Various Links

Posted on May 25, 2013 by woit
  • The Smithsonian has a long article about Lisa Randall here.
  • The Wall Street Journal has a shorter article about Randall’s high school classmate Brian Greene here. Brian’s World Science Festival will start here in New York on Wednesday.
  • I’ll probably skip the World Science Festival in favor of an event at the CUNY Graduate Center: a conference on the work of Jim Simons, in honor of his 75th birthday. The conference will start off Tuesday morning with talks by Witten and Deligne (for a recent piece about Deligne and the Weil conjectures by Ed Frenkel, see here)
  • One of many worthwhile things funded by Simons is Simons Science News, which now carries some of the best science journalism around. There’s a new interview with David Gross, who talks about the way QFT overcame those who wanted to do away with it in the sixties. About string theory:

    String theory is not as revolutionary as we once hoped. Its principles are not new: They are the principles of quantum mechanics. String theory is part and parcel of quantum field theory.

    About the multiverse:

    There are frustrating theoretical problems in quantum field theory that demand solutions, but the string theory “landscape” of 10500 solutions does not make sense to me. Neither does the multiverse concept or the anthropic principle, which purport to explain why our particular universe has certain physical parameters. These models presume that we are stuck, conceptually.

    About the current situation “Sometimes, he says, science is just plain stuck until new data, or a revolutionary idea, busts the status quo.”

    The latest article at Simons is one by Natalie Wolchover, who was at the same Nima Arkani-Hamed talk I recently attended. See her take here, mine here. Will write yet again about “naturalness” and some of the content of this article in a separate posting.

  • For the state of SUSY, and particle physics in general, check out recent talks here, especially Matt Reece’s SUSY theory overview. I think a fair description of the current state of affairs is that the only SUSY theories standing are either “fine-tuned” (removing the main argument of LHC-scale SUSY), or highly contrived (e.g. by going beyond the MSSM in various ways to escape LHC negative results). For the latest experimental results about SUSY, watch for this CMS talk on Tuesday.
  • For the latest in speculative theorizing about HEP and cosmology, see this past week’s Planck 2013 conference.
Posted in Uncategorized | 6 Comments

Eric Weinstein on Geometric Unity

Posted on May 23, 2013 by woit

Eric Weinstein is a Harvard math Ph. D. who has been working as an economist here in New York for many years, and someone I’ve often enjoyed talking to over the years. Going back to his days as a graduate student, he has been working on some of his own far out of the mainstream ideas about geometry and physics (which I’ve never seen the details of). Eric has finally gotten to the point where he is willing to talk about these ideas publicly, and he is giving a lecture today in Oxford, something that was arranged by Marcus du Sautoy. The Guardian has a long article about him and his work here.

There’s a bit of an analogy with the Garrett Lisi physics outsider story here, although I think Eric will get less media attention since he doesn’t have the surfing angle going for him. Both he and Garrett are pursuing what seems to me one of the deepest questions around: what is the relationship between the SU(3)xSU(2)xU(1) geometry of the Standard Model, and the 4d pseudo-Riemannian geometry of space-time and general relativity? Garrett was trying to understand this in terms of E(8) symmetry, and I’m looking forward to seeing what Eric’s ideas about this are. I’m not sure when he’ll have a paper out on the arXiv, or whether some sort of version of his lecture will be available.

Update: The Guardian now has a very enthusiastic article about this by Marcus du Sautoy, while New Scientist has a skeptical take here.

Update: See Jennifer Ouellette for a critical take on the Guardian coverage.

Update: It seems that claims that physicists were not invited to Weinstein’s talk are not true: an announcement and posters were sent to the physics department, but did not get widely disseminated. For a small amount of info about the talk, see the comment here from “Leaker”.

Posted in Uncategorized | 76 Comments

Hard Evidence for the Multiverse Found, but String Theory Limits the Space Brain Threat

Posted on May 22, 2013 by woit

In recent years there have been many claims made for “evidence” of a multiverse, supposedly found in the CMB data (see for example here). Such claims often came with the remark that the Planck CMB data would convincingly decide the matter. When the Planck data was released two months ago, I looked through the press coverage and through the Planck papers for any sign of news about what the new data said about these multiverse evidence claims. There was very little there; possibly the Planck scientists found these claims to be so outlandish that it wasn’t worth the time to look into what the new data had to say about them. One exception was this paper, where Planck looked for evidence of “dark flow”. They found nothing, and a New Scientist article summarized the situation:

“The Planck team’s paper appears to rule out the claims of Kashlinsky and collaborators,” says David Spergel of Princeton University, who was not involved in the work. If there is no dark flow, there is no need for exotic explanations for it, such as other universes, says Planck team member Elena Pierpaoli at the University of Southern California, Los Angeles. “You don’t have to think of alternatives.”

