If the photon did have a non-zero mass, would that imply the existence of anti-photons?
And then, wouldn’t these hypothetical anti-photons be easy to spot in cyclotron experiments?
A photon has a baryon number of zero (no quarks or anti-quarks), and therefore is its own anti-particle. If we did find the photon to have a rest mass, it would cause some serious revising (if not chucking out) the Standard Model of particle physics.
Stranger
It is rare that one can pick any nits in your posts, but there are a couple here…
Baryon number being zero doesn’t exactly mean “no quarks or antiquarks” but rather that the difference in the number of quarks and antiquarks is zero. (The mesons, of course, are each made of a quark/antiquark pair, yet they have zero baryon number.)
Also, a particle’s having zero baryon number does not imply it is its own antiparticle. The counterexamples, as it were, are all charged mesons, the charged leptons (electron, muon, tau), the W boson, and possibly the neutrinos (we don’t actually know yet on this one). All of these have distinct antiparticles and also have zero baryon number.
For the first part: they would probably continue to be their own antiparticle, in that there may not be any quantum numbers with which one could distinguish the photon and “antiphoton”. But, as Stranger notes, a massive photon causes enough havoc in the theory that this wouldn’t be the first issue to resolve.
For the second part: the hypothetical antiphotons might behave indistinguishably in all experiments to-date. In fact, they must, else we would already have evidence of them! (In other words, whatever theory you construct that says: “You should easily see antiphotons in past data”, I can instantly disprove by saying: “Yeah, but we haven’t.”)
As mentioned above, the neutrino is an example of a massive particle for which we do not yet know whether particle=antiparticle.
Thanks for the explanations.
Further nitpick: There are also neutral mesons that are not their own antiparticle, such as the K[sup]0[/sup]. Though those all have superimposed states of the particle and antiparticle states which are their own antiparticle.
And it wouldn’t really rock the foundations of the Standard Model all that much for photons to have mass. It would just mean that instead of a perfect symmetry, you have an approximate one, valid at high energies (where “high” means greater than the photon’s mass). This is already the situation we have with the Z; a massive photon wouldn’t be all that different.
But, once your model gets that complex, don’t you start really wanting a new paradigm? Not that I claim to understand any of it enough to say what qualifies as overly complex, but it just seems that superpositions of particles and their own antiparticles starts to look like holes in the math, filled with assumptions tailored to fit. Of course that doesn’t mean they are wrong, but elegant, it ain’t.
Tris
Actually, the neutral kaon system is extremely elegant, once you have a proper understanding of it. It’s only nonintutive because it’s inherently quantum mechanical, and quantum mechanics is as a rule nonintuitive.
Pasta! Great to see you back.
One thing I’ve always wondered about: Can a mass-less particle have a nonzero charge?
Oh boy oh boy oh boy!
There are no examples of such (or at least no known examples, but if there were any, we really ought to know about them by now).
“Yeah, but we haven’t” is emphatically NOT instant disproof.
It is the perennial counterpoint offered up by SETI proponents as they ask for yet more handouts from the gullibly affluent, or governments, which are gullibly bankrupt in our case.
Not disproof of what? You left out that part of my quote:
Not having seen evidence for antiphotons doesn’t mean they aren’t there. Not having seen evidence for antiphotons is damning evidence for any theory that says there are antiphotons that we should have seen already.
Right. Whether or not “I don’t see a cat in the box” is evidence of the presence of a cat depends entirely upon if I looked in the box yet.
1. Look in box.
2. Fail to see cat.
Conclusion: There is no cat in the box
That’s a completely valid argument, while:
1. I don’t see a cat in the box.
Conclusion: There is no cat in the box
isn’t.
Yes, of course.
A better way to phrase my question:
Is there anything we know of that would seem to completely rule out the possibility of such charged entities?
For instance, is it a theoretical impossibility with the Standard Model? Alternative models (already extant)?
- Og-zero (with positive strangeness)
It would take an extremely complicated model to explain how such objects might exist without ever having been detected. They’d be completely stable, since any decay of one would necessarily violate either the conservation of momentum or the conservation of charge, and they’d definitely couple to the electromagnetic force, since we’re assuming they’re charged. What this means is that any interaction at all involving the electromagnetic force, at any energy (since they have zero mass), there’d be a chance of producing some, and any such particles, once produced, would last indefinitely, so we should be swimming in them by now. And with a charge, they’d be even easier to detect than photons, which I don’t think I need to tell you are very easy to detect. I can’t think of any way one would account for a lack of detections.
It sounds like you are more interested in the philosophy of science – try checking out some intro to philosophy of science books. In particular, check out the sections on explanations: one way to put your question is, ‘do the laws of nature explain anything? if so, how?’
Scientists don’t really deal in these questions.
Note that it’s not limited to relativity. You can push similar questions in Newtonian mechanics, and sooner or later you get the answer, it just does.
pdts
I must say that the above is a fascinating expansion of the previous argument. Yes, short of minature “cloaking devices” 
I’m sure that the experience of all studies is that there are no such entities.
It’s just that everything you’ve said so far has been a directly empirical argument in the direction of non-existence. Don’t get me wrong. I don’t believe in “absence of evidence is not evidence of absence” as a general rule. (I’ve seen fundies throw this one forward frequently, as if they were speaking against a formal logical fallacy.) It’s a matter of the situation, and the sheer volume of possible occasions to observe something. In this case, surely the physicists have been at this long enough for the non-observation to be convincing.
What I was wondering about, though, was some sort of deduction from the quantum general study of particles. Something that would say that, hey, we can deduce from the Standard Model (if only that one) that in order for a particle to have a charge, it must first have mass. Here are the previous conclusions from the model, just do the math.
Our posts crossed. Would I be correct in figuring that your observation applies to the OP and his repeated questions? 
- Og
We can deduce from the Standard Model that all charged particles have mass, for the simple reason that the Standard Model includes a list of elementary particles. And the Standard Model was, after all, ultimately derived from experiments.
There are some models simpler than the full Standard Model which do, in fact, derive the fact that charged particles should all have mass from other principles, without assuming it from the outset. Unfortunately, the mass derived this way is infinite for point particles such as electrons. So there’s clearly something wrong with these simpler models.
After looking through more than a dozen journal articles, I conclude that this is a complicated question.
The short answer seems to be: yes, you can have massless charged particles. There are a couple of papers demonstrating that you can get self-consistent classical behavior. There are some showing that the “usual” electromagnetic charge is incompatible with spin 1 or higher particles, lest angular momentum conservation be damaged. (I have not reconciled this with the fact than a version of the standard model in which SU(2) remains unbroken leads to massless charged vector bosons, but this may have to do with the next sentence.) Some argue that you can solve some of the Lorentz invariance issues for the high-spin cases by giving the particle other couplings, and then there are some that show these solutions typically (in turn) introduce mass terms indirectly anyway. I should note that the original paper demonstrating that charge is incompatible with spin-1+ particles does not itself prove that it is compatible with spin-0 or spin-1/2 particles.
However, I’m sticking with a summary answer of: yes, because the standard model with unbroken gauge symmetries (or, alternatively, a Higgs field vacuum expectation value taken to zero) would lead to massless charged particles, and I know of no reason why you can’t do either of these things.
Do you just mean massless spin 1 particles, there? Because the W would be a counterexample otherwise.