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

Short Book Reviews

Posted on January 24, 2013 by woit

There’s now a fairly long list of books that I’ve found worthwhile recently and wanted to write about here, making it unlikely I’ll have time to write in detail about them. Instead, here are some short reviews:

  • More than seven years ago I wrote very critically here about Leonard Susskind’s The Cosmic Landscape. That book struck me as embodying the worst aspects of where string theory has ended up, promoting to the public in a high-profile way a dangerously pseudo-scientific excuse for string theory’s failure. Debate about the anthropic landscape has now been going on for nearly a decade, with mixed results. This ideology still has its believers and gets taken seriously, but I think it’s fair to say that interest has dwindled as it has become clear that no one has a serious idea about how to use it to make any kind of scientific prediction. For both proponents and opponents, it’s now old news, hard to get interested in talking about, especially since the lack of any evidence (pro or con, now or forever) seems guaranteed.

    Luckily for all of us, Susskind has moved on to much more promising topics. He has a new popular book out which is quite good, entitled The Theoretical Minimum. It’s basically a textbook on classical mechanics, written at a level appropriate for someone who has had a calculus class, but not necessarily any more physics or mathematics than that. The style is breezy and colloquial, with lots of nice explanations of some of the basic concepts of physics. It’s wonderful to see Poisson brackets appearing and nicely explained in a popular book destined to be displayed at bookstores everywhere.

    The book is based on one of several series of lectures given by Susskind as part of Stanford University’s Continuing Studies program, all of which are available on video at YouTube (see here for a list). The writing of the book is a joint effort of Susskind and George Hrabovsky, who started the project of turning Susskind’s lectures into book form.

  • While selling popular books with equations in them is a new concept in the US, it’s not so unusual in France. When last in Paris I picked up a copy in a non-scientific bookstore of Cédric Villani’s Théorème Vivant, which includes equations I can’t even follow. It’s basically a fascinating journal he kept during 2008-2011, focused on a problem he was working on during this period with his collaborator Clément Mouhot. It provides a good picture of what it’s like to be a top-class analyst working on a difficult problem. During this period, Villani was very much aware that he might be a candidate for a Fields Medal, which provided some motivation for him to push forward. If you want to know what it’s like to really want a Fields Medal, to work hard to get it and succeed, this is the book for you.

    A large part of this work took place during a year when Villani was holed up at the Institute in Princeton, and this is described in detail. Difficult working conditions included lack of access to good bread or cheese, a major reason Villani turned down efforts by Princeton to keep him there and returned to France, where he is now Director of the Institut Henri Poincaré in Paris. He also maintains a blog here where you can keep up with his activities.

  • Steven Weinberg’s Lectures on Quantum Mechanics is based on graduate-level quantum mechanics courses he has taught over the years. It covers concisely and well most of the standard topics that are make up a quantum mechanics course at this level (this is definitely not a beginning QM book). It does differ from most QM books though in providing a high-level and serious discussion of the question of interpretations of quantum mechanics, a topic about which Weinberg has thought deeply. After explaining carefully the issues, he ends up with:

    My own conclusion (not universally shared) is that today there is no interpretation of quantum mechanics that does not have serious flaws, and that we ought to take seriously the possibility of finding some more satisfactory other theory, to which quantum mechanics is merely a good approximation

    I fear I’m with those who don’t share this conclusion, but his arguments are well-worth paying attention to. For someone else who has thought deeply about all this, and come to conclusions closer to my own less well-considered ones, see this recent blog entry by John Preskill (don’t miss the discussion in the comments).

    The book ends with a modern but very short chapter on entanglement, Bell inequalities and quantum computation.

  • I’ve recently gotten a copy of a wonderful new quantum field theory textbook, Anthony Duncan’s The Conceptual Framework of Quantum Field Theory. It’s a long, fat book, packed with material that doesn’t appear in other QFT books. Most modern QFT books stay focused on the goal of writing down the Standard Model and giving the details of how to do perturbative calculations in the theory. Duncan instead devotes most of the book to a careful investigation of the basic issues raised when one works with a theory of quantized fields and tries to understand exactly how such objects are connected to the particle states and their scattering that we see in the real world.

    Besides the close attention to thorny conceptual problems normally glossed over, Duncan also gives a long discussion of the early history of the subject, a time in which the conceptual problems were being thought about by the leading figures in the field. Probably every one who has learned quantum field theory in one way or another could benefit by going through this book and picking up some insight into all the questions that were ignored in whatever other book they learned the subject from.

  • Finally, for those already fluent in quantum mechanics and quantum field theory, there is Mikhail Shifman’s Advanced Topics in Quantum Field Theory, published last year. It concentrates on methods for understanding the non-perturbative behavior of QFTs, especially gauge theories. A major topic is semi-classical methods and the art of extracting non-perturbative information about the QFT from interesting solutions to the classical equations of motions (e.g. instantons and solitons). The latter part of the book focuses on supersymmetric theories, where supersymmetry can be used to get further insight into the non-perturbative behavior. In recent years, much of the research interest in SUSY has moved away from the idea of using it for Beyond Standard Model physics (a trend likely to accelerate with the failure of SUSY to show up at the LHC), and towards thinking of it as a tool for studying QFTs. Shifman’s book gives a good introduction to the basic examples of how this works.

Update: Lev Okun sent me a copy of his ABC of Physics: A very brief guide. It’s a remarkable document, managing to cover all of fundamental physics in about 120 pages, from the simplest topics in high school physics to the Higgs and superstring theory (the latter treated with appropriate skepticism). If you want an overview of the subject that is as short as possible, this is for you.

