A quarter-century or so ago, one of common arguments for string theory research was that it was “the only game in town”, in the sense that it was the only possible way to get a unified theory. For instance, back in 1987 David Gross had this to say:
So I think the real reason why people have got attracted to it is because there is no other game in town. All other approaches of constructing grand unified theories, which were more conservative to begin with, and only gradually became more and more radical, have failed, and this game hasn’t failed yet.
As years went on and string theory unification went nowhere, this often was replaced by a new “only game in town” argument, that string theory was the only possible quantum theory of gravity. This argument got strong disagreement from people pursuing Loop Quantum Gravity or any number of other ideas.
This week, Quanta magazine has a new version of the argument, reporting that “Researchers are demonstrating that, in certain contexts, string theory is the only consistent theory of quantum gravity. Might this make it true?” The new argument (based on this and this) seems to be that string theory is the only possible theory of quantum gravity because if you look at a certain class of CFTs (based on orbifolds by permutation groups) and invoke the AdS/CFT conjecture for AdS3/CFT2, the 3d gravity theory in the large N limit would have a density of states more characteristic of string theory than a conventional particle theory.
The most obvious problem here is that it is in 3 space-time dimensions, where there are no physical gravitational degrees of freedom. The S-matrix of quantum gravity is exactly calculable in flat 3d space: it’s zero. There’s a very long history of studying 3d quantum gravity, as a toy model without gravitons, but with just topological degrees of freedom. For more about this, see for instance Steve Carlip’s 1998 book on 3d quantum gravity, which works out a large number of different ways of quantizing 3d gravity (not including string theory). One problem with the argument that string theory is the only way to quantize gravity because it is the only way that works in 3d is that, as Carlip shows, there’s a long list of other completely different ways to do this (all arguably not that relevant to the problem since none have gravitons). This is also quite different than the usual argument that string theory is needed to quantize gravity, which is based on the occurrence of a spin 2 graviton in the spectrum of the string theory.
Ignoring the obvious problem of no gravitons and being in the wrong dimension, there are other problems with the argument, for instance the claim that looking at permutation orbifolds tells you about all CFTs, or the claim that a large density of states at high energy means you have to have a string theory. The article quotes Matt Strassler about this:
But these aren’t really proofs; these are arguments. They are calculations, but there are weasel words in certain places… And just finding a stringy density of states — I don’t know if there’s a proof in that … This is just one property.
Carlo Rovelli sums up the issue with using this to hype string theory and excuse its failures:
They should try to solve the problems of their theory, which are many, instead of trying to score points by preaching around that they are ‘the only game in town.’
I haven’t followed closely work on AdS3/CFT2, but it is a quite interesting topic, although not because it promises a proof of the “universality” of string theory. Chern-Simons theory is based on a very similar relation between a topological 3d qft and 2d CFTs, and there we have some idea what is going on, although many fascinating questions remain. One might hope that AdS3/CFT2 provides a context where one could understand things using some ideas from the Chern-Simons context. This is what Witten did back in 2007 in his paper Three-Dimensional Gravity Revisited (I wrote about this before the paper here). My understanding is that problems with Witten’s proposal later surfaced, I’d be curious to hear from an expert on the latest state of that (perhaps Witten can write a “Three-Dimensional Gravity Revisited Revisited” paper).
There are a lot of wonderful questions still not understood about this story, but I don’t see that using it to argue that string theory is the “only game in town” does anything other than throw one more thing on the pile of outrageous hype generated by string theory partisans over the last 30 years.
Update: There’s been a change to the Quanta article, adding to the quote from Lee Smolin, who is making much the same point I was making in this posting:
“And even in that case [2+1 d], there have existed for a long time counterexamples to the string universality conjecture, in the form of completely worked out formulations of quantum gravity which have nothing to do with string theory.” (String theorists argue that these particular 2+1 gravity theories differ from quantum gravity in the real world in an important way.)
This whole thing really is very strange: on the one hand string theorists are arguing that only string theory can give you quantum gravity, based on an argument in 2+1 d. When you point out to them that there are well-known counterexamples to their argument in 2+1 d, they say “well, things are different in 2+1d than in other dimensions”. Just bizarre…
Thanks for responding to my piece with your hallmark spirit. 🙂 A couple of minor points: I’m not sure whether you’re calling the article itself “hype” or claims presented therein, but I would like to note that most of the points you raise about the limitations of this research are explored in the piece. I would also note that you’ve curated your excerpts a tad misleadingly. Strassler’s quote began “It’s clear that these papers are an interesting attempt.” Carlip, whom you mentioned, also had a mildly positive take. Most experts I spoke to (even some LQG theorists) consider these papers worth taking seriously. No one is anywhere near claiming that string universality is proven, of course. But it seems reasonable to call these consistency conditions “evidence” — a new kind of evidence, possibly, and certainly not of the smoking gun variety.
I also think it’s worth pointing out that there are a couple of papers discussed in the story that go beyond 3D gravity. The most significant is Maldacena and colleagues’ from last summer: http://arxiv.org/pdf/1407.5597.pdf.
Always a pleasure.
Peter, what exactly is the “only game in town” claim? Is it something like:
If I begin with a set of desiderata including
* Low-energy limit is general relativity in 3+1
* Low-energy limit is QFT/QM
* Lorentz invariance
I would (if I could do the math etc) find a unique solution: string theory. What is the full list of desiderata?
If it were proven, it would be quite compelling, but not the final word (eg Lorentz could be broken at a small level in nature).
Sorry not to make it clear, but the “hype” reference was to the claims physicists are making, not the article. You do quote appropriately skeptical voices. Just thought I’d add some context though about how 3d is different….
About ignoring the Maldacena et al 4d reference, that just seemed to be a completely different topic and sort of calculation, one I’m not familiar with and don’t have time to become well-enough informed to write about it. Taking a quick look I see that even they don’t make any strong claims for their result:
“We should mention the grand dream of deriving the most general weakly coupled consistent theory of gravity. It is quite likely that the only such theory is a string-like theory, broadly defined. We are certainly very far away from this dream, but hopefully our simple observation about three-point functions could be useful.”
Some string theorists would like to try this style of argument, there are a variety of sets of hypotheses you could imagine. See my previous quote though, where Maldacena et al acknowledge “we are certainly very far away from this dream” and I think that’s accurate.
The problem with No-Go theorems (here string theorists are hoping for a No-Go theorem about alternatives to string theory) is that very often someone finds a way around them. They can be very helpful at focusing attention on precisely what assumptions lead to an obstruction to doing something, giving ideas about what needs to be done to get around the obstruction. In this case though, I think one is so far away from anything one could reasonably call a No-Go theorem that it’s just hype to make claims about one. In particular, arguments about 3d quantum gravity are just pretty much irrelevant to the topic.
The strength of the proof is inverse to how many assumptions they make.
While we don’t live in a multiverse, we do live in a world with many towns, and a lot of league sports.
Maybe you can explain why the Maldacena, Zhiboedov 2011 paper provides “evidence that string theories are the only quantum gravity theories with a particular feature that reproduce general relativity at large scales.” as the article claims?
On the contrary this paper as I see it provides evidence of Vasiliev’s HS theories by confirming the famous HS_AdS₄/Free_O(N)_vector_model correspondence of Klebanov-Polyakov (http://arxiv.org/abs/hep-th/0210114).
But maybe I’m missing something.
On the other hand if your aim was to claim that a Supersymmetric version of HS_AdS4₄ is a limit of String theory you should have mentioned in addition the accompanying papers that make this claim i.e. in this case http://arxiv.org/abs/1207.4485. Otherwise the above statement in your article is meaningless and confusing.
But again maybe I’m missing something so I would be grateful if you can elaborate on this statement of your article.
The quote “the only game in town” is exceptionally apt. It’s most associated with con man “Canada Bill” Jones, master of the Three Card Monte, who ironically was himself addicted to gambling and lost his money to better professionals as fast as he took from marks. Supposedly, on being advised by a friend that the Faro game he was losing money at was rigged, he replied “I know, but it’s the only game in town!”.
The weird thing is that many string theory proponents are well aware of the origin of “only game in town”, but use this anyway. See for instance
For many years, it was regarded as a hallmark of a successful theory of quantum gravity that it should automatically exclude spacetimes that are known, on general grounds, to be unphysical. For example, it simply should not work on Goedel’s famous spacetime. Indeed, if somebody came up with a theory of quantum gravity that *demanded* that spacetime must be globally hyperbolic, as all realistic spacetimes are, that would be counted as strong evidence in its favour. String theorists used to argue like this: in the heady days of 1985, some of us thought that [at least] the internal manifold could have only a tiny number of geometries, and that was [rightly] hailed as strong evidence in favour of string theory.
If the correct theory of quantum gravity only works on globally hyperbolic spacetimes, then it won’t work at all on AdS. Conversely, a theory that works spectacularly well on unphysical spacetimes like AdS, indeed that is the only game in town there….. well, that is pretty powerful evidence that it is *not* the correct theory of quantum gravity. Yes, I know that things like gauge theories work quite well on AdS, but we don’t look to gauge theories to select our spacetimes for us.
So the conclusion here should be precisely the opposite of the one being drawn.
Well, I don’t know whether I really believe all that. But it shows how tricky, and perhaps how futile, this sort of argument can be……
Even with a short list of desiderata for a QG theory (well-defined theory, semiclassical limit is GR + QFT), string theory is not a unique solution, because the spin-cube models (categorical generalizations of spin-foam models) also satisfy the list. However, the complete list of conditions must include a generalization of the QM formalism to the whole universe (no external observes, the meaning of the measurement and the wave-function collapse in quantum cosmology, the role of time).
There’s no doubt that the sentence in question is very vague and probably confusing to an expert. As the paper was explained to me, Zhiboedov and Maldacena considered a class of AdS4 theories with higher spin symmetry; when they deformed these theories in a way that made the higher spin states heavy, the only theories that ended up looking like GR in the right limits were string theories. So the “particular feature” would be heavy higher spin states, as opposed to massless higher spin states, which of course our universe does not have. Hope that helps.
Thanks for the answer.
I think I understand now what have happened.
Are you sure you have referenced the correct paper?
From your description I understand that the correct paper is
Check the motivational introduction in appendix G and the relevant conjecture.
I have another game, but nobody wants to play it! Oh well, that is the fate of alternative approaches against the string/brane/susy juggernaut and one must just accept it. That is life.
The S-matrix cannot be zero, it is a unitary matrix, it must be the identity. When Einstein invented special relativity, SR survived on its own merits. It didn’t matter if anyone wanted to play Einstein’s game, or stick to the juggernaut of the luminiferous ether. Whatever new game anyone wishes to offer today will have to prove itself on its own merits.
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Regarding experimental “confirmation” of the String theory, there will be a Gordon Research Conference in June. Its title is “String Theory & Cosmology (New Ideas Meet New Experimental Data)”. I’m rather interested to see what the new experimental data are.
Thanks. More about this here
I see the participants are going to explain about
“Testing String Theory Through Observational Cosmology”
Wonder what new “test of string theory” they’ve got?
Once they’re finished in Hong Kong, many of them will fly to Aspen to discuss the same topics