Higgs Update

Posted on March 6, 2013 by woit

Updated results about the Higgs are being reported at Moriond today, slides available here. The organizers have given the talks titles with “BEH Boson” replacing the usual “Higgs Boson” (to promote Englert’s shot at a Nobel), but the speakers have mostly ignored this, titling their slides with the usual “Higgs” or maybe “Standard Model Scalar”. Some more details are starting to appear at the CMS site here, presumably ATLAS will soon update their site here.

The only surprise so far is that the CMS results for the gamma-gamma channel are not ready yet. Philip Gibbs has very good coverage of the latest news here, including this about CMS:

Rumour puts the CMS diphoton excess at 1.0 +- 0.2, to be shown at Moriond QCD next week perhaps.

As mentioned here a couple weeks ago, the size of the ATLAS excess in that channel has gone down since last year, now at 1.65 +/- 0.24(stat) +/- 0.21(syst) (where 1.0 is the SM prediction). If you believe Philip’s rumor, the combined ATLAS + CMS result for the gamma-gamma channel would be 1.32, consistent with the SM prediction at the level of 1-2 sigma.

In the ZZ channel, CMS reports a cross-section relative to SM of .91 +/- 0.27, ATLAS 1.7 +/- 0.5. Combining them gives 1.30, again quite consistent with the SM. For the WW channel, CMS has .76 +/- 0.21, ATLAS 1.5 +/- .6, averaging out to 1.13, again very much consistent with the SM. For channels with bottom quarks, CMS has 1.3 +/- .6 and for channels with taus CMS has 1.1 +/-.4, ATLAS says the expected signal is still to small in these channels for them to say much.

Some more talks this afternoon may give a bit more detail.

All in all, the story is that this is looking very much like a garden variety SM Higgs, which is discouraging for hopes of hints about how to get beyond the Standard Model. The experiments will continue working on improving their analyses of this data, but it seems unlikely that the picture will change much. There’s going to be a long drought now until we see significantly better data for these numbers. Probably not until 2016 until the LHC has been operating long enough to produce significant luminosity at higher energy.

The New York Times yesterday put out a wonderful special issue of its Science Times section, devoted to an excellent long article by Dennis Overbye telling the story of the Higgs discovery from the point of view of the ATLAS and CMS scientists (and emphasizing their rivalry). Highly recommended reading. The article does credit a certain blog with being the venue where a mistaken early Higgs claim was leaked (I’m sorry to hear that that ruined some people’s vacations), although the fact the the actual Higgs discovery news broke somewhere else than in the Times doesn’t get mentioned…

Professor Matt Strassler has a posting about the Times article, explaining how it shows that other particle physics bloggers were wrong to think that the 3 sigma signals reported by ATLAS and CMS back in late 2011 were strong evidence that the Higgs had been found, and that he had been right to be skeptical.

Update: New ATLAS results are here. Do not miss the extremely cool animated gifs of the evolution of the Higgs signal as data accumulated.

Update: Valuable commentary at Resonaances.

Posted in Experimental HEP News | 11 Comments

Quick Links

Posted on March 3, 2013 by woit

This should be a month with quite a bit of experimental news, including

  • Latest Higgs news from the LHC experiments here on Wednesday.
  • Release of data from AMS-02 was advertised as “two to three weeks away” back on February 17.
  • Planck data release on March 21.

A couple weeks ago, Arianna Borrelli of the Epistemology of the LHC project gave a talk at CERN (slides here). It includes some interesting data from surveys of HEP physicists in September 2011 and September 2012.

The Simons Foundation website keeps having some of the best writing on math and physics around. Natalie Wolchover has an excellent story about a complex subject, that of the role of computers in proof. The also have an essay by Barry Mazur about another complicated related subject, the nature of evidence in mathematics.

Finally, if you want to watch a very good introduction to D-modules, see video from David Ben-Zvi at MSRI here and here.

Update: Edward Witten will be speaking here in New York this evening (Monday March 4) at Hunter College, for more information see here. Unfortunately I have other plans and will have to miss this.

Posted in Uncategorized | 13 Comments

The State of SUSY

Posted on February 27, 2013 by woit

Results putting new limits on SUSY based on the entire first run of the LHC are starting to emerge (see for example this from CMS) with more likely at Moriond next week. Since one is dealing with a theory with a large number of parameters, these are hard to characterize in a simple way. One thing to focus on is the limits on gluinos, since just about every popular version of SUSY says these should be about the easiest thing for the LHC to see. Very roughly, the Tevatron was able to set typical limits of about 300 GeV on such things, and the LHC at 8 TeV (4 times the Tevatron) is now giving limits around 4 times higher, 1.2 TeV. This is not likely to change much for the next few years until after the LHC comes back at 13 TeV in 2015. One can with some confidence predict that the gluino mass limit will then go up to about (13/8)*1.2 TeV or around 2 TeV, maybe a bit more in years after that with a high-luminosity LHC. Farther out in time, the next machine under discussion that could raise the limit is the HE-LHC, at 32 TeV, giving limits around 5 TeV. The time scale for this though is something like 2030-40, even assuming such a project ever were to get funded. I suspect the right characterization of that project might be “not in my lifetime”.

There is a new paper out claiming to see evidence of a gluino in the data around 1000-1100 GeV. The same authors (see here), have been claiming to see such gluinos since the early LHC data, first at around 7-800 GeV, with a mass getting higher with each round of new data and higher mass limits.

Few are likely to pay attention to this, but what is getting taken much more seriously is the case that Nima Arkani-Hamed has been vigorously making recently (see for example his talk at the Higgs Symposium). Arkani-Hamed is now by far the most influential theorist in this area, with slides from his latest talks often appearing in many other people’s presentations, functioning as the embodiment of the conventional wisdom of the field. He also is the only phenomenologist with a $3 million Fundamental Physics prize, awarded for his work on models that have had great influence, although zero success experimentally.

One of these, split supersymmetry, is what he is now promoting as the explanation for the negative LHC results. In this model, which he developed with Savas Dimopoulos back in 2004, the main argument for SUSY, the hierarchy argument, gets abandoned in favor of anthropics. The Higgs mass and the electroweak scale are what they are not because of SUSY, but because otherwise physics would be different and we wouldn’t be here. Once one abandons the hierarchy argument, the remaining arguments for SUSY are extremely weak (I’ll try and explain these in more detail in a separate posting), but for some reason Arkani-Hamed still thinks the idea is worth promoting and that vindication for his $3 million may yet be had.

Split SUSY works by moving all scalar superpartners up to unobservably high energies, but a few particles including the gluino are supposed to be at potentially observable masses. Back in 2004, Arkani-Hamed and Dimopoulos were hopeful about the possibilities for the LHC seeing a split SUSY gluino, writing:

However, at peak luminosity of 30 fb-1 per year, the LHC may well be a gluino factory producing roughly a gluino per second(for m_g ∼ 300 GeV).

These hopes have now been dashed, and at the Higgs symposium talk, illustrative spectra show gluino masses at 2.1 and 2.3 TeV (this may just be because that’s about the limit of what the LHC could see). Arkani-Hamed and co-authors have a recent paper out discussing Simply Unnatural Supersymmetry, i.e. “the simplest picture of the the world arising from fine-tuned supersymmetric theories”. Here calculations are done for gluino masses ranging from 1.5 to 15 TeV, and the story is that we’ll have to be lucky to get any experimental evidence for this model. They end with:

If Nature has indeed chosen the path of un-natural simplicity, we will have to hope that she will be kind enough to let us discover this by giving us a spectrum with electroweak-inos lighter than ∼ 300 GeV or gluinos lighter than ∼ 3 TeV.

