# Are photons the antiparticle of itself?

Hey guys, I was thinking about this for a while now and I seem to be on a dead end. So here it is, this is my speculation. I would love some feedback, tell me which parts are correct/false and if they are false, guide me towards the right path! :)

1. Photons can technically be their own antiparticle.

2. They don't annihilate because they are bosons and can occupy the same quantum state.

3. In many Feynman diagrams we see a particle & antiparticle annihilation producing another particle and antiparticle pair. I was thinking thats why we get 2 photons in some other annihilation interactions (one photon, one antiphoton)

I must be missing something. Is my logic flawed?

## Answers and Replies

D H
Staff Emeritus
Science Advisor
Sure they are, and they can be viewed as being their own antiparticle.

WannabeNewton
Science Advisor
3. In many Feynman diagrams we see a particle & antiparticle annihilation producing another particle and antiparticle pair. I was thinking thats why we get 2 photons in some other annihilation interactions (one photon, one antiphoton)

You cannot have a QED Feynman diagram with two photon lines at a vertex. The fundamental vertex of the QED interaction consists of two electron (charged particle) lines and a photon line. You can certainly have the scattering ##\gamma + \gamma \rightarrow \gamma + \gamma##, which is called Delbruck scattering, but this is a four vertex diagram at the lowest order and consists of 4 external photon lines and 4 internal electron lines since higher order QED diagrams can only be built up from the fundamental vertex.

None of this is contradictory to the (correct) statement that photons are their own antiparticles.

Bill_K
Science Advisor
1. Photons can technically be their own antiparticle.
Photons ARE their own particle. Literally, not just technically.

2. They don't annihilate because they are bosons and can occupy the same quantum state.
Two photons don't annihilate because there is no direct interaction between two photons. Indirectly, through their interaction with other particles such as electrons, two photons can scatter, or turn into a neutrino-antineutrino pair.

3. In many Feynman diagrams we see a particle & antiparticle annihilation producing another particle and antiparticle pair. I was thinking thats why we get 2 photons in some other annihilation interactions (one photon, one antiphoton)
It's conservation of momentum, angular momentum and parity that requires two photons to be produced, or sometimes three of them.

Thanks a lot to everyone for the responses!

Two photons don't annihilate because there is no direct interaction between two photons. Indirectly, through their interaction with other particles such as electrons, two photons can scatter, or turn into a neutrino-antineutrino pair.

Can two boson pairs (boson - antiboson) annihilate in general? Since they can occupy the same quantum state I thought they wouldn't.

Bill_K
Science Advisor
Can two boson pairs (boson - antiboson) annihilate in general? Since they can occupy the same quantum state I thought they wouldn't.
Sure, no problem. The simplest example comes from e+ + e- → γ + γ. Just run it backwards: γ + γ → e+ + e-.

Most examples are unobserved, but that's just because it's more difficult experimentally to collide two bosons, since they tend to decay! But in principle, π+ + π-, or W+ + W- are annihilation examples.

Having two bosons just means that their two-particle wavefunction is symmetric, so e.g. they could orbit each other in an S state.

ChrisVer
Gold Member
Most examples are unobserved, but that's just because it's more difficult experimentally to collide two bosons, since they tend to decay! But in principle, π+ + π-, or W+ + W- are annihilation examples.

just a remark:
Wouldn't it be better to avoid the pions? fundamentally the annihilation still occurs between two fermions.
The rest OK

Bill_K
Science Advisor
just a remark:
Wouldn't it be better to avoid the pions? fundamentally the annihilation still occurs between two fermions.
The rest OK
Are you saying that pions are not bosons??

ChrisVer
Gold Member
Are you saying that pions are not bosons??

No I am not saying that.. they are not fundamental (consisting of (anti)quarks which are fermions), and the annihilation happens between the quarks (fermions).

Also
I am not sure whether they are bosons or not...since they have inner structure, they will tend to behave like bosons, but only effectively...there are also bosonic nuclei but they also behave like that effectively because they have fermionic inner structure (nucleons)
maybe I'm wrong or missing something

I am not sure whether they are bosons or not...since they have inner structure, they will tend to behave like bosons, but only effectively...there are also bosonic nuclei but they also behave like that effectively because they have fermionic inner structure (nucleons)
maybe I'm wrong or missing something

Pions obey Bose-Einstein statistics - > Bosons :)

Pions obey Bose-Einstein statistics - > Bosons :)

Yeah, but only at energies so their constitute fermions aren't relevant - a composite boson can not break the exclusion principle for its constituents!