2006 was another year with a couple of very interesting directions in our field. The level of excitement in the theoretical physics community couldn't match 1905, 1984, or 1996, but many people would surely say that we have seen worse years, not only 1942 or 1991. Let me mention what happened in a couple of the active research directions in 2006.
Adjacent fields: Poincaré's conjecture
The Science magazine has chosen the proof of Poincaré's conjecture to be the scientific event of 2006. The main hero behind the proof, Grigori Perelman, declined the Fields medal. The work of the Chinese scholars was essential in transforming the status of Perelman's proof from a promising work to an almost certainly correct proof.
You might ask: why is this proof counted as a result in science? ;-) The best answer is, as in many similar situations, string theory. In this article of the American Institute of Physics, David Morrison explains the research of the Ricci flows within the context of string theory that links names such as Dan Friedan and Richard Hamilton.
The proof of Poincaré's conjecture won't bring us much closer to the theory of everything but it is a cute result. Also, some great string theorists including Edward Witten, Anton Kapustin, and Sergei Gukov have focused on the Langlands program as real mathematicians. They have undoubtedly done a lot of amazing work although its importance may sometimes be insufficiently comprehensible to those who think as physicists.
Adjacent fields: WMAP & Hubble
The most heavily cited papers of the year are associated with the three-year data of WMAP. One of them approaches 1,000 citations which is not bad after less than 1 year. We have mentioned many aspects of this work, including the bizarre critics of the CMB who have not yet been given too much room in the media even though they are arguably more scientifically achieved and more technically focused than the weird critics of string theory who will be discussed at the end of this text.
Already at the beginning of the year 2006, we learned from a Hubble telescope picture that CSL-1 was not a cosmic string. It was never terribly likely that such a thing would turn out to be a cosmic string - Joe Polchinski estimated the probability that such an object is going to be observed in similar fashion to be 10% - but it would have been more exciting for most of us if CSL-1 were a cosmic string: let's admit it.
Some of the other experimental results announced in 2006 are still waiting to be settled. The PVLAS collaboration has observed a rotation of light in the magnetic field that shouldn't happen according to Maxwell's equations and that would point to axions. Although I think that it is unlikely that this experiment is correct, it still seems much more likely to be correct and correctly interpreted than the recent "axion" observations of Singh and Jain.
RHIC and holography
There have been two most alive realms of activity associated with the AdS/CFT correspondence in 2006: the quark-gluon plasma and integrability. We have introduced the reader to heavy ion physics and the AdS/QCD correspondence in a special long article. Here we just mention that the AdS/CFT methods of string theory have become one of the most natural techniques - if not the most natural technique - how to deal with the complex strongly coupled phenomena. In some cases, they lead to more accurate predictions of the collisions of heavy ions than other approaches to calculate. In other cases, gaps, and uncertainties survive because the exact stringy equivalent description of QCD is not known. And I find it plausible that it will never be exactly known. If that is the case, AdS/QCD will remain a framework to calculate some quantities rather accurately but every time we want to increase the accuracy, the corresponding model has to be refined.
Jacques Distler thinks that one of the most intruiging papers of 2006 was a paper relating the Choptuik scaling and Regge saturation. I tend to agree.
Another fascinating and heavily technical field relevant for the AdS/CFT correspondence are the steps to fully solve the N=4 supersymmetric gauge theory in four dimensions. In the 't Hooft planar limit, there should exist a worldsheet description of the physics of this "maximally supersymmetric" gauge theory. What do we mean by integrability in this case? The goal is really to focus on the excitations that can live on this 2D worldsheet that approximates the Feynman diagrams in the planar limit - neglecting the "stringy" interactions that change the topology of this worldsheet - and write down the exact energies and scattering amplitudes of the excitations of this worldsheet theory.
Although this theory is derived from an interacting non-Abelian theory with a lot of matter, the task from the previous sentence has been essentially solved by people like Niklas Beisert. The excitations are called magnons and their scattering amplitudes are now known exactly. This technical work is impressive. But the Reference Frame is still more amazed by the new ideas that arise in this field that are relevant for our understanding of geometry, fundamental physics, and even some unexpected insights of numerology. We have discussed these topics e.g. in the texts about giant magnons, Juan Maldacena's talk, and transcendentality that makes the counting of powers of pi and a closer inspection of the arguments of the Riemann zeta function physically relevant.
Several very powerful papers about metastable vacua and supersymmetry breaking have been written by people like Nathan Seiberg, David Shih, but also Cumrun Vafa et al. Some of them could have been written 20 years but the technology is much better than it was 20 years ago, even in these conceptually old-fashioned topics.
