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

Kreimer-Connes & Wilson-Polchinski

  1. Apr 11, 2010 #1

    atyy

    User Avatar
    Science Advisor

    How are the Kreimer-Connes and Wilson-Polchinski views of renormalization related? Especially with regards to the idea that QED has an infrared fixed point and is only an effective theory, versus QCD which has a UV fixed point is well-defined to arbitrarily high energies?
     
  2. jcsd
  3. Apr 11, 2010 #2
    Since I'm not a mathematician, the Kreimer-Connes work is hard for me to follow; below is only a statement of my understanding, not necessary the truth.

    Disclaimer aside, the work of Kreimer-Connes has very little relevance for your average working physicist, at least at the moment. What they have done is to put the usual methods often used in HEP (i.e. tracking Feynmann diagrams and coming up with suitable counterterms to cancel infinities) on a rigorous mathematical basis. Prior to that, it was not clear whether the procedures being used actually constituted a self-consistent set of rules. They show that it is, and in fact it is a sort of beefed up construction of a type that mathematicians have already known for a while. However, that "beefed up" part still means that they don't have any new ways (unknown to the physics community) for actually doing calculations. No doubt someday someone will exploit this fact and do something amazing, but right now that's not happening. In fact, I'd laid bets going the other way --- some heuristic, ad hoc procedure dreamt up by a physicist will get turned into a valuable theorem on the pure maths side first.

    As such, the relation to the Wilson-Polchinski view is as usual for the HEP vs condensed matter points of view. At the end of the day, we have the fact that we need to measure some parameters. These are the "physical" ones. Our theory, on the other hand, deals with "bare" quantities, and it is up to us where we choose our cut-off. Changing the cut-off changes the precise relationship between bare and physical parameters. For some calculations it's easier to put the cut-offs very low, so that bare and physical parameters are almost the same. For others, it's better to put it high, and apply some combinatorial cleverness to cope with the diagrams.

    The usual feeling is that the condensed matter view is clearer on what the various relationships are, but that the HEP methods are strictly more powerful. Specifically, it's hard to do momentum-shell renormalisation if you need Lorentz invariance. Gauge covariance only makes things worse. Whereas dimensional regularisation has the wonderful benefit of being an entirely orthogonal choice to any physical symmetry.
     
  4. Apr 11, 2010 #3

    atyy

    User Avatar
    Science Advisor

    Wow, you must be a condensed matter theorist! :smile: But really, where do you get to say the condensed matter view is clearer when Wilson was a HEP guy who "saved" the condensed matter field :tongue2: (ok, ok, Kadanoff should get a lot of credit too :smile:)
     
  5. Apr 11, 2010 #4
  6. Apr 12, 2010 #5

    atyy

    User Avatar
    Science Advisor

    Thanks! Yeah, it would not have been apparent that you and Genneth agree - good to know - I understand Genneth's viewpoint - the Connes-Kreimer stuff is Sumerian to me. I guess I have no choice but to try to read it carefully.
     
  7. Apr 12, 2010 #6
    :wink: However, it's rather telling that HEP no longer uses the Wilsonian methods of momentum shell renormalisation --- it's hard to find a suitable shell if you have a Lorentzian metric. We squidgy stuff people are always working with a Wick rotated system, so we can. I don't consider the combinatorial counting of counterterms at each order to be particularly physical --- it's a very neat piece of mathematics which implements at the end the same thing, but is more abstracted from the system. Clearly I'm just not clever enough to cope with the extra step...

    Btw, the idea of renormalisation in condensed matter is a little broader than this strict parameters-moving-with-cut-off. It's biggest use to us is to justify, however weakly, huge leaps of logic through its consequence of universality. Specifically, when we dream up some effective theory, very rarely do we actually proceed from a microscopic theory and then integrate out degrees of freedom we don't care about. Usually, we jump straight to some theory, with some unfixed parameters, which do not necessarily have any simple relationship to the microscopic ones, i.e. mass of Landau quasi-particles in Fermi liquid theory vs the bare mass. Our view is that we *will* throw away information, but hopefully in a maximally helpful manner, whilst retaining the physical phenomenon we are currently interested in, and that this is an unavoidable part of dealing with complexity.
     
  8. Apr 12, 2010 #7

    atyy

    User Avatar
    Science Advisor

    Yes, I wonder whether there is any conceptual clarification beyond Wilson, or if it mainly organizes the calculation more efficiently.

    A paper suggesting the former is:
    http://arxiv.org/abs/gr-qc/0203036
    Coarse graining in spin foam models
    Fotini Markopoulou

    Looking through the cites, this seems interesting:
    http://arxiv.org/abs/0806.4309
    Wilsonian renormalization, differential equations and Hopf algebras
    Thomas Krajewski, Pierre Martinetti
     
  9. Apr 12, 2010 #8

    George Jones

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    I don't of a relationship between QED and QCD fixed points, but there is a relationship between Kreimer-Connes and Wilson-Poilchinski. From Quantum Field Theory A Bridge Between Mathematicians and Physicists: I Basics in Mathematics and Physics "The renormalization groups used by physicists in specific situations are representations of one-dimensional representations of the motivic Galois groups."

    See pages 859-850 section 15.4.6 The Importance of Hopf Algebras,

    http://books.google.com/books?id=XY...r&dq=zeidler+quantum&cd=1#v=onepage&q&f=false
     
  10. Apr 13, 2010 #9
    Am I right in thinking that Kreimer-Connes renormalisation is a purely perturbabtive approach or can it be used for non-perturbabtive physics like in infra-red QCD?
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook