What happens when light hits light?

[andrien]

You can count individual gamma photons by the clicks they give on a GM tube.

I meant there that you can not follow a single photon's path.Photon is a result of quantization of electromagnetic field.It is just a quantum of EM field.In large occupation number limit,you can treat photons as light.Photons and light are same thing.

 

[Dale]

From a practical standpoint, I don't get the obsession with throwing the photon concept into situations which are perfectly adequately described classically. Unless the light the OP is beaming is such high frequency that you get photon-photon interaction or such low amplitude that you get single quanta, then just use Maxwell's equations and superposition.

 

[sophiecentaur]

I meant there that you can not follow a single photon's path.Photon is a result of quantization of electromagnetic field.It is just a quantum of EM field.In large occupation number limit,you can treat photons as light.Photons and light are same thing.

That's right, more than that starts to become a strreeeettch in thinking. The only actual evidence for photons is when they are are formed or detected. What goes on in between is a total mystery. To describe the nature of a photon whist energy is being transferred (in the wave) is, to my mind, a bit glib. And I think this applies however low the flux happens to be.

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'?

 

[Dale]

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'?

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.

 

[sophiecentaur]

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'.

 

[Dale]

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]

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]

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/Div844/publications/migdall/psm96_twophoton_interference.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]

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]

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- m which is too small to observe.