Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Most Influential Paper first quarter 2007

  1. Fairbairn-Livine

    0 vote(s)
  2. Freidel-Girelli-Livine

  3. Krasnov

  4. Magueijo-Singh

  1. Mar 23, 2007 #1


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    Each quarter last year we put down our predictions about which recent papers will have most impact on future QG research: efforts to reach a fundamental empirical understanding of spacetime geometry and matter. For example, here's the forecast poll from third quarter 2006.
    The idea of this poll is to continue the series.

    Here are the four candidate papers:

    3d Spinfoam Quantum Gravity: Matter as a Phase of the Group Field Theory
    Winston Fairbairn, Etera R. Livine
    17 pages, 1 figure
    "An effective field theory for matter coupled to three-dimensional quantum gravity was recently derived in the context of spinfoam models in hep-th/0512113. In this paper, we show how this relates to group field theories and generalized matrix models. In the first part, we realize that the effective field theory can be recasted as a matrix model where couplings between matrices of different sizes can occur. In a second part, we provide a family of classical solutions to the three-dimensional group field theory. By studying perturbations around these solutions, we generate the dynamics of the effective field theory. We identify a particular case which leads to the action of hep-th/0512113 for a massive field living in a flat non-commutative space-time. The most general solutions lead to field theories with non-linear redefinitions of the momentum which we propose to interpret as living on curved space-times. We conclude by discussing the possible extension to four-dimensional spinfoam models."
    PF discussion of the F-L paper:

    The Relativistic Particle: Dirac observables and Feynman propagator
    Laurent Freidel, Florian Girelli, Etera R. Livine
    14 pages
    "We analyze the algebra of Dirac observables of the relativistic particle in four space-time dimensions. We show that the position observables become non-commutative and the commutation relations lead to a structure very similar to the non-commutative geometry of Deformed Special Relativity (DSR). In this framework, it appears natural to consider the 4d relativistic particle as a five dimensional massless particle. We study its quantization in terms of wave functions on the 5d light cone. We introduce the corresponding five-dimensional action principle and analyze how it reproduces the physics of the 4d relativistic particle. The formalism is naturally subject to divergences and we show that DSR arises as a natural regularization: the 5d light cone is regularized as the de Sitter space. We interpret the fifth coordinate as the particle's proper time while the fifth moment can be understood as the mass. Finally, we show how to formulate the Feynman propagator and the Feynman amplitudes of quantum field theory in this context in terms of Dirac observables. This provides new insights for the construction of observables and scattering amplitudes in DSR."
    PF discussion of the F-G-L paper:

    Non-Metric Gravity I: Field Equations
    Kirill Krasnov
    This continues Krasnov's short introductory paper of November 2006
    Renormalizable Non-Metric Quantum Gravity?
    "We argue that four-dimensional quantum gravity may be essentially renormalizable provided one relaxes the assumption of metricity of the theory. We work with Plebanski formulation of general relativity in which the metric (tetrad), the connection as well as the curvature are all independent variables and the usual relations among these quantities are only on-shell...There is a new coupling constant that controls the non-metric character of the theory...non-metricity becomes important in the infra red. The new IR-relevant term in the action is akin to a curvature dependent cosmological 'constant' and may provide a mechanism for naturally small 'dark energy'. "
    Link to video of Krasnov explaining the idea at Perimeter Institute:

    Here is the abstract of the longer paper, first of a series, that appeared this quarter:
    "We describe and study a certain class of modified gravity theories. Our starting point is Plebanski formulation of gravity in terms of a triple of 2-forms, a connection A and a 'Lagrange multiplier' field Psi...The equations are of second order in derivatives. An analog of the Bianchi identity is still present in the theory, as well as its contracted version tantamount to energy conservation equation. The arising modifications to the later are possibly of experimental significance."
    related PF discussion:
    https://www.physicsforums.com/showthread.php?t=158874 (thread started by Jal)
    https://www.physicsforums.com/showthread.php?t=162078 (Bengtsson response to Krasnov)

