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Q & A: photon-photon scattering

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Most recent answer: 07/30/2013
Since light can be reflected off surfaces, can it reflect off itself?
- Jess (age 13)
Sydney, NSW, Australia

To the extent that light can be described by Maxwell's equations (the classical equations for electromagnetism), no it can't. Those equations are purely linear, which means they obey superposition. The outgoing wave for two incoming parts is just the sum of what each would have done separately, so they can't scatter.

There are, however, quantum effects that can become important for high-frequency light. Photon-photon scattering events can occur. You won't ever notice them under ordinary circumstances. 

Light waves propagating in materials are dressed in other effects, mainly involving the electrons in the materials. These effects not only slow the propagation speed but also can introduce non-linearities that allow rather strong light-light scattering. However, that's not really pure light but a combination of light and those electron motions.

I guess you could say that even the quantum photon-photon scattering involves something similar, since high energy photons start to couple significantly to electron-positron fields even in a vacuum.

Mike W.

(published on 07/30/2013)

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