Quantum Supremacy

Posted on August 26, 2018 by woit

Hasn’t been much that I’ve heard about worth discussing here recently. Presumably everyone is on vacation. I’ll try and gather some things that may be of interest, starting first with the hot topic of “quantum computation”. It looks like this will be drawing an increasing amount of attention and resources in the field of physics research. For instance:

  • Slides and videos from the summer IAS program From Qubits to Spacetime are available here.
  • There will be a graduate seminar at Harvard this fall, blog post about it here.
  • In a few weeks Fermilab will host a workshop on Next steps in Quantum Science for HEP.
  • Moving through the US Congress is a National Quantum Initiative Act, which would provide over a billion dollars in funding for things related to quantum computation.
  • At the NSF MPS (Mathematics and Physical Sciences) they’re promoting NSF’s Quantum Leap. This is the first of four “Big Ideas” (discussed here) which will influence what gets funded. The other three are multi-messenger astrophysics, big data, and things related to biology.
  • Launching this week is The NSF 2026 Idea Machine, which is a competition for suggesting research questions, more “Big Ideas” for the NSF to fund. If you want to enter the competition that opens Friday, I’m guessing that invoking the word “Quantum” will help.
Posted in Uncategorized | 11 Comments

On Status of KKLT

Posted on August 16, 2018 by woit

(Warning, this is just more about the topic of the last posting, which for most people will be a good reason to stop reading now. On the other hand, if you’re obsessed with the controversy over string theory, you might find this interesting).

I finally got around to watching some more of the Simons Center Workshop on the Swampland talks, and noticed a remarkable exchange at the end of Thomas Van Riet’s talk On Status of KKLT (starting at 1:30). The first commenter (a German, Arthur Hebecker?) starts off saying “I think you are doing something that is very dangerous”, with the danger being that KKLT will get thrown out and people will think that it is a “theorem” that string theory has no dS vacua. He is interrupted by Vafa who tells him that “your statement is defamatory, let’s calm down”. The German goes on to explain to Vafa the significance of the danger he is concerned about:

Maybe for you in the US it’s fine at Harvard, for me it will be a pain because people will turn against me. The little standing that string theory and new physics at all has in Germany will be harmed by a backlash on us that we have been talking nonsense all the time, which is not true.

Van Riet after a while interjects that there is an even worse danger:

The opposite happened and actually back-reacted very badly. We had the books by Woit and Smolin and it was based on the existence of the multiverse as a correct statement, right? And that’s when the criticism of string theory took off, right?

Someone else in the audience (Iosif Bena?) comes in on the Vafa/Van Riet side of the argument, criticizing multiverse mania:

I think the main problem was that at the beginning people in the KKLT camp, they came up with, “OK string theory has the multiverse, we’re not going to do physics anymore, the anthropic principle…” They came up with all these ideas that hurt string theory much much worse, at least in Europe, at least in my part of Europe. And you know, essentially hurt us heavily… Then there were these books by Woit and Smolin that were very popular…

It’s remarkable to see publicly acknowledged by string theorists just how damaging to their subject multiverse mania has been, and rather bizarre to see that they attribute the problem to my book and Lee Smolin’s. The source of the damage is actually different books, the ones promoting the multiverse, for example this one. A large group of prominent theorists, especially many from the West Coast, including the group at Stanford and the late Joe Polchinski at Santa Barbara, used the existence of the KKLT construction to push very hard a pseudo-scientific excuse for why string theory wasn’t working out. I’ve often point this out, and I do think this has been very damaging to the public perception of string theory. But the underlying problem is the takeover of string theory by multiverse pseudo-science, not that I and Lee Smolin criticized it.

A striking fact about the Stony Brook workshop is that none of the participants were from Stanford, and none of the many prominent figures responsible for promoting KKLT were there. It looks like there is now a dramatic split going on, with Vafa leading the charge to try and fight back against what in recent years has been a seeming dominance of string theory by the pro-multiverse faction. I think such a split is long overdue, that most string theorists for years now have been making a terrible mistake by going along with multiverse pseudo-science. As Hebecker(?) explained though, fighting back publicly at this point carries its own dangers. In particular, many observers will be asking: “for years you told us about the 10500 vacua”, now you say that maybe there aren’t any. Which is it? Why can’t you tell? And do you really have a serious alternative for how to connect string theory to the real world?

Vafa tries to not take sides, to portray this as a simple technical question that will yield to further calculations by theorists. Where I disagree with him is that I’m very skeptical that this is a technical question with a well-defined answer. This is not a new controversy: theorists have been arguing about moduli stabilization and this de Sitter/no de Sitter issue for twenty years or so, without coming to any firm conclusions. If you watch the technical talks at the Stony Brook workshop, the degree of technical complexity of the arguments is striking, as is their often rather vague nature. What you don’t see is a specific set of equations that everyone agrees on. We’ll see what happens in coming months and years, there are likely to be a large number of papers written on this subject. Also to look out for, likely the efforts of Vafa and others to throw doubt on KKLT will not be taken lying down. The West Coast Empire will strike back…

Update: At CNN, Don Lincoln has an article about this, which ends with:

It’s not quite a WWE cage match, but it’s going to be fun to watch these theories fight it out.

Update: Tonight the West Coast Empire has struck back, defending here and here their dS vacua against the Swampland attack, and going on the offensive, accusing the conjecture of their attackers as being “ruled out by cosmological observations, at least at the 3 sigma level”.

Posted in Multiverse Mania, Swampland | 15 Comments

Theorists with a Swamp, not a Theory

Posted on August 13, 2018 by woit

In recent weeks string theory has been again getting a lot of press attention, because of claims that new progress is being made in the study of the relation of string theory and the real world, via the study of the “swampland”. This is a very old story, and I’ve often written about it here. I just added a new category, so anyone who wants to can go follow it by clicking on the Swampland category of posts.

Recent press coverage of this includes an article by Clara Moskowitz at Scientific American, entitled String Theory May Create Far Fewer Universes Than Thought. This motivated Avi Loeb to write his own Scientific American piece highlighting the dangers of string theory speculation unmoored to any possible experimental test, which appeared as Theoretical Physics is Pointless without Experimental Tests. Loeb reports:

There is a funny anecdote related to the content of this commentary. In my concluding remarks at the BHI conference we held at Harvard in May 2018, I recommended boarding a futuristic spacecraft directed at the nearest black hole to experimentally test the validity of string theory near the singularity. Nima Arkani-Hamed commented that he suspects I have an ulterior motive for sending string theorists into a black hole. For the video of this exchange, see

https://www.youtube.com/watch?v=WdFkbsPFQi0

Last week Natalie Wolchover reported on this controversy, with an article that appeared at Quanta magazine as Dark Energy May Be Incompatible With String Theory and at the Atlantic as The Universe as We Understand It May Be Impossible (the Atlantic headline writer misidentifies “we” as “string theorists”).

Wolchover accurately explains part of this story as a conflict between string theorists over whether certain solutions (such as the KKLT solution and the rest of the so-called “string theory landscape”) to string theory really exist. Vafa argues they may not exist, since the proposed solutions are complicated and “Usually in physics, we have simple examples of general phenomena.” In response Eva Silverstein argues:

They [Vafa and others] essentially just speculate that those things don’t exist, citing very limited and in some cases highly dubious analyses.

On Twitter, Jim Baggott explains the problem

Let’s be clear. This is not a ‘test’ of string theory. There is no ‘evidence’ here. This is yet another conjecture that ‘might be true’, on which there is no consensus in the string theory community.

and in a retweet, Will Kinney accurately notes that

The landscape is a conjecture. The “swampland” is a conjecture built on a conjecture.

and points to an earlier tweet thread of his about this. Sabine Hossenfelder replies with the comment that

The landscape itself is already a conjecture build on a conjecture, the latter being strings to begin with. So: conjecture strings, then conjecture the landscape (so you don’t have to admit the theory isn’t unique), then conjecture the swampland because it’s still not working.

The Simons Center summer workshop this year has been devoted to Recent Developments in the Swampland, videos are here (this was also the case in 2006, see here). Next month in Madrid a conference will be devoted to Vistas over the Swampland, and I’m sure many more such gatherings are planned.

Unfortunately I think the fundamental problem here somehow never gets clearly explained: String theorists don’t actually have a theory, what they have is an approximation to an unknown theory supposed to be valid in certain limits, and a list of properties they would like the unknown theory to have. If this is all you have, there’s no way to distinguish when you’re on dry land (a solution to string theory) from when you’re in the swamp (a non-solution to string theory). Different string theorists can generate different opinions, conjectures and speculations about whether some location is swamp or dry land, but in the absence of an actual theory, no one can tell who is right and who is wrong. I don’t know why Vafa back in 2005 chose “Swampland” as the metaphor for this subject, but it’s an unfortunately apt one: string theorists are stuck in a swamp, with no way of getting out since they can’t tell what’s dry land and what isn’t.

Posted in Swampland | 15 Comments

Back

Posted on August 6, 2018 by woit

Back now from vacation. On the global warming front, I can report that Northern Norway has gotten rather warm, Svalbard is still pretty cold.

While I was away the big mathematics news was from the ICM. As everyone expected, one of the Fields medalists was Peter Scholze. I was surprised to find a blog post of mine quoted about this in the NY Times, since normally the way this works is that journalists are told who the winners are in advance, and then contact experts in the field (which I’m definitely not one of) for quotes. Some tweets from Davide Castelvecchi at Nature about the unusual embargo rules may provide some explanation:

The whole situation was surreal from the beginning: the organizers gave reporters advance notice of the winners, but on condition that we would not contact them — even though the winners had already been told long in advance.

They also made no other sources available. In other words, we were supposed to write about these difficult concepts without talking to any experts.

…

Oh and I forgot to say: The email with the names of the winners had no information whatsoever on why they won – in other words, no prize citations.

I suspect one reason for the unusual rules is that the ICM people had decided to concentrate on getting stories out through Quanta magazine, which ran the results here. The stories are very well done, and Quanta magazine is great, but a more usual process involving the rest of the science journalism press would have been a good idea.

One other big piece of news from the ICM was the choice of St. Petersburg over Paris as the site for the 2022 ICM. I was sorry to hear this. Perhaps it’s just that I’d rather have an excuse to go to Paris than one to go to St. Petersburg. It does seem to me though that in these worrisome times, when offered the choice between the world’s most active opponent of liberal democracy and one of the great remaining healthy liberal democracies, the other choice than the one the IMU made would have been the better one. My understanding is that Russia offered twice as much money, and that many feel that was the deciding factor.

Update: I hadn’t realized that the problems with the IMU embargo this year were not new, they were much the same as the problems four years ago with the announcement of the 2014 prizes. See here for discussion of the 2014 story (which, when reading it, I first mistook for a discussion of 2018), and here for a discussion of 2018.

The writer of the new story suggests that “Next time the IMU offers up an embargo agreement, reporters should just refuse” which I’d also semi-jokingly suggested in a comment. Actually, given the history of this, it seems to me that journalists seriously should plan to do this next time, and that sympathetic and well-informed mathematicians should help them find out in advance who the winners are. This would allow journalists to contact experts and do proper reporting, with no reason to wait until the ICM to write their stories.

Update:

  • The ICM Youtube channel still doesn’t have plenary talks from the ICM posted. Peter Scholze’s talk on Period maps in p-adic geometry is available now on a different channel. It’s an excellent overview of, not the technology of perfectoid spaces, but some of the results achieved using them.
  • The reason there are relatively few comments here about the decision to have the next ICM in Russia is that I’ve deleted most of them as they come in. Many commenters don’t believe Russia is any unusual threat to liberal democracy (or, if it is, that US/European liberal democracy is anything worth saving). Most also disagree with the idea that such a threat should have any effect on what mathematicians do. I agree that in general it’s best to keep mathematics and the ICM out of politics. A question to think about though for those who know the history of the 1930s is that of whether there was some point during the rise of Fascism that one would stop thinking it was a good idea to have the ICM in a Fascist capital. We’re not yet far along the horrific path of the 1930s, but maybe that just means that all should be thinking about what can be done to keep the world from going down that path again.

    Another frequent comment is “but, by your logic, the US would not be a good place to hold the next ICM!”. I fear the answer to that is that yes, Paris would be a much better choice than the US at this point.

Posted in Uncategorized | 32 Comments

Leaving Town

Posted on July 19, 2018 by woit

About to head out on vacation tonight, back in a couple weeks. On the hot topics in fundamental physics, two items are:

  • Concerning the “no dS string vacua” conjecture, a new preprint begins by explaining why the existence or non-existence of such vacua is a question that has not been resolved (and, it seems likely to me, can’t be resolved):

    classical no-go theorems such as [12] indicate that realizing de Sitter vacua in string theory requires quantum and/or stringy ingredients. The fact that corrections to classical 10d low energy supergravity are qualitatively important implies that dS compactifications, in contrast to AdS or Minkowski compactifictions, must live in a regime in which these corrections cannot be made arbitrarily small [13], hence perturbation theory cannot be made arbitrarily accurate. Moreover the absence of supersymmetry in dS, and perhaps more fundamentally the lack of a complete, nonperturbative formulation of string theory, make it hard to obtain exact results beyond perturbation theory. Thus a completely rigorous, parametrically controlled construction of individual de Sitter vacua in string theory has remained out of reach.

    The paper also explains why if you try and get known physics with a quintessence field rather than a CC, you immediately run into serious problems with coupling to the Standard Model.

  • Slides and video of the talks at this year’s PiTP summer school on “From Qubits to Spacetime” have started to appear. Once I get back from vacation I’ll try and watch some of the talks and hope to figure out how one is supposed to get our spacetime and its physics out of qubits.

Recently people have contacted me suggesting I blog about two physics-related topics likely to lead to vigorous debate. I’d begged off in both cases, since engaging in such a debate or moderating it would be on a short list of things I’d most like to avoid doing. This afternoon though, it struck me that there is an excellent, if cowardly, way to deal with this. I’ll mention the two topics briefly here, then shut off comments on the blog and leave town. So, some may find interesting and want to argue elsewhere about:

  • One of the PiTP lecturers, Aron Wall, has a blog on physics and theology, called Undivided Looking. Wall’s theological views include thinking he has a pretty good idea about how God wants people to behave, in particular he’s pretty sure that God doesn’t want them having homosexual relations. He wrote extensively about this in a blog entry (now deleted) back in 2015. He’ll be soon taking up a faculty position at Cambridge University, and some people are not happy about this, see for example this statement from the Cambridge University Student’s Union.
  • If you’d like to attend an early universe conference this September, one place you could do so is in the Israeli-occupied West Bank settlement of Ariel, where Ariel University is hosting a workshop on Inflation, Alternatives and Gravitational Waves.
Posted in Uncategorized | Comments Off on Leaving Town

abc News

Posted on July 17, 2018 by woit

[For those not up to speed on this story, see blog posts here and here from last December, as well as comments to those posts.]

The last couple months I’ve heard reports from several people claiming that arithmetic geometers Peter Scholze and Jakob Stix had identified a serious problem with Mochizuki’s claimed proof of the abc conjecture. These reports indicated that Scholze and Stix had traveled to Kyoto to discuss this with Mochizuki, and that they were writing a manuscript, to appear sometime this summer. It seemed best then to not publicize this here, better to give Mochizuki, Scholze and Stix the time to sort out the mathematics and wait for them to have something to say publicly.

Today though I saw that Ivan Fesenko has put out a document entitled Remarks on Aspects of Modern Pioneering Mathematical Research. It refers in footnote 18 to:

two recent texts by Sh. Mochizuki, ‘Report on discussions, held during the period March 15–20, 2018, concerning inter-universal Teichmüller theory (IUTCH)’and ‘Comments on the manuscript by Scholze–Stix concerning inter-universal Teichmüller theory (IUTCH)’, July 2018

I haven’t seen these two texts, or the Scholze-Stix manuscript. What I have heard about them is that Scholze-Stix identify what they see as a specific, serious flaw in the proof, and that Mochizuki denies that this is a problem or that his manuscript needs to be revised. Presumably, after the two sides try and sort this out amongst themselves, at some point we’ll see something publicly available describing the details of their disagreement.

Fesenko’s document has a lot of unpleasant things to say about those who have written anything at all skeptical concerning Mochizuki’s claimed proof, mostly without naming names. He refers to journalists and “US bloggers” as having produced “ignorant absurd articles and posts”, presumably has someone other than me in mind since the information posted here about this I believe has been quite accurate and of reasonably high quality. The one negative reference to identified mathematicians is in the text with footnote 18 pointing to Scholze and Stix, which says:

Several researchers, who could have become potential learners of IUT and then progressed to become experts, declined invitations to participate in the IUT workshops. Some, affected by negative emotions, broke professional rules of conduct and made public their ignorant and sometimes intolerant opinions. Tellingly, the only questions produced were shallow and misplaced and they were communicated only after several years of requests to do so.

Peter Scholze is by far the most talented arithmetic geometer of his generation, a sure thing to receive a Fields Medal at the ICM in a couple weeks. That his questions about Mochizuki’s proof were “shallow” seems highly unlikely, to me at least.

Much of Fesenko’s article concerns the question of whether contemporary mathematical research is too narrow and unambitious, devoted to minor improvements and producing lots of publications. This is a serious issue, one though where other fields than arithmetic geometry (e.g. fundamental physics) are in a much worse state. Fesenko tries to make the difficulties mathematicians have had with Mochizuki’s claims about his IUT research an exemplar of this problem, but this seems to me misguided. There are quite good reasons for why experts have been skeptical about IUT and the supposed abc proof, reasons which will be conclusively vindicated if Scholze and Stix turn out to be right. Ironically, an excellent example of the kind of fundamental breakthrough that Fesenko is asking for is Scholze’s own ground-breaking work over the past few years.

Posted in abc Conjecture | 24 Comments

Various Links

Posted on July 5, 2018 by woit

A random assortment of possibly interesting links:

  • New videos from the IHES include new interviews, and talks from the recent Ofer Gabber conference. If you want to know more about prisms than what you can get from the video here, I hear rumors that Bhargav Bhatt will be our Eilenberg lecturer this fall at Columbia.
  • Another talk at the Gabber conference was the latest on local (quantum) geometric Langlands from Gaitsgory. Also on this topic, there’s a July 4 preprint from Arakawa and Frenkel (advertised here).
  • John Horgan has an interview with Jim Holt, headlined Why Does Jim Holt Exist?. For reviews of Holt’s two most recent books, see here and here.
  • For two interesting blog posts from HEP experimenters about news from their field, see Jonathan Link at SciAm on neutrinos, and Tommaso Dorigo on the Higgs self-coupling. Dorigo’s posting is the more technical one, explaining a new CMS result bounding the Higgs self-coupling.

    The LHC experiments still seem to be a long ways away from actually measuring the Higgs self-coupling, but may be able to do so in future higher-luminosity stages of the LHC program. The Higgs remains the least understood part of the SM, responsible for most of the undetermined parameters of the theory. Any measurement of its self-interactions is an important goal.

    While it often seems that experimental results relevant to going beyond the Standard Model are inaccessible due to the necessity of higher energies, these two blog posts point to important open questions about the SM that are hard to study not because of fundamental limits on collision energies, but because of small event rates and high backgrounds.

  • The question recently came up here (see this posting) of how good the SUSY GUT coupling constant unification prediction is. At a recent summer school lecture, Ben Allanach says the prediction is off by 5 sigma, i.e. that if you try and predict the strong coupling at the Z mass this way, you get 0.129 +/- 0.002, whereas the measured value is 0.119 +/- 0.002. Someone should tell Frank Wilczek…

Update: For more on the dS vacua issue, see this blog posting by Ulf Danielsson. Danielsson refers to criticism of the landscape at my blog, but ignores the point I’ve often made that string theory is in just as much trouble if the advertised dS vacua don’t exist, since then it has no known way to connect to the real world and its positive CC. Somehow he sees this as a virtue, that “These are exciting times”, which I find mystifying.

He also claims that, all is well, since:

By studying the mathematics of the theory we will find out what it predicts, and by comparing with observations we will learn whether it has anything to do with reality.

But the underlying problem here is that the mathematics of the theory is unknown. Read the responses above from two experts to the question of why this issue has not been conclusively determined. Neither of them present any possibility of a conclusive determination. For both of them, this is about weighing the plausibility of various conjectures about possible solutions to unknown conjectural equations. At this point I seriously doubt it is possible for this to be resolved one way or the other.

Update: ICHEP 2018 is winding up, talks available here. There was one plenary summary talk on “Formal Theory Developments”, by Tadashi Takayanagi. It begins by promoting string theory, then goes on to address the problem of how it relates to fundamental physics with:

However, please do not ask me questions like:
How to derive Standard Model from string theory?
Why do we live in 4 dimensions?
How to realize de-Sitter spacetimes in a well-reliable way?

String theory is still too infant to give complete answers to them.

The “complete” is intentionally misleading, since string theory now gives no answers at all to these questions. While celebrating the 50th anniversary of string this year, it seems that “string theory is too new an idea to evaluate” is the standard answer to anyone who points out its failures.

Posted in Uncategorized | 30 Comments

Strings 2018 Panel Discussion

Posted on June 30, 2018 by woit

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.

Posted in Strings 2XXX, Swampland | 18 Comments

This Week’s Hype

Posted on June 27, 2018 by woit

New Scientist today has a feature article headlined

How to think about… The multiverse

The idea of an infinite multitude of universes is forced on us by physics.

It starts off quoting Sean Carroll:

“One of the most common misconceptions is that the multiverse is a hypothesis,” says Sean Carroll at the California Institute of Technology in Pasadena. In fact, it is forced upon us.”It is a prediction of theories we have good reason to think are correct.”

The problem with this claim is that it’s simply not true. There is no model that “we have good reason to think [is] correct” that predicts a multiverse of universes with different physics (i.e. fundamental constants). I’ve written about this many times, see for instance Theorists Without a Theory. In case you were thinking of interpreting Carroll’s claim in some other way, the article goes on to invoke Alexander Vilenkin:

“The so-called constants of nature, like the mass of the electron or Newton’s gravitational constant, will have different values in different bubbles,”

To be fair to New Scientist, I haven’t read beyond the headline and first few paragraphs of this article, since the rest is behind a paywall. Maybe the later part of the article (which most people can’t read) explains what is wrong with the Carroll and Vilenkin claims.

For the latest on the models supposed to give us different physics in different parts of the multiverse, you might want to take a look at this new paper on the arXiv, and Cumrun Vafa’s talk about it this week at Strings 2018. The paper and talk conjecture that the supposed metastable dS solutions of the string landscape don’t really exist (they are in the “swampland” of things that aren’t solutions of string theory). If this is true and you want to save string theory, as Vafa explains, you need to invoke different sorts of supposed solutions to string theory, with the CC replaced by a “quintessence” mechanism.

