QFT and Electrons: Is Instantaneous Orbital Change Possible?

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Discussion Overview

The discussion revolves around the nature of electron transitions between orbitals in quantum field theory (QFT) following photon absorption. Participants explore whether these transitions can be considered instantaneous and the implications of such a characterization on relativistic principles.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions if the time for an electron to move to a higher orbital after absorbing a photon can be considered instantaneous, suggesting that the quantized nature of wavefunctions implies no intermediate states.
  • Another participant notes uncertainty about the instantaneous nature of the electron state change, mentioning that photon absorption and re-emission are not instantaneous.
  • A different participant raises a question about whether light re-emitted experiences a phase shift, indicating a lack of clarity on the relationship between absorption and emission processes.
  • One participant asserts that the speed of light in a vacuum is constant, but acknowledges that light travels slower in a medium due to interactions with matter, providing an example from solar physics.
  • Another participant critiques the simplification of light's speed in a medium, emphasizing the focus on a single absorption event rather than cumulative effects.
  • A later reply clarifies that the transition probability for the electron varies over time according to the Rabi frequency, which is influenced by the dipole moment and the drive field amplitude, indicating that the transition is not instantaneous.

Areas of Agreement / Disagreement

Participants express differing views on the instantaneous nature of electron transitions, with some suggesting it may not be instantaneous while others explore the implications of such a claim. The discussion remains unresolved regarding the characterization of these transitions.

Contextual Notes

Participants highlight limitations in understanding the relationship between photon absorption, electron transitions, and the speed of light in different media. There is an acknowledgment of the complexity involved in these processes, particularly in relation to quantum mechanics and relativistic principles.

maverick_starstrider
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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?
 
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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 itself is instantaneous, but I know the photon absorption and re-emission is not.
 
Is this necessarily true (I don't know) or is it rather that light re-emitted experiences a phase shift?
 
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:
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.
 
Sorry. I tried.
 
maverick_starstrider said:
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?

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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