This is a followup of our text about the year 2006 in theoretical physics. Many things have happened in science and the review below simply can't be complete; we can't even recall all topics that have been discussed on this blog during the year. But I will try to make it somewhat representative anyway.
By citation count, the most successful 2007 paper both in theory and phenomenology was Howard Georgi's Unparticle Physics, something that also produces unusual interference patterns. What is unparticle physics? We are used to describe high-energy phenomena in terms of particles that are weakly coupled. Even quarks are weakly coupled at short distances, according to QCD. But in theoretical physics, we also know many inherently strongly coupled theories.
More concretely, free theories that describe these ordinary free particles contain operators with simple integer and half-integer dimensions: the dimensions can be calculated classically. If we add weak interactions, the dimensions change by small amounts: by the anomalous dimensions. But the total dimensions are still close to integers or half-integers. However, strongly coupled theories may have operators whose dimensions are completely fractional. We know these things in conformal, scale-invariant theories.
Georgi proposed a new phenomenological way to look at such a situation. He explained that if you look at the LHC collisions, new physics described by a strongly coupled conformal field theory will look like a production of a fractional number of particles: the number is the dimension itself. Why? Because the cross sections involving a fractional-dimension operator scale like a fractional power of the momentum. The phase space of K particles also scales like the 2K-th or 3K-th (relativistically vs not) power of the momentum or energy. And you might misinterpret where the power of energy actually comes from.
It's my understanding that they are trying to be deliberately phenomenological i.e. vague about any details of the theory that actually describes such a situation. On the other hand, "conformal windows" and other conformal theories relevant for phenomenology have been discussed many times in the past. Some of them have nice AdS duals - something that unparticle physicists are not too interested in. So if I summarize, I really don't understand what is so new and fascinating about the new Georgi framework.
Nevertheless, 100+ physicists followed it. For example, the observations of the D0-D0bar mixing by the BABAR collaboration, which is itself technically famous paper from 2007, has been linked to unparticle physics, too.
String phenomenology and landscape
Let me move to string phenomenology. There have been some relatively interesting developments in the analysis of the stringy vacua. Cosmology in several classes of the vacua was studied. Among many results, it was argued that inflating vacua might be highly non-generic. Because of wrong cosmological properties, the whole class of type IIA vacua was claimed to be excluded.
Some people have sketched a strategy to decode the shape of the hidden dimensions from details of the WMAP data. I would say that these attempts remain highly preliminary, to say the least.
Some physicists finally started to study the possibility that our previous understanding of the stability of a large number of vacua in the landscape could be highly inaccurate. The paper about resonance tunneling was arguably the most interesting representative of this line of reasoning.
The previous three paragraphs are three different particular realizations of a prophecy of David Gross, your humble correspondent, and a few others who have said that a more detailed analysis of consistency of cosmology might significantly reduce the landscape of realistic backgrounds that is only so large because these "advanced" features were being largely ignored and only the maximally symmetric flat or (anti) de Sitter solutions were considered. Quite obviously, a lot of work remains to be done. There are many pretty well-defined questions that are waiting to be answered here.
The landscape and even statistical anthropic reasoning has been applied to other classes than the traditional type IIB flux vacua, for example heterotic backgrounds. Some results of the heterotic statistics is very different from the type IIB case but some of the differences are due to personal differences of the authors' approaches.
Conventional, non-anthropic stringy phenomenology has made some progress, too. People started to look for fertile patches of the heterotic landscape and some special vacua, see also the paper about the tridophilia that updates some discussion about the realistic heterotic model building that was intensely covered back in 2006.
Besides the heterotic vacua, interesting things have been found in the context of F-theory - see e.g. a paper about three families and Higgsing of F-theoretical vacua - and the intersecting braneworlds - see e.g. some developments in the calculation of particle masses in these models. Again, these are places where manhours could be invested rather meaningfully.
Among the competing backgrounds, there is no clear winner. We still can't say for certain whether the heterotic vacua are clearly superior over others. We can't say the opposite statements either.
The discussion about the proper way of counting the probabilities of different vacua has been kind of merged with the research of the Hartle-Hawking wave function of the Universe. Hartle, Hawking, and Hertog have updated some volume factors in order to argue that the Hartle-Hawking wave function could predict a long enough inflation, after all. On the other hand, Leonard Susskind presented the census taker's hat, a meta-observer strategy to count events in an eternally inflating multiverse.
Some authors have argued that even ordinary field theories may have a vast landscape, showing that the large number of possibilities is not as characteristic a feature of string theory as some people argue.
