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Why electron absorbs photon costs FINITE time?

  1. Feb 26, 2010 #1
    Hi,all, why electron absorb photon costs FINITE time? Not only for electron, when phonon
    interact with photon(absorbs it) also costs finite time.

    As I think, it should be instant, can not find any reason for finite time. Helps!

    Thanks In advance!
  2. jcsd
  3. Feb 26, 2010 #2


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    Nothing in the universe is instant, everything takes a finite time.
    Not very long - but still finite

    Things happening instantly leads to infinite energies or faster-than-light causality problems
  4. Feb 26, 2010 #3
    An electron absorbing a photon and moving up from a stable bound atomic state (e.g., hydrogen 1s state) to a higher unstable bound state (e.g., 2p state) requires an absorption time

    1) Δt ΔE ≈ h-bar/2

    where ΔE is the natural width of the 2p state.

    Divide both sides by h-bar and get (using Ephoton = h-bar ω)

    2) Δt Δω ≈ 1/2 for electrical circuits..

    Derive Eqn 2 first for electrical circuits (doesn't require quantum mechanics), then multiply both sides by h-bar to get Eqn 1.

    Bob S
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