    Thermal fluctuations in loop cosmology
    Joao Magueijo, Parampreet Singh
    10 pages
    from the abstract:
    "Quantum gravitational effects in loop quantum cosmology lead to a resolution of the initial singularity and have the potential to solve the horizon problem and generate a quasi scale-invariant spectrum of density fluctuations. We consider loop modifications to the behavior of the inverse scale factor below a critical scale in closed models and assume a purely thermal origin for the fluctuations...
    ...more fully work out this complex aspect of loop cosmology, since the full picture would not only fix the free parameters of the theory, but also provide a model for a non-inflationary, thermal origin for the structures of the Universe."

    from the conclusions section:
    ...Loop quantum cosmology has the potential to relate observational physics and quantum gravity, allowing concrete calculations to be made in the quantum gravity regime as long as a minisuperspace approximation is assumed to be valid. The approach is known to modify the equation of state of ordinary matter, thereby permitting a solution of the horizon problem without resorting to scalar fields. It is then natural to ask whether in such scenarios thermal fluctuations could be behind the observed structure of the Universe..."
    related PF thread:

    UPDATE: In addition to the forecasts that show up on the poll, we have a "write-in" choice of the recent three-paper series by E.E. Ita. See the next post by ensabah and my post #4.
    Last edited: Mar 23, 2007
  2. jcsd
  3. Mar 23, 2007 #2
    I'm surprised you didn't include Eto's paper on Kodama
  4. Mar 23, 2007 #3


    User Avatar

    Already said too much.
  5. Mar 23, 2007 #4


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    OK let's make that a write-in vote for Eyo Ita!!!!!

    As long as you don't vote for any of the 4 others, ensabah, I'll count you as predicting major impact on future research for the recent series of papers by E.E. Ita, and we will include that in the poll. Other people can choose Eyo Ita and their post will be tallied as well.

    But if you don't want this choice, tell me. (UPDATE: no objection :-) )
    here are the three papers:
    Existence of generalized semiclassical Kodama states. I. The Ashtekar--Klein--Gordon model
    32 pages
    "This is the first in a series of papers aimed at outlining an algorithm to explicitly construct a finite quantum theory of gravity in Ashtekar variables. The algorithm is based upon extending some properties of a special state, the Kodama state for pure gravity, to more general models. In this paper we analyse a simple case, gravity coupled to a Klein-Gordon scalar field in the minisuperspace Ansatz, in order to derive a criterion for a new semiclassical state and its corresponding semiclassical orbits of spacetime. We then illustrate a presciption for nonperturbatively constructing the analog of the Kodama state for a general case, in preparation for subsequent works in this series."
    Existence of generalized quantum Kodama states. II. The minisuperspace Ashtekar--Klein--Gordon model
    41 pages
    "This is the second in a series of papers outlining an algorithm to consistently construct a finite quantum theory of gravity in Ashtekar variables. In Part I we constructed a generalized semiclassical Kodama state by solving the classical Hamiltonian constraint under the condition of a broken semiclassical-quantum correspondence due to a Klein-Gordon scalar field. In Part II we will demonstrate a method of restoring this correspondence by generalizing the self-duality condition for the Ashtekar electromagnetic field. The end result will be to establish the existence of a generalized quantum Kodama state devoid of quantum corrections in the minisuperspace model. We also derive the equations needed to solve for the full theory of a finite theory of quantum gravity within the context of this new interpretation."
    Existence of generalized Kodama quantum states. III. A new approach to finite, full quantum gravity
    18 pages
    "This is the third in a series of papers outlining an algorithm to consistently construct a finite quantum theory of gravity in Ashtekar variables. This paper is a first attempt at the quantization of the full theory coupled to matter, in this case to a spatially inhomogeneous Klein-Gordon scalar field. We delineate the conditions required to construct a solution to the quantum Hamiltonian constraint under the Ansatz of an isotropic, but spatially inhomogeneous, Ashtekar connection, and highlight some differences relative to the minisuperspace case."
    PF discussion of E.E. Ita paper (ensabah thread):

    We have 4 forecasts so far:

    one for Freidel-Girelli-Livine paper (me)
    one for Kirill Krasnov non-metric gravity paper (Jal)
    one for the E.E. Ita papers (Ensabah)
    one for Magueijo and Singh's paper (.Ultimate)
    Last edited: Mar 24, 2007
  6. Mar 24, 2007 #5