At the end of the talk (1:01), Eva Silverstein tries to explain what is wrong with Vafa’s arguments. He responds “I’m not saying you’re wrong, you might be right, this might be also be right”. This shows clearly the fundamental problem of the subject: there is no well-defined theory here, just a bunch of conjectures about what one might be, with no way to tell whether Vafa or Silverstein is right, and no way to extract well-defined predictions from the mass of possible conjectures.

Update: Thanks to those who sent me a copy of the full New Scientist article. It’s short, and the part behind the paywall is even worse than the part publicly available, just adding to the confusion by invoking “many-worlds”, with more from Sean Carroll. Our doppelgangers doing exciting stuff in other universes make an appearance, although Carroll expresses a lack of interest in what they’re up to.

Update: The Strings 2018 talks and videos are available here or at this Youtube channel, and 4 gravitons has a blog posting. As usual with Strings 20XX conferences, very little about actual string theory there. For an overview of the state of the field, you might want to watch this video of the 50 years of string theory session, moderated by David Gross. Dan Harlow was the only speaker raising the elephant in the room question: “is string theory still a useful candidate as a theory of HEP physics?” (and also asked whether they should finally rename the conference series). Gross read off submitted questions for the panel, most of which were asking about the elephant in the room. The panelists each found a different way of avoiding dealing with the question. Other questions asked about the hot “is there a dS string vacuum?” issue, responses were “maybe yes, maybe no”, with no indication of any way to resolve this.

Posted in Fake Physics, Multiverse Mania, Strings 2XXX, Swampland | 17 Comments

Knockin’ On Heaven’s Door

Posted on June 21, 2018 by woit

I’ve been thinking about what to write about this essay by Ben Allanach, which gives his take on the current state of HEP theory. Allanach is a specialist on the phenomenology of SUSY models, but here he announces that he’s basically giving up on these models:

The trouble is that it’s not clear when to give up on supersymmetry. True, as more data arrives from the LHC with no sign of superpartners, the heavier they would have to be if they existed, and the less they solve the problem. But there’s no obvious point at which one says ‘ah well, that’s it – now supersymmetry is dead’. Everyone has their own biased point in time at which they stop believing, at least enough to stop working on it. The LHC is still going and there’s still plenty of effort going into the search for superpartners, but many of my colleagues have moved on to new research topics. For the first 20 years of my scientific career, I cut my teeth on figuring out ways to detect the presence of superpartners in LHC data. Now I’ve all but dropped it as a research topic.

While most HEP physicists still try and end their talks with some sort of optimistic expression of hope that things will change soon, I was struck by a recent talk by John Iliopoulos, which was more somber and more realistic:

No coherent picture emerges

We were expecting new physics to be around the corner…..
But we see no corner

The easy answer: We need more data

Two problems: (i) We do not know what kind of data
(ii) They will not come for quite a long time

A rather frustrating problem!

and he ends with

The Future of Particle Physics will undoubtedly be bright, but

I will not learn the answer

While thinking about this I happened to look at an old posting of mine, a review of Lisa Randall’s Knocking on Heaven’s Door written back in 2011. There I wrote

One odd thing about the book is the title, which for Randall carries a positive meaning that she acknowledges doesn’t correspond to the very dark one of the Bob Dylan song from the soundtrack of the Sam Peckinpah film. It’s a beautiful song, but one not about finding truth, but about getting shot in the gut and facing death, hopefully not relevant to particle physics in the LHC era:

Mama, put my guns in the ground
I can’t shoot them anymore.
That long black cloud is comin’ down
I feel like I’m knockin’ on heaven’s door.

It does seem like much of the last 40 years of HEP theory is now “knockin’ on heaven’s door”, deeply wounded by negative results from the LHC. What this means for the future is still up in the air: what story about what has happened will become the conventional wisdom?

Posted in Uncategorized | 17 Comments