This Time It’s Really for Real

Posted on June 14, 2017 by woit

Twice now I’ve thought I had a finished version of the book I’ve been writing forever (see here and here). Each time it turned out that, the way the publishing process was going, I ended up having more time to work on the manuscript and deciding I could do better, especially with some of the basic material about quantum field theory. I do think the latest version has a much improved treatment of the basics of that subject.

This version will go off to Springer in a day or so, and they plan to publish it late this year/early next year. I’m setting up a web-page for the book, there may be more material there later.

One thing ensuring that I will stop working on this is that in a couple days I’m heading off on vacation, for a two-week or so trip to Europe. Blogging during that time is likely to be light to non-existent. Back around the Fourth of July, and looking forward to thinking about other projects, anything but this book…

Update: If you’re wondering why the document length changed today, no change in content, just a minor change (improvement) in formatting.

Posted in Quantum Theory: The Book | 12 Comments

The Dangerous Irrelevance of String Theory

Posted on June 12, 2017 by woit

Eva Silverstein has a new preprint out, entitled The Dangerous Irrelevance of String Theory. The title is I guess intended to be playful, not referring to its accurate description of the current state of string theory, but to the possibility of irrelevant operators having observable effects.

The article is intended to appear in the forthcoming Cambridge University Press volume of contributions to the Munich “Why Trust a Theory?” conference held back in December 2015. The impetus behind that conference was a December 2014 article in Nature entitled Scientific method: Defend the integrity of physics. In that article, Ellis and Silk explained the problems with string theory and with the multiverse/string theory landscape.

The organizing committee for the Munich conference was chaired by Richard Dawid, a string theorist turned philosopher who has written a 2013 book, String Theory and the Scientific Method. For a fuller discussion of that book, see the linked blog post. To oversimplify, it makes the case that the proper way to react to string theory unification’s failure according to the conventional understanding of the scientific method is to change our understanding of the scientific method. Much of the Munich conference was devoted to discussing that as an issue in philosophy of science.

One aspect of the Munich conference was that it was heavily weighted towards string theorists, with contributions from Dawid, David Gross, Joe Polchinski, Fernando Quevedo, Dieter Lust and Gordon Kane all promoting the idea that string theory was a success. Polchinski explained a computation that shows that string theory is 98.5% likely to be correct, going on to claim that the probability is actually higher: “something over 3 sigma” (i.e. over 99.7%). The only contribution from a physicist that I’ve seen that argued the case for the failure of string theory was that from Carlo Rovelli, see here. Silverstein’s article says that it was commissioned by Dawid for the proceedings volume, even though she hadn’t been at the meeting. I’m curious whether Dawid commissioned any contributions from string theory critics who weren’t at the meeting.

Silverstein begins her article explaining how physics at a very high energy scale can in principle have observable effects. This of course is true, but the problem with string theory is that, in its landscape version, it has a hugely complicated and poorly understood high energy scale behavior, seemingly capable of producing a very wide range of possible observable effects, none of which have been seen. The article is structured as a defense of string theory, without explaining at all what the serious criticisms of string theory actually are. The list of references includes 53 items, only one critical of string theory, the Ellis/Silk Nature article. Some of the arguments she makes are:

  • It is sometimes said that theory has strayed too far from experiment/observation. Historically, there are classic cases with long time delays between theory and experiment – Maxwell’s and Einstein’s waves being prime examples, at 25 and 100 years respectively… One thing that is certainly irrelevant to these questions is the human lifespan. Arguments of the sort ‘after X number of years, string theory failed to produce Y result’ are vacuous.

    I think the comparison to EM or GR is pretty much absurd. For one thing it’s comparing two completely different things: tests of a particular prediction of a theory (EM or GR) that made lots of other testable, confirmed predictions to the case of string theory, where there are no predictions at all. More relevant to the argument over how long to wait for an idea to pay off is that the real question is not the absolute value of the amount of progress, but the derivative: as you study the idea more carefully, do you get closer to testable progress or farther away? I don’t think anybody can serious claim that, 33 years on, we’re closer to a successful string theory unification proposal than we were at the start, back in 1985. I’d argue that the situation is the complete opposite: we have been steadily moving away from such success (and thus entered the realm of failure).

  • About supersymmetry Silverstein writes:

    In my view, the role of supersymmetry is chronically over-emphasized in the field, and hence understandably also in the article by Ellis and Silk. The possibility of supersymmetry in nature is very interesting since it could stabilize the electroweak hierarchy, and extended supersymmetry enables controlled extrapolation to strong coupling in appropriate circumstances. Neither of these facts implies that low-energy supersymmetry is phenomenologically favored in string theory.

    It is true that Silverstein has never been one of those arguing that the usual string theory scenarios with supersymmetry and 10 or 11 dimensions show that string theory is testable. See for instance her comment here back during a “String Wars” discussion in 2006. Her current take on whether string theory implies supersymmetry is just

    Much further research, both conceptual and technical, is required to obtain an accurate assessment of the dominant contributions to the string landscape.

    The problem with this is that there is no sign of any possibility of progress towards deciding if the string theory landscape implies low-energy SUSY or not (quite the opposite). If you give up the assumptions of SUSY and 10/11 dimensions, you give up what little hope you had of any connection with experiment. She doesn’t mention the LHC at all, especially not the negative results about supersymmetry and extra dimensions that it has produced. The significance of these negative results is not that they disconfirm a strong prediction of string theory, but that they pull the plug on the last remaining hope for connecting standard string theory unification scenarios to anything observable. Pre-LHC string theorists could make an argument that there was good reason to believe in electroweak-scale SUSY, that such a scenario fit in well with string theory unification, and that LHC discovery of SUSY would point a way forward for string theory unification. That argument is now dead. All that’s left is basically the argument that “maybe a miracle will happen and we’ll be vindicated” which in her version is:

    In principle one could test string theory locally. In practice, this would require discovering a smoking gun signature (such as a low string scale at colliders, or perhaps a very distinctive pattern of primordial perturbations in cosmology), and nothing particularly favors such scenarios currently.

  • Silverstein’s main argument is basically that string theory is valuable because it leads to the study of models that have various observable signatures that people would not otherwise look for. One example here is supersymmetry, the study of which has had a huge effect on collider physics, strongly shaping the analyses that the experimentalists perform. She gives some detailed other examples from her field of cosmology, in particular about possibly observable non-Gaussianities.

    String theory participates in empirical science in several ways. In the context of early universe cosmology, on which we have focused in this article, it helped motivate the discovery and development of mechanisms for dark energy and inflation consistent with the mathematical structure of string theory and various thought-experimental constraints. Some of these basic mechanisms had not been considered at all outside of string theory, and some not quite in the form they take there, with implications for effective field theory and data analysis that go well beyond their specifics.

    I think this is the best argument to be made for “phenomenological” string theory research (as opposed to “formal” string theory, where there are other arguments). Yes, coming up with new models with unexpected observable effects is a valuable enterprise. If your speculative idea generates such things, that’s well and good. The problem though is how to evaluate the situation of a speculative idea that has generated a huge number of such models, none of which has worked out. At what point do you decide that this is an unpromising line of research, better to try just about anything else? Silverstein makes the argument that

    Whether empirical or mathematical, constraints on interesting regions of theory space is valuable science. In this note we focus on string theory’s role in the former.

    Since information theory is currently all the rage, it occurred to me that we can phrase this in that language. Information is maximized when the probabilities are equal for a set of outcomes, since one learns the most from a measurement in that case. The existence of multiple consistent theoretical possibilities implies greater information content in the measurements. Therefore, theoretical research establishing this (or constraining the possibilities) is directly relevant to the question of what and how much is learned from data. In certain areas, string theory plays a direct role in this process.

    The problem here is that of what is an “interesting region of theory space”. At this point the failures of string theory unification strongly indicate that it’s not such an interesting region. It seems likely that we’d be better off if most theorists focusing on phenomenology of this failed program were to pick something else to work on.


Update: Will Kinney has a Twitter commentary here.

Update: For another relevant recent Will Kinney Twitter storm, see here and here.

Update: Silverstein gave some lectures to the public about this at Stanford recently, video here and here, slides here. A large part of the lectures were an advertisement for string theory, with the summary at the end

Quantum gravity (string theory) plays a subtle but important role, even contributing to our understanding of empirical measurements of early universe dynamics.

Crediting string theory with “contributing to our understanding of empirical measurements of early universe dynamics” is a peculiar way of saying that string theory produces lots of cosmological models that don’t work (see a better summary by Will Kinney at the end of this presentation).

Update: Renata Kallosh is Silverstein’s colleague at Stanford (and Andrei Linde’s wife). In an interview here she makes much simpler and stronger claims about string cosmology than does Silverstein:

string theory ideas help us to build cosmological models which fit the data from observations. Moreover, we have produced relatively simple predictions from string theory and related theories which will be testable with future detectors of primordial gravity waves.