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    I hope you are just kidding. You didn't say too much already!
    Yr reactions are a big help. Most people aren't curious enough about what is going on---which a lot is.
  7. Mar 24, 2007 #6


    User Avatar

    You make it easier to learn.
  8. Mar 24, 2007 #7
    Hi Marcus,

    Well a common string-theory inspired criticism of LQG is that LQG isn't a theory of gravity -- it doesn't have a large volume limit described by GR, and until you have a BI that does, you do not have a theory of "gravity" and the Kodama state is what purports to show this. Eyo is the first paper I know of that cites Andy Randanomo's results on the generalized Kodama state. Eyo attempts to make contact with work already done on minisuperspace LQC.

    The argument is that LQG does not have a good low energy limit, and Randamo generalizes the Kodama state and Eyo attempts to answer this.

    Last edited: Mar 24, 2007
  9. Mar 25, 2007 #8


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    I'm not sure how critical E.E.Ita's work actually is.
    with so many people working on Spinfoam and on DSR, does anyone need the Kodama state cured of its ailments?
    One person who is nominally doing LQG is Bojowald (he is working outside of LQC with LQG sometimes) and he mentioned last year that LQG has a good classical limit in those cases that interest him that he needs for his work.

    Of course LQC has a good classical limit and that is where a lot of Loop people's current research is.
    But I think I understand your point. You think that the LQG community is mainly focused on standard LQG and therefore they should be very interested in Kodama state. I understand why you would think that.

    But I don't believe they are focused the way you seem to think. So on reflection I dont see such a big impact for E.E. Ita's present work. However I have nothing but admiration for Ita as a young researcher! We are seeing his PhD thesis basically. In future he will probably take on different problems. And I could certainly be wrong and this PhD thesis work of his could turn out to be extremely important (so then you would be proven right!)
    Last edited: Mar 25, 2007
  10. Mar 25, 2007 #9


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    we seem to have gotten in to saying why some of this research might be especially important (or might not be) so I will quote some FGL stuff and discuss it a little tomorrow. Here's F-G-L conclusions. DSR is really really important now so they are explaining how their work fits in:

    We have looked at the relativistic particle from the perspective of its algebra of (Dirac) observables. We have identified a set of Lorentz covariant position observables, which turn out to be non-commutative. This non-commutativity reflects the fact that one can not localize a massive quantum particle with a precision better than its Compton length. We then showed that the particle admits in this context a natural representation in five dimensions and could be quantized in terms of wave functions on the 5d light cone. This allows a direct comparison with the free particle in DSR (e.g. [6]), which evolves in a non-commutative space-time and whose momentum lives in the curved de Sitter space. Moreover, it turns out that the 5d light cone formulation is subject to divergencies, which are naturally regulated by DSR. This allows a clear understanding of how DSR arises as an extension of Special Relativity. In particular, it lead us to an interpretation of the fifth coordinate as proper time and of the fifth moment as generating the Hamiltonian flow of the particle...

    Finally, we also described a new representation of the Feynman diagram evaluations in term of Dirac observables. We now hope to extend this approach to DSR and use these new tools in order to build the scattering amplitudes and S-matrix of a Quantum Field Theory in DSR in a consistent way.

    Here is a kind of paraphrase just to say it a different way. What we always wanted was NOT A SPECIAL relativistic QFT and Std Mdl---we wanted a GENERAL relativistic QFT Std Mdl.
    (that's a way of saying the goal of Background Independent Quantum Geometry-and-Matter or non-string QGM or whatever).
    So for a long time the goal has been a General Relativistic QFT or as Rovelli expressed it in the preface to his book, a "General Relativistic quantum physics."

    But maybe that is actually the wrong goal, because Gen Rel has flat tangent spaces and we could need a DSR form of Gen Rel.

    So a possible scheme or program is
    1. QFT is already special relativistic, that is not good enough because nature is not Minkowski.
    2. So let's rebuild QFT in a DSR version (look at Freidel's recent papers and the conclusions just quoted) then maybe it will be right and we can proceed. (there will be scattering amplitudes to test)
    3. We can also readily construct DSR versions of Gen Rel and Einstein equation. So we attempt a DSR-General Relativistic quantum physics.
    That might work better than trying for an ordinary General Relativistic quantum physics.
    Last edited: Mar 25, 2007
  11. Mar 25, 2007 #10

    String theorist Lubos Motl wrote in reply to Lee Smolin on BH entropy and LQC


    "They don’t even follow from loop quantum gravity. One can’t of course answer any of these questions because LQG probably doesn’t contain smooth space (or, if I am very optimistic, the existence of space can’t be derived at this moment) which also implies that it can’t contain (or one can’t derive) objects such as black holes that require smooth space around."

    Whether one is working with spin foam or LQG, deriving a low energy limit of smooth space is what is required for a theory to be even a theory of gravity.
  12. Mar 26, 2007 #11
    Hi ensabah,

    Forgive me, but let me just remind that very very few people bother with lqg since nearly everyone knows that it`s wrong. In fact this was known from the very beginning. What drives the paucity of research that is carried out in lqg is not the theory, but rather the personalities of the researchers themselves. They simply need to work on ideas that are different from most others, even when they know the difficulties with these ideas probably cannot be overcome for fundamental reasons like the one you gave above.

    This is why the most important papers in QG will never be papers on lqg so that the poll in this thread and others like it are a bit of a joke. In fact courses in string theory at the graduate level are now quite standard. There are even schools now who are planning to introduce the subject at the undergraduate level within the next year or two. The university of toronto teaches them at both levels. Part of this has to do with the many fine textbooks on the subject that are available now, including some that are on the undergraduate level.

    I suggest you forget about lqg, these polls and similar threads and spend your time on learning what physicists are actually working on rather than wasting your time with crap.
  13. Mar 27, 2007 #12
    Thanks for your suggestion.

    I'm entirely open to the possibility that astronomers may discover cosmic strings, or HEP will see SUSY or extra dimensions when LHC comes online, which I accept as experimental validation of key string theory principles.
    So I'll suspend judgment on string theory until several years after LHC collects data and possibly SUSY.

    I infer you believe LHC will see SUSY. How would you feel about string theory if LHC does not find SUSY? (Nor any other observational or experimental evidence for string theory in the decades to come?)
    Last edited: Mar 27, 2007
  14. Mar 27, 2007 #13
    My point is about what you should spend your time on. If you`re dead-set on giving equal time to studying the ooga-booga religion of ten guys in tahiti your opinions in my view should not be taken seriously because if you really want to understand the impact of religion on world affairs, you would begin with the great religions of the east and west. After that you would be in a much better position to assess the situation in the middle east and other parts of the world, including the fringe belief systems that threaten chaos.

    You need to be honest with yourself about whether you know enough physics to choose what you think is important to learn about. If not, then you have to rely on the opinions of the experts, and they`ll tell you to work towards learning string theory. Of course string theory is much more complicated then lqg which in my view is part of the latter`s appeal to certain people at PF. But learning lqg isn`t a cake walk either and you have to ask yourself whether all that work is really worth it if when you take your head out of the sand you find you still don`t really understand why twenty-first century physics is the way it is.

    You`ve probably heard that string theory implies supersymmetry. This isn`t true. Also, although there are strong arguments from field theory that supersymmetry if it exists should be broken at just above the electroweak scale, this also is not necessarily the case. But I`d be disappointed (though perhaps not as much as the people who paid for the collider).
  15. Mar 27, 2007 #14


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    In this prediction poll we have four main papers to evaluate (plus a write-in fifth which has already been discussed a fair amount).
    I'm going to review some of the signal features of those papers, to help in getting perspective on which is likely to be the most influential on future research.

    The Magueijo-Singh is clearly about TESTING. At this stage in his career Magueijo is intensely focused on QG phenomenology---he is not a partisan of this or that. he really wants to find ways to test and maybe SHOOT DOWN various QG ideas. His recent papers include an idea of using LISA pathfinder probe to test MOND within the solar system (!) and an idea for a laboratory experiment using high-power lasers to test something in QG.

    and in this case he is proposing to test an ALTERNATIVE TO INFLATION (!) to explain largescale structure formation. This does not mean he favors inflation or disfavors inflation or is partisan in any way. It means the guy is instensely into the very hard problem of uncovering ways to test stuff that other people don't think is testable.

    Also Parampreet Singh has been working some in LQC phenom over the past 3 years at least. Investigating for possible QG signatures that might be sought in the CMB and GRB data. It is a good sign that Singh and Magueijo collaborate on a paper.
    A noteworthy factor is that Magueijo-Singh ASSUME COSMOLOGICAL BOUNCE as part of the idea to be tested and this removes one of the arguments for inflation

    the stuff before the bounce had plenty of time to thermalize---get into thermal equilibrium. One of the main things that got people started being interested in inflation is that it was the only solution to the "horizon problem" they could think of---how come is the CMB approximately the same temperature in all directions, when two opposite regions couldn't have been in contact? or could they? Inflation is a rather Baroque, far-fetch proposal to respond to that puzzle but for a long time was the only available response. In the meantime it has gotten entrenched.

    LQC is quite compatible with inflation. Indeed Ganashyam Date has written a paper explaining that inflation is "generic" in LQC---a certain amount of it just naturally tends to happen
    Maybe not always 60 e-foldings, maybe not as MUCH as in typical scenarios---but some inflation just generically happens even without assuming an inflaton---according to Date. Other people have also studied inflation in the LQC context.

    But Magueijo is looking to see if structure formation can be attributed to other stuff in LQC besides inflation, and what would it look like, and how to test.

    It is a key paper, not least because it challenges the assumption of inflation.
    Last edited: Mar 27, 2007
  16. Mar 27, 2007 #15


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    On the other hand there is the Fairbairn-Livine paper. I'll try to pick out some signal features of it too.
    Here was some discussion with f-h (who is at the Zakopane QGQG workshop at the moment)
    I already mentioned these papers in preparation, by
    The papers, mentioned in the other thread, are:

    Non-perturbative structures for Spinfoams: Instantons for the Group Field Theory

    Quantisation of string-like sources coupled to BF theory : Physical scalar product and topological invariance

    String theory as a phase of the four dimensional group field theory

    We are seeing work on string-like and brane-like structures in a LOW-DIMENSION and background independent context. these structures do not vibrate in a fixed-geometry target space, as in string-thought. they don't live in Calabi-Yau-land:smile:
    they are structures living in the usual 4D and 5D world of spinfoam, BF theory, GFT (Group Field Theory) that people in the Loop Gravity community normally deal with.
    But they are nevertheless string-LIKE and brane-LIKE, so that is an interesting feature.

    That is where the Fairbairn-Livine paper is tending, and its main thrust is what has become a major theme in LQG research namely DERIVING MATTER AS A FACET OF THE QUANTUM GEOMETRY OF SPACE.

    f-h had some things to say about that "derivation" in one of his posts in the other thread.

    It is interesting that nobody so far picked the F-L paper as likely to have a major impact on future research. We may be showing ourselves up as dull-witted guessers :smile:
    Last edited: Mar 27, 2007
  17. Mar 27, 2007 #16
    Really? How so?
  18. Mar 27, 2007 #17

    well bosonic string theory does not need SUSY, but it only has bosons in 26 dimensions, not fermions.

    I am aware of the landscape issue is such that while it is highly desirable SUSY breaking scale hopefully is just above the electroweak scale, and that the LHC should have both the energies and luminosity to see this, the SUSY breaking scale could be at any energy level above this. So not seeing SUSY at LHC would not rule out SUSY.

    I'm skeptical of SUSY and higher dimesions being a part of our world so until they are actually experimentally verified, I will suspend any judgment on string theory.
  19. Mar 28, 2007 #18
    But even string theory with fermions need not have supersymmetry, broken or unbroken. This is something that is often not recognized by people, but is nonetheless true.


    I`m aware of no special relation between the landscape and the scale of supersymmetry breaking that holds generically.