So, the current state of the conventional wisdom about SUSY from its most influential proponent is pretty much the following. It’s still the thing to try and sell to the public as the best bet for the future of physics, but the hierarchy argument is gone, and at a fundamental level it’s anthropics, the landscape and the multiverse. He’s pretty much given up hope of ever getting any experimental evidence for this, other than the outside possibility of maybe the gluino mass being just low enough to be visible in rare LHC events late in the decade.

The interesting question about all this I think is a sociological one: will this untestable and rather ugly theory based on anthropic reasoning become widely seen as the “best hope” for fundamental particle theory? In a post-LHC world where mankind has abandoned the high-energy frontier, will the conventional wisdom of the textbooks be that SUSY and those gluinos must be there, but unfortunately happen to be just out of reach?

Update: For a survey article that just came out this evening, which tries to show that the main argument for SUSY (the hierarchy problem) is not quite dead yet, see here.

Update: New Scientist has a special section this week about “Crunch time for physics” (unfortunately mostly behind a paywall). On SUSY, Frank Wilczek is still a believer, based on the renormalization group calculation he was a co-author of back in 1981. If no SUSY turns up at the next LHC run though, even he will throw in the towel:

I cannot believe this success is an accident. But in science faith is a means, not an end. Supersymmetry predicts new particles, with characteristic properties, that will come into view as the LHC operates at higher energy and intensity. The theory will soon undergo a trial by fire. It will yield gold – or go up in smoke.

He has a bet with Garrett Lisi that superparticles will be detected by July 8, 2015.

Posted in Uncategorized | 65 Comments

Higgs News

Posted on February 21, 2013 by woit

Official announcements won’t come out until the Moriond conference first week of March, but reliable rumors are starting to trickle out about what the Higgs news will be. ATLAS will report (based on about 21 fb-1 of 8 TeV data + the 2011 7 TeV data) that the gamma-gamma excess has gone down slightly (from 1.8 to 1.65 times the SM value). Still about 1.5 standard deviations high, but this isn’t encouraging if you want something that disagrees with the SM.

At the AAAS 2013 meeting in Boston this past week, a press conference was held to update the media on the Higgs. What the media got from the press conference was the news that the Higgs may spell doom, unless supersymmetry saves us. This isn’t just doom for HEP physics research, it’s doom for the entire universe:

“At some point, billions of years from now, it’s all going to be wiped out…. The universe wants to be in a different state, so eventually to realise that, a little bubble of what you might think of as an alternate universe will appear somewhere, and it will spread out and destroy us,” Lykken said at AAAS.

This is based on a renormalization group calculation extrapolating the Higgs effective potential to its value at energies many many orders of magnitude above LHC energies. To believe the result you have to believe that there is no new physics and we completely understand everything exactly up to scales like the GUT or Planck scale. Fan of the SM that I am, that’s too much for even me to swallow as plausible.

If you are being kept awake by the Higgs metastability issue, you’ll want to know the Higgs mass as accurately as possible. The rumor from ATLAS is that the difference in best fit masses between the gamma-gamma and ZZ channels has narrowed, with gamma-gamma moving up slightly to 126.8 GeV, ZZ quite a bit, to 124.3 GeV.

Posted in Experimental HEP News | 18 Comments

SUSY and Quantum Mechanics

Posted on February 19, 2013 by woit

You may have read somewhere today that Columbia professor strips down to underwear in bizarre lesson to help baffled students learn quantum mechanics (first-hand sources here and here). That wasn’t me, but I have been talking to my class for the last couple weeks about quantizing fermionic variables, and some simple quantum mechanical examples of supersymmetry. Notes on the supersymmetry stuff are here, earlier notes on the fermionic version of quantization and what it has to do with Clifford algebras and spinors are on this page.

