Light and the electron

[Peron]

In QED (Quantum Electrodynamics) virtual photons are absorbed by electrons and this is what we feel as force. But how does a electron physically absorb a photon?

[Bob_for_short]

There is no virtual particles and their absorption. But there are a Coulomb and magnetic interactions that we feel as forces.

[swansont]

In QED (Quantum Electrodynamics) virtual photons are absorbed by electrons and this is what we feel as force. But how does a electron physically absorb a photon?

 

I'm not sure that's an answerable question. How would you possibly study it? It's not a macroscopic effect that can be studied at a more fundamental level.

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There is no virtual particles and their absorption. But there are a Coulomb and magnetic interactions that we feel as forces.

 

Virtual particles are part of mainstream physics models.

[Bob_for_short]

...But how does a electron physically absorb a photon?

 

Just as a footbol gamer "absorbs" a ball: it acquires additional energy and momentum with it.

Edited October 23, 2009 by Bob_for_short

[ajb]

In QED (Quantum Electrodynamics) virtual photons are absorbed by electrons and this is what we feel as force. But how does a electron physically absorb a photon?

 

I agree with swansont, it is not really an answerable question. The best I can offer is that the electron takes on the energy and momentum of the photons. (It is important to realise that virtual particles are in general off mass-shell.)

 

I think of virtual photons (in this context) as just "terms" in a mathematical expansion that describes the process. A little more technically, they correspond to internal propagators in Feynman expansions of correlation functions.

[Bob_for_short]

Virtual particles are part of mainstream physics models.

But:

I'm not sure that's an answerable question. How would you possibly study it? It's not a macroscopic effect that can be studied at a more fundamental level.

Also:

...I think of virtual photons (in this context) as just "terms" in a mathematical expansion that describes the process. A little more technically, they correspond to internal propagators in Feynman expansions of correlation functions.

Why to fool a poor guy? The "internal propagator" is reduced to the Coulomb and megnetic interactions in most of cases. This is more comprehensible.

[ajb]

My viewpoint is that virtual photons (or other "exchange particles") appear only as an interpretation in perturbation theory. That is they appear as mathematical terms in a mathematical expansion.

 

The "particles" they correspond to may be "external" and be observed in nature, such as the photon or they may never appear "externally" as example FP ghosts.

 

virtual photon <-> internal propagator <-> mathematical term [math]\sim \frac{1}{p^{2}}[/math] where [math]p[/math] is the momentum.

 

 

I have no idea how one could interpret any of this outside perturbation theory. Any ideas welcomed.

Edited October 23, 2009 by ajb

[Bob_for_short]

virtual photon <-> internal propagator <-> mathematical term [math]\sim \frac{1}{p^{2}}[/math] where [math]p[/math] is the momentum.

 

I have no idea how one could interpret any of this outside perturbation theory. Any ideas welcomed.

 

There is no need to go outside the perturbation theory. Just fulfil the calculation to the end or make transition to the configurational space. You will obtain 1/r + magnetic terms from your propagator. That's the force terms.

[swansont]

Why to fool a poor guy? The "internal propagator" is reduced to the Coulomb and megnetic interactions in most of cases. This is more comprehensible.

 

I don't think anyone here is trying to fool anyone. Saying that a coulomb interaction exists is not the same as explaining what actually goes on when a virtual particle is absorbed by an electron, which is what was asked. When one asks an advanced science question, one has to allow for the possibility that there is no simple answer.

 

"Comprehensible" is in the mind of the individual; I don't think asserting one model over another on this basis has any foundation whatsoever. Certainly asserting that a particular model doesn't exist, when it clearly does, gets anyone anywhere.

[ajb]

There is no need to go outside the perturbation theory. Just fulfil the calculation to the end or make transition to the configurational space. You will obtain 1/r + magnetic terms from your propagator. That's the force terms.

 

Been a while since I have done any perturbation theory calculations. Anyway, that was not my point. My point is that I do not know how to discuss virtual particles outside perturbation theory. Indeed, I am not really sure how to discuss physical particles outside perturbation theory.

 

So, to get to any sensible answer to the opening question we would need to examine Feynman diagrams and their interpretation.

 

Really, we should think of the virtual particles as "carrying" the energy and momentum between the vertices. But this is just an interpretation of the mathematical expressions.

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When one asks an advanced science question, one has to allow for the possibility that there is no simple answer.

 

I think there is no answer, for the reasons stated.

[Bob_for_short]

So we arrive to the conclusion that we cannot, despite a huge number of textbooks, give an answer to the question in the frame of virtual particles.

 

On the other hand, there is the Coulomb gauge where the Coulomb potential is clearly written. It leads to bound states an other obvious effects. That is why I have given my answer in terms of potential energy terms.

[ajb]

So we arrive to the conclusion that we cannot, despite a huge number of textbooks, give an answer to the question in the frame of virtual particles.

 

I think the question is ill-posed, but a natural one if you do not understand the role of virtual particles and Feynman diagrams.

 

On the other hand, there is the Coulomb gauge where the Coulomb potential is clearly written. It leads to bound states an other obvious effects. That is why I have given my answer in terms of potential energy terms.

 

Yes, the propagator has what is known as the instantaneous Coulomb term.

(See for example Kaku's book Quantum Field Theory).

 

But, this does not address what was initially asked. We still have perturbation theory and the opening question about virtual photons.