What happens when light hits light?

sophiecentaur

Quote by DaleSpam

Two oppositely-travelling 511 keV photons could interact to produce an electron-positron pair. If the photons were just barely 511 keV then the resulting electron and positron would have very little KE and so they would attract each other, anhilate, and produce two 511 keV photons. The net result would be scattering of the photons.

So, is 511keV the minimum? This would make 511keV a very significant energy quantity, wouldn't it? It would seem to be some sort of threshold value for the production of 'free mass', rather than just 'mass defect'.

DaleSpam

Quote by sophiecentaur

So, is 511keV the minimum? This would make 511keV a very significant energy quantity, wouldn't it?

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.

sophiecentaur

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.

Vanadium 50

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?

Bboy Physics

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.

andrien

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.

Cthugha

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.

sophiecentaur

Quote by Vanadium 50

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?

Does this statistic basically reflect a kind of scattering cross section of a photon?

sophiecentaur

There seem to be two parts to this thread. There is Interference, which gives a pattern of probabilities of a photon being detected by some detector at different points in space and there is Interaction between two photons. These are, surely, two distinct things and they seem to be used interchangeably here.

andrien

Quote by sophiecentaur

There seem to be two parts to this thread. There is Interference, which gives a pattern of probabilities of a photon being detected by some detector at different points in space and there is Interaction between two photons. These are, surely, two distinct things and they seem to be used interchangeably here.

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[itex]-[/itex] m which is too small to observe.

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Bboy Physics	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.

sophiecentaur Recognitions: Gold Member Science Advisor	There seem to be two parts to this thread. There is Interference, which gives a pattern of probabilities of a photon being detected by some detector at different points in space and there is Interaction between two photons. These are, surely, two distinct things and they seem to be used interchangeably here.