These notes are not quite finished, mainly because I’ve been trying to sort through the hairy issue of sign conventions that comes up when you start dealing with a Hermitian inner product on anti-commuting variables, something you need to do to get unitary representations. There’s a detailed treatise on the subject by Deligne and Freed, who are very smart and sensible, but I’d like to understand this better. The choices they make end up leading to odd self-adjoint operators having eigenvalues proportional to a square root of i, which is consistent, but not exactly intuitively clear. The best source for finding details of the mathematics used in SUSY is probably the IAS volume that Deligne/Freed is part of. The first part includes valiant efforts by Bernstein, Deligne, Freed, and Morgan to get the mathematics right, including the signs (they say “Writing this has been an absolute cauchemar de signes!”). One sign they get wrong is a typo on page 91 (equation 4.4.5).

The parallel stories of bosonic and fermionic oscillators are among the deepest things in theoretical physics, and involve just spectacularly intricate and deep mathematical ideas (symplectic geometry, rotation and spin groups, Heisenberg groups, the metaplectic representation, Clifford algebras and Weyl algebras, spinors, etc., etc…). I hope the course notes I’ve been writing give a little insight into this and the way Lie groups, Lie algebras, and their representations are involved. Generically, “supersymmetry” refers to generalizing the notion of a Lie algebra to include odd generators, and thus get a “super” Lie algebra, sometimes acting in an interesting way that mixes even and odd variables. In the notes I describe two very simple examples, showing how one gets a “square root” of the Hamiltonian operator.

There are all sorts of interesting structures one can get by looking for supersymmetrical versions of QFT, and the IAS volume describes a lot of them. One wonderful example is the N=2 susy gauge theory that gives a TQFT with observables four-manifold invariants. This is an unphysical theory, but tantalizingly close to physical theories. It involves a “twisting” mixing the space-time and internal symmetries which might be the sort of thing needed to avoid the problem of the kind of “superpartners” that is deadly for SUSY extensions of the standard model.

Perhaps the most compelling example though is the way the fact that the Dirac operator is a square root of the Laplacian can be thought of as an example of SUSY. This is one of the deepest ideas in mathematics, something whose implications I suspect we still don’t completely understand.

Posted in Favorite Old Posts, Uncategorized | 18 Comments

Quick Links

Posted on February 17, 2013 by woit

Posting has been light recently, partly since I’ve been working on writing up notes for my course (more about that soon), but largely because there hasn’t been a lot of news to write about in the math-physics world. The LHC shutdown yesterday, with the latest online machine status report now saying:

No beam for a while. Access required. time estimate: ~2 years

It will take about that long to replace magnet interconnections and do other work required to get the LHC working at an energy close to the design energy of 7 TeV/beam (seems likely they’ll be trying for 6.5 TeV/beam).

Results from the full 2012 data set for the Higgs are likely to be released soon, at Moriond in early March. Not much in the way of rumors available about this, which may have something to do with no surprises in the data. I hear there will also be, as expected, yet more stringent limits on SUSY reported.

For a US-centric series of reports on HEP and future plans, see talks here at a meeting this week at Fermilab.

On the cosmology front, there should be big news next month with Planck finally reporting results on March 21 (see here), to be followed by a conference dedicated to the results a couple weeks later.

No matter how cosmology is doing as a science, the Templeton Foundation is doing its part to promote its non-scientific aspects, with major funding for projects designed to promote and institutionalize the subject of “Philosophy of Cosmology”. Just before the Planck data release, DAMTP will host a Templeton-funded conference on “Infinities and Cosmology”, which will include two lectures by Michael Douglas on “Can we test the string theory landscape?”. Templeton is also funding a three-week summer institute in Santa Cruz to “promote understanding and research” on topics like “reasons for believing in a multi-verse, anthropic arguments, the metaphysics of laws and chance, why anything at all exists.” If you want to spend three weeks this summer among the redwoods discussing such topics, and collect a check for $2500 from Templeton, apply now.

Sometimes I make fun of pseudo-scientific research favored by some Northern California physicists by speculating about the role of marijuana in their research efforts. On a much more serious note, Southern California’s John Schwarz and his wife Patricia have been involved in admirable efforts to change US policy against investigating medical uses of marijuana, with Schwarz writing an editorial here last year, and speaking at a conference in DC next week.

For more evidence of how ideas about string theory have worked their way into US general cultural life, a couple people have pointed me to Adam Gopnik’s piece about Galileo in last week’s New Yorker, which contains the following:

Contemporary historians of science have a tendency to deprecate the originality of the so-called scientific revolution, and to stress, instead, its continuities with medieval astrology and alchemy. And they have a point. It wasn’t that one day people were doing astrology in Europe and then there was this revolution and everyone started doing astronomy. Newton practiced alchemy; Galileo drew up all those horoscopes. But if you can’t tell the difference in tone and temperament between Galileo’s sound and that of what went before, then you can’t tell the difference between chalk and cheese. The difference is apparent if you compare what astrologers actually did and what the new astronomers were doing. “The Arch-Conjuror of England” (Yale), Glyn Parry’s entertaining new biography of Galileo’s contemporary the English magician and astrologer John Dee, shows that Dee was, in his own odd way, an honest man and a true intellectual. He races from Prague to Paris, holding conferences with other astrologers and publishing papers, consulting with allies and insulting rivals. He wasn’t a fraud. His life has all the look and sound of a fully respectable intellectual activity, rather like, one feels uneasily, the life of a string theorist today.

Posted in Uncategorized | 21 Comments

Existence of Kähler-Einstein Metrics

Posted on February 7, 2013 by woit

An important recent development in geometry has been the announcement of two claimed proofs of a long-standing conjecture about the existence of Kähler-Einstein metrics. Simon Donaldson is talking about this at MIT this week (see here and here), and the last in a series of his papers with Xiuxiong Chen and Song Sun giving details of their proof appeared on the arXiv earlier this week, see here. For the earlier papers in the series, see here and here, as well as the original announcement of the proof in outline here. Gang Tian also has a preprint with a proof, see here. As usual in mathematics, one might want to wait for these preprints to be refereed by experts before being sure that a proof is in hand.

Given any manifold, there’s an infinity of ways of putting a metric on it. A major theme in modern geometry and topology has been the pursuit of the idea that in many cases there may be a unique “best” choice for such a metric. The proof of the Poincaré Conjecture involved just this sort of idea, showing that starting with any metric on a simply-connected three-manifold one could deform it in a specific way to end up with certain special possibilities that could be completely analyzed.

For Kähler manifolds, the big open question of this kind has been that of whether one can find a unique metric that is both Kähler and Einstein (thus “Kähler-Einstein”). For negative first Chern class this was shown by Aubin and Yau, and for zero first Chern class by Yau in his proof of the Calabi conjecture (these are the “Calabi-Yau” manifolds). For positive Chern class there are counter-examples, but the conjecture has long been that Kähler manifolds satisfying an appropriate notion of “stability” will have such a unique Kähler-Einstein metric, and it is this conjecture that apparently has now been proven.

The details of this are far beyond my expertise, so I refer you to the papers quoted above, as well as some expository articles about the problem by Donaldson and Tian, as well as a series of blog posts (here, here, and here) by Terry Tao based on lectures by Yau.

Posted in Uncategorized | 7 Comments

Short Items

Posted on February 3, 2013 by woit
  • Resonaances has an excellent posting about the latest WMAP9 CMB measurements, and the value Neff for the number of implied light degrees of freedom. When the WMAP numbers were released late last year, they quoted

    Neff=3.89+/-.67, 3.26+/-.35, 2.83+/-.38

    for the results of fits to their data and others (see section 4.3.2). Jester described this as “like finding a lump of coal under the Christmas tree”: the value Neff=3 implies no new light degrees of freedom beyond the known 3 light neutrinos. A rumor soon appeared on his blog that this result was in error and would be corrected.
    The corrected version is now out, with new results

    Neff=3.89+/-.67, 3.84+/-.40, 3.55+/-.49

    and a note about the correction: “slight correction to Neff for case with BAO.”
    which seems reasonable if you regard the difference between finding no unknown degrees of freedom and discovering a new unknown one as “slight”.