If we look at some unusual realizations of the Standard Model and physics beyond it, Alain Connes continued to promote his non-commutative "derivation" of the Standard Model. He claimed that the Standard Model is a unique solution to a list of conditions in a non-commutative setup. However, I argued that the conditions are so contrived that it is more insightful and honest to write the SM Lagrangian itself. Amusingly enough, Gia Dvali has thought about theories with 100 quintillion new particle species.
Needless to say, neither string theorists nor phenomenologists know the right particle spectrum so far or a method to calculate the parameters of the Standard Model.
AdS/CFT and its global impact
The AdS/CFT correspondence continued to be a very active area of research. The most famous example of this duality involves the N=4 supersymmetric gauge theory. Its planar limit has been essentially solved.
The AdS/CFT correspondence has found its applications in unexpected scientific disciplines including condensed matter physics, further emphasizing that Nature likes to recycle Her great ideas at many scales and many places. While the laws of condensed matter physics are emergent and not fundamental, you might also adopt a viewpoint of condensed matter physicists and consider them as fundamental. If you study them in detail, an increasing toolkit of results found by string theorists is gonna become relevant for you. The insights searched for and found by string theorists are important beyond physics at the Planck scale: many of them are key ideas that every physicist should know.
The holographic description remained a relevant and rather intensely studied approach to heavy ion physics and the Sakai-Sugimoto model arguably remained the most promising realization of QCD within string theory.
Progress was made in the construction of a proof of the AdS/CFT correspondence analogous to the case of topological string theory.
Black hole microengineering
Increasingly complex objects with event horizons were constructed and investigated, including bizarre things such as the black Saturn, a black hole surrounded by a stable black ring. Samir Mathur, Iosif Bena, and their friends continued to describe the internal degrees of black holes in terms of complicated fuzzballs without horizons.
Physicists continued to refine the OSV conjecture and some of them such as Gaiotto and Sen were computing the genus two partition functions.
Meanwhile, the ability of some people in the topological string industry to calculate went out of control. For example, the Gopakumar-Vafa invariants may be rather easily computed to 50+ orders in the topological string coupling.
People were getting used to the fact that the information loss paradox has been essentially solved and the answer is that the information is not lost. However, the detailed dynamics that preserves the information is still not understood fully satisfactorily. Hayden and Preskill offered a rather transparent text showing how relatively easy it is to retrieve the information from a black hole, at least in principle.
For a popular modern video about black holes, see e.g. a talk by Leonard Susskind.
Finiteness of supergravity
A significant amount of work was dedicated to the possible perturbative finiteness of the maximally supersymmetric supergravity in four dimensions. I find the evidence for the perturbative finiteness to be rather strong. I would bet that the N=8 SUGRA is more likely than not to be perturbatively finite. At the same moment, it became very clear that such a result can't extend non-perturbatively. The supergravity sector can't decouple from the rest of physics of M-theory.
Conceptual progress in geometry of physics
String theory includes its own modern generalization of geometry in the same way as general relativity incorporates its own, Riemannian generalization of the Euclidean geometry. We know some of its properties but clearly not all of them. In an interesting 2007 paper, it was shown that supercritical string theories are equivalent - T-dual, in fact - to theories on negatively curved manifolds such as higher-genus Riemann surfaces.
Certain brave minds continued to connect supercritical string theories with each other and with more conventional vacua. An old question has been whether the "less orthodox" vacua of string theory such as supercritical string backgrounds belong to "the" theory that we actually love. It became clear that they are usually not connected on the moduli space - you can't interpolate between static backgrounds in distinct kinds of theories - but once you allow sufficiently violent cosmology to be a part of the things that you study, non-critical theories and similar stuff inevitably become a part of the picture.
Another group of brave minds studied the relevance of E11 and related groups generalizing the U-duality group of M-theory on tori for the full formulation of M-theory. There exists a certain body of extended numerological evidence that something non-trivial is going on. But a complete picture doesn't seem to exist so far.
Monster group and pure AdS3 gravity
Recently, Edward Witten has dedicated a lot of his powerful brainhours to the Langlands program, surface operators in gauge theories, and related mathematical topics. Although I don't follow all details, I think that this work is perfectionist as Witten's work usually is. But I don't think that the work has any far-reaching consequences for theoretical physics.
On the other hand, I am kind of excited about his CFT dual of pure gravity in AdS3 that hypothetically involves an exotic - and extremal - conformal field theory with the monster group symmetry! We have covered this topic, one that has led to some previously unexpected observations, at many places:
Two well-known experiments that are 1 level smaller than the LHC have led to tantalizing but unconvincing evidence of new physics. AUGER has observed some extremely high-energy cosmic rays that have a certain potential to contradict existing explanations based on well-known particles.
The MAGIC experiment has measured something that can be interpreted as a relativity-violating energy-dependence of the speed of gamma rays. But more conventional explanations exist, too. The observations from this team have been misinterpreted, hyped, anti-hyped, and politicized.
LIGO has seen no gravity waves, at least so far. Axion rumors from PVLAS have evaporated. The resolution of Gravity Probe B turned out to be twice worse than needed to see frame-dragging. Miniboone refuted the simplest model with a sterile neutrino, suggested by some previous observations.
Cosmology: Attempts to explain the cosmological constant away have been made in literature. Weird people have suggested that the research of cosmology helps to kill the Universe. The imagination of many people was waken up by the huge hole in the WMAP cold spot. Guth and Turok exchanged their opinions and inflation remained by far the more convincing explanation of cosmology beyond the Big Bang theory. The king of supernovae was observed. Add various habitable planets and planets with water. Both theoretical and experimental progress in cosmoclimatology, the influence of cosmic rays and solar activity on the terrestrial climate, took place. But this article is not about global warming.
Everyone is waiting for the LHC - a largely completed machine with triplet assemblies fixed, a silicon tracking detector installed, and cooling system completed - that should be activated in May of this year. Michael Dine wrote a decent popular text about string theory and phenomenology in the era when everyone waits for the LHC to be turned on.
Supersymmetry & Higgs: false alarms & limits
Because the relevant life of the Tevatron was approaching its final moments, the intensity of speculations about possible discoveries logically intensified during the last year. A question was whether the Tevatron had a chance to discover the Higgs boson (the God particle) or supersymmetry; see a popular introduction to MSSM. A 2.1 sigma signal of an MSSM Higgs was claimed to be found at the Fermilab: the mass of the particle was 160 GeV. The signal went away but you may still want to read how a careful treatment of a possible excitement from the pen of John Conway (FNAL) looks like.
Instead of these big discoveries, the analyses of the LEP and Fermilab data led to the standard, small, incremental progress such as new limits. LEP and Tevatron seem to imply that the Higgs should be pretty light. New limits on fermionic superpartners were set. Cascade b, a not exceedingly important particle, was discovered.
The LHC will be more poweful - it can even see black holes - and a similar text already next year could tell you many more interesting things than today.
An experiment linked to Taiwanese physicists was claimed to have a new particle at 214.3 MeV [sic], identified as an NMSSM Higgs boson, but I am deeply unconvinced by these press releases and articles.
Concerning the supersymmetric model building, we talked about the mirage mediation as a representative of the 2006 developments. I choose the sweet spot supersymmetry as an example of the things studied in 2007.
Several new textbooks were published in 2007. Two of them were written by Becker, Becker, Schwarz and by Dine, respectively. Another one was written by Kiritsis. In a few days, we should see a new book by Veneziano and Gasperini about somewhat different topics.
Controversies and public relations
Some hateful primitive books against the very basic principles and methods of theoretical physics that were published in 2006 have already made their damage - and it was not undone - but no additional damage was created in most of the year of 2007. After their fifteen minutes of fame, these non-physicists and pseudo-physicists returned to their conventional status of irrelevance.
Nevertheless, at the beginning of the year, we still saw some debates of serious physicists with the less serious ones, for example Duff vs Smolin and Polchinski vs Smolin. In the second part of the year, the focus of the media partly returned to the substance instead of the hostile conspiracy theories.
Physics World is the magazine that is closest to the actual research which is why it was exactly this source that published an extensive and pretty good up-to-date story about the state of string theory. Meanwhile, it was said that an elegant physicist made string theory sexy.
There have been many other controversies. For example, some people tried to force Steven Weinberg to admit that we couldn't live without the holy torsion, something that obviously doesn't exist and doesn't have to exist in the real world. Steven Weinberg hasn't surrendered. ;-)
The Discover Magazine organized its string theory in two minutes video contest.
Two physicists from Pilsen and France shared the Nobel prize for physics of gigabyte hard disks.
Joel Shapiro, Pierre Ramond, John Schwarz, and others have written their memoirs about the early days of string theory. David Gross delivered some cool popular lectures in Israel and Berkeley and he summarized the Strings 2007 conference so that I don't have to do it again. Hermann Nicolai wrote his own popular article and some videos from TASI 2007 are available online.
To summarize, 2007 was another average year in physics with a lot of incremental progress. In 2008, the LHC might become the main reason to revolutionize such conclusions.