it is a “a close cousin of the theory of quarks” is just naive. I often compare such statements to a man who wants to go to the Moon, and who after climbing a tree says “I’ve made a first step”. It’s just a lack of perspective.
HTH, 🙂
Marko
Ha, I will refrain from listing all the reasons I think consciousness research is a dead end 🙂 As for “let’s study string theory since some gauge theories are string theories” I will point out that generalization is not always a good thing. Also, the fact that you can compute something does not make it interesting, or useful. If only – one of the joys of being a mathematician is working on some problem, realizing you can compute something related to what you want, and at the same time realizing it won’t do you a lick of good…
Peter,
Ethan, who edits the collection, asked me to write a piece on the question. (He also picked the title and changed a few paragraphs here and there.) My first impuls was to say, well, the answer is obviously no. On second thought, I managed to write “no” in 1600 words 😉 To be fair, I find the AdS/CFT approach to strongly coupled matter very interesting, though the applications to strange metals seem better motivated. Besides, I’m trying to remain open-minded. As the saying goes, predictions are difficult, esp about the future. (And I’ve been told it’s wrongly attributed to Niels Bohr, so I won’t.)
Thanks Bee,
I should have made clearer that I think it was a very well-done article, except maybe for the title….
John McVirgo,
I did write something at that Quora question. Best if people who want to discuss that do it there, although I fear participating in discussions at sites like that is not something I have the time for.
I think Mr. Carroll’s problem is more with Mr. Occam than with Mr. Popper. If I come up with a scientific model that fits the existing data, but inevitably also suggests that there are unicorns that I can never see, should I take the unicorns seriously, or should I doubt that perhaps there’s an equally good, yet to be found model that doesn’t suggest unicorns?
Unless logicians demonstrate that this is the only mathematical model we can ever find, I’m not comfortable with taking unicorns seriously. Even then, my next move would be to question the foundations of math or its use in physics itself.
I think if there’s a theory that fits the data, then successfully predicts the observation of something new, AND also contains a mechanism within itself that generates unicorns that can never been seen, but the mechanism follows from the mathematics and from the principles of the theory, THEN we should take those unobservable unicorns seriously.
Koray/Hilbert,
I think that what Carroll is doing is trying to shift the argument from actual theories we know about (where the unicorns are a dubious feature) to some hypothetical unknown theory (where the unicorns are a sure thing). At least this zealot isn’t very interested in such hypotheticals, happy to admit there’s logically some unicorn theory out there which I’d sign on to.
This is a standard move in string theory defenses, to try and discuss some completely successful unknown string theory that string theorists wish existed, not the one that actually does.
From Frank Close quotation:
“Some scientists are proposing that the definition of what is “scientific” be loosened… towards an era in which elegance will suffice and into the domain of theories that are beyond the reach of experimental proof? ”
Reminds me of the talk about new methods of valuating corporations at the top of a market boom, like the dot.com boom, when “elegant” corporations that had never made a cent were sky high. Could this mean the career ending bust is near?
From Sean Carroll quotation:
“It would be a terrible shame if we gave up on string theory when maybe next year someone will figure out how to bring it in connection to observations, or maybe ten years from now it will happen.”
Definition of insanity: doing the same thing over and over again and expecting different results. – Albert Einstein
“moving into research into complexity and consciousness.”
Oh no!
“the next six months will be critical for string theory”
– Tom Friedman
I thought this was interesting, from the “not the only game in town” department:
https://www.quantamagazine.org/20150527-a-new-theory-to-explain-the-higgs-mass/
Also of possible interest to you, Peter, though OT for this posting:
https://now.dartmouth.edu/2015/05/major-byrne-gift-launches-new-program-mathematics
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Hi Peter Woit and John McVirgo,
It seems to me that you are talking apples and oranges here. The David Simmons-Duffin quote sums up some of the very strange and remarkable results now available for large-n, N=4 Super Yang Mills theory. That’s some exotic conformally invariant, in some sense “integrable” QFT that keeps leading to new surprises, from AdS/CFT to the Amplituhedron. It’s possible that one can learn some deep aspects of quantum field theory from studying this “exactly solvable” (?) model. The same can be said about many aspects of quantum gravity and string research over the years; sometimes seemingly completely unrealistic models of fundamental physics end up having surprising and testable consequences for effective condensed matter systems–for example, 1+1-D Liouville field theory, which has connections to membrane and glass physics as well as being a toy model of quantum gravity.
Irrespective of the “hype” they use to sell their work, I view the efforts of string theorists from a pragmatic perspective, similar to my view of manned space exploration. Absent the race to put weapons platforms in space that was a big cold war motivation, one might ask what the compelling scientific purpose of manned exploration has been or will be in the near future, beyond useful near-earth orbit missions to deploy and service scientific instruments, satellites, etc. Advocates stress the indirect benefits; perhaps if string theorists were better-acquainted with the mathematics and physics “spin-offs” that have paid off in condensed matter, one could make a better argument than the one to abandon scientific falsifiability. I’m afraid that the latter will just isolate high-energy theory from the rest of science, and probably have a chilling effect on funding.
At the same time, if you acknowledge that unexpected byproducts are a main benefit of a large line of research, one has to evaluate the value of those compared to the investment. I’m curious where you come down on that one, Peter, given your interest in Langlands, CFT, and the mathematical aspects of QFT more generally. Are these less impressive or important than the benefits of further accelerator research, assuming no sparticles or other non-SM physics is found at LHC?
Matthew Foster,
I don’t see any useful way to compare accelerators and pure theory research, they’re just two very different things. The HEP experiment people have a very tough job ahead figuring out how to go beyond the LHC. There isn’t an obvious best choice, and all choices are expensive. Theorists are not expensive at all on that scale. I don’t think moving money from one side to the other can help: diverted theory funding would be too small to solve the problem of the experimenters, diverted experimental funding I don’t think will solve the problems of the theorists (I don’t think they are now due to insufficient funding).
I’m obviously all in favor of mathematically based QFT research, which I think one can justify either by “spin-offs” or simply by the argument that deeper understanding is inherently of value. What I see as problematic is selling one narrow part of this research (that related to string theory) by bogus claims that string theory will unify physics, when that’s a failed idea. One part of the problem is that if you justify your field by an overhyped idea that doesn’t work, as it becomes clear the idea doesn’t work, people will pull the plug on your field (already happening: in the US right now very few young theorists can get a job in this area).
The other part of the problem is that this leads to a focus of research into certain narrow areas based purely on the fact that they are connected to the failed program. Sure, N=4 SYM is an interesting model, but there are a huge number of other poorly understood questions at the intersection of qft and math. The Simmons-Duffin quote doesn’t seem to recognize this is not the only model out there with a fascinating and not completely understood structure. Instead of more people working on AdS/CFT, justifying themselves by the failed idea of string theory unification, I think we’d be better off if questions not related to AdS/CFT could also get attention. These might seem harder to justify than just invoking string theory and the theory of everything, but making a more honest case is likely to work better in the long run anyway.
“Liouville field theory, which has connections to membrane and glass physics as well as being a toy model of quantum gravity”
This sounds fascinating. Could you give me a reference on applications of Liouville theory to membrane and glass physics?
Hi Totally off topic,
There is a generalized version of Derrida’s Random Energy Model (a supersimplified, exactly solvable model of the glass transition) in which many-body energies have logarithmic correlations, instead of being independent as in the original. The partition function can alternatively be thought of as that for a classical particle in a log-correlated random potential. At high temperatures, the particle is “delocalized” throughout the volume, while it tends to get stuck in one of a “few” minima below a freezing transition at which the extensive component of the entropy vanishes. There is yet another interpretation of the model in terms multifractal wavefunctions of disordered Dirac fermions in 2D.
The connection to Liouville is as follows. The partition function is a random variable, and exhibits log-normal fluctuations at low temperatures. This is in contrast to the free energy, which is non-fluctuating (self-averaging) in the thermodynamic limit. You can write a generating functional to calculate moments of the partition function, averaging over potential configurations. The result has the form of Liouville field theory.
In the high temperature phase, you can calculate statistics using scaling dimensions of local (vertex) operators. This fails at low temperatures, however, because the OPE generates higher moment operators from lower ones.
There is a mapping to the KPP equation for non-linear diffusion, in which one keeps the entire infinite tower of moment operators. In the end, the glass transition (or transition for the moments) is encoded in the “velocity selection” of the front solution to KPP, and this allows an exact solution. This part is well-understood and has been checked numerically in various contexts.
The part that I don’t understand that well is how to interpret the results in terms of LFT. There are some interesting ideas that the fusion of this infinite tower of operators somehow produces new “macroscopic” operators, and it is the dimensions of these that one is computing with KPP.
This whole business is due to Carpentier and Le Doussal, although the result was actually conjectured earlier by Chamon, Mudry, and Wen, using a schematic mapping to the problem of directed polymers on the Cayley tree.
Try:
“Glass transition of a particle in a random potential, front selection in nonlinear renormalization group, and entropic phenomena in Liouville and sinh-Gordon models”
Carpentier and Le Doussal, PHYSICAL REVIEW E, VOLUME 63, 026110
Thanks so much, Matthew Foster!
Interesting piece in June 7th New York Times on this topic:
http://www.nytimes.com/2015/06/07/opinion/a-crisis-at-the-edge-of-physics.html?ref=todayspaper&_r=0
In case you haven’t seen it, there is a comment in the major local Berlin newspaper: http://www.tagesspiegel.de/wissen/mathematik-ein-universum-aus-zahlen/11770478.html