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

Two Questions about Photon/Electron Interaction

  1. Nov 9, 2011 #1
    Richard Feynman writes, on page 97 of his primer of quantum electrodynamics, QED: The Strange Theory of Light and Matter, about the interaction of a photon and an electron. He writes: "One way this event can happen is: a photon is absorbed by an electron, the electron continues on a bit, and a new photon comes out."

    FIRST QUESTION: When Prof. Feynman says "continues on a bit," what sort of time scale is involved? Is it on the order of 10-30 to 10-15 second, or can it also be whole seconds, minutes, years or even eons?

    I take it that an electron in free space cannot interact with a photon because there is no way an electron can store a photon's energy. But for an electron in an atom, the energy of the photon can be stored in the atom, i.e., with the electron being pushed a little farther from the nucleus.

    SECOND QUESTION: What drives, provokes, or stimulates the process by which "a new photon comes out"?

    I'm not a physicist, just an interested layman. Thanks for any guidance.
    Last edited by a moderator: Dec 4, 2011
  2. jcsd
  3. Nov 9, 2011 #2


    User Avatar
    Science Advisor

    The original electron obeys the usual special-relativity relationship between energy and momentum: E2 = p2c2 + m2c4. When the photon is absorbed, energy and momentum are conserved in the collision, and so the electron that emerges from the collision has greater energy E' and momentum p'. Furthermore E' and p' do not obey the above relationship. We say it is a virtual particle, and is "off the mass shell."

    The virtual electron "continues on a bit." How long it does so is a matter of chance, but is approximately given by the Heisenberg uncertainty principle.

    What drives the particle to decay back into a photon plus electron is the same thing that drove them to combine in the first place: the interaction between the electromagnetic field and the charge of the electron.
  4. Nov 10, 2011 #3
    Like on the order of 10-34 second?
    Last edited by a moderator: Dec 4, 2011
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook