What happens when light hits light?

sophiecentaur
Gold Member
yes,I know what he has said.He said to give photon a mass in low energy limit to avoid infrared divergence and then combining it with cut-off provided by bethe in non-relativistic limit gives a fully divergenceless result i.e. no infrared divergence!and take mass zero in the end.mass is invariant.
I don't really understand that except that it confirms that photons do not exhibit mass. (I'll take "infra red" to include LF radio too.) When I asked what he said about low frequency photons, I was wondering about the situation with interference from two independent synchronous, RF sources. This is such an easily produced phenomenon at RF and not that difficult with two phase locked lasers, apparently.
If, indeed, it were really necessary for actual photons from each RF source to interfere with each other in order to produce an interference pattern then surely we would expect an awful lot of photons from each source not to interfere ('cancel' in one direction) - producing a very diluted interference pattern. In fact, very deep, stable nulls (0.1% amplitude) can be formed from two independent RF sources. If the sources are actually not phase locked at all but just very stable and slowly drifting past each other, a very clean interference pattern can result. (All that is necessary is that the amplitudes are equal, of course)
So the choice is between saying that, magically, pairs of matched photons are always present in the two 'beams' so that they can combine into a perfect pattern or that the interference is nothing to do with photons at all and the photons only turn up when they're actually detected (which is in line with the Copenhagen interpretation, I think).

I wish someone could comment on this and find the flaw - if there is one.

sophiecentaur
Gold Member
No, two-photon interference is exactly not that. It is very different. This is why you need to be careful. An interesting discussion on TPI can be founs in "Can Two-Photon Interference be Considered the Interference of Two Photons?", Phys. Rev. Lett. 77, 1917–1920 (1996). You can also find free copies of it all over the web.

In a nutshell, TPI arises not due to superposition of waves, but due to superposition of indistinguishable probability amplitudes associated with the simultaneous detection of two photons. I do not know your background, so it is somewhat hard to tell, whether it is sensible to go into details or not. If you are not interested in very special stuff, that does not occur in eveyday life, "different light sources do not interfere" is rather correct.

Typically you need indistinguishable photons for two-photon interference. That also means you need the same polarization. Interference terms will also cancel out on average, if you do not have a fixed phase relationship between the two fields of interest. To maintain a fixed phase relationship over a longer timescale, you need the fields to be as similar as possible. Just taking two arbitrary light beams will therefore not create interference. Two-photon interference is therefore a rather rare thing happening only under lab conditions.
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.

you can not count about every photon,in most precise definition a photon interferes with itself.All those classical ideas fails,there i no intuition.

sophiecentaur
Gold Member
you can not count about every photon,in most precise definition a photon interferes with itself.All those classical ideas fails,there i no intuition.
You can count individual gamma photons by the clicks they give on a GM tube.
You are right enough about intuition failing. I think this is the general problem that people have when talking of photons. When Feynman asserted that they were particles, I think he did no one any favours because the particle word that he used was not the particle that comes to most people's minds. There are so many apparent paradoxes involved when we compare what happens with photons of different energies and those paradoxes should be taken as a strong message that photons are nothing like most people think.

I don't think it helps that most specialists on photons seem to be concerned with optical photons which are only a small sub-set of the beasts.

jim hardy
Gold Member
2019 Award
Dearly Missed
Interesting thoughts there. Thanks i learned quite a bit

From my experiments as a teenager installing aircraft landing lights in our automobiles-
superposition makes the best sense.
When you place two of them face to face and energize both , one of the filaments soon melts.

Ohh, nostalgia!

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
Mentor
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
Gold Member
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
Mentor
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
Gold Member
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
Mentor
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.

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sophiecentaur
Gold Member
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.

Staff Emeritus
2019 Award
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?

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.

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
Gold Member
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
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.