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

I If electromagnetic waves propagate, do photons as well?

  1. Jul 27, 2017 #1
    In classical physics, EM waves propagate this is one of the main features of all waves in general. Usually for mechanical waves the elements (like molecules) that vibrate do some little motion. For example a string can move up and down, but the waves travel further through propagation. The information or a signal carried by these waves travels by propagation. For EM waves there is not even vibration (mechanical) of the elements involved but only a variation of E and B fields. For these waves there seem to be absolutely no physical motion involved. What about the quantum mechanical photons? Is there any physical motion involved?
    Because if photons propagate and not move then I suppose it is likely that all particles do that. Does the motion we know from classical physics where an object keeps its identity exist in QM or QFT?
  2. jcsd
  3. Jul 27, 2017 #2


    User Avatar
    Gold Member

    Photons don't propagate at all. EM waves propagate and if they encounter something a photon can be the result of that interaction.
  4. Jul 27, 2017 #3
    From that I understand the waves propagate and the photons simply pop-up (where they are observed for example) at fixed locations , but they don't propagate of travel. Is that correct?
  5. Jul 27, 2017 #4


    User Avatar
    Gold Member

    Yes, although "pop up where they are observed" is a bit casual. They come into existence when an EM wave excites something (like an atom)
  6. Jul 27, 2017 #5


    Staff: Mentor

    In QM whats going on when not observed the theory is silent on. So the idea of propagation, which implies something going on regardless of observation, is not applicable.

  7. Jul 27, 2017 #6
    What about the other particles (leptons and quarks)?
  8. Jul 28, 2017 #7


    User Avatar
    Science Advisor

  9. Jul 28, 2017 #8


    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    First of all in classical theory the electromagnetic field to our present understanding is a fundamental entity. You cannot describe it by more fundamental other entities. It is defined operationally by its action on charged bodies.

    Now when quantizing it, what's provided by the theory as by any quantum theory are probabilities to have a certain "reaction", i.e., a scattering cross section. A photon is a special state of the electromagnetic field, namely a socalled single-photon Fock state. Photons are very difficult to explain in plain English. There's no way out: To really understand photons you need some mathematics. In any case you must not think about photons as something like a classical particle. This picture even more wrong than for massive quanta like electrons, protons, etc. E.g., one can prove from the formalism of relativistic QFT that a photon has no observable you could call "position of the photon". The only thing you can say, given the single-photon state at its preparation, is the probability to register it with some photodetector at a given place and a given time. In this sense the "electromagnetic field propagates" in the same sense as any other quantum system propagates, i.e., given the initial state you can calculate the probabilities for measuring an observable on this system at any time (provided you know the dynamics of the system with sufficient accuracy and are able to solve the equations of motion).
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted