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DrZoidberg
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Is it possible for photons to interact with each other directly?
haael said:Speaking of fermion loops - what does it mean for us? Can photons attract each other? Is there some change in the usual Coulomb force?
This is misleading as it applies to massless gluons as well; but gluons DO interact.Karl Coryat said:DrZoidberg: One way to think of it is, photons cannot interact because each photon occupies a unique point in relativistic spacetime. In that context, photons do not move at all relative to one another and therefore could never achieve local contact.
Why?threadmark said:But the idea of two photons being in phase is not valid in quantum theory.
threadmark said:I am merely saying that I cannot see photons in phase with one another. I won't say they do interact in this context but if two photons can't be in phase I would think there could be an affect preventing the two to be in phase.
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Coherence in relation to a laser is a rough description of its properties. Being its beam is in phase achievable with optics. But the idea of two photons being in phase is not valid in quantum theory.
tom.stoer said:Interaction means that there is a term in the time evolution operator (coming from the Hamilton operator) that changes an initial state. So if there should be an interaction in a two-photon state that means that at least the two photons
1) exchange momentum,
2) change into two different particles, e.g. an electron-positron pair
...
Symbolically
[tex]U|\gamma_{p_1}, \gamma_{p_2}\rangle = \alpha_1 |\gamma_{p^\prime_1}, \gamma_{p^\prime_2}\rangle + \alpha_2 |e^-_{p^\prime_1}, e^+_{p^\prime_2}\rangle + \ldots[/tex]
Interference means that there is an interference pattern when observing e.g. a single-photon state or its wave function
Symbolically
[tex]\psi_\gamma(x) = \langle x|\gamma\rangle[/tex]
That can happen w/o a second photon and even in a free theory where nothing "happens" with a two-photon state. So interference is an "intrinsic property" of the state.
In practice, observation of interference patterns of course requires interaction with a screen; w/o this screen the interference pattern does not become visible.
tom.stoer said:1) there is no wave function for a photon (and no Schrödinger equation)