# Why electron absorbs photon costs FINITE time?

1. Feb 26, 2010

### luxiaolei

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!

2. Feb 26, 2010

### mgb_phys

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

3. Feb 26, 2010

### Bob S

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