1. Simple question, with a probably complicated answer :wink: If there were 2 perfectly identical photons that collided directly head on, how would they react? Which direction would they bounce? Would their energy change? Whatever else you could think of too.

2.

3. Photons do not bounce off each other. However, according to the wave description, they could interfere.The result of this would depend, again 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 would expect the average behaviour (taken over many such collisions) to be the same as is predicted by wave theory. Small numbers of photons appear to comply with the wave description of light only in a statistical sense.

What goes on inside a Fabry-Perot interferometer at very low light intensities would have some relevance to the situation you describe.

4. On the other hand, there is the particle based approach. The probability of an event is equal to the square of the length of an arrow called the "probability amplitude." An arrow of length .4, for example, represents a probability of the entire event. The genersal rule for drawing arrows if an event can happen in alternative ways is to draw an arrow for each way, and then combine the arrows, or "add" them, by hooking the head of one to the tail of the next. A "final arrow" is then drawn from the tail of the first arrow is the one whose square gives the probability of the entire event. (the partial reflection by glass, for example)

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