Today’s Hype

Posted on December 9, 2015 by woit

Joe Polchinski’s contribution to the ongoing Munich meeting has now appeared on the arXiv, with the title String Theory to the Rescue. Evidently he’s not actually to be at the meeting, I’m not sure how his paper will be presented.

It’s pretty much the usual hype about the string theory and the multiverse, with untestable ideas about quantum gravity the only topic. The one innovation is that it contains a calculation: the probability of a multiverse is 94%:

To conclude this section, I will make a quasi-Bayesian estimate of the likelihood that there is a multiverse. To establish a prior, I note that a multiverse is easy to make: it requires quantum mechanics and general relativity, and it requires that the building blocks of spacetime can exist in many metastable states. We do not know if this last is true. It is true for the building blocks of ordinary matter, and it seems to be a natural corollary to getting physics from geometry. So I will start with a prior of 50%. I will first update this with the fact that the observed cosmological constant is small. Now, if I consider only known theories, this pushes the odds of a multiverse close to 100%. But I have to allow for the possibility that the correct theory is still undiscovered, so I will be conservative and reduce the no-multiverse probability by a factor of two, to 25%. The second update is that the vacuum energy is nonzero. By the same (conservative) logic, I reduce the no-multiverse probability to 12%. The final update is the fact that our outstanding candidate for a theory of quantum gravity, string theory, most likely predicts a multiverse. But again I will be conservative and take only a factor of two. So this is my estimate for the likelihood that the multiverse exists: 94%.
This is not to say that the multiverse is on the same footing as the Higgs, or the Big Bang. Probability 94% is two sigma; two sigma effects do go away (though I factored in the look-elsewhere effect, else I would get a number much closer to 1). The standard for the Higgs discovery was five sigma, 99.9999%.

My problem with a lot of the West Coast theorists is that I don’t seem to have the same sense of humor, so often I have trouble telling when they’re joking (does anyone know when Andrei Linde is joking?). Here though it seems quite clear that Polchinski is pulling the leg of the philosophers gathered to hear him speak. My calculations show that the chance the above text could be a serious contribution to a philosophy of science conference cannot be above .1%.

The other evidence that something comical is going on here comes from some of the over-the-top claims about the virtues of string theory. In particular, we’re told

A remarkable feature of string theory is that the dynamics, the equation of motion, is completely fixed by general principle. This is consistent with the overall direction of fundamental theory, describing the vast range of phenomena that we see in terms of fewer and fewer underlying principles. Uniqueness would seem to be the natural endpoint to this process, but such theories are truly rare…
Indeed, when I assert that the equations of string theory are fully determined by general principle, I must admit that we do not yet know the full form of the equations, or the ultimate principle.

Polchinski is quite right that it is “remarkable” to know that you have a unique theory, with unique equations, fixed by a general, ultimate principle, but you don’t know what the theory is, what the equations are, or what the principle is. I’m curious to hear from people at the conference what gets the bigger laughs: this or Kane’s argument that the unknown theory predicts a 1.5 TeV gluino (unless it’s not found, in which case it doesn’t).

Update: See here for the latest coverage of the meeting from Massimo Pigliucci, who comments at one point:

[Yet another string theorist. I must say, there does seem to be a stacking of them at this conference, and no experimentalists have been invited either]


Update: For more on Polchinski’s paper there’s Sean Carroll on Twitter, here and here, who tells us

So strange how the public perception of string theory has been warped by a few contrarian voices. Good topic for some future PhD thesis.

and that Polchinski’s new paper

lays out the case for string theory, and how unexpectedly successful it’s been.

I’m assuming by “contrarian voices” he’s referring to Polchinski. That string theory has been “unexpectedly successful” and the multiverse is a 94% sure thing is a highly contrarian point of view among physicists.

Update: More coverage of the workshop is available from Sabine Hossenfelder and Massimo Pigliucci.

Posted in Multiverse Mania, This Week's Hype | 39 Comments

Next Week’s Hype

Posted on December 2, 2015 by woit

Next week there will be a workshop in Munich with the title Why Trust a Theory? Reconsidering Scientific Methodology in Light of Modern Physics. It’s organized by Richard Dawid, to discuss his ideas about “non-empirical theory confirmation”, developed to defend string theory research against accusations that its failure to make any testable predictions about anything make it a failure as a research program.

I guess the idea of such a workshop is to bring together string theory proponents and critics to sort things out, but looking at the program and talk abstracts, this doesn’t look promising, with the central issues to be evaded, and speakers likely to just talk past each other.