    It`s not written in stone that all aspects of the correct theory will be able to be experimentally verified. Then you would have to judge the theory in terms of it`s explanatory power. Any skepticism would then have to be based on theoretical reasons. So what`s your skepticism based on?
  20. Mar 28, 2007 #19
    There is to date no experimental evidence for either SUSY or extra dimensions, and there are internal theoretical difficulties with both proposals, (i.e SUSY breaking mechanism, the large cc vacuum energy required) string theory itself did not predict neutrino masses nor dark energy, and the most interesting sector of string theory research, AS-Desitter/CFT correspondence does not match up with our Desitter space, and to get desitter space out of string theory requires a KKLT mechanism, which results in the landscape, resulting in many continously adjustable parameters. I think these are valid reasons to be skeptical of the string theory program. Fermilab Tevatron has not seen either the Higgs or SUSY, which is a bit worrisome. SUSY itself does not relate known bosons with known fermions, but insteads double the number of particles, which again is not as elegant as I would have liked to see. String theory itself has not reproduced the SM, nor offered explanations or deeper insights of the 19 free parameters (nor neutrino physics), and of course MSSM has added nearly another 100 free parameters.

    Despite these difficulties, if LHC does see SUSY, or tests for violation of Newton's gravity at submillimeter scale or violations of the strong equivalence principle are forthcoming, or astronomers sees cosmic strings, I'll take that as a good sign string theory is on the right track.

    Again, while MSSM increases the number of free parameters of SM, if it is experimentally discovered so be it. We just have to accept the additional free parameters. But until it is discovered, I think a healthy dose of skepticism is warranted. In most fields of science, skepticism that demands experimental evidence is the norm (or explaining more completely previous unexplained facts).

    My current skepticism is such that I think we live in a 4D universe that is not SUSY since there is no empirical evidence otherwise.
    Last edited: Mar 28, 2007
  21. Mar 28, 2007 #20
    The reason that you will only accept experiment as indicating that string theory "is on the right track" is that you don`t understand (or perhaps even want to understand) string theory.

    Nobody knows how to break supersymmetry, either in string theory or in any other theory, and the problem goes far beyond the positive cosmological constant that appears when supersymmetry is broken. In any event there is nothing known about string theory that excludes the possibility of a mechanism of supersymmetry breaking that works.

    These questions will have to wait until we do learn how to break supersymmetry, but again, there`s nothing known about string theory that says it can`t succeed on both of these issues.

    Firstly, it`s known as the AdS/CFT correspondence. Secondly, just because the observable universe has a positive cosmological constant doesn`t mean the entire universe is de Sitter. Thus we don`t say the universe is De Sitter space but rather that it (at least the observable part) is in a De Sitter phase, though the universe may indeed be De Sitter.

    The KKLT mechanism stabilizes the moduli of string theory, and it does so without breaking supersymmetry since the cosmological constant so produced - this being a sum of the potential minima of the moduli- is negative. This is achieved through flux compactifications which are obtained by including nonperturbative degrees of freedom. The different ways this can be done then gives the landscape after...

    De Sitter solutions are obtained by inserting D-branes in a way that supersymmetry is broken. Unlike in dynamical supersymmetry breaking, this allows the model builder to control how supersymmetry is broken.

    The very first thing one learns about moduli is that although they do "parameterize" the different possible background geometries in string theory, they are not adjustable parameters but rather scalar fields whose physics must be governed by the equations of the theory just like any fields is. Thus we do not choose the values of the moduli, but rather stabilize them to sit at the bottom of their respective potentials.

    The statistics of particles express the way particles behave under Spacetime-transformations, e.g. Poincare transformations. In order to relate particles with bosonic statistics to ones whose statistics are fermionic one must find a new set of generators that can be added to those generating Poincare-transformations, these latter being tensorial. Since the statistics change when going from boson to fermion, the new set of generators cannot be tensorial but rather spinorial. When added to the generators of the Poincare group, we get the supersymmetry algebra. There simply is no other sensible way to relate bosons and fermions.

    Your wrong, but to understand why you`re going to have to learn enough about string theory to understand why 99% of the researchers in high energy theory disagree with you.

    In fact, even on general grounds your position is ridiculous since it is likely that theory is going to precede experimental confirmation in this field of physics.
    Last edited by a moderator: Mar 28, 2007
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook