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New crack in QED?

  1. Jul 8, 2010 #1

    Apparently the proton is about 4% smaller than thought, which is somewhat challenging for QED. It could be new effects or experimental error of some sort. Any such guessing is speculative at this time, but what would you think?
  2. jcsd
  3. Jul 8, 2010 #2


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    (New crack? I didn't know there was an old one ;) )

    QED has successfully calculated the Lamb shifts for hydrogen and helium to a high degree of accuracy, so it would appear QED 'works'. And has for some time - it's not very new stuff, calculating the Lamb shift of hydrogen - but muonic hydrogen, on the other hand, is relatively new stuff. And there's room for mistakes when going from hydrogen to muonic hydrogen, since the relative masses are very different, and the Lamb shift energy includes mass-dependent recoil corrections.

    So that's my offhand guess - errors in the recoil corrections.
  4. Jul 8, 2010 #3
    Yeah well it was just sensationalism :devil:
    I suppose the vacuum catastrophe might sort of qualify for one :biggrin:
  5. Jul 9, 2010 #4


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    Hmm, looking deeper into this, I'm not so sure - it seems the recoil corrections are overall too small to be able to account for the discrepancy. Of course, if a guy like me could find a correction that could possibly account for this, it should be immediately obvious to the people working on this thing, and we might not be having this discussion ..

    The opinion I'd most like to hear is that of http://www.fuw.edu.pl/~krp/" [Broken], since he's the one who's done most of the calculations used for the muonic Lamb shift.

    Me, I'm just a chemical physicist, so as far as I'm concerned the nucleus is a point charge, and the Lamb shift is just a rounding error! :biggrin:
    Last edited by a moderator: May 4, 2017
  6. Jul 9, 2010 #5
    It's not something I'm seeing any obvious way to correct either. My first thought was binding energy, but... still seems the large mass difference highly likely has something to do with it. I'm stuck on this one without even any really reasonable guesses.
  7. Jul 9, 2010 #6
    If the size of the proton is actually smaller does this imply that other quantities such as proton mass are different?
  8. Jul 9, 2010 #7
    QED does not deal with protons.

  9. Jul 9, 2010 #8


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    Not likely. Basically they're talking about the charge radius here. This is measured using scattering (typically), and this number is then used calculating the Lamb shift. The mass is also used, but this is determined independently, and better. (For instance, you have to use it to get the unshifted values of hydrogen to begin with, it's several orders of magnitude larger).

    What on Earth do you mean by that?

    Lamb-shift calculations, which are done using QED include a whole host of corrections which are dependent on the properties of the nucleus/proton. (charge radius, mass, polarizability, etc) Everyone agrees it's not likely to be a fundamental failure of QED here, but it's at least a distant possibility.
  10. Jul 9, 2010 #9
    Look up form factors. The Lamb shift calculation does not use QED, but effective field theory where the proton is a "black box" with some phenomenological parameters associated to it.
  11. Jul 9, 2010 #10
    Thanks alxm.My feeling is that it's early days yet and we can't rule out the possibility of other surprises cropping up.Interesting stuff.:biggrin:
  12. Jul 9, 2010 #11


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    I know about the form factors (in fact I thought it was the experimental determination of them we were talking about), I don't know why you think that that means it doesn't use QED. What would that imply?

    All I can say that your definition of 'using QED' must be different from everyone else's, because it certainly seems everyone else thinks that QED corrections are used, including the people who wrote the paper in question, and the people who did the Lamb shift calculations that they refer to, and the textbooks, and Wikipedia.
  13. Jul 9, 2010 #12
    I would even argue that QCD has very little theoretical relevance in this context : not only most of the Lamb shift corrections are just QED, but in addition form factors which could in principle be calculated from QCD in fact are not manageable (non-perturbative), and are just experimentally measured using QED probes (DIS) !
  14. Jul 9, 2010 #13
    Size can take on different meanings depending on the context. It's not necessarily a simple geometric volume in space. Would you call a charge radius a size? It really makes no difference what ontological notions you attach to size in this case, it's about 4% smaller than what we can account for.
  15. Jul 9, 2010 #14
    Is it true that the Rydberg constant is used to calculate the size of the proton?
  16. Jul 9, 2010 #15
    The charge radius is defined consistently in electron scattering, ordinary hydrogen Lamb shift and muonic hydrogen Lamb shift. The definition chosen is the round-mean-squared of the charge 1-D distribution as a function of radius.

    We already expect from models that the mass radius (by which I mean : the RMS of the mass 1-D distribution; also other radii, such as from angular momentum density or force density) are different BTW.
  17. Jul 9, 2010 #16
    The root-mean-square charge radius is, and that goes into defining the Rydberg constant.
  18. Jul 9, 2010 #17
    A qoute from the introduction to the paper:

    The references used here are:

    • 3. Mohr, P. J., Taylor, B. N. & Newell, D. B. CODATA recommended values of the
      fundamental physical constants: 2006. Rev. Mod. Phys. 80, 633–730 (2008)
    • 11. Pachucki, K. Theory of the Lamb shift in muonic hydrogen. Phys. Rev. A 53,
      2092–2100 (1996)
    • 12. Pachucki, K. Proton structure effects in muonic hydrogen. Phys. Rev. A 60,
      3593–3598 (1999)
    • 13. Borie, E. Lamb shift in muonic hydrogen. Phys. Rev. A 71, 032508 (2005)
    • 14. Martynenko, A. P. 2S Hyperfine splitting of muonic hydrogen. Phys. Rev. A 71,
      022506 (2005)
    • 15. Martynenko, A. P. Fine and hyperfine structure of P-wave levels in muonic
      hydrogen. Phys. At. Nucl. 71, 125–135 (2008)

    I see three (of the many) authors:

    Laboratoire Kastler Brossel, École Normale Supérieure, CNRS, and Université P. et M. Curie-Paris 6, 75252 Paris, Cedex 05, France

    Eric-Olivier Le Bigot, Paul Indelicato,

    Institut für Teilchenphysik, ETH Zürich, 8093 Zürich, Switzerland

    Franz Kottmann

    who 'did work on QED theory', according to the paper.
  19. Jul 9, 2010 #18
    Yes, but by how much and at what variance from the muon data?
  20. Jul 9, 2010 #19
    Here's something that may help:
    http://engineering.library.cornell.edu/node/4510 [Broken]
    Last edited by a moderator: May 4, 2017
  21. Jul 9, 2010 #20
    Wow the implications here are really interesting.:tongue2:
    Last edited by a moderator: May 4, 2017
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