A panel discussion at the Strings 2018 conference ended by addressing audience questions, and this seemed to me to give some insight into where string theory is now. I noticed that the Youtube video comes with an auto-generated transcript, so decided it was worth the time to clean that up a bit and post it here.
David Gross: The plurality of questions had to do with the connections of string theory to real-world experiment. Some of these were raised by the panelists already. Let me just give you a sense of those questions, and see if you would like to briefly address them:
How long can string theory survive without experimental verification? At what point does it become mathematics?
Can string theory survive as a theory of the physical world if de Sitter space can’t be accommodated and supersymmetry ruled out?
In the next fifty years do you expect any physically relevant result (i.e. worthy of a Nobel Prize) to come out of string theory?
Has string theory given up on particle physics? [Gross:”no names attached” (laughter)]
When and where do you foresee a real confirmation of string theory in particle physics or cosmology (other than the existence of gravitons)?
In what area of physics do you expect the first observation of string theory?
Do you think string theory is closer to phenomenology than 50 years ago?
What could be a prediction of stringy models that can be verified in the lab and it cannot be an effective field theory model that gives you the same prediction?
How much of the string community has given up on the goal of connecting string theory to the standard model and observational cosmology?
Susy isn’t observed, what should we do? Is it okay to believe that string theory is still the theory of everything?
David Gross: So, this is symptomatic of this community. I imagine [for] the younger members of the community perhaps a bit worrisome.
Eva Silverstein: My answer is yes. (laughter) To the very last question you posed.
Juan Maldacena: I think the main virtue of string theory is to be a consistent theory of quantum gravity and maybe we shouldn’t be… I mean of course it would be wonderful to have a comparison to experiment but it’s a complicated theory and it might take many years until we understand how to compare it to experiment. I think an important thing is to understand the theory, understand basic things like the singularity because it might be that we’ll understand that experimental connection by understanding the Big Bang singularity and some predictions from there, or something in this direction.
John Schwarz: One of those questions had to do with “is string theory just mathematics?” I’ve heard this question many times and I find it puzzling that someone would consider mathematics to be a pejorative term. Where would physics be without it?
David Gross: I don’t think that was the question. The question was “if string theory without experimental verification goes on and on is it indistinguishable”. There was nothing pejorative.
John Schwarz: I’m sure the person in this audience who raised that question didn’t mean it that way but I’ve heard it used by others in that way.
Dan Harlow: I just want to give a sociological data point so I mean I won’t repeat what I said in my talk but it’s a true fact that you know every every month or two I am contacted by an experimentalist. Usually this or that atomic physics experimentalist who is looking for things to do in their lab and somehow thinks that talking to me will help. I’m not sure if it will or not, but I think that there’s this fantasy that you find on the blogs that string theory is something that exists independent of the rest of physics and I think really nothing could be further from the truth. I mean I feel we’re really part of physics I talk to physicists all the time and not just string theorists. (laughter)
David Gross: There were a few other versions of this question that perhaps reflected the anxiety of some of you here, which had to do with funding and and having to defend yourselves in your academic departments and universities and that I think is a real issue. I think Daniel addressed that but let me give you some other points of advice to defend string theory or what we call the activities of this crowd, with respect the funding agencies or department chairmen. String theory was attacked bitterly in the eighties for being not even science and but now it’s truly impossible to make that argument. It is continuously connected to the standard model after all through our dualities and the standard model is certainly part of nature and verified experimentally. So string theory and field theory are not distinguishable and certainly not the standard model. String theory has given us many insights into the standard model, condensed matter theory, information theory, mathematics etc. It is easy to defend it intellectually, aside from the fact that it’s addressing these deep conceptual problems of unifying quantum gravity with the other interactions, or just understanding gravity. So you should feel no shame in defending this field and arguing for both funding and positions.
Gabriele Veneziano: One mistake we made in the early days of the atomic theory was to think that the hadrons were elementary and to which we had to find a string idea. One of the big assumptions of the new 80s interpretation is that the particles we consider elementary today are indeed so. Maybe the fact that we so far failed to find a model is that we try to find a string theory for the wrong thing.
David Gross: There are also many questions about de Sitter space:
The existence of dS solutions appears to be controversial. What are the technical obstacles to resolving that controversy?
Is de Sitter space in the swampland? Can we get de Sitter in string theory? If yes why haven’t we succeeded? If no, why not?
David Gross: Igor [Klebanov] (sorry Igor) asked an even broader question:
Is there a stable non-supersymmetric compactification of superstring theory whose existence has been established using known controlled approximations? Should have Poincaré or dS or AdS symmetry corresponding to the two or more non-compact dimensions?
David Gross: This is an interesting topic where there’s clearly controversy. I’ve been unable to find a strong statement on the negative side. Juan has offered to defend it or at least he has been put forward to defend the existence of dS solutions.
Juan Maldacena: There are constructions that I think are reasonable, there are scenarios for how the solutions should work. They involve complexity in an essential way in the sense that you have to invoke complexity to find this fine-tuning that [?] was talking about, and they are reasonable so if you’re going to say that they don’t exist you also should argue with comparably strong arguments. Also no one guarantees us that the physical theory will have very simple solutions, so if you want to solve for the oxygen atom you can decide whether it will exist or not. Even in QCD if you try to decide what’s the last stable nucleus you will not beable to predict it probably from pure theory.
I’ll say one more thing, but this is more speculative. So our understanding of the vacuum in string theory many times relies on having an asymptotically simple situation: asymptotically flat space, asymptotically AdS, and if we ask “where does AdS arises from?” then “Oh well it’s a brane embedded in a bigger space and so on”. So we have this kind of “turtles upon turtles upon turtles” picture of the theory, so everything is defined by a bigger simpler asymptotic space. But where did this asymptotic space come from? de Sitter is different, de Sitter is a bit like a sphere, so it has no edge or anything and we need to think now “We’re theorists, how to describe that?” So maybe we’ll understand another framework where we understand the fact that it has no boundary is more crucial and essential and we’ll see that those equations might have a different nature than the types of equations we think about.
David Gross: That’s a defense of KKLT. Trivedi isn’t here, I was going to ask him to defend it. There are many people who are confused as to whether this is a crisis for string theory or not, and Hiroshi volunteered to give some criticism of these compactifications.
Hirosi Ooguri: I was asked to say something about it probably because I posted a paper with Cumrun earlier this week about this which Cumrun talked about. So, the last 20 years or so, especially after dark energy was identified, there have been enormous attempts to construct de Sitter space and other accelerated universes, with various degrees of rigor, and this is really a very important part of string theory research. So, many of the things we do is to look at a set of these constructions and try to deduce lessons from these data. This is like experimental science where we are given a set of this data and then try to understand it. But of course depending on how much rigor you ask, how much sort of control you would like, the set of data you look at can be different. Just like experimenters look at the different sigmas and then select reliable data. I should say in the case of string theory it’s particularly difficult because string theory doesn’t have parameters so all the low-energy parameters are the vacuum expectation value of some field. So if you successfully stabilize all the scalar fields then by definition these are numbers and not controlled. This is in contrast to the case of say, QED, where we have the fine structure constant which you can dial in a given theory, in our world a given number you can dial, so we can trust it, so the situation seems to be different. When the KKLT compactification first appeared I was hoping that maybe since there are so many ends around the flux that you can actually find a series of models where you have control over that, which we have not seen. I think this is difficult and so you can draw different lessons from this and I think it’s very important to sort of develop tools to make more sort of finer predictions out of this existing situation.
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