Posted: Dec 13, 2013 8:35 pm
newolder wrote:Adco wrote:I am thinking about photons. ...
What about when photons bump into each other? That must happen all the time. I guess nothing happens, they just carry on flying around until they get absorbed or reflected. OK, there's one answer sorted.
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Nope. http://physics.aps.org/synopsis-for/10. ... 111.080405Despite what movie lightsabers suggest, light beams pass through each other without effect. However, two photons will, on rare occasion, bounce off each other. This elastic photon-photon scattering, which occurs via intermediate particles, has never been observed directly, but a new analysis in Physical Review Letters shows that the Large Hadron Collider (LHC) at CERN could detect around 20 photon-photon events per year...
This is a phenomenon that only occurs with photons with at least 0.51MeV in the centre-of-mass frame. IMO, it is the reason that the EM force can be united with the weak force, as the electro-weak force. With visible light (photon energy a few eV), for example, it cannot occur.
EDIT: I suppose a virtual electron-positron pair could exist very briefly without sufficient energy, but then the scattering cross-section would be minute.