[ben_w]

> Also, do other particles behave like this?

Yes, as you note, photons are their own antiparticles.

The maths doesn't have a preferred time direction, so two photons can annihilate into an electron-positron pair.

I'm not sure if this has actually been observed given how hard it is. That said, my favourite type of supernova is caused by pair creation, though I don't know the proportion of that which comes from 2-photon interactions: https://en.wikipedia.org/wiki/Pair-instability_supernova

There's also Majorna particles, but as I understand it the only known particles that are definitely Majornas are also quasiparticles:

https://en.wikipedia.org/wiki/Majorana_fermion

[kamilner]

Notably it does bound the energy of gamma rays over long distances (as the higher the energy the more likely it will annihilate with other photons along the way.)

The wikipedia article on the Breit-Wheeler process has some history of the work on experimental observations, although I don't know how accurate or up to date it is https://en.wikipedia.org/wiki/Breit–Wheeler_process

[AnimalMuppet]

> The maths doesn't have a preferred time direction, so two photons can annihilate into an electron-positron pair.

But only if their energy is high enough. So by that reckoning, photons below 511 keV don't have antiparticles, and those above it do. That's pretty weird. So maybe it's better to say that photons aren't really their own antiparticle, but they might theoretically destroy each other in some rare circumstances.