Is there such thing as an anti-photon?

[DrChinese]

It was my assumption that 'anti-particles' meant that the particles annihilate (as in - not there any more, and their energy is transmuted somehow), and it wasn't a comment anyone picked up on, so I assumed it was right. This might be the source of my confusion.

That assumption is the part that is not correct. It happens to be essentially correct for massive particles, like electrons or protons, but not for photons.

Whenever any particle and its anti-particle interact (these are the inputs), the combined output has all fundamental quantities conserved. Only output combinations with such conservation can be observed. These occur according to chance, although the probabilities can be determined in some cases. In the case of photons, an output of a photon and an anti-photon is nearly 100% certain. Whether you consider these the same particles or not is just a matter of semantics. Such semantic issues often arise in the area of QM.

The general name for experiments in this area is scattering experiments, and that is why scattering was mentioned by some of the other posters. If I recall correctly, ZapperZ works at a facility where such experiments are done daily.

 

[vanhees71]

Whether a particle is strictly neutral or not, i.e., whether particles are indistinguishable from their anti-particle partners or not is a very clearly answerable question. A particle is called strictly neutral if its one-particle states are eigen states of the charge-conjugation operator. The eigen values of the charge-conjugation operator are +1 and -1 since $C^2=1$.

A photon is strictly neutral, and its charge parity is -1. Thus a photon and an anti-photon are indistinguishable.