# Do anti-photons exist?

I know what anti-matter is and how when it collides with matter both are annihilated. However, what about anti-photons? Are there such things as anti-photons?

I initially thought the idea preposterous. However I am curious because, if anti-photons don't exist, then anti-matter could theoretically transfer its energy to normal matter - through the mechanism of light...right?

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## 2 Answers

Well, they do and don't. Depends on your point of view. Here's the story.

Quantum field theory requires for consistency reasons that every charged particle has its antiparticle. It also tells you what properties will the anti-particle have: it will have the same characteristic from the point of view of space-time (i.e. Poincaré group) which means equal mass and spin. And it will have all charges of opposite sign than a matter particle.

If the particle is not charged then QFT doesn't impose any other constraint and so you don't need antiparticles for photons (since they are not charged). But you can still consider the same operation of keeping mass and spin and swapping charges and since this does nothing to photon, you can decide to identify it with an antiphoton. Your call.

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There are uncharged Antineutrons –  Helder Velez Aug 17 '11 at 10:21
I would say that QFT requires that every particle has an antiparticle, but that some uncharged particles (like photons) can be their own antiparticles. –  Peter Shor Aug 17 '11 at 11:06
Doesn't the need for antiparticles also come from the CPT symmetry? I guess maybe that's the same thing you're saying, but from a mathematician's point of view. –  Peter Shor Aug 17 '11 at 11:25
@Helder: Antineutrons have baryon charge -1 (corresponding to a $U(1)$ baryon symmetry). One should understand charges here more generally than just electrical ones. After all, properties of arbitrary QFT (such as Yang-Mills SU(N)) surely can't depend on electromagnetism but will depend on the corresponding charges of the theory (for Y-M the color charges). –  Marek Aug 17 '11 at 20:02
More simply: Neutrons are made of quarks. An antineutron is made of antiquarks, which do have the opposite charge. –  Ben Crowell Aug 17 '11 at 20:26

Actually as per physics text books, photons are its own antiparticle. Have a look here

this ought to be a comment but I don't have the privilege to comment .. :(

Regards,

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