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When an atom or electron absorbs a photon, before the interaction you have an electron and a photon (possibly virtual). Afterwards, you have an excited electron (or atom). How much time does it take for the change?

According to my simple understanding,the change, when it happens, is instantaneous. It would seem that conservation of energy requires that at an exact time the photon goes away and the electron/atom goes to a higher energy state. This does not seem in the spirit of quantum mechanical variables that are usually associated with an uncertainty.

Is there in fact some delta-t for the event duration?

It seems strange that such a classical mechanics notion of a variable, time, carries over into QM.

metrovavin
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It could be argued that the amount of time required to emit a photon is essentially equal to the "coherence length" of the photon. Set up an interferometer, send identical photons through it one at a time, and find out how much path length difference is required to prevent the photons from forming an interference pattern. The photon can be thought of as a wave packet of that length. That path length difference is the coherence length of the photons. The photon can be thought of as a wave packet of that length. It is not unusual these days for a laser to have several meters of coherence length, corresponding to ten or more nanoseconds of travel time.

S. McGrew
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