I’m not sure what specifically she is referring to, but suspect that “prediction” here means something like the “predictions” of string cosmology that Kinney describes (see above) whose failure to be observed has in no way affected string cosmologists enthusiasm for the subject.

Update: For some more context about string theory inflation models and the issue of their testability, you could consult Silverstein’s guest post at Lubos Motl’s blog from 2014, explaining how the BICEP2 observation of that era could provide evidence for “axion monodromy inflation”.

Posted in Uncategorized | 22 Comments

2018 US HEP Budget

Posted on June 6, 2017 by woit

HEPAP has been meeting the past couple days, with presentations available here. Much of the discussion is about the President’s 2018 budget proposal recently submitted to Congress, which contains drastic cuts to all sorts of programs, including for support of scientific research. In particular the proposal is to cut the total NSF budget from \$7.5 billion to \$6.65 billion (-11.3%), and the DOE science budget from \$5.4 billion to \$4.47 billion (-17%).

At the DOE, for HEP physics, the cut would be from \$825 million to \$673 million (-18.5%). For topics less popular with the new administration the cuts are even larger, e.g. a 43% cut for biological and environmental research.

At the NSF (numbers with respect to FY 2016), the proposed cut for DMS (Mathematics) is 10.3%, for Physics 8.5% (-\$23.6 million) and for Astronomy 10.3%. The FY 2016 budget number for Physics was \$277 million, of which \$13.2 million went to HEP theory.

Budget cuts on this scale would be extreme and unheard of, requiring shutting down major planned experimental projects. For some sorts of spending, this sort of cut is painful but manageable, but cutting out 18.5% of the spending on an experimental apparatus under construction may likely mean you don’t have an experiment anymore.
The HEPAP presentations are from people working for DOE/NSF and under orders to plan for these cuts and not complain about them, so I think don’t reflect at all what the real implications of such cuts would be.

There’s a summary of discussion here, including a discussion of last year’s HEP theory letter. It sounds like nothing much has been done about that, and it may not get much attention given the current situation.

It’s important though to keep in mind that this budget proposal may very well already be dead on arrival at Congress. Take a look at slide 22 of this presentation that reports that of the staffers and representatives asked about (a preliminary version of) this, only 8.4% were in favor. In recent years the US budgeting process has been quite dysfunctional, with actual budget numbers only appearing at the last minute of an opaque process leading not to a budget but to a “Continuing Resolution”. I doubt anyone has any idea what is going to happen this year, with the passing of something close to this budget probably one of the least likely eventualities. Physicists and mathematicians up in arms about these proposed budget cuts need to keep in mind the context: this budget is an extremely radical proposal of an unparalleled sort, with even larger cuts aimed at groups that are far needier than scientists (for one random example, food stamps are to be cut by 25.3%). Yes, scientists should be organizing to fight this budget, but the impacts on them and their research are one of the less important reasons for doing so.

I’m setting all comments to go to moderation. If you just want to rant pro or con about the awful situation the US is now in, please do it elsewhere. If you have any actual information about the effects of this on the physics and math communities as the budget process gets underway, that would be worthwhile and interesting. Two people tweeting about this are Kyle Cranmer and Matthew Buckley.

Updates: Details of the DOE HEP budget proposal are here. It explains that about 20-25% of the research positions funded by DOE at all levels would be eliminated. There would be an “extended shutdown of the Fermilab accelerator complex”.
About 1/3 of DOE HEP theory funding would be eliminated, but it would be replaced by an equal amount of funding for quantum information science as a subfield of HEP. Looks like someone in the Trump administration is a great believer in “It From Qubit”…

Update: According to this story, if this budget passes about 700 jobs at Argonne and Fermilab would be eliminated.

Posted in Uncategorized | 18 Comments

Multiverse Politics

Posted on June 1, 2017 by woit

The political campaign for the multiverse continues today with a piece by Amanda Gefter at Nautilus. It’s a full-throated salvo from the Linde-Guth side of the multiverse propaganda war they are now waging, with Linde dismissing Steinhardt’s criticism as based on “a total ignorance of what is going on”. All of the quotes for the article are on the pro-multiverse side. There is a new argument from them I’d never heard before: Guth comes up with this one:

You can create a universe from nothing—you can create infinite universes from nothing—as long as they all add up to nothing. Not only is that a deep insight, it also creates a testable prediction. “Eternal inflation certainly predicts that the average density of all conserved quantities should be zero,” Guth says. “So if we ever became convinced that the universe has a nonzero density of electric charge or angular momentum, eternal inflation would no longer be an option.”

The article is subtitled “Why the majority of physicists are on one side of a recent exchange of letters”. One way to interpret this claim is just that 33 is more than 3, but the reason for this is clear: while Guth, Kaiser, Linde and Nomura decided to go on a political campaign, drumming up signatures on their letter, Ijjas, Loeb and Steinhardt didn’t do this, but instead put together a website discussing the scientific issues.

Where the majority of physicists stand on the Guth-Linde claims is an interesting question, one that I don’t think is addressed anywhere by hard numbers. My anecdotal data is that the majority of those I’ve ever talked to about this don’t think the Guth-Linde multiverse claims are science, but don’t see any reason to waste their time arguing with pseudo-science. They hope it will just go away by itself, as it becomes ever clearer that the multiverse is, scientifically, an empty idea.

Unfortunately, I don’t see this going away and I think it’s now doing very serious damage to physics and its public image. There’s a political campaign now being waged, and one side is very determined to win and putting a lot of energy into doing so. Those on the other side need to step up and make themselves heard.

Update: Guth and Linde brought their publicity campaign to New York this past Saturday (video here) where they told a large World Science Festival audience that string theory is beautiful, it predicts the multiverse, inflation has made lots of predictions that have all worked out, and they have (more or less…) the full theory that does all this wonderful stuff. Nothing from them about any thing less than utterly glorious and well-defined about the string theory landscape/eternal inflation product they were pushing. Also on the panel were three philosophers (Jim Holt, David Albert and Barry Loewer) who did an admirable job of trying to push back by pointing out obvious inconsistencies. They at least got Guth and Linde to admit that there was this “measure problem” thing still to be fixed. Physics is in a very weird state indeed now that physicists have adopted untestable metaphysical speculation as their program, with philosophers the ones trying to engage in more normal scientific practice.

There was one multiverse skeptic, George Ellis, who unfortunately didn’t engage with this and was diverted onto other topics. One string theorist was there (Veronika Hubeny), who explained about AdS/CFT duality, without anybody bringing up the fact that this has nothing at all to do with what Guth and Linde were promoting in the rest of the discussion.

Posted in Multiverse Mania | 37 Comments

A Few Quick Items

Posted on May 31, 2017 by woit

I’ve had little time for blogging, and coincidentally, there seems to be little to blog about recently. Here though are a few quick items:

  • Several people had asked me about this paper about the CC, and I had to tell them that this was not something I could evaluate. Luckily, Sabine Hossenfelder has read it and thought about it carefully, and discusses the problems with this sort of thing here. The physics community owes her a great debt.
  • The LHC is back in business, with intensity starting to ramp up. You can follow progress here. This summer should see release of more results based on last year’s run, results from this year’s run likely will not appear until early next year.
  • Inference magazine has a thoughtful piece in the latest issue by Adam Falkowski (AKA Jester) about prospects for the future of HEP physics. The same issue also has a piece by Aurélien Barrau about the implications of the failure to find the “natural” physics some expected SUSY to provide.
  • Hironaka has recently put on his website a document intended to give a proof of resolution of singularities in characteristic p. For some background and links to explanations of what this is about, see mathoverflow. Evidently Hironaka has been working on this proof for quite a few years, this is the first complete version to be made public. Sometime in the next few months it should become clear whether this proof will really work, as experts get a chance to go through it carefully. If it does work, it will be a remarkable story, especially since Hironaka is now 86.
  • Maybe I’m the last one to find this out, but for quite a few years now MIT has been making public detailed course materials including lecture notes from many courses in mathematics and physics.

Update: For the obligatory Multiverse Mania item, see this interview with Lord Martin Rees. Rees is rather proud of himself for leading the field of theoretical physics to embrace Multiverse Mania, quoting Frank Wilczek as claiming at the end of a conference that:

five years ago we were a beleaguered minority, whereas now, he and I and others had led many other people into the wilderness.

Besides his belief in the multiverse, he also believes this is what we have in our future:

I don’t think Elon Musk is realistic when he imagines sending people a hundred at a time for normal life because Mars is going to be far less clement than living at the South Pole, and not many people want to do that. I don’t think there will be many ordinary people who want to go, but there will be some crazy pioneers who will want to go, even if they have one-way tickets.

The reason that’s important is the following: Here on Earth, I suspect that we are going to want to regulate the application of genetic modification and cyborg techniques on grounds of ethics and prudence. This links with another topic I want to come to later about the risks of new technology. If we imagine these people living as pioneers on Mars, they are out of range of any terrestrial regulation. Moreover, they’ve got a far higher incentive to modify themselves or their descendants to adapt to this very alien and hostile environment.

They will use all the techniques of genetic modification, cyborg techniques, maybe even linking or downloading themselves into machines, which, fifty years from now, will be far more powerful than they are today. The posthuman era is probably not going to start here on Earth; it will be spearheaded by these communities on Mars.


Update: Another black hole merger detection from LIGO announced today, some commentary here and here.

Posted in Multiverse Mania, Uncategorized | 10 Comments

GAMBIT

Posted on May 23, 2017 by woit

The LHCP 2017 conference was held this past week in Shanghai, and among the results announced there were new negative results about SUSY from ATLAS with both ATLAS and CMS now reporting for instance limits on gluino masses of around 2 TeV. The LHC has now ruled out the existence of SUSY particles in the bulk of the mass range that will be accessible to it (recall for instance that pre-LHC, gluino mass limits were about 300 GeV or so).

Over the years there has been an ongoing effort to produce “predictions” of SUSY particle masses, based on various sorts of assumptions and various experimental data that might be sensitive to the existence of SUSY particles. One of the main efforts of this kind has been the MasterCode collaboration. Back in 2008 before the LHC started up, they were finding that the “best fit” for SUSY models implied a gluino at something like 600-750 GeV. As data has come in from the LHC (and from other experiments, such as dark matter searches), they have periodically released new “best fits”, with the gluino mass moving up to stay above the increasing LHC limits.

I’ve been wondering how efforts like this would evolve as stronger and stronger negative results came in. The news this evening is that they seem to be evolving into something I can’t comprehend. I haven’t kept track of the latest MasterCode claims, but back when I was following them I had some idea what they were up to. Tonight a large collaboration called GAMBIT released a series of papers on the arXiv, which appear to be in the same tradition of the old MasterCode fits, but with a new level of complexity. The overall paper is 67 pages long and has 30 authors, and there are eight other papers of length totaling over 300 pages. The collaboration has a website with lots of other material available on it. I’ve tried poking around there, and for instance reading a Physics World article about GAMBIT, but I have to confess I remain baffled.

So, the SUSY phenomenology story seems to have evolved into something very large that I can’t quite grasp anymore, perhaps a kind reader expert in this area can explain what is going on.

Posted in Uncategorized | 20 Comments

This Month’s Hype

Posted on May 16, 2017 by woit

It seems that a couple of the authors of the recent Cosmic Controversy letter (discussed here) are going on a campaign to embarrass the 29 physicists who were convinced to sign their letter. Andrei Linde has gone to Lubos Motl’s blog to thank him for his blog entry which lauded Linde as having eaten from the biblical tree of knowledge and which denounced his critics as imbeciles. To deal with Linde and his claims, Ijjas, Steinhardt and Loeb have added a new webpage to their website called Fact Checking. It lists the four “predictions” of inflation claimed to agree with experiment by Linde et al. and gives four references to papers published by Linde touting different “predictions” for the same quantities, predictions not agreeing with experiment.

This month’s Scientific American has a remarkable cover story, The Quantum Multiverse from one of the other four letter authors, Yasunori Nomura. I’ve seen some fairly bizarre stories about fundamental physics in Scientific American over the years, but this one sets a new standard for outrageous nonsense, and I’m wondering whether it too may cause some of the 29 co-signers of the letter co-authored by Nomura to question the wisdom of joining with him and Linde. Nomura is well known for a definite prediction based on the multiverse: in 2009 he co-authored a paper claiming that the multiverse predicted the Higgs mass would be 141 GeV +/- 2 GeV. This played a major role in the film Particle Fever. That three years later the Higgs was discovered at 125 GeV seems to have had no effect on his multiverse enthusiasm.

The new SciAm cover story is not about anything new, but is based on a 6 year old paper by Nomura discussed here. At the time I wrote about this “I’m having trouble making sense of any of these papers” and quoted Lubos’s evaluation: “They’re on crack”. Nothing I’ve seen about this over the past six years seems to me to make any sense at all, including the new SciAm cover story, which just seems even more content-free and meaningless than previous efforts to explain this “multiverse interpretation of quantum mechanics”. On the obvious question: how would you test this, Nomura just has this to say:

Evidence so far indicates that the cosmos is flat, but experiments studying how distant light bends as it travels through the cosmos are likely to improve measures of the curvature of our universe by about two orders of magnitude in the next few decades. If these experiments find any amount of negative curvature, they will support the multiverse concept because, al­­though such curvature is technically possible in a single universe, it is implausible there. Specifically, a discovery supports the quantum multiverse picture described here because it can naturally lead to curvature large enough to be detected, whereas the traditional inflationary picture of the multiverse tends to produce negative curvature many orders of magnitude smaller than we can hope to measure.

This paragraph manages to put together three different misleading and unsupported claims:

  • “If these experiments find any amount of negative curvature, they will support the multiverse concept because, al­­though such curvature is technically possible in a single universe, it is implausible there.” This is just nonsense.
  • “the traditional inflationary picture of the multiverse tends to produce negative curvature many orders of magnitude smaller than we can hope to measure”. What is the inflationary multiverse “prediction” for negative curvature? As far as I can tell it’s compatible with pretty much any level we might observe.
  • “the quantum multiverse picture described here because it can naturally lead to curvature large enough to be detected.” I can’t find anywhere a calculation of the negative curvature expected by the “quantum multiverse picture”, and I don’t believe any such calculation is possible.

Given some of the outrageous hype I’ve seen in recent years in respectable publications, it’s gotten rather hard to shock me with this sort of thing, but I do find this Scientific American cover story shocking.

Update: For some reason this was not mentioned in the SciAm article, but the paper justifying Nomura’s claims about negative curvature is here.

Update: A modest proposal: Given the situation, I think someone needs to write a letter to SciAm complaining about the Nomura article and get leaders of the community to sign in support of it. They could start gathering signatures by writing to the 29 signers of the earlier letter. If these people were willing to object to the Steinhardt et al. article, they should be willing to object to the far worse Nomura article.

Posted in Multiverse Mania, This Week's Hype | 49 Comments

A Cosmic Controversy

Posted on May 10, 2017 by woit

A couple months ago Scientific American published an article by Ijjas, Steinhardt and Loeb (also available here), which I discussed a bit here. One aspect of the article was its strong challenge to multiverse mania, calling it the “multimess” and accusing multiverse explanations of being untestable and unscientific.

Yesterday Scientific American published, under the title A Cosmic Controversy, a rebuttal signed by 33 physicists, together with a response from the authors, who have also set up a webpage giving further details of their response. Undark has an article covering this: A Debate Over Cosmic Inflation (and Editing at Scientific American) Gets Heated.

As Ijjas, Steinhardt and Loeb point out on their webpage, the story of this letter is rather unusual. It was written by David Kaiser and three physicists well-known for their outspoken promotion of the multiverse (Guth, Linde and Nomura). Evidently these authors decided they needed reputational support on their side, and sought backing from other prominent names in the field (I’m curious to know who may have refused to sign if asked…). Their letter starts out with a claim to represent the “dominant paradigm in cosmology” and notes the large number of papers and researchers involved in studying inflation.

If you read carefully both sides (IS&L and GKL&N) of this, I think you’ll find that they are to a large degree speaking past each other, with a major problem that of imprecision in what one means by “inflation”. To the extent that there is a specific identifiable scientific disagreement, it’s about whether Planck data confirms predictions of the “simplest inflationary models.” IS&L write:

The Planck satellite results—a combination of an unexpectedly small (few percent) deviation from perfect scale invariance in the pattern of hot and colds spots in the CMB and the failure to detect cosmic gravitational waves—are stunning. For the first time in more than 30 years, the simplest inflationary models, including those described in standard textbooks, are strongly disfavored by observations.

whereas GKL&N respond:

there is a very simple class of inflationary models (technically, “single-field slow-roll” models) that all give very similar predictions for most observable quantities—predictions that were clearly enunciated decades ago. These “standard” inflationary models form a well-defined class that has been studied extensively. (IS&L have expressed strong opinions about what they consider to be the simplest models within this class, but simplicity is subjective, and we see no reason to restrict attention to such a narrow subclass.) Some of the standard inflationary models have now been ruled out by precise empirical data, and this is part of the desirable process of using observation to thin out the set of viable models. But many models in this class continue to be very successful empirically.

I take this as admission that IS&L are right that some predictions of widely advertised inflationary models have been falsified. Of course, if these had worked they would have been heavily promoted as “smoking gun” proof of inflation, as was demonstrated by the BICEP2 B-mode fiasco. After BICEP2 announced (incorrectly) evidence for B-modes, Linde claimed this was a “smoking gun” for inflation (see here) and the New York Times had a front page story about the “smoking gun” confirmation of inflation vindicating the ideas of Guth and Linde. A couple months later, before the BICEP2 result was shown to be mistaken, Guth, Linde and Starobinsky were awarded the $1 million Kavli Prize in Astrophysics.

GKL&N don’t mention the sorry story of the BICEP2 B-modes, what they have to say about this is

the levels of B-modes, which are a measure of gravitational radiation in the early universe, vary significantly within the class of standard models…

The B-modes of polarization have not yet been seen, which is consistent with many, though not all, of the standard models.

About the IS&L “unexpectedly small (few percent) deviation from perfect scale invariance” all GKL&N have to say is

The standard inflationary models… predict the statistical properties of the faint ripples that we detect in the cosmic microwave background (CMB). First, the ripples should be nearly “scale-invariant”

This doesn’t seem to address at all the IS&L claims, which they make in more detail as

The latest Planck data show that the deviation from perfect scale invariance is tiny, only a few percent, and that the average temperature variation across all spots is roughly 0.01 percent. Proponents of inflation often emphasize that it is possible to produce a pattern with these properties. Yet such statements leave out a key point: inflation allows many other patterns of hot and cold spots that are not nearly scale-invariant and that typically have a temperature variation much greater than the observed value. In other words, scale invariance is possible but so is a large deviation from scale invariance and everything in between, depending on the details of the inflationary energy density one assumes. Thus, the arrangement Planck saw cannot be taken as confirmation of inflation.

GKL&N argue for three other confirmed predictions of inflationary models:

Second, the ripples should be “adiabatic,” meaning that the perturbations are the same in all components: the ordinary matter, radiation and dark matter all fluctuate together. Third, they should be “Gaussian,” which is a statement about the statistical patterns of relatively bright and dark regions. Fourth and finally, the models also make predictions for the patterns of polarization in the CMB, which can be divided into two classes, called E-modes and B-modes. The predictions for the E-modes are very similar for all standard inflationary models

On these issues I don’t see anything from IS&L and would love to hear from an expert.

The main issue here comes down to the question of the flexibility vs. rigidity of inflationary models. Is the inflationary paradigm rigid enough to make solid predictions, or so flexible that it can accommodate any experimental result? GKL&N are making the case for the former, IS&L for the latter, and they point out the following quote from Guth himself:

when asked via email if they could name any pro-inflation scientists who believe that the theory is nonetheless untestable, the trio pointed to a video of a 2014 panel during which Loeb asks Guth directly whether it’s possible to do an experiment that would falsify inflation.

“Well, I think inflation is a little too flexible an idea for that to make sense,” Guth replied.

A fair take on all this would be to note that it’s a complicated situation, and I doubt I’m the only one who would like to see an even-handed technical discussion of exactly what the “simplest” models are and a comparison of their predictions with the data. Claims to the public from one group of experts that Planck data says one thing, from others claiming it says the opposite are generating confusion here rather than clarity about the science.

I’m strongly on the side of IS&L on one issue, that of the danger of theories that invoke the multiverse as untestable explanation. I don’t think though that they make a central issue clear. The simple inflationary models whose “predictions” for Planck data are being discussed involve a single inflaton field, with no understanding of how this is supposed to couple to the rest of physics. One is told that eternal inflation implies a multiverse with different physics in different universes, but in a single inflaton model this physics should just depend on a single parameter, and such a theory should be highly predictive (once you know one mass, all others are determined). What’s really going on is that there is no connection at all between the simple single field models that GKL&N and IS&L are arguing about, and the widely promoted completely unpredictive string theory landscape models (involving large numbers of inflaton-type fields with dynamics that is not understood).

I think IS&L made a mistake by not pointing this out, and that Guth, Linde, Nomura and some of the signers of their letter (e.g. Carroll, Hawking, Susskind, Vilenkin) have long been guilty of promoting the defeatist pseudo-scientific idea that “evidence for inflation is evidence for a multiverse with different physics in each universe, explaining why we can’t ever calculate SM parameters”. By defending the predictivity of “inflation” while ignoring the “different physics in different parts of the multiverse” question, I think many signers of the GKL&N letter were missing a good opportunity to make common cause with IS&L on defending their science against an ongoing attack from some of their fellow signatories.

Update: There are sources with technical details of the arguments being made by both parties:

I’ll try to find time to read these carefully and try and understand exactly what claims are being made. Would love to hear from experts who may have looked at these and are better placed to evaluate what the arguments are.

This controversy continues to involve an unusual level of PR rather than science. The Stanford press office has just put out this, where Linde makes it clear that he sees this as a political and PR fight:

Linde added that he worries about the younger generation of scientists getting the wrong impression from this story. “I don’t want them to read this article and think that they are spending their time on inflationary theory in vain. But the enthusiastic support that we are receiving makes us optimistic that this is not going to happen,” he said.

There’s no mention in this press office story of their last press office story about Linde and inflation, which promoted the BICEP2 “smoking gun” vindication of Linde and inflation.

Some more takes on this story can be found here, here and here.

Update: Another article about this is at the Atlantic. A crucial issue here is whether inflation has now entered the realm of unfalsifiability. Given any likely new data that could appear, is there any way it could falsify inflation, or can one just come up with some version of inflation that will match it? Guth and Linde seem at times to be taking the attitude that this is fine, I take Steinhardt et al. as pointing out that this is no longer conventional science. Replacing falsifiability by arguments about how many prominent people have signed your letter is a worrisome development. From the article:

In 2014, for example, Loeb asked Guth during a panel discussion if inflation was falsifiable—whether you could design an experiment to disprove it. Guth called that a “silly question,” suggesting that inflation was an umbrella over many theories, making it very hard to knock them all out at once. The hope right now, he says, is to use observation and further theory to winnow inflation down to just one specific version.

“Our point is that this kind of reasoning is inconsistent with normal science and cannot be resolved by invoking authority,” Ijjas, Steinhardt, and Loeb wrote to The Atlantic.


Update: More about this from John Horgan.

Update: It seems that Andrei Linde is a Lubos Motl fan. This is getting very weird. It’s not normal to respond to a scientific argument by enlisting letter writers on your behalf, even less normal to put your university press office to work on a response, and truly far out there to think that it’s helpful to have Lubos announce that you have eaten from the tree of knowledge and that your opponents are imbeciles.

Update: For a sensible take on this that I think likely reflects well the views of most mainstream cosmologists, see Peter Coles.

Update: IJS have put together a Fact-Checking page. It lists the four “predictions” of inflation claimed to agree with experiment by Linde et al. and gives four references to papers published by Linde touting different “predictions” for the same quantities, predictions not agreeing with experiment.

This month’s Scientific American has a bizarre cover story by Nomura on “The Quantum Multiverse”, see here.

All in all, I don’t know what other people’s reactions to this have been, but before this started I was a lot more sympathetic to the argument that Steinhardt was treating the case for inflation unfairly.

Update: Yet more coverage trying to make sense of this, from Nick Stockton at Wired.

Posted in Favorite Old Posts, Multiverse Mania | 53 Comments

Some Quick Items

Posted on May 10, 2017 by woit

A few quick items, I may use this posting to add a couple more later, the next posting will discuss today’s letter to Scientific American about inflation.

  • Today’s LHCC meeting at CERN had reports from the LHC machine and experiments. About two weeks to go before collisions and data-taking start again.
  • Physics Today has a report this month on the LHeC proposal, something that has not gotten as much attention as it deserves. This is a proposal to collide protons and electrons, by building a new electron machine and a detector at a collision point with the LHC beam. Unlike proposals for a 100 TeV proton-proton machine that are getting a lot of attention, this would not push the energy frontier, but it would cost a great deal less (estimate is half a billion to a billion, vs. multiple tens of billions for the 100 TeV machine). In a few years when the question of a follow-on machine to the LHC starts to get very pressing, this idea and the HE-LHC idea (higher field magnets in the LHC tunnel, maybe doubling the energy) may get a lot more attention as the only financially viable ways forward.
  • The Université de Montpellier today has started to make accessible about 18,000 pages of its archive of Grothendieck’s mathematical writings. For anyone interested in Grothendieck’s work, this should keep you busy for a while…

Update: A few more.

  • I was sorry to hear of the recent death of Cecile DeWitt-Morette, a mathematical physicist responsible for the Les Houches physics summer school. Her books on geometry and physics (with Yvonne Choquet-Bruhat) were influential, and her more recent book with Pierre Cartier on Functional Integration contains a lot of interesting material.
  • Every so often I’d wondered what the Chudnovsky brothers have been up to, some information about this emerged today.
  • For a story about problems at the science magazine, Nautilus, which is having trouble transitioning from its original Templeton Foundation funding, see here.
Posted in Uncategorized | 12 Comments

Theories of Everything

Posted on May 2, 2017 by woit

I’ve written a review for the latest issue of Physics World of a short new book by Frank Close, entitled Theories of Everything. You can read the review here.

As I discuss in the review, Close explains a lot of history, and asks the question of whether we’re in an analogous situation to that of the beginning of the 20th century, just before the modern physics revolutions of relativity and quantum theory. Are the cosmological constant and the lack of an accepted quantum theory of gravity indications that another revolution is to come? I hope to live long enough to find out…

Posted in Book Reviews | 26 Comments