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Comment by halosghost | Hacker News Reader

Disclaimer: I am not a particle physicist

In the theoretical physics courses I took in college, we spent a while on anti-matter, and one of the things that the professor mentioned was that it is widely accepted that photons are their own anti-particles as well. Researching this now (read: googling for sources that seem legitimate), that seems to still be widely accepted.

However, I have never read anything that suggests that photon collisions would lead to annihilation. Is there a reason such might be the case for {anti-,}neutrinos?

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> I have never read anything that suggests that photon collisions would lead to annihilation.

You may at least have heard that a particle antiparticle collision can lead to the production of two photons. Viewed in reverse this would be two photons annihilating and producing a particle antiparticle pair.

Since the laws of physics work equally well in reverse two photons must therefore be able to annihilate.

Two photons "annihilating" to two photons is... well.. photon-photon scattering, but you can also produce other particles. High energy gamma rays pair produce into electron positron pairs with the cosmic microwave background, so at high enough energies, they don't go very far through the universe.

https://en.wikipedia.org/wiki/Two-photon_physics

Yes. One quick and not-exactly-honest answer is that photons are gauge bosons and thus are automatically their own antiparticle, while neutrinos are fermions and the symmetry is not guaranteed. Beyond that, though, I don't know how to explain it without delving into some of the structure of quantum field theory and the Standard Model.

Edit: and yes of course, as aroberge points out, two photons can collide and produce a particle/antiparticle pair. The real question is: can two neutrinos collide and pair produce? Or do you need a neutrino and an antineutrino (assuming the are Dirac in nature)?

> photons are gauge bosons and thus are automatically their own antiparticle

So is W^+, but it is charged, hence not its own antiparticle. W^- should in this case be considered the antiparticle.

And before electroweak symmetry breaking?

Before EWSB you have four massless bosons: W+, W−, W0 and B0

The W0 and B0 mix to give the photon and Z0

The W+, W− and Z0 gain mass and the photon is left massless

Physicist here: yes, photon collisions can lead to annihilation ... and production of a particle--anti-particle pair.

Then why don't things happen when you point two lasers at each other?

Because photon density in your little laser is far below the levels required to make this phenomenon observable.

It can be done, but it requires very high power laser beams, focused very tightly, and even then the events are pretty rare.

Scattering happens in this situation, but for ordinary lasers, it's an undetectably tiny effect. You need particle accelerator scale energies and very high densities to see anything.

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