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QFT Conundrum

  1. Jun 17, 2008 #1
    My mathematical knowledge of QFT is nonexistant (I'm only just starting grad school) so I was wondering if anyone could clear up a questions I have:

    1) Can the time it takes for an electron (which has just absorbed a photon of the correct frequency) to move to a higher orbital be said to be instantaneous?

    Naively one would expect so since the wavefunction of the two orbitals are quantized and thus one would think that there is no 'intermediate' quantum state through which it can time-evolve through. However, if it is instantaneous would that not potentially lead to a relativistic violation?
  2. jcsd
  3. Jun 17, 2008 #2
    I am aware that the presence of matter slows down light's effective speed due to the time spent during refraction.
    I don't know if the electron state change its self is instantaneous, but I know the photon absorption and re-emission is not.
  4. Jun 17, 2008 #3
    Is this necessarily true (I don't know) or is it rather that light re-emitted experiences a phase shift?
  5. Jun 17, 2008 #4
    It is very true. The value of c is the speed of light in a vacuum. The effective speed a light beam takes to travel through matter is slower because of the cumulative time spent refracting off atoms. For example, it takes tens of thousands of years for photons generated inside the Sun's core to complete their random walk and escape because they interact so frequently with the dense plasma.
    Last edited: Jun 17, 2008
  6. Jun 17, 2008 #5
    That's a rather simplistic model of why the speed of light is less in a medium. I believe there's a sticky about that. I'm considering a single absorption event.
  7. Jun 17, 2008 #6
    Sorry. I tried.
  8. Jun 17, 2008 #7


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    No, not quite. Remember that the only thing we can calculate is the probability to find the electron in the higher orbital as a function of time. That probability varies between 0 and 1 at the Rabi frequency, which in turn depends on the dipole moment for the transition and the amplitude of the drive field.
    Note that this is true even if you consider a single atom and a single photon, the best example of this can be found in cavity-QED experiments where the Rabi frequency is simply twice the coupling strength/h between the cavity and the atom.
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