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Every second there are photons flying in all directions in front of me, a vast majority of which are invisible to me as they just don't happen to strike the cornea of my eye.

Why don't any of these photons interfere? If they behaved like particles, wouldn't they collide and be obstructed from their original straight paths? If they behaved like waves, wouldn't they interfere constructively and destructively? Why can't I see the results of these interactions?

If I have a red block a fixed distance directly in front of me, and one source of light lighting up the room directly above me (which is a coherent source of light), wouldn't there be points on the cylinder which appear black to me because of destructive interference with other points on the block?

StopReadingThisUsername
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1 Answers1

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Regarding the photons zinging around in front of you: as particles, they do interact in photon-photon scattering with a negligible cross section in the visible. As electromagnetic waves, their field strengths add linearly, which is the mechanism for con/de-structive interference; however, that only occurs when the waves are coherent. These waves are incoherent and "pass though" one-and-other.

Regarding the red laser on the red block (or white-why not use a white block?), yes, you will see spots. It's called speckle noise. Shine a laser on the wall, you will see tiny dark and bright spots modulating the uniform/gaussian laser spot. Speckle is cause by interference of many scattering centers adding with random phase, which is a random walk in transverse field amplitude, which leads to a Rayleigh distributed amplitude, which squares to an exponential brightness distribution.

JEB
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