One of those promoting the idea that “dark flow” was evidence for a multiverse was Mersini-Houghton, who in a 2008 paper with Holman wrote:

Our contention, then, is that these observations of bulk flow can be construed as evidence for the birth of the universe from the landscape multiverse imprinted on the superhorizon sized nonlocal quantum entanglement between our horizon patch and others that began from the landscape. When we calculate the size of the induced dipole in our theory and convert it into a bulk velocity dispersion, we will see that for the constrained values of our parameters we arrive at a velocity dispersion of order 670 km/sec, remarkably close to the observed value of 700 km/sec.

One might think that the refutation of their prediction by the Planck data would be a problem. Instead though, the Sunday Times reported a few days ago that Scientists believe they have found the first evidence that other universes exist. The story got picked up by other news outlets, and appeared in the Daily Mail as “The first ‘hard evidence’ that other universes exist has been found by scientists”. The source for the story was Mersini-Houghton:

Laura Mersini-Houghton, theoretical physicist at the University of North Carolina at Chapel Hill, and Richard Holman, professor at Carnegie Mellon University, predicted that anomalies in radiation existed and were caused by the pull from other universes in 2005.
Now that she has studied the Planck data, Dr Mersini-Houghton believes her hypothesis has been proven.
Her findings imply there could be an infinite number of universes outside of our own.
She said: ‘These anomalies were caused by other universes pulling on our universe as it formed during the Big Bang.
‘They are the first hard evidence for the existence of other universes that we have seen.’

She will be in Britain soon promoting this at the Hay Festival on May 31 and at Oxford on June 11.

According to a New Scientist story just out, this hard evidence for the multiverse should be welcomed, since it (together with string theory) has just been shown to have the power to save us from “Legions of disembodied brains floating in deep space”. The story, which appeared in print as String Theory Limits Space Brain Threat starts with

LEGIONS of disembodied brains floating in deep space threaten to undermine our understanding of the universe. New mathematical modelling suggests string theory and its multiple universes may just provide our salvation – and that could win the controversial theory a few more backers.

It goes on to explain about Boltzmann brains and a recent paper by Bousso and Zukowski, and ends with news of yet another experimental success for string theory:

“This is potentially an added experimental success for string theory and eternal inflation,” says Daniel Harlow, a physicist at Princeton University. “We need to understand it better – [but] the fact that it potentially explains something is motivation to understand it better.”

Update: More here on how string theory will save us from the space brains.

Update: I’ve appended a response from Laura Mersini-Houghton and Richard Holman about this to a later posting, see here.

Posted in Favorite Old Posts, Multiverse Mania, This Week's Hype | 34 Comments

One Ring to Rule Them All

Posted on May 16, 2013 by woit

This week in Sweden the Nobel Foundation is running a symposium on LHC results. It’s invitation only, but the slides of the talks are available here.

One of the scheduled talks today was about string theory, and I was wondering how that would fit into the “LHC results” framework since string theory has nothing to say on the topic. Now that the slides are available I don’t see anything about the LHC, but there are some remarkable revelations. The first is that string theory is not science but “Magic”, with several slides describing the “Magic of String Theory”. The relationship to mathematics is that in string theory “No concept in Math remains unambiguous”, which I guess is about what you would expect when you’re dealing with magic.

An even bigger revelation comes later in the talk: string theory is Sauron’s Ring of Power! It is described as “Concentrated Power” and it seems that the markings on the ring are a SUSY Lagrangian. Part of the Ring Poem is quoted

One Ring to rule them all, One Ring to find them,
One Ring to bring them all and in the darkness bind them.

To put this back in context, recall that this is Sauron’s ring he created in order to control everything from Mordor. Here’s more of the poem, including the original language

Ash tug Shakhbûrz-ûr Ulîma-tab-ishi za,
Uzg-Mordor-ishi amal fauthut burgûli.
Ash nazg durbatulûk, ash nazg gimbatul,
Ash nazg thrakatulûk, agh burzum-ishi krimpatul
Uzg-Mordor-ishi amal fauthut burgûli.

One for the Dark Lord on his dark throne
In the Land of Mordor where the Shadows lie.
One Ring to rule them all, One Ring to find them,
One Ring to bring them all and in the darkness bind them
In the Land of Mordor where the Shadows lie.

This interpretation of string theory as Sauron’s ring I suppose could explain a lot…

Posted in Uncategorized | 24 Comments

String Theory and the Scientific Method

Posted on May 14, 2013 by woit

There’s a new philosophy of science book out, Richard Dawid’s String Theory and the Scientific Method (available online here if your institution is paying Cambridge University Press appropriately or if you have a credit card). It comes with endorsements from string theorists David Gross and John Schwarz, with Schwarz writing:

Richard Dawid argues that string theory plays a novel role in the scientific process that has been neglected by philosophers of science. I believe that this book is a valuable contribution to the philosophy of science, which should interest practicing scientists as well as those who are more interested in the methodology of science.

Dawid is a particle theorist turned philosopher, and as you might guess from the endorsement, he approaches string theory from an enthusiast’s point of view. The fundamental question addressed is how one can reconcile string theory’s failure in terms of conventional methodology of science with its continuing hold on at least part of the physics community. He explains how the book came about as follows:

This book has been on my mind ever since I left physics and turned to philosophy in the year 2000. A core motivation for making that step at the time was my feeling that something philosophically interesting was going on in fundamental physics but remained largely unappreciated by the world outside the departments of theoretical physics – and underappreciated even within. Twelve years of grappling with the specification of that general idea have considerably changed my perspective on the issue but left the overall idea intact. This book is the attempt to present it in a coherent form.

Reading the book in an odd way reminded me of my recent experience reading Gordon Kane’s reissued book on supersymmetry and string theory written in 2000, especially Witten’s essentially unchanged introduction. The degree of self-confidence of string theorists at that time was much different than now: AdS/CFT was a new idea, with a solution to QCD on its way, SUSY a sure thing at the LHC if not at the Tevatron or LEP, and at least half of the new jobs in the field going to string theorists. No landscape or multiverse pseudo-science was around to sow dissension in the ranks. No failure of SUSY to show up anywhere. No discouraging numbers like those for the last two years which show, in the US at least, 9% of jobs going to string theorists, with more jobs going to lattice gauge theorists in 2011 than to string theorists. And of course, a uniformly positive press, with no naysayers like Smolin and Woit causing trouble.

In Dawid’s description, string theorists are still partying like it’s 1999:

String theory has attained a pivotal role in fundamental physics and has been treated as a well-established and authoritative theory for quite some time by the community of string theorists and by physicists in related fields. As we have described above, large parts of fundamental physics are influenced by string theoretical analysis. The string community is one of the largest communities in all of theoretical physics and for many years has produced the majority of the field’s top-cited papers. Moreover, many string theorists express a remarkably strong trust in their theory’s viability.

For the actual list of last year’s top-cited papers in HEP, see here, and “remarkably strong trust in their theory’s viability” seems to me more 2000 than 2013. He does go on to mention skeptics, but to him a majority of the field is behind string theory, with the skeptics only coming from outside particle theory:

On one side of the divide stand most of those physicists who work on string physics and in fields like inflationary cosmology or high energy particle physics model building, which are strongly influenced by string physics. That group represents a slight majority of physicists in theoretical high energy physics today. Based on an internal assessment of string theory and the history of its development, they are convinced that string theory constitutes a crucial step towards a better and more genuine understanding of the world we observe. On the other side stand many theoretical physicists of other fields, most experimental physicists and most philosophers of physics. They consider string theory a vastly overrated speculation.

Dawid’s main thesis is that string theory critics fail to recognize that a new paradigm of scientific methodology is now needed:

String theory thus should not be taken to announce an end of science but rather to represent a new phase of scientific progress. In this new phase, progress in fundamental physics is no longer carried by a sequence of limited, internally fully developed theories, but rather by the discovery of new aspects of one overall theoretical scheme whose general
characteristics identify it as a candidate for a final theory, yet whose enormous complexity bars any hope of a full understanding in the foreseeable future.

What is the reason you should accept this final theory that no one can understand? Obviously the lack of any empirical support is a problem, so Dawid turns his attention to a detailed study of the subject of “non-empirical theory assessment”: how do you assess scientific progress absent connection to experiment? This is a real and serious problem, which Dawid studies in detail, although from a point of view which just naively accepts all arguments made by string theorists. He considers three main reasons for studying a theory with no empirical support:

  • The No Alternatives Argument. This is the best argument for string theory: there aren’t a lot of viable unified theories out there. Of course, the way science progresses is that there always are unsuccessful ideas with no good alternatives, until the day someone come up with a better idea.
  • The Unexpected Explanatory Coherence Argument. This is the idea that if a theory holds together better after you start studying it and understand it better, that’s a good thing. Dawid repeats uncritically claims of some string theorists that this is the case for string theory. I think you could make an equally good case for string theory unification becoming a more and more dubious idea as it became better understood (see, the Landscape).
  • The Meta-Inductive Argument. Here the idea is that if a theoretical research program worked before, so will a similar later one. Dawid claims that the string theory research program is just like the research program that led to the Standard Model:

    Given the entirely theoretical motives for its creation, the lack of satisfactory alternatives and the emergence of unexpected explanatory inter-connections, the standard model can be called a direct precursor of string theory.

    Honestly, this I just find bizarre, and have no idea what he’s talking about, with the history of the Standard Model and the history of string theory two radically different subjects.

In one crucial respect, this book is very different though than Kane’s. Kane is well aware that the idea of an inherently experimentally untestable theory is something he can’t sell to his colleagues and the public, so he devotes his book and its argument for string theory to supposed experimental tests. More savvy string theorists than Kane though are seeing the writing on the wall: no SUSY at 8 TeV means almost surely no SUSY at 13 TeV, and thus no prospects for experimental evidence for SUSY during any of our lifetimes. To prop up the string theory unification program past SUSY null results from the 13 TeV LHC in 2016 is going to require relying on Dawid’s “non-empirical theory assessment” and convincing people that string theory and the multiverse represent a new paradigm for how to pursue fundamental science. This book will be welcomed by those pursuing such a goal.

Update: For a different take on the book you can see Lubos Motl’s review (as you might expect, he’s a big fan). The case of the most prominent string theorist blogger reminds me of one of the funnier things in the Dawid book that I forgot to mention, this footnote:

It should be emphasized that physicists on both sides of the divide are aware of the slightly precarious character of the “non-physical” arguments deployed in the debate. Lee Smolin has applied the concept of groupthink to the community of string physicists (which, incidentally, seems a quite accurate representation of what many critics of string physics do think about string physicists) but is careful not to present it as a core argument. String theorists, when entering a discussion with their critics (see e.g. Polchinski in his reasoning against Smolin), try to keep the debate at an entirely physical level.

Posted in Book Reviews | 47 Comments

Number Theory News

Posted on May 12, 2013 by woit

A special seminar has been scheduled for tomorrow (Monday) at 3pm at Harvard, where Yitang Zhang will present new results on “Bounded gaps between primes”. Evidently he has a proof that there exist infinitely many different pairs of primes p,q with p-q less than 17,000,000 70,000,000.

Whether this proof is valid should become clear soon, but there still seems to be nothing happening in terms of others understanding Mochizuki’s claimed proof of the abc conjecture. For an excellent article describing the situation, see here.

Update: The “bounded gaps” talk is now on the Harvard seminar listing with abstract

The speaker proves that there are infinite number of pairs of primes whose difference is bounded by 70 million.

For more on the significance of this, see this Google+ posting by David Roberts.

I haven’t seen a paper, but rumor is that one exists and two referees at a major journal have found it to be correct.

Update: The most recent version of Mochizuki’s lecture notes for a general talk about his work is here. As mentioned in the Caroline Chen article, Go Yamashita has been talking to Mochizuki. Yamashita has now posted a short document FAQ on “Inter-Universality” and promises “For the details of the theory, please wait for the survey I will write in the near future.” He also notes:

I refuse all of the interviews from the mass media until the situation around the papers will be stabilised.

Update: In a weird coincidence, another major analytic number theory result is out today, a proof by Harald Helfgott of the ternary Goldbach conjecture. This says that every odd integer greater than 5 is the sum of three primes. The result had been known for all integers above e3100, and Helfgott’s proof reduces that bound to 1030 which is small enough so that all smaller values can be checked by computer.

Update: Nature has a story up about the Zhang result, including details of one of the Annals referee reports (I gather the paper will be published there).

Update: For some background to the methods being used by Zhang, see here. For Terry Tao on Zhang, see here, on Helfgott, here.


Update: New Scientist has a story about the Zhang result here, with quotes from Iwaniec, who has reviewed the paper, finding no error.

Update: A report from the talk at Harvard is here.

Update: There’s more about the Zhang proof at Emmanuel Kowalski’s blog, including a link to the Zhang paper.

Update: Nice piece about this in Slate from Jordan Ellenberg.

Posted in Uncategorized | 57 Comments

Miscellaneous Links

Posted on May 6, 2013 by woit
  • There’s an interesting discussion amongst philosophers at Brian Leiter’s blog about the effects of Templeton money (and I contributed my two cents…). In other Templeton news, they’re funding a new “literary science magazine” called Nautilus. Also via Leiter, they have awarded $3 million to two philosophers at Saint Louis University (“one of the largest grants SL has ever received in the areas of the humanities or the sciences”) for them to study the subject of intellectual humility.
  • In the category of rumors I’ve heard from so many reputable sources they must be true and I can’t really be violating confidentiality, W. Hugh Woodin is moving from Berkeley to Harvard, and Simon Donaldson from Imperial College to the Simons Center at Stony Brook.
  • Via Simon Willerton at the n-Category Cafe, Edinburgh now has a gallery with a wonderful collection of portraits of seventy mathematicians, including commentary from Michael and Lily Atiyah, an online version is here.
  • The publisher sent me a copy of Tony Zee’s new GR textbook, Einstein Gravity in a Nutshell, which I very much enjoyed looking through. Zee takes the textbook concept to new levels of informality, so it includes a wealth of interesting and amusing comments, spread throughout the text, footnotes and endnotes, including quite a few about quantum gravity. At over 800 pages, it’s a pretty huge book, including a lot of conceptual material (as in his QFT textbook), but more calculational detail than the QFT book. Undergraduate physics students should find this quite an approachable text (unlike the QFT one, which I think you need graduate level training to really follow).

    This definitely is a text for physicists, not mathematicians, with the geometry taking a back-seat. Differential forms and orthonormal frames don’t appear until nearly the end of the book. Personally I’ve found using the same language of connections on principal bundles to do gauge theory and gravity to make the most sense, but this involves getting familiar with quite a bit more formalism than most physicists are willing to deal with.

  • I’d been curious to hear more about recent work of Jacob Lurie and Dennis Gaitsgory on Tamagawa numbers, and had been waiting to see a paper from them. Turns out there’s something much better: Lurie has been teaching a course about this at Harvard this semester, with notes appearing here. For some indication of why you might take an interest in this if your interest is gauge theory, see here.
  • While about the only bipartisan agreement in Washington these days is that something must be done to deal with the terrible problem of the shortage of STEM graduates in the US, someone has noticed that there actually is no shortage, see here.
  • This past weekend there was a conference in honor of Bruno Zumino’s 90th birthday at Berkeley, and one can hope that some version of the talks might become available online here.
  • A hot topic in HEP remains that of when the failure of the SUSY picture that has been heavily over-sold for several decades will finally be acknowledged. From the list of titles at the Zumino conference, Maiani’s was “Supersymmetry: not time to give it up, yet”. Cormac O’Raifertaigh reports here that Nati Seiberg is saying “only certain aspects of minimal models had been ruled out so far. As for the future, who knows?”. (now corrected, see Cormac’s blog). Physics World has a story here, with Ben Allanach claiming that “data taken at the LHC have excluded roughly half of supersymmetry’s parameter space” and that now one has to wait until 2015 when, if SUSY is right, it will be found nearly immediately:

    “My hopes are pinned on the next run,” he says. “The energy jump now is going to make the big difference. And if supersymmetry is the correct theory of nature, I would be expecting to see a big signal within the first month. If it doesn’t crop up, I’ll then be getting pretty depressed.”

    Bill Murray of ATLAS makes the excellent point that

    “Proving [supersymmetry] wrong would be as important as proving it right,” he says. “Null results are hard to sell to newspapers, but they are really important to scientific progress.”

    Killing SUSY will be one of the great achievements of the LHC, and complaining about this might be kind of like being upset that Michelson-Morley didn’t find the ether.

Update: I’ve been pointed to an impressive photo of Robbert Dijkgraaf that unfortunately did not make the Atiyah Gallery.

Update: The New York Times has a story today about the new Templeton-funded science magazine Nautilus.

Update: The news from Britain is that Stephen Hawking has joined the academic boycott of Israel, cancelling plans to attend a conference there this month. Please discuss your views on the Israeli/Palestinian conflict elsewhere. There’s no way I’m going to moderate such a discussion, and there are now surely dozens of other sites carrying this story where comments are encouraged.

Posted in Uncategorized | 37 Comments

Supersymmetry and Beyond

Posted on May 1, 2013 by woit

Back in the year 2000, Gordon Kane published Supersymmetry: Unveiling the Ultimate Laws of Nature, a popular book promoting supersymmetry and string theory. The thrust of the book was that there was already indirect evidence for SUSY, with confirmation by discovery of superpartners due to come soon from LEP (which was running at energies near 100 GeV/beam) and the Tevatron (where Run II at high luminosity and nearly 1 TeV/beam was to start in 2001). The LHC was also discussed, mainly as the place that would confirm and extend the LEP/Tevatron superpartner discoveries.

Thirteen years later, with no hint of SUSY showing up as promised, not only at LEP/Tevatron energies, but also at the much higher energies and luminosities of the 8 TeV LHC, Kane has a new popular book promoting supersymmetry and string theory, entitled Supersymmetry and Beyond. It includes his claim to have predicted the Higgs mass using string theory (see Matt Strassler’s take on this here, mine here). Much of the book though consists of exactly the same text as the 2000 version.

How does Kane handle the detailed failed predictions of the 2000 edition in the new 2013 version? Basically by editing them out, with no indication to the reader that this has been done.  What’s the right word to describe the result of an Orwellian exercise like this? You can make up your mind about that yourself, since I’ve gathered together here some examples of the book text, showing the edits that were done to create the new version.

Pages xvii/xviii

Supersymmetry is still an idea as this book is being written (mid-1999) in late 2012. There is considerable indirect evidence that it is a property of the laws of nature, but the confirming direct evidence is not yet in place. That is not an argument against nature being supersymmetric; rather, the accelerator collider facilitiesy that could confirm it (the LHC) are  is just beginning to cover the region where the signals could appear (Chapter 5)[about LEP and Fermilab].

Pages 2-3/3

If we understand supersymmetry and its implications correctly, direct experimental evidence for supersymmetry will be found in the next few years – possibly soon after this book is published (or, with great luck, even before).

Pages 13/16

Only now are colliders and detectors at laboratories are now achieving the energies and luminosities (amounts of data) and sensitivities needed to explicitly detect the superpartners explicitly, at least if our thinking about their properties is more or less right.

Pages 70-71/77-79

The manner in which supersymmetry explains the Higgs physics is elegant and has important consequences for how we expect to test supersymmetry experimentally. It is rather technical. A more detailed description is given in Appendix B; here I will give a short version. There are three parts…

Therefore, the supersymmetric Standard Model explanation of the Higgs mechanism would not make sense unless the some superpartner masses were not much larger than the Standard Model masses they explain. That gives us an estimate of the masses we should expect the superpartners to have as we search for them, and it tells us at what stage we should question the validity of the theory if the superpartners have not been detected. Such estimates are only approximate, but luckily the expected masses are small enough that they imply the superpartners should be detected soon.

Appendix B was deleted entirely, it contained the text (page 156)

Therefore, the superpartner masses cannot be very much larger than the Z boson mass if this whole approach is valid. This is the only place where we can use the theory to relate the unknown superpartner masses to known masses, so on the one hand, it is a major test of the correctness of the supersymmetry explanation of the Higgs physics, and on the other, it is the most significant reason whey we expect the masses of the superpartners to have values that allow them to be produced at Fermilab or even LEP. This connection also suggests that if the superpartner masses are much larger than the Z boson mass, then the apparent success of the supersymmetry theory in explaining the origin of the Higgs physics of the Standard Model could be an accident.

Pages 88-89/89

Several arguments imply that some sparticles are within the reach of Fermilab the LHC. The strongest One of the most appealing is based on the explanation supersymmetry gives for the Higgs mechanism of the Standard Model, as described in the last chapter Chapter 7. Basically the qualitative argument is that because since supersymmetry provides the Higgs mechanism that accounts for the masses of W and Z, the some sparticle masses cannot be much heavier than the W and Z masses themselves. Fermilab has already produced and detected thousands of W’s and Z’s. When this argument is framed put in a technical form, it implies that gluinos and probably charginos and neutralinos and stops should be in the Fermilab within the LHC reach. If they are not, the impressive successes of supersymmetry listed at the beginning of Chapter 4 may be meaningless coincidences. There are some arguments, both theoretical and phenomenological, suggesting that squarks and sleptons will be too massive to produce at the LHC.

Chapter 8, on SUSY implications for matter/anti-matter asymmetry, proton decay, rare decays like mu to e-gamma, and CP violation has been deleted. Appendix D, on large extra dimensions, has also been completely deleted.

Witten’s preface has been edited:

Experimental clues suggest that the energy required to produce the new particles is not much higher than that of present accelerators. If supersymmetry plays the role in physics that we suspect it does, then it is very likely to be discovered by the next generation of particle accelerators, either at Fermilab in Batavia, Illinois, or Large Hadron Collider (LHC) or its upgrades, at CERN in Geneva, Switzerland.

Posted in Book Reviews | 51 Comments

Arkani-Hamed Colloquium

Posted on April 30, 2013 by woit

Nima Arkani-Hamed was here at Columbia yesterday to give the physics colloquium, which clocked in at a bit over 1 hour and 45 minutes. He did reveal the secret of why his talks are this long: when invited to give a 1 hour colloquium, he plans on talking for at least 1 hour 30 minutes. The content of the talk was similar to many others he has given recently that are available on the web, see for instance this one at the IAS, this recent one at BNL, or for a written version, see here.

As a performer, he’s a powerful speaker: smart, vigorous, and supremely self-confident. His arguments lead to “inevitable” conclusions, not just implying results but “nailing” them. It’s clear why he’s the most influential person in the field these days. With most theorists made worried and unsure by 40 years of failure to get anywhere in their efforts to improve on the Standard Model, he knows exactly what he thinks and will tell you forcefully what you should think. The fact that none of the ideas about BSM physics he is famous for (large extra dimensions, split SUSY, Little Higgs, etc…) have ever worked out doesn’t seem to slow him down, and he has a professorship at the IAS and a $3 million prize from Yuri Milner to back him up.

Despite his long-time advocacy of SUSY, according to Arkani-Hamed, the negative results from the LHC are “not making many of us worried about SUSY”. He (accurately) points out that he’s not one of those like Gordon Kane who for decades has been predicting the discovery of superpartners to be six months away. It has long been clear that the simplest versions of SUSY should have shown up at LEP and the Tevatron, and pre-LHC the lack of any indirect evidence for SUSY indicated to him that it was unlikely to show up at the 8 TeV LHC. So, by his lights, there’s no reason that LHC results so far should cause any new worries about SUSY, beyond those he already had pre-LHC. On the more limited question of whether a “natural” version of SUSY will work out, one where the superpartner masses just barely avoid large amounts of fine-tuning, a year ago (see here) he was saying we were at the “eleventh and a halfth hour” for this possibility. Now that the 8 TeV results are here (and negative), he argues that it is only with the 2015 data that the results will be decisive. The current wisdom about “natural SUSY” I guess is summarized in slide 8 here: Keep Calm and Wait for 14 TeV.

The main point of the talk was one that Arkani-Hamed has been consistently making for nearly a decade, that the role of the LHC is to decide between two possible futures for fundamental physics:

  • The small value of the Higgs mass (in Planck units) has a “natural” explanation, most likely using SUSY, in which case we spend the rest of our lives unraveling the complexities of a SUSY-extended Standard Model.
  • The small value of the Higgs mass (in Planck units) indicates “fine-tuning” that can only have an anthropic explanation, just like the one for the CC. In that case, we live in a multiverse, with physics determined by something like the string theory landscape. About this whole conceptual framework, he says the “ideas are so poorly defined, not clear if they make any kind of mathematical sense”, and it’s “not clear progress will happen anytime soon” but, no need to worry or get discouraged, since this is an “attractive problem”.

Based on the LHC results so far, it looks like all evidence is that we’re headed to the second alternative.

Arkani-Hamed’s talk was structured so as to present a long chain of argument (needing at least 1h 30 min to explain) leading to these two alternatives. One of the alternatives (SUSY naturalness) is essentially already dead, with the die-hards intent on hanging on a couple more years. The other is essentially what David Gross has called “giving up”: you just announce that the problems you haven’t been able to solve can never be solved. In this vision, the 20th century with its huge success at finding a highly predictive, mathematically beautiful fundamental theory was an aberration caused by only being able to see physics at energies way below the Planck scale. In this new 21st century physics, you just postulate that at higher energies things are much more complicated, in ways we can’t hope to ever know, and theorists devote their lives to making excuses, not predictions. Witten may end up being right that “string theory is 21st century physics that fell into the 20th century”, in a much more negative way than he intended.

If a long, complicated argument leads you to the conclusion that the only viable alternative is to give up, then it seems to me you have two choices: give up, or examine more carefully your argument. A much more interesting and more useful talk than Arkani-Hamed’s would be one less devoted to forcefully insisting on the conventional chain of argument based on the technical problem of sensitivity of the Higgs potential to the cut-off, instead looking carefully for weaknesses in the argument (one possibility is discussed here). Arkani-Hamed is a brilliant physicist, but this may be a time when what is needed is not self-confidence in the power of one’s arguments, but instead a suspicion that one has been making a mistake somewhere for quite a while now.

Posted in Favorite Old Posts, Uncategorized | 45 Comments

Time Reborn

Posted on April 23, 2013 by woit

Lee Smolin’s new book, Time Reborn, is out today. For more about the ideas in the book, see video of a talk here, and an interview here.

While I mostly vehemently agreed with what Smolin had to say in his last book, The Trouble With Physics, I find myself equally vehemently in disagreement with this one. On some of the topics covered, I’m indifferent to his arguments mostly as a matter of taste. While my views on human society are likely similar to Smolin’s, I’ve never found the scientific insights of fundamental mathematics or physics to have anything significant to tell me about this part of life. Similarly, while I’ve spent some time studying philosophy, I’ve mostly found this of little help in gaining deeper understanding of math or physics. Others though have a very different experience than me, and I’m not about to argue against people looking for enlightenment wherever they happen to find it.

On some of the scientific issues dealt with in the book, again I’m mostly just indifferent. Smolin accurately explains how the lack of predictivity makes typical multiverse models empty, but I’m not convinced that his favored alternative (“cosmological natural selection”) does much better. While I understand well the human appeal of wondering about what came before the big bang, I’ve yet to see any specific models of this that carry enough explanatory power about anything to make them particularly attractive or interesting.

Many of the ideas Smolin is arguing for are clearly labeled as what they are: speculative challenges from a very much minority point of view to some of the received wisdom of this kind of science. Unfortunately, parts of his argument that are most problematic are ones which are in danger of becoming the new received wisdom of the subject. The refusal to admit the failure of the idea of string/M-theory unification has left many of our most prominent theorists pushing the idea that fundamental physics is based on some new and very different degrees of freedom, with dynamics that just happens to be too complicated to allow them to find vindication by seeing how the Standard Model emerges at low energies. For his own reasons, Smolin signs on to a version of this point of view, writing:

I’m inclined to believe that just about everything we now think is fundamental will also eventually be understood as approximate and emergent: gravity and the laws of Newton and Einstein that govern it, the laws of quantum mechanics, even space itself…

A large part of the elegance of general relativity and the Standard Model is explained by understanding them as effective theories. The beauty is a consequence of their being effective and approximate. Simplicity and beauty, then, are the signs not of truth, but of a well-constructed approximate model of a limited domain of phenomena.

The notion of an effective theory represents a maturing of the profession of elementary-particle theory. Our young, romantic selves dreamed we had the fundamental laws of nature in our hands. After working with the Standard Model for several decades, we are now simultaneously more confident that it’s correct within the limited domain in which it has been tested and less confident of its extendability outside that domain.

This notion that the SM is “just an effective theory”, with its fascinating and deep mathematical structures nothing but an artifact of low-energy approximation has become the reigning ideology of the last few decades. One impetus for this has been string/M-theory, with its conjectured very different physics at short distances. This has been put together with our modern understanding of renormalization, according to which non-renormalizable theories make perfect sense as effective theories. The argument is then made that this is all there is to the SM, neglecting to note that to a large degree the SM couplings are asymptotically free, meaning that (most of) the quantized geometric degrees of freedom make perfectly good sense at all energy scales.

Smolin’s view that the recent history of particle physics makes us “less confident of its [the SM’s] extendability outside that domain [where it has been tested]” is one I strongly disagree with. Despite endless “naturalness” and “fine-tuning” predictions based on the “nothing but an effective theory” argument, the SM has not only been vindicated at the LHC over a large new energy range, but the discovery of the Higgs has shown it to have just the right characteristics to make perfectly good sense up to extremely high energies, far beyond anything we can test.

I’ve been teaching a course this past year on quantum mechanics for mathematicians, emphasizing the role of Lie groups, unitary representations and symmetries in providing not only useful calculational methods, but governing the underlying structure of the theory. Smolin argues instead that, based on Leibniz’s “identity of the indiscernibles”, symmetries cannot be fundamental (although a footnote says this doesn’t apply to gauge symmetries):

Symmetries are common in all the physical theories we know. Several of the most useful tools in the physicist’s toolbox exploit the presence of symmetries. Yet if Leibniz’s principles are right, they must not be fundamental.

This applies to the very structure of quantum mechanics:

Quantum mechanics, too, is likely an approximation to a more fundamental theory.

since it is linear, and he bets thus just a linear approximation to some fundamentally non-linear theory. Again, mathematical simplicity is seen as an artifact of approximation, not indication of something fundamental.

Smolin ends with a vision that is pretty much the exact opposite of mine, one with a vastly diminished role for mathematics in understanding the nature of reality:

The most radical suggestion arising from this direction of thought is the insistence on the reality of the present moment and, beyond that, the principle that all that is real is so in the present moment. To the extent that this is a fruitful idea, physics can no longer be understood as the search for a precisely identical mathematical double of the universe. That dream must be seen now as a metaphysical fantasy that may have inspired generations of theorists but is now blocking the path to further progress. Mathematics will continue to be a handmaiden to science, but she can no longer be the Queen.

Unfortunately it seems possible that Smolin’s arguments about mathematics will resonate well with the current backlash against sophisticated mathematics that one sees at many physics departments in the wake of the failure of string theory. In a footnote he explicitly argues that the problem with string theory was too much symmetry:

Indeed, we see from the example of string theory that the more symmetry a theory has, the less its explanatory power.

I don’t understand this argument at all. The problems with string theory are something I’ve written about endlessly here, but too much symmetry is not one of these problems.

Smolin has been quite right to point out in recent years that fundamental physical theory is in a state of crisis, but I think his diagnosis in this book is the wrong one. Abandoning the search for a more powerful mathematical understanding of the world because the huge success of this in the past has made further progress more difficult is the wrong lesson to draw from recent failures (the nature of which he lucidly described in his previous book).

My own interpretation of the history of the Standard Model is that progress came not from finding more, larger symmetries, but from a deeper appreciation of the various ways in which gauge symmetry could be realized (spontaneous symmetry breaking, confinement, asymptotic freedom). The arrival of string theory pushed the study of gauge symmetry into the background, and these days one often hears arguments against its fundamental nature, such as this one from Arkani-Hamed

What’s as a misnomer called gauge symmetry, whose beauty is extolled at length in all the textbooks on the subject, is completely garbage. It’s completely content free, there’s nothing to it.

Smolin’s arguments against the fundamental nature of symmetries, even if gauge symmetry is let off the hook in a footnote, just reinforce some of the attitudes at the root of our present-day crisis. The problems that remain in fundamental theory are difficult, but denigrating now the powerful ideas that have led to success in the past won’t help find a way forward.

Update: For more about this, there’s a review in the NYRB, and a piece at edge.org (with responses).

Posted in Book Reviews | 83 Comments