# Why take FINITE time for an electron to observe a photon?

## Main Question or Discussion Point

Hi,all

Q1: Why take FINITE time for an electron to observe a photon?

(Q2: Why not much people answer my questions? Am I put them in a wrong place? whats the definition of General Physics? Classical only?)

Quuote from Jeff Reid:

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I think he means how long does it take for an electron to absorb (capture) a photon, and why is that interaction not instant, but instead takes a finite amount of time?

Yet, thats the question in porper word, thanks

Last edited:

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Pengwuino
Gold Member
Well it depends on how you word your questions. If you word them as if they were homework questions and don't put them in homework sections, you probably won't get much of a response. Also if your questions are like this, you aren't giving a lot of context to the problem. What do you mean by your question? I assume the photon is being emitted from somewhere else, so why wouldn't it take a finite amount of time until it's observed?

rcgldr
Homework Helper
I think he means how long does it take for an electron to absorb (capture) a photon, and why is that interaction not instant, but instead takes a finite amount of time?

I think he means how long does it take for an electron to absorb (capture) a photon, and why is that interaction not instant, but instead takes a finite amount of time?
Thanks for replay, and indeed your got my meaning, sorry for my poor english.

When an electron absorbs a photon, in order for it to release the photon again it must gain a higher energy level in the atom and then drop. This process must take finite time because the electron can only move so fast.

This is wrong tho, or at least a half truth.

I mean, a good question is what processes take no time? None of them I think.

rcgldr
Homework Helper
I found what I hope is a decent starting article about electron transition times, which are in the nano-second range. There a much higher frequency oscillation time during the transition period in the femto-second range. Again this is just a web site, so without confirmation from another site, I don't know how accurate it is.

When an electron absorbs a photon, in order for it to release the photon again it must gain a higher energy level in the atom and then drop. This process must take finite time because the electron can only move so fast.

This is wrong tho, or at least a half truth.

I mean, a good question is what processes take no time? None of them I think.
Thanks for replay. However, your answer is a classical view which electron spatially changed position when excited, what if thinking electron in terms of a wavefunction, jump to a higher state simply means change its wavefunction, i cant see why take finite time for an electron change its wavefunction.

Thanks for replay. However, your answer is a classical view which electron spatially changed position when excited, what if thinking electron in terms of a wavefunction, jump to a higher state simply means change its wavefunction, i cant see why take finite time for an electron change its wavefunction.
Ok, imagine that the transition between wavefunctions is 'continuous' in that one is morphed into the other in finite time as it interacts with the photon. Or, barring that, do quantum electrodynamics and rid yourself of the idea of wavefunctions altogether.

Ok, imagine that the transition between wavefunctions is 'continuous' in that one is morphed into the other in finite time as it interacts with the photon. Or, barring that, do quantum electrodynamics and rid yourself of the idea of wavefunctions altogether.
Yes, I think I got it:) It is continuous as you said. Then it must be depend on the frequency of electron and the photon then.. Thanks a lot!

Honestly the question is still not answered. But I don't know the answer.

Honestly the question is still not answered. But I don't know the answer.
I though you were kind of mean: say at t=0, I move my hand up and down in a water causing waves continually, then at t=t', you start to move your hand up and down to cause another wave. Then your wave takes finite time to fully influence my waves.

Dale
Mentor
This is a result of an uncertainty principle relationship between energy and time. I think it is $\Delta E \, \Delta t = \hbar$. It arises directly from the wavefunction, but I don't remember the details precisely. It is something like highly forbidden transitions have a very precise energy and take a very long time.

This is a result of an uncertainty principle relationship between energy and time. I think it is $\Delta E \, \Delta t = \hbar$. It arises directly from the wavefunction, but I don't remember the details precisely. It is something like highly forbidden transitions have a very precise energy and take a very long time.
@DaleSpam; Thanks for replay, your answer is great! and it becomes more interesting. I am wondering when we talk about uncertainty principle, one key feature is when WE ''observer'' measure the particle, however, as you described, it seems uncertainty principle not only applied when WE observe BUT also when that particle observe. i.e when that particle observe a photon wave came in, and wanna measure its energy, the particle precise d the energy of the photon but caused an uncertainty on the time, and for us, the particle HAS NOT collapsed its wavefunction yet during this process. That's so fascinating! am I right?

Dale
Mentor
That is correct. Also, I don't think that "we" as in conscious human beings are important in the observation. An observation is any interaction that measurable alters the state of the system. So usually the observer is some sort of measuring device like a photodetector.