Comments on Why is it forbidden for two photons to turn into one?
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Why is it forbidden for two photons to turn into one?
In the context of quantum field theory, why is it impossible for two photons (or other massless bosons like gluons) to collide and produce a single photon? This kind of a process is supposed to be forbidden by momentum conservation, but it was not immediately obvious to me why this is.
Imagine two equivalent (e.g. same frequency) photons colliding with each other head-on. The linear momentum of the syste …
2y ago
Think about how that is supposed to work. It seems you want two photons to somehow combine into a single photon and not …
2y ago
Olin and Derek have covered the biggest part of this: the combination of energy and momentum conservation rules out the …
11mo ago
Suppose you have two photons A and B on an x, y plane: Photon A is at (-20, 9) and traveling towards (20, 0). Phot …
9mo ago
With Maxwell's equations being linear and superposition working, isn't there no mechanism (in free space) for photons to …
1mo ago
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Imagine two equivalent (e.g. same frequency) photons colliding with each other head-on. The linear momentum of the system is
If these two photons collide and form a single photon (and nothing else), then conservation of linear momentum would mean that this photon has
Even for a more glancing collision, we'd either lose energy from the cancelled out parallel portions of the momentum, or we'd need to increase momentum in the perpendicular direction to make up for it which would violate conservation of linear momentum in that direction.
Thus, to satisfy conservation of energy and conservation of linear momentum simultaneously, we either need multiple particles moving at the speed of light afterwards, or the photons need to annihilate and produce a massive particle which can have zero momentum and absorb the energy as its mass (or some mixture of massive and massless particles whose net momentum is zero).
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