Posted: Dec 13, 2013 8:35 pm
by DavidMcC
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.


Nope. ... 111.080405
Despite 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.