Photon-photon collision confusion

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    Collision Confusion
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Discussion Overview

The discussion revolves around the concept of photon-photon collisions, exploring the apparent contradiction between traditional understandings of photons as probability functions and the possibility of their interactions as suggested by recent experiments and quantum electrodynamics (QED).

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant expresses confusion about the concept of photon-photon collisions, noting that traditional teachings suggest photons do not have precise locations until absorbed, which seems to conflict with the idea of collisions.
  • Another participant suggests that collisions in the quantum realm should not be thought of as classical collisions, proposing that wave-packages can interact, which allows for scattering formalism.
  • A different participant explains that while photons do not interact at tree level in QED, they can interact at loop level through virtual electrons, indicating that photon scattering is theoretically possible.
  • One participant emphasizes that the non-linearity of QED is very weak, suggesting that photons are unlikely to "see" each other during a collision.
  • Another participant confirms the previous point about photon interactions and suggests looking up "Delbrück scattering" for further information.

Areas of Agreement / Disagreement

Participants express varying levels of understanding and acceptance of photon-photon collisions, with some agreeing on the theoretical possibility while others remain confused about the implications of quantum mechanics on this phenomenon. No consensus is reached regarding the fundamental nature of photon interactions.

Contextual Notes

The discussion highlights limitations in understanding the implications of quantum mechanics, particularly regarding the nature of photon interactions and the conditions under which they may occur. There are unresolved aspects related to the interpretations of QED and the behavior of photons as probability functions.

jnorman
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okay, i have to ask a stupid question. a short while ago, there was a thread discussing photon - photon collisions. being an amateur, i had always understood that two photons could not collide, and i stuck in a comment to that effect, indicating that since photons, once they are emitted, only exist as a probability function, and have no exact location until they are absorbed.

i was corrected, and told that the wiki article supporting what i had said was wrong, and pointed at some papers referring to current experiments with photon-photon collisions.

so, apparently, photons can collide with each other, but i simply cannot understand how that corresponds with what i have been taught, regarding the photon probability function and no precise location of a photon. for photons to collide, they must have some precise location, but to have a location, the probability function must be collapsed, which seems in conflict with the basic premise of QED.

can someone please try to provide some explanation of this for me? what am i misunderstanding? thanks.
 
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You have to stop thinking of collisions in the quantum world as two tiny balls colliding as in pool or snooker...

You have wave-packages, which is enough to build up a scattering formalism (you can do it with plane waves too if you want)

If you don't think electron-electron scattering is strange, or electron-photon scattering; then you should not think photon-photon scattering be strange...
 
jnorman said:
so, apparently, photons can collide with each other, but i simply cannot understand how that corresponds with what i have been taught, regarding the photon probability function and no precise location of a photon. for photons to collide, they must have some precise location, but to have a location, the probability function must be collapsed, which seems in conflict with the basic premise of QED.

can someone please try to provide some explanation of this for me? what am i misunderstanding? thanks.

Not having a precise location is a synonym to a wave. Two waves on water surface, created with different sources, propagate differently (scatter) after collision if the wave equation is non linear. It is the case of QED. But the QED photon non linearity is very very weak so most likely photons will not "see" each other while "collision".

Bob.
 
Looking at the QED Lagrangian we can see that photons don't interact at tree level, but they can interact at some order at loop level through virtual electrons. So the possibility for two photons two scatter is there. So they should be able to interact, right?
 
Yes, exactly.
 
And a Google search on "Delbrück scattering" should lead you to some information...
 

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