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

How does QED explain charge interaction in the macro world?

  1. Oct 8, 2014 #1
    I have personally been deeply fascinated by the processes and principles that underlie the QED and have done some recent research myself. However, recently my teacher posted me a question that stunned me. He went,"Why do like charges repel and unlike attract?" In QED I understand that this is due to electromagnetic interactions between charged particles by exchanging virtual photons. However, what left me hanging in curiosity was how the virtual photons cause the difference between attraction and repulsion. Does it have something to do with the momentum of photons? And how does this translate to the macro world? Essentially, why do "unlikes" attract and "likes" repel?
  2. jcsd
  3. Oct 8, 2014 #2

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper

    QED explains macro=world dynamics in terms of the statistics of the quantum world. The macro laws of physics appear as the average behavior of many particles in the quantum world. But I don't think that's what you mean.

    You mean, "how can a particle exchange lead to an attractive force?"

    It's down to the symmetry of the wavefunction of the virtual particle.
  4. Oct 8, 2014 #3


    Staff: Mentor

    Its to do with gauge invariance - it's not even really quantum in nature although its nice looking at it that way:

    For the detail see if you can get a hold of the following:

    The reason lies in the coupling to the EM field can take on positive and negative values. There is no more fundamental reason for that except you really would like some way to have zero net charge (charge is basically a measure of how strongly a particle couples to the EM field).

    Chug through the math and low and behold like charges repel, opposites attract.

  5. Oct 8, 2014 #4


    User Avatar
    Science Advisor

    There is also a nice and interesting discussion in Zee's book "Quantum field theory in a nutshell", which shows that whether a theory leads to repulsion or attraction of particles of like charge depends on the spin of the mediating particle being odd or even.
    Last edited: Oct 8, 2014
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook