- matter, anti-matter pair
Main Question or Discussion Point
I seek an explanation as to how a particle can be its own anti-particle. I would think the instant such a particle comes into existence, it would self-annihilate.
There is the maxim that "if a process does not violate a conservation law, it should happen".A photon (which is its own antiparticle as well) can't decay, it is stable.
Yes. Since an electron-positron pair can annihilate to two photons, two photons (given sufficient invariant mass for the pair) can reverse this process. What the cross section is is another matter.Can two photons annihilate?
They can react (and we have found reactions), but I don't think "annihilation" is a good name for that.Can two photons annihilate? I would think not because they don't interact very strongly. But they can interact. Delbruck scattering is an example. I'm confused.
There is some symmetry that tells you the cross section is exactly zero, I forgot the name.Classically it´ s obvious - a plane wave of a given frequency cannot spontaneously change its frequency. But viewing it as a photon subject to conservation laws only, which one specifically forbids it to do such an absurd thing?
In QED a positron and electron can annihilate to form 2 photons. Just run the reaction diagrams backward in time and I'm good to go. So yeah, looks like annihilation of photons to me.They can react (and we have found reactions), but I don't think "annihilation" is a good name for that.
Then what are the rules? Particle meets antiparticle to produce other particles is the definition of particle annihilation hocked up by Google when queried. Is there a more technically correct one?There is no rule that would give time-reversed processes the same name.
But ultimately it doesn't matter. You can call annihilation what you want as long as it doesn't lead to confusion.If the reaction products are massless or much lighter (which is again possible but not guaranteed) we typically call this reaction "annihilation".