Is there such thing as an anti-photon?

[PAllen]

I'm not aware I discussed or at all raised a single-photon interference. I have been trying to ask what happens when a photon and an anti-phase photon intersect.

Well, I think what Zapper Z is getting at is that all inteference is a single particle phenomenon (at least since it was understood in the early 1900s that single particles can produce interference patterns). The reason why I deliberately included atoms (parenthetically) in my discussion of interference is to emphasize that interference has nothing whatsoever to do with particle/antiparticle issues - no one would claim that an atom is its own antiparticle.

 

[Mordred]

http://www.google.ca/url?sa=t&sourc...C-cy_Z_PnX0xGk5Lw&sig2=WGm50ZdVJDIAXNKN_lMW_g

hope that link posted correctly seems a bit long, Its a decent post regarding anti-photon characteristics compared to photons and describes several of the provided answers in more detail.

 

[cmb]

Thanks for the link.

Sorry, but it, and the further links from it, raise more [of the same] questions for me that are unanswered!

If photons can scatter off each other and they are anti-particles, then is that process of scattering always an annihilation-then-reemission of photons, or can they interact without annihilating, and if so what are the conditions when they do annihilate, and when they don't?

 

[DrChinese]

Thanks for the link.

Sorry, but it, and the further links from it, raise more [of the same] questions for me that are unanswered!

If photons can scatter off each other and they are anti-particles, then is that process of scattering always an annihilation-then-reemission of photons, or can they interact without annihilating, and if so what are the conditions when they do annihilate, and when they don't?

You are sort of getting stuck on the words. "Annihilate" usually means that the original particles are no longer there. But fundamental quantities, like total energy or spin, are ALWAYS conserved. But there isn't much, if anything, for 2 typical photons to turn into in which those quantities are conserved. (I guess conceptually, you could have "up conversion" to a single photon.)

So generally, there is no annihilation in the sense I just defined.

 

[cmb]

So generally, there is no annihilation in the sense I just defined.

Oh, OK. I can see that, but does that mean that my question/definition above was not right?

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.

What defines a particle as an anti-particle, then?

 

[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.