  • Martin Perl has an interesting blog entry entitled What Me Worry About The Future of High Energy Physics? He describes his views about the problems facing HEP, what he thinks of the Fundamental Physics Prize, and some comments on the history of physics (as well as some kind words about this blog).
  • On the Beauty front, you can watch a video of Enrico Bombieri’s lecture at the IAS on Beauty in Mathematics. On February 15 in Boston the big AAAS annual meeting will include a session on Is Beauty Truth? Mathematics in Physics from Dirac to the Higgs Boson and Beyond.
  • viXra log has a posting about video released by CMS of the session on June 15th where their convincing evidence for the Higgs in gamma-gamma decays was first unveiled to the larger collaboration. It was at this point that most of the 3000 or so physicists in CMS knew for sure they had a Higgs discovery. One can speculate about what the graph of number of people in the world aware of this would look like as a function of time, but I’m sure by June 17th when I first heard about it, it was already much more than 3000, and growing exponentially.

    This was about three weeks before the public announcement on July 4. Of course now what we all want to know is what the full 2012 CMS dataset says about gamma-gamma, and whether it agrees with the SM or not. The general assumption is that this will be made public at the March 2-9 conference in Moriond. So, based on the timetable last time, one can guess that within the next week or two such results will be disclosed to the full CMS collaboration.

  • As every year, one can follow the latest trend in US particle theory hiring at the tenure track level here. Lubos Motl describes the current situation as one of hep-th being subjected to terrorism, I guess by hep-ph.
Posted in Uncategorized | 18 Comments

Arthur Wightman 1922-2013

Posted on January 30, 2013 by woit

I just heard today that mathematical physicist Arthur Wightman passed away earlier this month, at the age of 90. Wightman was one of the leading figures in the field of rigorous quantum field theory, the effort to try and make precise sense of the often heuristic methods used by physicists when they deal with quantum fields. He was a well-liked and very respected professor at Princeton during the years 1979-84 that I was a graduate student there, but unfortunately I don’t think I ever made an effort to talk to him, to my loss. The university has something about him here, the department here.

Wightman is most well-known for the “Wightman Axioms”, which are an attempt to formalize the fundamental assumptions of locality and transformation under space-time symmetries that any sensible quantum field theory should satisfy. His 1964 book with Raymond Streater, PCT, Spin, Statistics and All That, explains these axioms and shows how they lead to some well-known properties of quantum field theories such as PCT invariance and the Spin-Statistics relation. When this work was being done during the 1950s and early 60s, quantum field theory was considered something that couldn’t possibly be fundamental. All sorts of discoveries about strong interaction physics were being made, and it seemed clear that these did not fit into the quantum field theory framework (this only changed in 1973 with asymptotic freedom and QCD). In any case, problems with infinities of various sorts plagued any attempt to come up with a completely consistent way of discussing interacting quantum fields, providing yet another reason for skepticism.

Wightman was one of a small group of mathematical physicists who reacted to this situation by trying to come to grips with the question of exactly what a quantum field theory was, in an attempt to find both the implications of the concept and its limitations. After the early 60s, attention moved from the axioms and their implications to the question of “constructive quantum field theory”: could one explicitly construct something that satisfied the axioms? Examples were found in 2 and 3 space-time dimensions, but unless I’m missing something, to this day there is no rigorous construction of an interacting QFT in 4 space-time dimensions. There is every reason to believe that Yang-Mills theory, constructed with a lattice cut-off, has a sensible continuum limit that would provide such an example, but this remains to be shown (and there’s a one million dollar prize if you can do this).

Thinking back to the early 1980s and my days as a graduate student, it’s clear what some of the reasons were why I didn’t spend time going to talk to Wightman. With the triumph of the Standard Model, attention had turned to questions about quantum gravity, as well as questions about the non-perturbative behavior of QCD. I certainly spent some time trying to read and understand the Streater-Wightman volume, but its emphasis on the role of the Poincaré group meant it had little to say about QFT in curved space-time, much less how to think about quantized general relativity. Gauge theories in general did not seem to fit into the Streater-Wightman framework, with the tricky issue of how to handle gauge symmetry something their methods could not address. For non-perturbative QCD, we had new semi-classical computation methods, and I was happily programming computers do numerical simulations of Yang-Mills theory. Why pay attention to the difficult analysis needed to say anything rigorous about quantum fields, when the path integral method seemed to indicate one could just put them on a computer and have the computer tell you the answer?

In later years I became much more sensitive to the fact that quantum fields can’t just be understood by a Monte-Carlo calculation, as well as the importance of some of the questions that Streater and Wightman were addressing. As particle theory continues to suffer deeply from the fact that the SM QFT is just too good, anything that can be done to better understand the subtleties of QFT may be worthwhile. It remains true that gauge theories require new methods way beyond what is in Streater-Wightman, but looking back at the book I see it as largely devoted to understanding the role of space-time symmetries in the structure of the theory. The importance of such understanding of how symmetries govern QFT may be a lesson still not completely absorbed, with gauge symmetries and diffeomorphism symmetries part of a story extending Streater and Wightman to the Standard Model, in a way that we have yet to understand.

Posted in Obituaries | 38 Comments

This Week’s Finds

Posted on January 26, 2013 by woit

Twenty years ago this past week, John Baez posted the first of his "This Week's Finds in Mathematical Physics" to the sci.physics newsgroups, inaugurating internet blogging about Mathematical Physics, many years before anyone even knew what a blog was. For his first posting, try looking at
http://math.ucr.edu/home/baez/twf_ascii/week1
and for all the rest of them see
http://math.ucr.edu/home/baez/TWF.html

There's a huge amount of interesting material in John's TWF postings, and the amount of effort that he has put into providing detailed, clear explanations on all sorts of topics is kind of staggering. While I often try to emulate what John has done (and "This Week's Hype" of course is a sort of homage, one he may not appreciate...), I feel I'm doing well if I can manage to put together a few sentences of comments about a "Find", with John's much more useful detailed expository work something beyond my capabilities.

For a "Find" from this past week in mathematical physics, I can recommend Hermann Nicolai's "Quantum Gravity: the view from particle physics" (arXiv:1301.5481) a write-up of his lecture this past summer at a conference in Prague. He makes a point about quantum gravity that I very much agree with: the problem with the subject is not so much that of finding a consistent quantum gravity, but of finding one that fits together with the SM and tells us something new that we can check. He writes:

Being exposed to many talks from the different ‘quantum gravity camps’ I am invariably struck by the success stories I keep hearing, and the implicit or explicit claims that ‘we are almost there’. I, for one, would much prefer to hear once in a while that something does not work, and to see some indications of inconsistencies that might enable us to discriminate between a rapidly growing number of diverging ideas on quantum gravity [27, 28]. If, however, the plethora of theory ambiguities were to stay with us I would conclude that our search for an ultimate explanation, and with it the search for quantum gravity, may come to an ignominious end (like in Breughel’s painting).

and

To conclude let me restate my main worry. In one form or another the existing approaches to quantum gravity suffer from a very large number of ambiguities, so far preventing any kind of prediction with which the theory will stand or fall. Even at the risk of sounding polemical, I would put this ambiguity at 10^500 (or even more) – in any case a number too large to cut down for any conceivable kind of experimental or observational advance.

Included in his talk are various more specific comments about these issues, well worth pondering. If I were John Baez, I'd have the energy to describe them in detail and explain clearly exactly what is going on, but for this I fear someone will have to get John more interested in quantum gravity again...

Update: I should mention other tributes to TWF here, here, and here.

Posted in Uncategorized | 19 Comments