What has been happening with the vacuum selection problem? First of all, I think that most people will agree that the anthropic excitement has diminished considerably and the anthropic wave could be viewed as a fashionable wave - something that normally takes a year or so and it was also the case of the anthropic lack of principles. There have been quite a few rather confusing papers about the ways how the anthropic probability measure should be or shouldn't be calculated. Neither of them is really convincing and they don't agree with each other.
There are very smart people who believe the anthropic principle and I think it is a legitimate attitude that can be justified by some new insights. I personally encourage them to think about their networks of ideas. On the other hand, most of us are making it clear that almost no one wants theoretical physics to write quasi-religious papers with some mathematics that has nothing to do with the actual arguments or measurable quantities and whose goal is to encourage some philosophical viewpoints by permanent repetition.
That's why some of the papers that would bring no conclusive results were stopped. It is conceivable that the anthropic principle is right to one extent or another. But it is unlikely that someone will convince the physics community that this hypothesis is correct unless he will offer some non-trivial results that provide us with a strong consistency check or that are even directly related to observable quantities.
Swampland and general predictions of quantum gravity
The ideology that all important details about particle physics will have to remain unpredictable has been partially superseded by the observation that we already know that this "very strong" anthropic principle is wrong. Quantum gravity or string theory - choose any name you want, it is finally the same thing - can already be used to make some predictions that are independent of the particular vacuum where we choose to live - predictions that couldn't be derived from a low-energy effective field theory. The term "swampland" became a popular description of these general predictions of string theory and many of them - usually expressed as inequalities for masses, charges, coefficients in the effective action, volumes of moduli spaces, and similar quantities - have been described pretty quantitatively. Some of us including your humble correspondent worked in this direction. For example, the statement "gravity must be the weakest force" has been kind of quantified and some of its implications for the corrected black hole masses have been checked. Although we can't yet find the exact single vacuum at this moment, each such an insight increases our understanding of the theory, removes some fog, and makes the assumption of some colleagues that everything is just mess less correct.
Regardless of the philosophical implications of the large number of certain kinds of compactifications, much progress has been seen in the actual construction of these vacua and ever more accurate analyses of their properties. See Mariana Graña's review for more details what has been happening in this subfield before September 2005 and click at "cited by" to see about 100 papers that have extended our knowledge about it afterwards.
Although most of these papers deal with six-dimensional geometries and various complicated formulae for the superpotentials, there exist a few classes of very different papers about the stringy vacua. Many leaders of the field recently re-visited the non-Kähler vacua with an H-flux. Other people looked at non-geometric vacua. Some activity was dedicated to the singularity that would describe the MSSM or the Standard Model in the holographic fashion, while identifying the decoupling of the 4D gravity with the non-compactness of the geometry.
Other groups have been looking into rather old-fashioned heterotic Standard Models that became better, too - even though your humble correspondent loved them even before they got better. ;-)
High-precision black hole statistical physics
A significant incremental progress has occurred in the calculations of the entropy of new classes of black holes in string theory, including non-supersymmetric ones, and the exact calculation of the supersymmetric black hole entropy. The Ooguri-Strominger-Vafa conjecture was found to be more general than assumed at the beginning. The conjecture has been kind of proven. New geometric explanations of the appearance of certain fancy functions - such as modular functions for genus two surface - have been revealed by Davide Gaiotto, among others. Many of those who are spiritually close to Harvard have studied split attractor flows.
2006 was the year in which the statement that the black holes preserve the information became uncontroversial and there currently exist no serious reasons to think that this principle of quantum mechanics is violated in the presence of the event horizons.
There have been many undercited papers written in 2006 that deserve much more attention than what they have received, according to this blog. Puffed field theory is one of many examples.
There are surely many topics that are as interesting as the categories above if not more and if there is some urgency, the list will have to be updated. But the time of the reader is limited, so let us return to more general topics.
Final preparations for the LHC
The LHC seems to be doing fine and it will probably start trials in March or so. The world's largest superconducting magnet is on its place, too. The experimenters seem ready and so are the phenomenologists who became almost as skillful in analyzing the raw accelerator data as the experimenters (or more), after the successful LHC olympics. More than 99% of professional high-energy theorists think that the LHC will see the Higgs boson. I would say that about 40% of the people think that the most likely outcome is that the LHC will only see the Higgs. The remaining 60% are divided to various kinds of new physics and supersymmetry remains the clear front-runner, at least in the U.S., despite the fact that some people seem discouraged and hedging their bets because SUSY has not yet been seen.
New textbooks were completed in 2006. Becker-Becker-Schwarz will probably become a serious competitor for Polchinski's book while Michael Dine's phenomenologically flavored textbook of supersymmetry and string theory seems to be unique and unprecedented, at least in the literature of the last decade or so. Kiritsis is going to release his own textbook of string theory in April or so.
Great P.R. year for the anti-physics jihad
While the previous year has been an average year for physics itself, it has been a great year for the enemies of science and for the crackpots. Although I have been seeing these things for many years and it is time to learn from the experience :-), I am always surprised and overwhelmed how incredibly silly assertions the public - and indeed, sometimes it is even the scientific public - is ready to buy. Instead of the difficult technical arguments relevant for particular scientific questions whose list was sketched at the beginning of the article, these people prefer easy solutions based on unusual and emotionally loaded sociological theories, much like extreme political parties.
In the past, the newspapers and the Internet were tools to speed up science and make it more efficient. Today, it is sometimes the other way around. Take, for example, one of the infamous "critics". A bright high school student would need at most 3 minutes to get all the information he needs: the "critic" just doesn't like string theory. He also has some personal reasons to dislike it. But he has apparently learned the grammar well enough to be able to write a diatribe against physics - or one diatribe after another, depending on the demands of the market.
He is also able to extract bad words from any report and from some papers and add some additional sour stuff into it. But there won't be any interesting physics in it because the person doesn't really follow the modern physics so there is no reason to read more than one such diatribe. In fact, the "critic" says it himself: he doesn't want to decide whether the theory is right or wrong. He wants to write bitter things about it without any need of looking whether the theory is right or wrong. Why would anyone keep on reading this stuff more than once?
Today, many people need more than 3 minutes to see that there is nothing about the writings of this person. They need more than 1 month. More than one year. First year, second year, third year - the garbage is still thriving and many people are still not getting it. And it is not just about this kind of critics. There are also the crackpots who are recycling their theories that have been falsified 20 years ago within 7 minutes after they first published them if not earlier: this abandoning of the wrong theories is a key part of the scientific method. But they can circumvent this method, returning the waste back to the digestion system with the help of new and new silly journalists who still can't get the right answer.
When a crackpot proposes that the value of theories in physics should be judged by their agreement with some philosophical dogmas from the 17th century - that were a part of bad physics already in the 17th century but they are now proposed to control the 21st century (!) physics - and the only argument is that these misconceptions sound "nice" to the crackpot, no real physicist takes such a crackpot seriously. Physics hasn't worked like that at least for 300 years. Philosophical prejudices no longer dictate what is true and what is not true about natural sciences.
The closest thing to these prejudices we have is mathematical elegance that has been a pretty good guide for the theorists throughout the 20th century. But our "taste" required to judge this elegance has to be constantly refined, otherwise it would become misleading and obsolete: the theoretical physicists' sense of beauty is really just a different form of rational reasoning when it works well. The crackpots don't refine it because they are stuck in the 17th century. And 17th century philosophy is simply not a good method to decide about the 21st century physics.
Nevertheless, whenever such a crackpot makes a similar argument that is unsubstantiated by anything rational whatsoever, the listeners are split. And this includes not only random janitors. I am always amazed how incredibly dumb most people are. Is it really so difficult to figure out that the crackpot has learned how to construct popular sentences to earn some easy bucks but he has nothing that is both true as well as relevant to say about science, otherwise his insight(s) would already be noticed and extended by at least one of the tens of millions of readers of the conspiracy theories in the newspapers?
Shouldn't the public already learn that if someone dismisses all technical analyses and only builds on conspiracy theories, ad hominem attacks, carefully doctored compassion of the public, and 300-year-old philosophical dogmas, one should at least assume that the person is probably a crackpot? Is that really so incredibly difficult to see it?
And it's not just one crackpot. It is a whole industry of aggressive crackpots. All of them think that they don't have to study a single particular scientific question at the technical level but they were nevertheless given a paranormal ability to decide what answers are right, what answers are wrong, and what answers are not even wrong. Their "arguments" are as much valuable as the arguments of drunk teenagers who support a soccer team and who physically attack the managers of other teams but they will always find some audience that gets multiplied because some segments of the media are optimized to reproduce the garbage of the worst kinds.
These crackpots produce an astonishing amount of nonsense and hatred and an overwhelming amount of publicity because the newspapers are literally crowded by thousands of shoddy journalists who are clearly not up to their job. In 2006, these crackpots who are responsible for less than 0.01% of the insights in high-energy theoretical physics were given more room in the media than the actual science on which they're parasiting itself. Let's hope that 2007 will be the first year in which this garbage of the scientific discourse will start to be recognized as the garbage of the scientific discourse because otherwise we're heading towards some real trouble not only with physics.
And that's the memo.