# What happens when light hits light?

by junguo93
Tags: hits, light
PF Gold
P: 11,370
 Quote by DaleSpam It is pretty significant, it is the mass of an electron. EDIT: actually, I guess this could happen for neutrinos also, at much lower energies.
Yes, I thought so. We are really talking in terms of the entities with the lowest mass. Is there a lower limit then or is it that the likelihood of particles existing with lower and lower masses becomes less and less? It would be aesthetically more satisfying than just having some sort of cut-off.
 Mentor P: 15,613 Before we get too far into the realm of light-by-light scattering and pair production, it's worth pointing out that this, as a practical matter, does not happen. If I have two light bulbs a meter apart and I sit and wait anxiously for a single photon to be scattered, on average I will have to wait something like 10^32 years. If you want neutrinos to come out, add another 20 zeros on top of that. Or perhaps 40, or maybe even 80. Does it really matter?
 P: 9 When two waves collide, they get bigger as they go 'through' each other and then they just get to regular size again and move on. That's the way I learned about waves anyway. I believe they can cause interference with each other however.
P: 977
 At the high frequency end, where photons interact to produce matter, the situation can still obtain. I would like to know just what is the minimum frequency for this to happen, though, and what particle is involved. It seems here must be a major change in the Physics of EM at that point. Is there some kind of breakdown in the way 'space works' then or could it be looked upon as some sort of minimum quantum EM energy for a change of 'mass state'?
energy is same as mass,so why should one care about any physics change here.However after a certain cut-off limit there has to be some different physics(short-distances) and at that much distances(high energy) other interactions can interfere.
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 Quote by sophiecentaur Is there a real difference between these two things? Is it not just two ways of saying the same thing? Using RF sources tends to take care of the polarisation issue. I looked for that article but could only find sources that charge for it.
Sorry, I am replying somewhat late here. Can you access the following link hosted by NIST? http://physics.nist.gov/Divisions/Di...terference.pdf
I am not sure, whether it is free or I just have local access. There is also a good review article called "Quantum effects in one-photon and two-photon interference" by Mandel (Rev. Mod. Phys. 71, S274–S282 (1999)), but for this one I am not sure whether there is a free version or not.

Back to the original question. It may be similar under some circumstances, but there are differences. First, TPI also can take place for two beams which have a fixed phase relationship with respect to each other although both beams alone are incoherent (like in down conversion or for entangled light)., Second, you also need to take the detection events into account and therefore also the backaction of the detection event on the light field. Quantum effects without classical counterpart can come into play just through the simple fact that every photon can only be detected once.
agree with that,the so far scattering of light by light cross-section is too small.it is order of 10-31 cm2 at ω$-$ m which is too small to observe.