While the workshop title refers to “Modern Physics” in general, the talks are mostly focused on one very narrow part of the subject: quantum gravity. String theory is supposed to be something much more than a quantum gravity theory, explaining the Standard Model and low energy physics. This has been a complete failure, and the plan at the workshop seems to be to deal with this elephant in the room by ignoring it, or worse, claiming it isn’t there.

From the talk abstracts, about the only person discussing particle physics will be Gordon Kane (Quevedo may mention it, although his main interest is cosmology). Kane will be claiming that string theory makes testable predictions about particle physics, ones about to be tested. The problem is that he has been making the same claims for thirty years, arguing back in the 90s in the pages of Physics Today and a book that string theory would be tested at LEP and the Tevatron (by finding superpartners). As his predictions have been conclusively falsified, he just refuses to acknowledge this and starts advertising new ones. Perhaps the most outrageous case of this is his latest book (discussed here), which is a reissue of the old one, with falsified predictions simply deleted and replaced, without any acknowledgement of what happened. I don’t think this behavior raises any philosophical issues about theory confirmation, just the sociological issue of why the physics community tolerates this, or why he’s the one person invited to this workshop to address the largest problem of the subject.

Another central problem here is the hype problem. If you give up on testability, and allow theory confirmation based on claims that “my theory is just better” by some ill-defined metric, you open up the obvious problem of how to deal with people’s natural human inclination to praise the wonderful characteristics of their intellectual children. At the workshop, Joe Polchinski’s talk is entitled “String Theory to the Rescue”. I see nothing in the program about any planned examination of the significant string theory hype problem, or even any acknowledgement that it exists.

I’m actually in a way more sympathetic than most people to the idea that “non-empirical” evaluation of a theory is an important and worthwhile topic. Fundamental physics theory is facing a huge problem due to the overwhelming success of the Standard Model and the increasing difficulty of exploring higher energy scales. If it is to continue to make progress there is a real need to do a better job of evaluating theoretical ideas without help from experiment. There is a group of scientists who have a lot of experience with this problem, and have a well-developed culture designed to deal with it. They’re called “mathematicians”. Despite the fact that this workshop is hosted by the Munich Center for Mathematical Philosophy, the organizers don’t seem to have thought it worthwhile to invite any mathematicians or mathematical physicists to participate, missing out on a perspective that would be quite valuable.

Update: It’s now “this week”, not “next week”. Some tweeting from the conference going on, you can try the hashtag #WhyTrustATheory. Massimo Pigliucci comments from the Q and A session about a problem with this kind of thing: some people “very very much like the sound of their own damn voice”. I hear that David Gross claimed to have 20 possible observations that would invalidate string theory, but didn’t say what they were.

Update: Massimo Pigliucci is blogging a detailed account of the conference, see here (he’s also a speaker, slides here).

Update: I don’t think I’d noticed before that Lee Smolin has a very much to the point review of the Dawid book here.

Posted in Favorite Old Posts, This Week's Hype, Uncategorized | 54 Comments

Super and Great Colliders

Posted on November 30, 2015 by woit

I’ve recently finished reading two new books on huge collider projects, which make an interesting contrast.

The first is From the Great Wall to the Great Collider, by Steve Nadis and Shing-Tung Yau. It’s a very well-informed and topical book, a bit of a political document, designed to make the case for a Chinese “Great Collider”. This is a proposed machine of up to 100km in circumference, that would operate first as an electron-positron collider, designed to be a “Higgs factory”, allowing precision study of the Higgs. In a second stage the same tunnel would be used for a proton-proton machine with collision energies up to 100 TeV. This would be designed to explore the energy range above a TeV, in much the same way as the LHC, but with seven times the energy, thus a much higher energy reach. This energy would also allow study of Higgs self-interactions.

The Nadis-Yau book is an unusual document in many ways. Yau is a great geometer, but a main concern of the book is something completely different, the question of how one might construct such a huge physics and engineering project. There is a great deal of information in the book about the history and current state of experimental HEP, but from an unusual angle, that of the many Chinese contributions to the subject. I’ve read many histories of HEP, but learned a lot of new things from this one, with its very different emphasis.

This is a short, rather than encyclopedic, book, with about 130 pages of text. It functions well in explaining the case for a large new collider to anyone interested, but has a distinct focus on arguments for the proposal to do this in China. The Chinese government and people in coming years will be deciding whether to go ahead with this, and this book is the perfect place for them to read a serious account of what this proposal is and why it deserves to be taken seriously.

The current state of affairs is that an initial conceptual design has been completed recently, which was reported here. This gave rise to some mistaken reports like this one that the Chinese government had given its approval to the project. There’s still quite a ways to go before that happens, with a final conceptual design not due until next year, and even if there is a go-ahead, construction only starting in 2020-25.

For a detailed look at the physics to be done by such a collider, see this new review article. I was interested to see (page 32) that the previous description by one of the authors of the current situation as leading to only two possibilities (“natural” SUSY or some such, or the multiverse and the end of hope for explaining things) has been expanded to now include a more interesting third possibility: “correlation between the physics of the deep UV and IR”.

Just after finishing the Nadis-Yau book, I got a copy of a new history of the SSC project, Tunnel Visions, by Riordan, Hoddeson and Kolb. This book has been in the works for a long time, with the authors starting to gather material back in the 80s, before the project was cancelled in 1993. I’ve been hearing about the book for quite a while, glad to see that it has finally appeared.

The cancellation of the SSC had a disastrous effect on the US experimental HEP program, moving the center of research conclusively to CERN and its LHC project. A central concern of any book of this kind has to be the “what went wrong?” question. The conclusions drawn are similar to ones I remember often hearing back then in the wake of the disaster: the SSC was a juicy target for a Congress intent on budget-cutting, easily portrayed as out of control (its budget kept increasing from $3 billion early on, to maybe $12 billion at the end), with little support from non-Texas representatives. In some sense the surprising part of the story is that the project got as far as it did before being terminated by an overwhelming Congressional vote.

One part of the story I had never understood was that as the SSC budget expanded it was coming into direct conflict with the plan to keep funding the other HEP labs (Fermilab, SLAC, Cornell, Brookhaven), and that was part of the story of the politics of this within the scientific community. I also hadn’t appreciated the way the challenges of a project of this scale required bringing in companies and other parts of the US military-industrial complex, making it take on some of the aspects of a large defense spending project. A major topic of the book is that of the problematic interaction between this and the standard ways that physicists were used to doing business.

Unlike Nadis-Yau, Tunnel Visions is more of an academic book, with notes and references to a huge number of extensive interviews making up a large part of the text. It’s not at all an inspirational story, nor is there all that much physics discussed. At the same time, it’s the definitive work on a crucial part of the history of high energy physics in the US. One group of people who should definitely be reading it are those planning the Chinese project. Some of the difficulties they will face if they go ahead will be similar: the SSC was an 87km ring, of similar scale to the new proposal. That this almost got off the ground 20 years ago here in the US is a good argument that it is something that could be pulled off in China over the next 20 years if they want to do it.

Based on the fact that the SSC might have worked with more international support, the authors end with the conclusion

Despite the added difficulty of organizing and managing them, pure-science projects at the multibillion-dollar scale should henceforth be attempted only as international enterprises involving interested nations from the outset as essentially equal partners. Nations that attempt to go it alone on such immense projects are probably doomed to failure like the Superconducting Super Collider.

The Chinese proposal is still in its infancy, but there’s reason to expect it might be a “go it alone” project. Given the way the US budget operates, at this point no country is likely to look to the US as a reliable source of sizable funding. CERN has its own proposal for a 100 TeV collider, but it seems hard to believe that both projects will go ahead, although also hard to see the Europeans agree to give up energy frontier physics to China. Many of the lessons of the SSC funding debacle are rather specific to the US and the way budgets are done here. I have no idea what the considerations in China are for projects like this, I guess we’ll start to find out in coming years.

Posted in Book Reviews, Experimental HEP News | 13 Comments

Quick Items

Posted on November 25, 2015 by woit

A few quick items before the holiday:

  • I hear that Luis Alvarez-Gaumé will be the next Director of the Simons Center, starting next Fall, taking over from John Morgan, the founding Director. My understanding is that the hope was to have the directorship alternate between mathematicians and physicists, and with the hire of Alvarez-Gaumé, they’ve managed to achieve this. His early work on supersymmetric path integrals and the index theorem (see here) was characteristically lucid and this remains one of the great points of intersection between modern mathematics and the quantum theory. One of the best relatively short introductions to QFT is this one (with a shorter arXiv version here). I think he’s an excellent choice.
  • In physics blogger news, Tommaso Dorigo reports that he has found a publisher for the book he has been writing: Anomaly! – Scientific Discoveries and the Quest for the Unknown, and it should appear next year. I’m very much looking forward to seeing a copy. His insightful but irreverent take on experimental HEP I’m sure will make this a fascinating read for anyone interested in the subject.
  • Matt Strassler’s blog has been dormant for a while, but he has now been heard from. After a couple year visiting position at Harvard, he says he’s now “employed outside of science”, but working on a book about particle physics for non-experts.
  • Jim Holt has a review in the latest New York Review of Books of my colleague Michael Harris’s Mathematics Without Apologies.
  • By some accounting, today is the 100th anniversary of Einstein’s GR field equations, which he presented November 25, 1915 at a lecture in Gottingen Berlin. This anniversary has been celebrated in many places, in many ways this year, so there’s not any need for me to chime in. Among many excellent treatments of the topic, there’s also an unfortunate tendency of some to use Einstein to grind their particular axes. Sean Carroll I suspect has Einstein spinning in his grave, using the PBS NewsHour to enlist Einstein as a multiverse fan:

    The ability for seemingly constant things to evolve and change is an important aspect of Einstein’s legacy. If space and time can change, little else is sacred. Modern cosmologists like to contemplate an extreme version of this idea: a multiverse in which the very laws of physics themselves can change from place to place and time to time. Such changes, if they do in fact exist, wouldn’t be arbitrary; like spacetime in general relativity, they would obey very specific equations.

    Perhaps Carroll could enlighten the public by writing down these “very specific equations” he’s advertising, for comparison to the Einstein field equations.

    If I were to grind my own ax here, it would be to note that Einstein’s great breakthrough came about through close collaboration with some of the best pure mathematicians around, adopting difficult but deep ideas about geometry. Without the mathematicians, I’d guess that the theory of general relativity would have taken many more decades to come to fruition. Maybe there’s a lesson there…

Posted in Uncategorized | 45 Comments

This Week’s Non-Hype

Posted on November 23, 2015 by woit

Since I often post here complaints about articles produced by the press offices of various institutions that hype in a misleading way physicist’s theoretical work, I thought it a good idea to make up for this by noting a positive example of how it should be done. The SLAC press office this week has a Q and A with Lance Dixon, with the title SLAC Theorist Lance Dixon Explains Quantum Gravity which is quite good.

Dixon gives an informative explanation at a basic level of what the quantum gravity problem is. He includes an even-handed description of the string theory approach to the problem, and explains a little bit about the alternative that he and collaborators have been pursuing, one that has gotten much less attention than it deserves. This is a very technical subject, so there’s a limit to how much he can explain, but he gives the general idea, and includes a link to his most recent work in this area.

Many promotional efforts for string theory begin by making claims that quantum field theory cannot be used to understand quantum gravity, due to the divergences in the perturbation series. This has been repeated so often, for so many years, that it is an argument most people believe. The situation however is quite a bit more complicated than this, with one interesting aspect of the story the discovery in relatively recent times that long-held assumptions about divergences in perturbative quantum gravity calculations were just wrong. Such calculations turn out to have extra unexpected structure, and thus unexpected cancellations, making naive arguments about divergences incorrect. Continuing progress has come about as Dixon and others have developed new techniques for actually computing amplitudes, uncovering unexpected new symmetries and cancellations.

For a good summary of the current situation, see this talk by Zvi Bern, especially page 7, where Bern details how, going back to 1982, “So far, every prediction of divergences in pure supergravity has either been wrong or missed crucial details”. For N=8 supergravity, current arguments say that a divergence should show up if you could calculate 7 loop amplitudes, but Bern warns against betting on this. In that talk he also explains the recent work with Dixon and others that gets mentioned in the SLAC piece, about the surprising nature of the divergence in pure gravity at two-loops, making its physical significance and whether it really ruins the theory not so clear.

I was interested to read Dixon’s account of his thinking back in the mid-80s:

I began to be concerned that there may be actually too many options for string theory to ever be predictive, when I studied the subject as a graduate student at Princeton in the mid-1980s. About 10 years ago, the number of possible solutions was already on the order of 10500. For comparison, there are less than 1010 people on Earth and less than 1012 stars in the Milky Way. So how will we ever find the theory that accurately describes our universe?

Although this never made it into media stories, I think that by a couple years after the initial enthusiasm for string unification in 1984, many theorists had already started to notice that the idea likely had fundamental problems, with a serious danger that it would turn out to be an empty idea. This now has become clear, but the idea lives on, with “QFT must have divergences” the main argument for continuing to take it seriously. Now that argument isn’t looking so solid…

Update: A good explanation of the situation from 4 gravitons who, thankfully, is not overly worried that he might be giving succor to the Woits of the world…

Posted in Uncategorized | 37 Comments

Langlands Items

Posted on November 20, 2015 by woit

There’s an interesting development in the math-physics overlap, with a significant number of physicists getting interested in the theory of automorphic forms, often motivated by the problem of computing string scattering amplitudes. This has led to a group of them writing up a very long and quite good expository treatment of Eisenstein series and automorphic representations, which recently appeared on the arXiv. The emphasis is not on the physics applications (which an introduction explains come about when one is dealing with systems with discrete symmetries like the modular group or higher dimensional generalizations), but on the calculational details of the mathematics. There are quite a few expositions of this material in the mathematics literature but many (mathematicians included), may find the detailed treatment here very helpful.

Another aspect of this area is some overlap with the interesting of mathematicians studying Eisenstein series in the context of Kac-Moody groups. There’s a conference this week bringing together mathematicians and physicists around this topic.

Turning to recent developments in the Langlands correspondence itself, which relates automorphic forms to Galois representations, when I discussed David Nadler’s talk at the Breakthrough Prize symposium (the video is available here), I forgot to mention one thing he talked about that was new to me, the Fargues-Fontaine curve. Nadler explained that Fargues has recently conjectured that the local Langlands correspondence can be understood in terms of ideas from the geometric Langlands correspondence, using the Fargues-Fontaine curve. For more about this from Fargues himself, see materials at his website, which include lecture notes, links to videos of talks at the IAS and MSRI, and this recent survey article. Also informative is some explanation from David Ben-Zvi at MathOverflow.

In April there will be a workshop in Oberwolfach on geometric Langlands that will include this topic, for details of the planned discussions, see here.

Fargues was here today at Columbia, and gave a talk on “p-adic twistors”. Nothing much about Langlands, this was about the question of what the analog is for the Fargues-Fontaine curve when you take the real numbers as your field (the use of “twistors” is that of Simpson’s, see here, not the common use in physics, which is quite different).

I won’t display my extremely limited understanding of this subject by trying to provide my own explanations here. A big problem is that this is mainly about the p-adic Langlands correspondence, something I’ve never been able to understand much about. After making a renewed attempt the last few days, I at least started to get some idea of what are the biggest problematic holes in my knowledge of the math background. Interestingly, it seems many if not most of them have Tate’s name attached (Hodge-Tate, Lubin-Tate, etc, etc…). One pleasant discovery I made is that there are now some excellent expository pieces on this material available, often courtesy of some talented graduate students. One wonderful source I ran into is Alex Youcis’s blog Hard Arithmetic, which has given me some hope that with his help I might soon make a little progress on learning more about this kind of mathematics. I don’t know what’s in the water at Berkeley, but something there keeps producing high-quality blogging by mathematics students, another example is here.

Posted in Langlands | 8 Comments

This Week’s Hype

Posted on November 17, 2015 by woit

This week’s string theory hype comes to us from USC physicists Clifford Johnson and Nick Warner, courtesy of the USC press office (see here and here). It’s garden variety hype of this kind, exactly the same claims about strings and extra dimensions that were being made thirty years ago. There’s no acknowledgement these haven’t gone anywhere, instead we’re “closer than ever to an answer”.

When the question of testability comes up, the multiverse is not invoked as an excuse. Instead, it seems that dark matter is going to provide the test:

Observations show that dark matter and energy constitute more than 95 percent of the universe. Scientists have established that they are new forms of matter and energy, but so far their precise nature is unknown. They may hold the key to confirming the veracity of string theory, Johnson said.

“It’s really kind of amazing — and humbling. There are forms of matter that seem to show up naturally in string theory that could well be good candidates to be dark matter,” he said. “People are hoping that this could be a key to making contact between theory and nature.”

Hard to know what Johnson has in mind for his “show up naturally in string theory” claim. Presumably he’s thinking of the ancient “we’ll test string theory by finding superpartners” claim, somehow neglecting to mention that this hasn’t worked out.

I’m especially impressed by the description of string theory’s power to explain dark matter as “amazing – and humbling”, deftly pairing outrageous over-the-top hype with an invocation of the selfless humility of the research scientist.

Posted in This Week's Hype | 22 Comments

2016 Breakthrough Prizes

Posted on November 9, 2015 by woit

The 2016 Breakthrough Prizes were announced last night, discussed a bit in the last posting. Today there are programs going on at Berkeley, livestreams available here.

One thing that strikes me about these things is that the situation with the physics prize has changed dramatically since the first three years, when they went mostly to string theorists. Having a heavily promoted much larger cash prize than the Nobel, given largely to theorists for ideas many of which haven’t worked out, raised obvious questions about the wisdom of the whole thing. The last two years have seen a 180 degree turn, with the prizes going to experimentalists for successful experimental results. Even better, there has been an unusual emphasis on making an award to entire experimental collaborations, not just a small number of “great men” identified as collaboration leaders or spokespersons. I don’t know of any other major prizes that do this. The lack of an experimental Nobel for the Higgs discovery is one reflection of that problem, it’s great that the Breakthrough Prize people are doing something about it.

This is now just the second year of the math prize, which has never been as problematic as the early physics prize. However, the institution of cash prizes of this size, promoted in a Hollywood style, is something I don’t think anyone in the math community ever asked for, and it’s not at all clear it’s a good thing, or in keeping with some of the best values of the math research community. This year I think Peter Scholze set a remarkable example by turning down a prize, a move which unfortunately has gotten little attention in the media. I hope his action causes people to take a closer look at this gift horse. Instead of just celebrating the shower of cash and attention, research mathematicians may want to consider whether, just as they changed direction with the physics prize, Milner and Zuckerberg perhaps should be encouraged to listen to Scholze and move in a different direction.

Update: I just watched some of the talks at the afternoon symposia. Arkani-Hamed made the case for a Great Collider, mostly quite sensibly in terms of the desirability of better understanding the Higgs: is it pointlike? how does it self-interact? The argument is that addressing these questions goes beyond what the LHC can do, can be done by a large new collider.

On the math side, David Nadler gave a beautiful talk about Langlands/geometric Langlands, ending with a prediction for the future that a central role will be played by Peter Scholze’s work, including recent ideas on what Nadler calls “Arithmetic conformal field theory”. He suggested that 50 years from now, Hartshorne and other graduate textbooks on algebraic geometry will be replaced with new ones based on Scholze’s perfectoid spaces. Maybe if they hadn’t offered Scholze money they could have gotten him to talk about this stuff…

Update: The New York Times Science section has an article today about the goal of the Breakthrough prizes, to turn scientists into celebrities. Yuri Milner is quoted:

“We are at the very beginning of this journey,” he said, noting that if you were to look at a list of the top 100 celebrities in American society, there would not be a single scientist on the list.

“The question is why?” he added.

The question this raises, which may have something to do with Peter Scholze’s refusal to participate, is “what if good scientists don’t want to be celebrities?” The impulse to do science and mathematics at the highest level and the impulse to be a celebrity may just be two very different, incompatible things.

The New York Times article doesn’t mention the Scholze story, but it does discuss the young student, Ryan Chester, who was given a $400,000 award for making a film about special relativity. It turns out that doing a great job of making such a film has a lot more to do with interest in being a filmmaker than interest in being a scientist:

Mr. Chester, however, said in an interview that he was not planning to study science in college, but instead will probably study film, hopefully at a school like the University of Southern California or New York University.

Update: Videos from the symposia and panel discussions at Berkeley are available here.

Update: In other large-check news of the day, the IAS has announced a $20 million donation from Robert Rubenstein, CEO of the Carlyle Group, a top private equity firm. This completes a $212 million capital fundraising campaign.

Posted in Uncategorized | 52 Comments

Various News

Posted on November 8, 2015 by woit

Later tonight will be the 2016 Breakthrough Prize ceremony, broadcast live on the National Geographic channel. While mathematicians and physicists are getting their popcorn ready, waiting to find out which of their colleagues will be $3 million richer, they might want to check out Mathematics Without Apologies, where Michael Harris is writing about his experience on the red carpet at last year’s ceremony. In other news:

  • A few miles down the road from the event tonight at the NASA Ames Center, the Stanford Institute for Theoretical Physics has a new website. They have various videos you can watch, as well as this account of the history of the SITP:

    The gauge hierarchy problem was first addressed in the earliest days of SITP by Susskind and Dimopoulos, whose ideas eventually led to the introduction of supersymmetry into particle physics. The gauge hierarchy problem and the discovery experimental discovery of supersymmetry were principle reasons for the building of the Large Hadron Collider, but the puzzle remains. To this day Dimopoulos and his band of young postdocs and students have offered the most exciting proposals for discovering new physics in this area.

    Personally, I thought the Higgs was the principal reason for the LHC, but perhaps I was misinformed.

  • Several people wrote to tell me about a USA Today article reporting Study may have found evidence of alternate, parallel universes, noting that this needed a new installment of This Week’s Hype. I’m very pleased to see that the excellent science journalist Jennifer Ouellette has been on the case, debunking this much better than I ever could. The blame for this kind of thing is jointly shared by physicists and journalists, glad to see that at least some journalists are taking action to deal with the problem.
  • There have also been several suggestions that I write about Leo Kadanoff, the great theoretical physicist who passed away a little while ago at the age of 78. Unfortunately I never met him, and only had a general acquaintance with his work. A very good obituary by Kenneth Chang did appear in the New York Times.
  • I’ve also heard from lots of people with more stories about the Khrzhanovsky film about Landau described here. It seems that I’m the only one who didn’t know about this. In other performing arts/physics news, Lee Smolin discusses here a project he has been involved in.
  • The LHC has finished its 2015 run colliding protons at 13 TeV, will now turn to heavy ion physics. Integrated luminosity recorded by ATLAS and CMS is about 4 inverse femtobarns. Results of the analysis of this data may start to be available publicly around the time of Moriond in March. Consulting Jester to see what to expect, it looks like better limits on or evidence for stops, Z-primes and gluinos should be available. In particular, we’ll finally see a conclusive test of string theory (Gordon Kane argues that string theory predicts a 1.5 TeV gluino, see here).
  • In other news from the LHC, it seems that humanity narrowly missed a major problem back in August, when Simon Parkes, a Labour town councillor for Whitby in North Yorkshire, foiled a sinister plot by the Illuminati to “use the LHC to open an evil portal that would allow them to become more powerful.”

Update: Awards ceremony hasn’t started, but names are out: Ian Agol for mathematics, 5 teams of 1300 people for neutrino oscillations (with 7 specifically identified, including the 2 winners of this year’s physics Nobel).

Junior recipients in math are Larry Guth and Andre Neves, in physics Bogdan Bernevig, Liang Fu, Xiao-Liang Qi, Raphael Flauger, Leonardo Senatore, and Yuji Tachikawa.

Update: The big news, via Michael Harris, is that Peter Scholze (to my mind a better mathematician than any of those who got prizes) turned down a $100,000 prize. Good for him.

Posted in Uncategorized | 29 Comments

The Elephants in the Room – What every physicist should know about string theory

Posted on November 3, 2015 by woit

The string wars seem to still be going on, with the latest salvos coming from Ashtekar and Witten. In a very interesting recent interview, at the end Ashtekar has some comments about string theory and how it is being pursued. About claims that string theory is the only possible way to get quantum gravity he says:

I don’t know why science needs such statements; indeed, scientists should not make such statements. Let the evidence prove that it’s the only theory. Let the evidence prove that it is better than other theories or let its predictions be reproduced more than those of others. Science should not become theology. And, somehow such statements have a strong smell of theology, which I don’t like.

About AdS/CFT and the current state of its relation to quantum gravity:

We seem to be using these gravity ideas in other domains of physics rather than solving quantum gravity problems. I don’t think that the quantum gravity problems have been solved. And I have said this explicitly in conferences with panels – in which Joe Polchinski, Juan Maldacena and I were panellists – that, in my view, this is very powerful and these are good things. However, the AdS/CFT conjecture is the only definition of non-perturbative string theory one has – and it’s a definition, it’s not a proof of anything. It talks about duality, but there’s no proof of duality. To have a duality, A should be well defined, B should be well defined and then you say that A is dual to B. Since we don’t have another definition of string theory, we cannot hope to prove that string theory is dual to its conformal field theory. You can define string theory to be the conformal field theory. You have to construct a dictionary relating string theory in the bulk and conformal field theory on the boundary. That dictionary has not been constructed in complete detail.
Again, nobody is taking anything away from the successes that the AdS/CFT duality has had; but there is a big gap between the successes and the rhetoric. The rhetoric is at a much higher level than the successes. So, for example, in this conjecture, first of all the space-time is 10 dimensional. The physical space-time is supposed to be asymptotically anti-de Sitter, which has a negative cosmological constant. But we look around us, and we find a positive cosmological constant. Secondly, the internal dimensions in the conjecture, or this definition, are macroscopic. The Kaluza-Klein idea is that there are higher dimensions but because they are all wrapped up and microscopic, say, at Planck scale, we don’t see them. That’s plausible. But here, in AdS/CFT duality, they need the radius of the internal dimensions to be the same as the cosmological radius. If so, if I try to look up I should see these ten dimensions; I don’t. So, it can’t have much to do with the real world that we actually live in. These are elephants in the room which are not being addressed.
… there are these obvious issues and practitioners just pretend that they don’t exist. And that to me is unconscionable; I feel that that’s not good science. I don’t mean to say string theory is not good science, but publicizing it the way it’s done is not good science. I think one should say what it has done, rather than this hyperbole.

A good example of the problems Ashtekar is concerned about is provided by an article in the latest Physics Today by Witten with the title What Every Physicist Should Know about String theory. It’s devoted to a simple argument that string theory doesn’t have the UV problems of quantum field theory, one that I’ve seen made by Witten and others in talks and expository articles many times over the last 30 years. This latest version takes ignoring the elephants in the room to an extreme, saying absolutely nothing about the problems with the idea of getting physics this way, even going so far as to not mention the first and most obvious problem, that of the necessity of ten dimensions.

The title of the article is the most disturbing thing about it. Why should every physicist know a heuristic argument for a very speculative idea about unification and quantum gravity, without at the same time knowing what the problems with it are and why it hasn’t worked out? This seems to me to carry the “strong smell of theology” that Ashtekar notices in the way the subject is being pursued.

Witten is a great physicist and a very lucid expositor, and the technical story he explains in the article is a very interesting one, with the idea that most physicists might want to hear about it a reasonable one. But the problems with the story also need to be acknowledged and explained, otherwise the whole thing is highly misleading.
Besides the obvious problems of the ten dimensions, supersymmetry, compactifications, the string landscape, etc. that afflict attempts to connect this story to actual physics, there are a couple basic problems with the story itself. The first is that what Witten is explaining as a problematic framework to be generalized by string theory is not quantum field theory, but a first-quantized particle theory, with interactions put in by hand. This can be used to produce the perturbation series of a scalar field theory, but this is something very different than the SM quantum field theory, which has as fundamental objects fields, not particles, with interactions largely fixed by gauge symmetry, not put in by hand. For such QFTs, there is no necessary problem in the UV: QCD provides an example of such a theory with no ultraviolet problem at all, due to its property of asymptotic freedom.

Another huge elephant in the room ignored by Witten’s story motivating string theory as a natural two-dimensional generalization of one-dimensional theories is that the one-dimensional theories he discusses are known to be a bad starting point, for reasons that go far beyond UV problems. A much better starting point is provided by quantized gauge fields and spinor fields coupled to them, which have a very different fundamental structure than that of the terms of a perturbation series of a scalar field theory. A virtue of Witten’s story is that it makes very clear (while not mentioning it) what the problem is with this motivation for string theory. All one gets out of it is an analog of something that is the wrong thing in the simpler one-dimensional case. The fundamental issue since the earliest days of string theory has always been “what is non-perturbative string theory?”, meaning “what is the theory that has the same relation to strings that QFT has to Witten’s one-dimensional story?” After 30 years of intense effort, there is still no known answer to this question. Given the thirty years of heavily oversold publicity for string theory, it is this and the other elephants in the room that every physicist should know about.

Update: For another take on string theory that I meant to point out, there’s an article quoting Michael Turner:

Turner described string theory as an “empty vessel,” and added: “the great thing about an empty vessel is that we can put our hopes and dreams in it.”

The problem is that the empty vessel is of a rather specific shape, so only certain people’s hopes and dreams will fit…

Update: Many commenters have written in to point out this article, but I don’t think it has anything at all to do with the topic of this posting. There are lots of highly speculative ideas about quantum gravity out there, most of which I don’t have the time or interest to learn more about and discuss sensibly here.

Update: It is interesting to contrast the current Witten Physics Today article with a very similar one that appeared by him in the same publication nearly 20 years ago, entitled Reflections on the Fate of Spacetime. This makes almost the same argument as the new one, but does also explain one of the elephants in the room (lack of a non-perturbative string theory). It also includes an explanation of the T-duality idea that there is a “minimal length” in string, an explanation I was referring to in the comment section when describing what I don’t understand about his current argument.

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