• Question: Can you Please explain Quantum electrodynamic Theory?

    Asked by physicspunk to Arttu, Ceri, James_M, Monica, Philip on 20 Jun 2011.
    • Photo: Arttu Rajantie

      Arttu Rajantie answered on 20 Jun 2011:

      It is difficult to explain it in a few sentences, but it is basically a quantum field theory that describes photons and electrons. You can think of it as the quantum mechanical version of electromagnetism, but there are many subtle issues and it took physicists many decades to really understand how it needs to be done. One of the people who developed the theory, Richard Feynman, wrote a nice little book called “QED: The Strange Theory of Light and Matter”, which explains it nicely at a fairly simple level, so you could have a look at it.

      On the other hand, when Richard Feynman was asked this same question, he famously replied “If I could explain it to the average person, I wouldn’t have been worth the Nobel Prize.” 🙂

    • Photo: James M Monk

      James M Monk answered on 20 Jun 2011:

      QED is the theory that describes how charged particles interact by exchanging photons, and contains within it all of electromagnetism. It is a gauge theory, which means the following: In quantum mechanics we can describe the location of charged particles with a wavefunction, which tells us the probability that a particle is at a particular place or time. The wavefunction has an un-observable property called a phase – really the phase is a mathematical trick because it is un-observable. You can think of it like this – at each point in space-time the particle’s wavefunction is oscillating up and down (it is a wave, after-all), and its position in that imagined oscillation is the phase.

      QED is what happens when you insist that the phase (position in the oscillation) can be arbitrarily altered at any point in space-time. Clearly this changes the wavefunction, so we have to introduce another type of field to cancel out the change of phase – this field is the photon of light! Adding the photon means that the theory becomes invariant to (local) changes in the phase.

      So simply by insisting that the theory must not change because of changes in the (un-observable) phase we get the theory of electromagnetism. Neat, eh?

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