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This is something that I have wondered for a long time. How come when I walk around why do I not see random black spots where light has collided destructively and bright spots where it has collided constructively?

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3 Answers

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There are two answers to your question. Actually now that I read your question again, I also see two questions:

To answer the first question: Light does not collide - it can sail right through any other light beam - so light does not interact with itself - one light beam does not bend another light beam.

BUT light can form interference patterns, since the waves do add up together whenever they overlap.

Now for the two answers to the now slightly modified question: "How come when I walk around why do I not see random black spots where light has destructively or constructively added together?"

The first is that there is destructive interference - but the regions for it are too small to see, since usually the configuration for destructive interference is about the wavelength of the waves in size - this is a result of the waves coming at all angles, and furthermore having different frequencies. Since the wavelength of light is just less than a millionth of a meter - or less than 0.001 mm which is too small to see, you don't see it. Also light vibrates at billions and billions of times per second, and is coming in general from billions of places at once, so the whole effect gets washed out by our eyes and brain, which operate quite a bit slower than that.

The second answer is despite all these obstacles to observing a phenomenon like this, you can see it - usually under physics class experiments, but also in 'the wild'. For example if you are walking at night under monochromatic street lights and see an oil slick on a puddle - you can see constructive and destructive interference.

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Does this accurately represent the accepted wave-particle duality thinking? The 0.001 mm argument seems almost self-defeating since the number density should be enough to fill a cube that big with many overlapping photons. I imagine the full answer involves path integrals, E&M, maybe field theory, and all kinds of stuff I don't understand myself. –  AlanSE Oct 17 '11 at 2:05
    
@Zassounotsukushi wave-particle duality has nothing to do with it.. in the particle view, photons do not interact. Interference is entirely a wave phenomenon. –  user2963 Oct 17 '11 at 2:43
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I would just add that other reasons we don't see the interference is because the light sources in ordinary life are incoherent. Incoherence means that even if two photons happen to have the same frequency, they will not have the same phase. Finally, there are not just billions, but more like a billion billion photons per second from just a 60 watt light bulb. –  FrankH Oct 17 '11 at 3:15
    
Just a clarification on the dual wave particle picture. Photons do interact through higher order diagrams; of course this has nothing to do with interference and coherence which are wave properties . –  anna v Oct 17 '11 at 4:11
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As I understand the question, this phenomenon has to do with coherence. This is the concept behind Tom's answer. Light coming from a lightbulb is not coherent because the light waves are caused by the different oscillations of many different atoms. They have slightly different frequencies and wavelengths. So they can only overlap constructively/destructively for short periods. For example, try overlapping the graph of y = sin x with y = sin 1.01x. They only completely interfere for some intervals. The frequency of the light waves is so great that these intervals are very small and infrequent. Also, many light waves can completely add/cancel, while many other in the same place do not, resulting in no noticeable interference pattern.

Tom's mention of the thin films of oil brings up the topic of coherence distance (there are probably other terms for this). The waves are coherent over very short distances, much like sin x and sin 1.01x are coherent over small intervals. Then they can interefere constructively/destructively, as seen by these patterns. The irredescence of oil drippings is due to this phenomenon.

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""For example, try overlapping the graph of y = sin x with y = sin 1.01x. They only interfere for some periods"" Nonsense –  Georg Oct 17 '11 at 5:47
    
I have merged your accounts. Now you can edit directly. –  mbq Oct 17 '11 at 6:43
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There are numerous excuses why the interference of light can only be observed under certain circumstances. The reason in fact is that the constructive and destructive interference is not the explanation of the result. In short I was able to correct a few mistakes in Einsteins SRT paper. The corrections predicted a particle that spins at C and propagates at C. Two of these particles merge to become stable when they are 60 degrees out of phase. This creates with high probability a particle that is the photon. I have several hundred pages of math to demonstrate that the model is consistent with all observed photon properties to date. PLUS....

The model predicts 2 types of dispersion, one elastic, one not. Particle bounce arises when the particle traveling helically hits a parallel surface (slit or obstruction). The model predicts that when an array of photons meet an obstruction some bounce off the sides and others begin to loop. Bounce corresponds to a term we call scatter. The looping is caused by a negative gravity that exists between dissimilar photons. They are observed as EM forces and I refer to it as the Maxwellian dispersion factor) The results of the 2 different types of dispersion can be witnessed in an experiment I did. (search Chinstrap effect on YouTube) Using a cheap laser and hair obstruction and collapsing the pattern created by Maxwellian dispersion (loops) the scatter pattern is clearly visible. In the second part of the experiment I use diffraction grating or more specifically phase grating which isolates the Maxwellian or loop effects. In this section you can clearly see that the interference pattern is not line of sight and in fact one can take some of the looping photons and redirect them to a tangential target. The line of sight image intensity lowers while the tangential image increases with no shadow on either image. I in fact was able to calculate a zone where the reflective obstruction shadow can clearly be seen not in line of sight and as the shadow moves closer to the image on the target the intensity reduces and the tangential target increases. If you are interested please feel free to view the video. Experiment 2 will be posted soon. The results will likely be met with a lot of cynicism since it was demonstrated by an engineering math professor and it proves light is singularly a particle while also undermining the foundations of quantum, but since anyone can recreate the results at home with a swiss army knife and dollar store laser that makes pretty pictures it will be difficult for science to ignore it.

email me at John.blaszynski@mohawkcollege.ca for a free copy of the math.

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