Posted in Book Reviews | 16 Comments

The Anatomy of a Scientific Gossip

Posted on January 23, 2013 by woit

The University of Birmingham has put out a press release today about new research by their computer scientists, on the topic of the spread of gossip about the Higgs via Twitter. This is all based on an arXiv paper, The Anatomy of a Scientific Gossip, and has been picked up by New Scientist, Phys.org, and Aidan Randle-Conde.

Since I’ve been designated as one of the Best Physics Gossips on this topic:

If the Higgs boson was a dead celebrity, Woit would be your TMZ — first to the scene, first to break it, and have it be right.

I think I should perhaps comment on what this research actually shows. From what I can tell, it just provides evidence that Twitter is a worthless swamp full of people who have no idea what they are doing “re-tweeting” stale information to each other. Getting their information from tweets, according to these researchers things began with

Period I: Before the announcement on 2nd July, there were some rumors about the discovery of a Higgs-like boson at Tevatron;

and went on from there. They start looking at the data only from July 1 on.

Looking back at what actually happened, I started posting about the coming LHC results on June 17 (the Tevatron results were a side-show). On June 18th, Matt Strassler had the story, accusing me of ruining the CMS and ATLAS blind analyses, for top-secret reasons that could not be revealed. June 19th saw a New York Times story about this with a link to my blog entry and by June 20th Sean Carroll and Jennifer Ouellette were writing about #HiggsRumors being a “Trending Topic” on Twitter.

I suppose it’s true that a couple weeks later there were about a million tweets about this, but why would you conceivably want to look at any of them? While I was writing this blog posting, an incoming e-mail from Twitter popped up on my screen.

We’ve missed you on Twitter!

So much is happening right now on Twitter, and building a great timeline is the way to really enjoy the service. Get to Twitter and start building a timeline that reflects you and your interests, you’ll see how quickly Twitter becomes an invaluable part of your life.

I don’t think so…

Update: At his blog, Matt explains that he wasn’t accusing me of anything. It was CMS and ATLAS physicists who, by telling me me about the results after unblinding, were guilty of ruining the blinded analyses for still top-secret reasons.

Posted in Uncategorized | 12 Comments

CERN Briefing Book

Posted on January 18, 2013 by woit

This year the US and European HEP communities are engaging in exercises designed to put together plans for the future. In the US it’s Snowmass 2013, leading up to a big meeting in Minneapolis this summer. This past week has seen preliminary meetings at Irvine to discuss future prospects for experimental study of SUSY and other BSM ideas, and at Princeton for discussion of future prospects for study of the Higgs.

Over in Europe, there is a plan to update the 2006 European strategy for particle physics, This will officially take place in May/June, based on a document to be finalized by the CERN Council in March. Next week in Erice there will be a meeting to draft this document. To prepare for this, there was an open symposium last September, leading to the preparation of a briefing book, now available here.

The briefing book is a very interesting 220 page document covering in up-to-date detail the current experimental situation and future prospects, for all areas of HEP. One big issue is the Higgs: now that its mass is known, what can be learned about it using some new machine (a “Higgs factory”) beyond what can be learned from the LHC? As far as prospects for a new, higher energy machine, the document describes the possibilities, but no decision about such a thing is likely to be made until after results become available from seeing the LHC run at 13 TeV starting in 2015.

Posted in Experimental HEP News | 7 Comments

What *Should* We Be Worried About?

Posted on January 14, 2013 by woit

Back from vacation today, so regular blogging likely to resume. Will start with something quick, a link to material that was posted today.

The Edge web-site annual question feature is out today, with this year’s question What *Should* We Be Worried About?. I wrote something about the “Nightmare Scenario” that HEP is facing if the LHC finds a Standard Model Higgs and nothing else.

Others addressed the same issue, with Lisa Randall writing:

In my specific field of particle physics, everyone is worried. I don’t say that lightly. I’ve been to two conferences within the last week where the future was a major topic of discussion and I’m at another one where it’s on the agenda.

Her specific concern is motivated by her interest in large extra-dimensional theories, for which no evidence has shown up so far at the LHC. If the extra jump by a factor of 6.5/4 in energy that will arrive in 2015 after repairs still shows nothing, this may be the end of the line for such theories for a very long time. The prospects for a higher energy machine are problematic in terms of technology, as well as the political will to pay for them. The overselling of this that went on for many years pre-LHC won’t make it any easier to re-use these theories as an argument for building a new machine.

Amanda Gefter sees no reason to worry. Particle theorists will just move to making progress without experiment, through studying paradoxes of the current theory, with her final example for optimism the recent debate over the “firewall paradox”.

Carlo Rovelli’s contribution explains one problem with this: humans are very good at convincing themselves they have found some wonderful explanation of something (e.g. some resolution of a paradox, like the supposed SUSY solution to the hierarchy problem), when reality actually involves something quite a bit more subtle and unexpected:

A number of my colleagues in theoretical physics have spent their life studying a possible symmetry of nature called “supersymmetry”. Experiments in laboratories like Geneva’s CERN seem now to be pointing more towards the absence than the presence of this symmetry. I have seen lost stares in the eyes of some colleagues: “Could it be?”, how dare Nature not confirm to our imagination?

By the way, when I was in Paris last week I picked up a copy of Rovelli’s wonderful short book that has just come out in France Et si le temps n’existait pas?. It begins with a personal history of how he got into science, from a background in the 1970s disillusion following the flowering of radical ideas in the 1960, a story I found quite interesting, since I’m of the same generation as him. There’s also the story of how some of the ideas of loop quantum gravity developed, and some speculative material about time. Definitely worth looking for if you read French and are interested in these topics.

Posted in Uncategorized | 59 Comments