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

Does QED have any real-world applications?

  1. Dec 11, 2011 #1
    I'm trying to think of real-world applications of the standard model of particle physics, but can't come up with anything.
    Are there any technological applications in which neither non-relativistic quantum mechanics nor Dirac's relativistic wave mechanics is insufficient, and QED must be used? QED doesn't seem to be used in the electronics industry, but I'll be glad to be corrected.
    And how about QCD? I thought of nuclear power, but the fact that nuclear weapons were tested in 1945, decades before QCD, makes me suspect that QCD doesn't have any unambiguous applications in nuclear engineering, either.
    So I guess it's safe to say that the standard model doesn't have real-world applications, even though the seemingly more esoteric general relativity does have applications (GPS)?
     
  2. jcsd
  3. Dec 12, 2011 #2

    tom.stoer

    User Avatar
    Science Advisor

    I guess you don't accept the LHC as an application ;-)
     
  4. Dec 12, 2011 #3
    I also tried to come up with something related to atomic spectroscopy. I can even find applications of hyperfine energy levels, in nuclear engineering (uranium isotope separation), astronomy (hydrogen 21cm line), and quantum computing (hyperfine levels as qubits). But when it comes to the real hallmark of QED, the Lamb shift, I again found nothing...
     
  5. Dec 12, 2011 #4
    I have some clues again. Are there technological applications that rely on the fact that the electron Lande g-factor is slightly greater than 2? This is certainly a QED effect. Maybe some instruments that measure magnetic fields accurately need to take this into account?
     
  6. Dec 12, 2011 #5

    Bill_K

    User Avatar
    Science Advisor

    Gell-Mann used to tell us that his funding would be much greater if he claimed to be working on a quark bomb.
     
  7. Dec 12, 2011 #6

    Cthugha

    User Avatar
    Science Advisor

    Hmm, I am not sure whether that is what you are after, but in the semiconductor laser industry, the so-called cavity-QED regime is pretty attractive for low threshold lasers. Very often tailored quantum dot microcavity or photonic crystal lasers make use of tailored cavities and use the Purcell effect to create an enhanced spontaneous emission rate into the lasing mode.
     
  8. Dec 12, 2011 #7

    A. Neumaier

    User Avatar
    Science Advisor
    2016 Award

    The Lamb shift is only one of a huge number of verifiable consequences of QED, and has no special status.
    (It is famous only because it was the first calculation that made quantum field theory respectable. It is not even the most accurate prediction - this honor belongs to the anomalous magnetic moment.)

    Atomic spectroscopy is a consequence of QED, though it can be applied to the standard N-particle Coulomb Hamiltonian (which is the nonrelativistic limit of QED, and thus may be considered not to require QED). But QED is needed if you want to get high accuracy spectra. For heavier elements, it is even needed to get the right order of magnitude. For example, it requires QED to get the color of gold (a spectroscopic property) right. Calculations from the N-particle Coulomb Hamiltonian get the color completely wrong, since important correction terms derived from QED are missing.

    The fact that quicksilver is liquid at room temperature is also a genuine QED effect, as applying the usual statistical mechanics calculations to the N-particle Coulomb Hamiltonian gets the freezing point quite wrong.

    One also needs QED in laser-based chemistry, as the interaction between a laser and a molecule cannot be described without QED.
     
  9. Dec 12, 2011 #8

    Bill_K

    User Avatar
    Science Advisor

    I don't agree. The reflection spectrum of gold can be calculated correctly using a relativistic Hartree-Fock approximation. Requiring relativity is not the same as requiring QED!
    According to this article, the unusual melting point of Mercury is due to a contraction of the 6s orbital, again a result of relativity.
    This does not even require relativity, just Bose statistics.
     
  10. Dec 12, 2011 #9

    A. Neumaier

    User Avatar
    Science Advisor
    2016 Award

    How do you get the relativistic Hartree-Fock approximation? Of course from QED. Otherwise you don't
    have a guidance how to choose the terms in the Hamiltonian, and need to resort to extra experimental information beyond the mass and charge of a gold atom. Whereas from QED you get it with minimal input.
    And again, the Hamiltonian used for the relativistic orbital calculations comes from QED.
    One needs the concept of a photon and the form of its interactions with the electrons, both borrowed from QED.
     
  11. Dec 12, 2011 #10

    Bill_K

    User Avatar
    Science Advisor

    You write down and solve the Dirac Equation. Which was done in 1928, about 20 years before the development of QED. Contrast this with the other effects mentioned previously: the Lamb shift and the anomalous magnetic moment, which DO depend on QED.
    Einstein and Planck, who developed these concepts, would be surprised to hear that you are crediting them with the discovery of QED.
     
  12. Dec 13, 2011 #11

    A. Neumaier

    User Avatar
    Science Advisor
    2016 Award

    The Dirac equation is a single particle equation. It doesn't tell how different charged particles interact.
    Traditional Poincare invariant generalization to multiple particles suffer from inconsistencies involving the Dirac sea.

    One needs QED to specify relativistic interactions consistently from an ab initio perspective (i.e., with only masses and charges of the particles given).
    Where did Einstein or Planck specify the form of the interaction of a photon with an electron?
    I'd be too curious to know.
     
  13. Dec 13, 2011 #12
    I don't consider consistency in this context to be an important issue. For example, I don't consider nuclear beta decay, at least to the first approximation, to be an application of nonabelian SU(2)*U(1) gauge theory, because the old Fermi's 4-point interaction is already sufficient, even though it's manifestly inconsistent (nonunitary at high energy).

    Another example. The lifetime of hydrogen 2p state can't really be calculated consistently without QED, because in non-relativistic QM it's formally an exact eigenstate so shouldn't decay. But by Einstein's A-B coefficient argument, this can be calculated. Again this was done decades before QED. So QED doesn't have anything groundbreaking to say here.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook