# What happens when light hits light?

by junguo93
Tags: hits, light
 P: 8 As titled, lets say Source A is beaming light at Source B and Source B is doing the same to Source A, what is actually happening with the protons? My guess is that light photon collides and slows down o.o? But that is based on a wild guess, can someone come up with another theory/explanation? (:
 P: 11 Photons do not bounce off each other. However, according to the wave description, they could interfere.The result of this would depend, according to the wave theory, on their difference in phase. However, things aren't as simple as this implies because interference is known to take place even with single photons. If one imagines two photons "colliding" in the way you describe, and further imagines similar "collisions" many times, the outcomes would not be expected to be always the same. However, I think the average behaviour (taken over many such collisions) is the same as is predicted by wave theory.
 Math Emeritus Sci Advisor Thanks PF Gold P: 38,456 Note that the ''interference" is the same as if two water waves crossed. At the time of crossing, two "highs" and two "lows" sum while a "high" and a "low" cancel. After they have passed through one another, they will not have changed.
PF Gold
P: 5,550

## What happens when light hits light?

If,say, two automobiles headlights are pointed at each other. what happens?? Each is illuminated, right?

You know light is an electromagnetic wave, right? So its behavior is characterized by Maxwell's equations.

If you mean monochromatic light sources, like that produced in theory via a laser, I'm not sure exactly what happens...
in theory they can destructively interfere....but I do know that all electromagnetic waves travel at c in a vacuum. Photons don't slow down. You might also achieve some unique results in fiber optic cables between a pair of 'identical' light sources.

In general as noted already, wave interference via superposition should describe the interactions...see the diagram here for an illustration of constructive and destructive wave interference:

http://en.wikipedia.org/wiki/Interfe...ve_propagation)

and check the additional diagrams for different interference patterns.
P: 16
 Quote by HallsofIvy Note that the ''interference" is the same as if two water waves crossed. At the time of crossing, two "highs" and two "lows" sum while a "high" and a "low" cancel. After they have passed through one another, they will not have changed.
What exactly does it mean for two photons to interfere and cancel their amplitudes, or to double it? For example, if we place a detector right where they interfere, would they leave more bright or less bright dots, or something?

I always thought they actually do get changed after the interaction as well, at least their trajectory seem to change in double-slit experiments.
PF Gold
P: 1,717
 Quote by Br.S What exactly does it mean for two photons to interfere and cancel their amplitudes, or to double it? For example, if we place a detector right where they interfere, would they leave more bright or less bright dots, or something? I always thought they actually do get changed after the interaction as well, at least their trajectory seem to change in double-slit experiments.
Well for starters, the property of the interference is going to be part of the overall description of the detection of the photons, not with the individual photons themselves. I just saw a discussion the other day where a person was asking how the phase of one photon interferes with the phase of another photon. This is incorrect in thinking that we ascribe a phase to each photon. What we do is we solve for a wave function that has a phase associated with it and it is the wave function that experiences interference. Physically, this wave function can be thought of as the probability of detecting a photon in a given volume. The thing is, this interference is independent of the number of photons present at a given point in time. We can emit one photon at a time and still get the exact same interference pattern over time as if we emitted trillions at a time. So there is no difference in trying to solve the problem for one, two, or more photons (though this isn't true for every kind of particle). There's more to it than this but I want to try and keep this brief.

To answer your question though, since the wave function represents the probability density, then we find that the interference is associated with regions of greater or lesser intensity in the rate of photons being detected depending upon the kind of interference. In other words, the interference describes regions of darkness or brightness.

I think it's just easier to think of this in the classical model, where light is simply a wave. Now the electromagnetic field that describes light has a phase associated with it. So when two waves combine, the relative phase difference between the waves in time and space gives rise to the interference.
PF Gold
P: 5,550
 I always thought they actually do get changed after the interaction as well, at least their trajectory seem to change in double-slit experiments.
There IS a change...individual particles interfere with themselves because the method of detection [ slit] affects the outcome. Particles have a wave nature and even specific frequencies. If there is no image at a point, the quantum mechanical view is that the probability of a photon's appearing at that point is very low. We can't predict where the next particle image will appear....

But in open space, say when you turn on a light bulb, each emergent photon [wave packets of energy] follows a path [a geodesic] and none 'disappear'.
PF Gold
P: 10,211
 Quote by Br.S What exactly does it mean for two photons to interfere and cancel their amplitudes, or to double it? For example, if we place a detector right where they interfere, would they leave more bright or less bright dots, or something? I always thought they actually do get changed after the interaction as well, at least their trajectory seem to change in double-slit experiments.
Two photons cannot interfere. Photons in two light beams are Bosons and, as such, can all occupy exactly the same space location and Energy. If they are not actually interacting with matter in some way (passing through a transparent non-linear medium) then they have no way of affecting what the other does.
The effect of interference is essentially how the statistics of a single photon passing through a system give a probability distribution of where it will be found.
P: 16
 Quote by Born2bwire Well for starters, the property of the interference is going to be part of the overall description of the detection of the photons, not with the individual photons themselves. I just saw a discussion the other day where a person was asking how the phase of one photon interferes with the phase of another photon. This is incorrect in thinking that we ascribe a phase to each photon. What we do is we solve for a wave function that has a phase associated with it and it is the wave function that experiences interference. Physically, this wave function can be thought of as the probability of detecting a photon in a given volume. The thing is, this interference is independent of the number of photons present at a given point in time. We can emit one photon at a time and still get the exact same interference pattern over time as if we emitted trillions at a time. So there is no difference in trying to solve the problem for one, two, or more photons (though this isn't true for every kind of particle). There's more to it than this but I want to try and keep this brief. To answer your question though, since the wave function represents the probability density, then we find that the interference is associated with regions of greater or lesser intensity in the rate of photons being detected depending upon the kind of interference. In other words, the interference describes regions of darkness or brightness. I think it's just easier to think of this in the classical model, where light is simply a wave. Now the electromagnetic field that describes light has a phase associated with it. So when two waves combine, the relative phase difference between the waves in time and space gives rise to the interference.
The wave function can't be just an abstract description, those waves ought to actually be there, in whatever physical form, but one way or another they must be describing some real physical property. Wave-like interference is actual and real after all, so the question is then more along the lines whether this "wave" property is contained individually in each photon, or it emerges from some combination of their group existence, motion, or whatever else.

Take vertically polarized light for example, double slit wave-like interference is then supposed to impact photons only over their vertical oscillation frequency, their amplitude in the vertical plane should either increase or decrease, yet the effect we measure is that their trajectory would be bent horizontally, forming spacing between the pattern fringes. I think, maybe not, maybe vertically polarized light does not actually produce the pattern with horizontally spaced double slit?
P: 16
 Quote by sophiecentaur Two photons cannot interfere. Photons in two light beams are Bosons and, as such, can all occupy exactly the same space location and Energy. If they are not actually interacting with matter in some way (passing through a transparent non-linear medium) then they have no way of affecting what the other does. The effect of interference is essentially how the statistics of a single photon passing through a system give a probability distribution of where it will be found.
I'm pretty sure there are quite a few experiments demonstrating two beams from independent light sources interfere and can produce the pattern. Do you think then it's not two photons that interfere with each other, but something else is going on there, or perhaps you doubt in such experiments?
PF Gold
P: 10,428
 Quote by Br.S Take vertically polarized light for example, double slit wave-like interference is then supposed to impact photons only over their vertical oscillation frequency, their amplitude in the vertical plane should either increase or decrease, yet the effect we measure is that their trajectory would be bent horizontally, forming spacing between the pattern fringes. I think, maybe not, maybe vertically polarized light does not actually produce the pattern with horizontally spaced double slit?
I don't think polarization affects the interference pattern, only the shape/size of the slits and the wavelength of the light.
P: 16
 Quote by Drakkith I don't think polarization affects the interference pattern, only the shape/size of the slits and the wavelength of the light.
I think polarization must have some impact.

These guys say it influences visibility and some angles in the pattern. They also say this: "importance of the polarization characteristics is that only parallel oriented polarization modes interfere...", whatever is that supposed to mean.
PF Gold
P: 10,428
 Quote by Br.S I think polarization must have some impact. http://www.ibsen.dk/phasemasks/techn...tion-influence These guys say it influences visibility and some angles in the pattern. They also say this: "importance of the polarization characteristics is that only parallel oriented polarization modes interfere...", whatever is that supposed to mean.
Perhaps. I was thinking only about the double slit experiment itself, not about phase masks, which may be different. Maybe someone else here will know.
PF Gold
P: 1,717
 Quote by Br.S I'm pretty sure there are quite a few experiments demonstrating two beams from independent light sources interfere and can produce the pattern. Do you think then it's not two photons that interfere with each other, but something else is going on there, or perhaps you doubt in such experiments?
This is the problem when people try to think about in terms of photons. Quantum theory states that photons do not interfere with each other because they are bosons. This is why the interference pattern is independent of the number of photons. It has been shown experimentally that you still get the same interference pattern if you only use a single photon source.

 Quote by Br.S The wave function can't be just an abstract description, those waves ought to actually be there, in whatever physical form, but one way or another they must be describing some real physical property. Wave-like interference is actual and real after all, so the question is then more along the lines whether this "wave" property is contained individually in each photon, or it emerges from some combination of their group existence, motion, or whatever else. Take vertically polarized light for example, double slit wave-like interference is then supposed to impact photons only over their vertical oscillation frequency, their amplitude in the vertical plane should either increase or decrease, yet the effect we measure is that their trajectory would be bent horizontally, forming spacing between the pattern fringes. I think, maybe not, maybe vertically polarized light does not actually produce the pattern with horizontally spaced double slit?
The wave function is not physical in that it is not observable. The wave function does however give the physical results when we use it with an appropriate operator. By performing the desired operation on the wave function, we can find the physical properties of the system like energy or the probability density of where the particle would be detected. The phase is not assigned to the photons, it is assigned to the wave function that describes the specific state of the photon system that we are looking at. We could have multiple states combine and interfere with each other in that way, but we do not have photons coming together to interfere with each other.

 Quote by Br.S I think polarization must have some impact. http://www.ibsen.dk/phasemasks/techn...tion-influence These guys say it influences visibility and some angles in the pattern. They also say this: "importance of the polarization characteristics is that only parallel oriented polarization modes interfere...", whatever is that supposed to mean.
I also agree with Drakkith. Double slit interference is not affected by polarization. What you have linked to in your post is about gratings, not double slit. These gratings are made using dielectric fibers, not a perfect conductor as we assume in the double slit experiment.
P: 1,414
 Quote by Br.S I'm pretty sure there are quite a few experiments demonstrating two beams from independent light sources interfere and can produce the pattern. Do you think then it's not two photons that interfere with each other, but something else is going on there, or perhaps you doubt in such experiments?
One must be a bit careful when discussing that. Two-photon interference is real, but it is a bit different than your typical single-photon interference which is tested by double slits or Mach-Zehnder interferometers. The simplest version of two-photon interference is the Hong-Ou-Mandel effect or maybe photon bunching/bosonic final state stimulation.

If you use different sources to create two-photon-interference, it is necessary that you make these two modes indistinguishable, so they have exactly the same spectral, spatial and temporal properties (to be more exact, it is enough to make emission and detection events caused by these two light fields indistinguishable, but that is nitpicking). In other words, you rather have more complicated interference properties for modes which are occupied by more than one photon, rather than two individual photons interfering.

 Quote by Br.S I think polarization must have some impact.
Well, if you put polarizers at the positions of the slits, it can of course have an impact as you can create distinguishable fields which do not interfere. But the polarization you start with, does not matter unless you use slits which are larger than the wavelength of the light used.
P: 16
 Quote by Cthugha One must be a bit careful when discussing that. Two-photon interference is real, but it is a bit different than your typical single-photon interference which is tested by double slits or Mach-Zehnder interferometers. The simplest version of two-photon interference is the Hong-Ou-Mandel effect or maybe photon bunching/bosonic final state stimulation. If you use different sources to create two-photon-interference, it is necessary that you make these two modes indistinguishable, so they have exactly the same spectral, spatial and temporal properties (to be more exact, it is enough to make emission and detection events caused by these two light fields indistinguishable, but that is nitpicking). In other words, you rather have more complicated interference properties for modes which are occupied by more than one photon, rather than two individual photons interfering. Well, if you put polarizers at the positions of the slits, it can of course have an impact as you can create distinguishable fields which do not interfere. But the polarization you start with, does not matter unless you use slits which are larger than the wavelength of the light used.

I don't see what to be careful about, two independent light beams can either interfere or not. Sure, no slit is different thing than two slits, but it's supposed to be the same mechanics relating to their wave property and the principle of wave superposition. Right?

So anyway, are you saying two independent light beams can not interfere unless they are both polarized perpendicularly the same plane, for example both vertically polarized? Or can two independent light beams interfere where one is vertically and the other horizontally polarized?
 Sci Advisor P: 2,456 Interference does not affect light propagation. It's just superposition in action. You literally add the electromagnetic field of one beam with electromagnetic field of another. You don't need to drag up any quantum mechanics here. So long as we are not looking at entanglement, electromagnetic field IS the wave-function of photons. This, by the way, points out the fact that while wave-function need not be observable, it does not mean it has to be non-observable. For a single photon, its wave function is a measurable field. The electromagnetic field.
P: 1,414
 Quote by Br.S I don't see what to be careful about, two independent light beams can either interfere or not. Sure, no slit is different thing than two slits, but it's supposed to be the same mechanics relating to their wave property and the principle of wave superposition. Right?
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.

 Quote by Br.S So anyway, are you saying two independent light beams can not interfere unless they are both polarized perpendicularly the same plane, for example both vertically polarized? Or can two independent light beams interfere where one is vertically and the other horizontally polarized?
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.

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