# I got a question.

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Okay, how does a electron absorb a photon? Can anyone explain this to me, or is it one of those things in physics that can't be broken down into laymens terms? Or is their even a mathematical explanation of this phenomena?

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Charged particles, such as the electron interacting with an electromagnetic field is understood in terms of Quantum ElectroDynamics, QED for short.

This interaction can be understood in terms of photons via perturbation theory. This allows for the nice pictorial description of Feynman diagrams, which are really mathematical terms in a (maybe formal) asymptotic expansion.

This is really the best explanation to offer, without going into details.

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A photon has energy-momentum and simply increases the energy of the electron which causes the electron to go into a higher energy state, an excited state. When the electron loses it's energy, it returns to the ground state with the release of a lower energy, n. wavelength photon.

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Okay, how does a electron absorb a photon? Can anyone explain this to me, or is it one of those things in physics that can't be broken down into laymen terms? Or is their even a mathematical explanation of this phenomena?

A photon is a long wave-train propagating is space. An atom is a compound system also existing in space as a de Broglie wave. When two waves meet, the photon wave starts to push and pull the atomic electron. If the resonance conditions are approximately satisfied, the atom may get excited in the end and the photon energy is thus spent on increasing the atomic internal motion energy. The quantum mechanics is a wave mechanics so this works as I have just described.

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From a classical perspective, the electron and proton have opposite electrical charges and so attract. It takes energy to separate them. The electron also orbits the nucleus, and making it go faster also takes energy (and momentum). The photon has energy and momentum, which are transferred to the electron to make it go faster and pull it a bit farther from the nucleus.

In reality, the above description is faulty and the electron should really be thought of as being in a quantum mechanical orbital rather than a classical orbit.

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I am not sure if the electron in higher orbits moves faster.

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A photon has energy-momentum and simply increases the energy of the electron which causes the electron to go into a higher energy state, an excited state. When the electron loses it's energy, it returns to the ground state with the release of a lower energy, n. wavelength photon.

Yes, I understand this. What I'm asking is how this occurs, how does a photon form out of the energy of the electron?

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Yes, I understand this. What I'm asking is how this occurs, how does a photon form out of the energy of the electron?

This might get a bit technical. A quantum state at one time evolves into a new quantum state at a different time according to something called the time-evolution-operator.

This is generally true. So even a ball rolling down a hill has a time evolution operator which tells you where it will be some time after its start. In the ball's case, the time evolution operator will contain information about gravity.

For the electron, the time evolution operator contains objects known as creation and annihilation operators. A creation operator creates a particle in the system and an annihilation operator destroys a particle. Which creation and annihilation operators live in the time evolution operator depends on the physical laws of the system.

In this case, the relevant combination in the time evolution operator is one electron annihilation operator, one photon annihilation operator and one electron creation operator. The annihilation operators destroy both the electron and the photon, while the creation operator creates a 'new' electron with the combined energy of the electron-photon pair.

So in some sense, the electron does not absorb the photon at all - it is destroyed, along with the photon, and a new electron is created.

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Reading this I'm reminded of the following from Nature... 'almost impossible for the non-scientist to discriminate between the legitimately weird and the outright crackpot'

Peter Woit told the New York Times, 'but these days that doesn't much distinguish it from a lot of the rest of the literature'.

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This might get a bit technical. A quantum state at one time evolves into a new quantum state at a different time according to something called the time-evolution-operator.

For the electron, the time evolution operator contains objects known as creation and annihilation operators. A creation operator creates a particle in the system and an annihilation operator destroys a particle. Which creation and annihilation operators live in the time evolution operator depends on the physical laws of the system. [...]

So in some sense, the electron does not absorb the photon at all - it is destroyed, along with the photon, and a new electron is created.

Apart form this language (creation-annihilation operators) there is another, more appropriate one: the state amplitude depending smoothly on time. While absorbing the photon wave the old state amplitude fade and the excited atomic state amplitude grows up. In the end there is not the initial states but the final only. Similarly for scattering a photon. The state populations depending on time is more physical since there is no instant creation or destroying states in reality.

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The state populations depending on time is more physical since there is no instant creation or destroying states in reality.

Can you back that up? The initial state in this thought experiment is an electron and a photon, while the final state is a single electron. Have you ever observed something in between, which is neither initial nor final state?

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Can you back that up?

Yes, I can.

The initial state in this thought experiment is an electron and a photon, while the final state is a single electron. Have you ever observed something in between, which is neither initial nor final state?

I am not an experimentalist but I think any radio-wave attenuation in a conducting medium is a simple example of that. A photon has a finite wave-train to be more or less of certain frequency (10^4 oscillations, for example) so its absorption time is finite.

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This might get a bit technical. A quantum state at one time evolves into a new quantum state at a different time according to something called the time-evolution-operator.

This is generally true. So even a ball rolling down a hill has a time evolution operator which tells you where it will be some time after its start. In the ball's case, the time evolution operator will contain information about gravity.

For the electron, the time evolution operator contains objects known as creation and annihilation operators. A creation operator creates a particle in the system and an annihilation operator destroys a particle. Which creation and annihilation operators live in the time evolution operator depends on the physical laws of the system.

In this case, the relevant combination in the time evolution operator is one electron annihilation operator, one photon annihilation operator and one electron creation operator. The annihilation operators destroy both the electron and the photon, while the creation operator creates a 'new' electron with the combined energy of the electron-photon pair.

So in some sense, the electron does not absorb the photon at all - it is destroyed, along with the photon, and a new electron is created.

So, have any experiments been performed to show this happens, or is this some kind of postulate?

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So, have any experiments been performed to show this happens, or is this some kind of postulate?

It is a description of how one would calculate such things in the framework of quantum field theory.

An electron cannot absorb a photon in isolation. There needs to be some background field, this is provided by the Coulomb potential sourced by the nucleus of an atom. A slightly more simpler situation to think about is Thomson scattering where we have inelastic scattering of a photon off a free electron.

Now, all we can observe are the initial and final states. The stuff in the "middle" is not observable to us. In terms of Feymnan diagrams the internal lines do not correspond to physical particles. Only the external legs do.

I should say that Severian is thinking in terms of the operator formalism as where I am thinking of path integrals, both (for sure for QED) are equivalent.

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It is a description of how one would calculate such things in the framework of quantum field theory.

An electron cannot absorb a photon in isolation. There needs to be some background field, this is provided by the Coulomb potential sourced by the nucleus of an atom. A slightly more simpler situation to think about is Thomson scattering where we have inelastic scattering of a photon off a free electron.

Now, all we can observe are the initial and final states. The stuff in the "middle" is not observable to us. In terms of Feymnan diagrams the internal lines do not correspond to physical particles. Only the external legs do.

I should say that Severian is thinking in terms of the operator formalism as where I am thinking of path integrals, both (for sure for QED) are equivalent.

So, their really is no clear way of knowing what goes on when a electron absorbs a photon?

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I thought that when an electron absorbed a photon (I hate that word) it jumped to a higher state/orbit????

Is there anyplace outside of the old "mass/gravity" world where light acts like a particle?

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So, their really is no clear way of knowing what goes on when a electron absorbs a photon?

This is the trouble with any interpretation in physics. The best we can really do us describe a mathematical set-up or a calculation.

I know rudimentary QED so I can work with charges in electromagnetic fields. Given an initial state one can work out the final state. From the calculation one can place an interpretation on the process.

The interpretation of what is going on in say an atom or a metal in relation to the photoelectric effect is that the photon is absorbed by the electron. All we mean by this really is that we are studying a process

$e^{-} + \gamma \rightarrow e^{-}$

in which we know the initial and final particle content.

Merged post follows:

Consecutive posts merged
I thought that when an electron absorbed a photon (I hate that word) it jumped to a higher state/orbit????

Yes, if it is in a potential like in the case of an atom.

Peron knows this, but wants a clear explanation of how this happens. (I am not sure anyone can answer this completely)

Is there anyplace outside of the old "mass/gravity" world where light acts like a particle?

Yes, all over the place. You could start be looking up quantum optics (for example)

In general we have wave-particle duality of "particles". If you ask a photon a wave-question you get a wave answer, interference or diffraction for example. Ask a photon a particle-question you get a particle answer, scattering processes for example.

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Yes, all over the place. You could start be looking up quantum optics (for example)

In general we have wave-particle duality of "particles". If you ask a photon a wave-question you get a wave answer, interference or diffraction for example. Ask a photon a particle-question you get a particle answer, scattering processes for example.

I tried that but they never slow down to listen...

It's always rush, rush, rush...

Seriously though, scattering of light; isn't that explainable as waves too? light of a single wave-length scatters less (a lot less) than when you have light of all wavelengths involved. Don't waves propagate in a "scattering" form?

I really am trying to get this.

Paul, the 60 year old student

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Most EM phenomena can be explained (semi)classically, but it is far easier to do with quantum mechanics. There are some behaviors, though, that require a quantum treatment.

http://www.scienceforums.net/forum/showpost.php?p=496829&postcount=5

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• 2 weeks later...

I don't know either but maybe it could have something to do with the fact that electricity and light are closely related.

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I don't know either but maybe it could have something to do with the fact that electricity and light are closely related.

Of course, classically via Maxwell's equations we know how to describe all electromagnetic phenomena.

The trouble is that the interaction between electrons and the electromagnetic field is really quantum in nature. (Though, a lot can be explained semiclassically, i.e. quantum electrons in a classical electromagnetic background.)

The OP is asking how one arrives at the interpretation of an electron absorbing a photon.

One way we can see this is to think about an interpretation of n-point functions (which are then related to an S-matrix, this tells you how to go from in states to out states ) in terms of Feynman diagrams. To do this, follow Severian's post. Match the diagram (expressed in terms of classical fields) and terms in the n-point function (expressed in terms of creation and annihilation operators).

If you do this, say just for scalar field theory at first you see that we have creation and annihilation of particle states at specified positions and times (or momenta depending on how you look at it). (again see what Severian has said.)

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