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Hi everybody and happy 2019. In my teaching sessions sometimes someone asks questions i cannot truly answer (although i have many arguments on it) and here's one that really puzzles me:

A massless (real) particle moves at $c$; now, since electric charge is always associated with the presence of mass, if you move at $c$ you cannot carry any electric charge, nevertheless you can still carry color charge, why?

I think that a more fundamental question for this would be to ask why if you are massless you actually must move at speed $c$ plus to understand the nature of the e.m. interaction and strong force one. Any help to give my student an answer that is not a labirynth of theorical burning hell?

The point here is not if charged massless particle can or cannot exist but if in principle this apply to all kind of charges (e.g. elecrtic, color, etc.) that is to say if the property of a filed of self-interaction is somehow necessarely linked to the property of having no mass (or alternatively spin 0)

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  • $\begingroup$ Related physics.stackexchange.com/q/7905 $\endgroup$
    – mavzolej
    Commented Jan 1, 2019 at 15:38
  • $\begingroup$ How would you define the momentum of a massless particle that doesn't travel at speed c? $\endgroup$
    – Bill N
    Commented Jan 1, 2019 at 16:45
  • $\begingroup$ The part about moving at $c$ doesn't seem particularly relevant here. It's a separate fact about special relativity that massless particles move at $c$. $\endgroup$
    – user4552
    Commented Jan 1, 2019 at 23:24

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...if you move at $c$ you cannot carry any electric charge, nevertheless you can still carry color charge...

This seems to be assuming that there is some kind of massless particle that carries color charge. As far as I know, that is not correct. Unlike photons, gluons do not occur as isolated particles traveling at the speed of light, as explained here:

How do we know that gluons travel at the speed of light?

Many popular (and, confusingly, many technical) presentations do use the massless-particle language to describe gluons, but it's important to realize that what they're really describing is a computational procedure, not a physical prediction, as described in this post:

Why do nearby charges increase the probability of virtual particles?

In quantum chromodynamics (QCD), it is true that the gluon field doesn't have any associated mass term in the definition of the model. However, even if we also set the quark mass terms to zero in the definition of the model (the up- and down-quark masses are nearly zero anyway), what the model predicts is a spectrum of color-neutral particles: baryons, mesons, and color-neutral glueballs. For a simplified-but-defensible graphic depiction of each of these three types of object, I recommend figure 1.5 in

It's a technical paper, but the figure is simple and still manages to convey some legitimate intuition about the structure of color-neutral objects in QCD. Here's the idea: The electromagnetic field does not carry electric charge, so it does not interact directly with itself. It only interacts with charge particles (which all have non-zero mass). In contrast, the gluon field does carry color charge, so it does interact directly with itself. The interaction is attractive, so the gluon field tends to gather itself into a rope-like structure called a flux tube that connects the quarks to each other. This is related to why quarks are confined into color-neutral baryons and mesons: the energy in the rope is (roughly) proportional to its length (and the proportionality constant is called the string tension), so the force doesn't get any weaker with distance. And here's the important part: What does the gluon field do if there are no quarks? Well, it still gathers itself into a rope-like flux tube, but this time the rope doesn't have any ends: its more like a circle, a flux loop. This is a (somewhat legitimate) picture of a glueball. A glueball is color-neutral and has a non-zero mass.

The bottom line is that, as far as I know, there are no massless particles that carry color charge, with the caveats explained in the first post cited above.

(By the way, some candidates for real-life glueballs have been noted in particle-accelerator experiments, but as far as I know, none of them have been unambiguously confirmed. Here's a recent example of a paper analyzing one such candidate: https://arxiv.org/abs/1810.08067)

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  • $\begingroup$ Thank you! this part of the theory is quite obscure to me and i always wanted to dig more in. This explains more things to me and also that at the end of the game it is wrong to focus on the "quality" of the charge rather than on the pure interaction... $\endgroup$
    – Pietro Oliva
    Commented Jan 1, 2019 at 18:52
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    $\begingroup$ @PietroOliva You're welcome. I have a confession, though: the reason I didn't directly address the question about why an electrically-charged particle can't move at the speed of light is because I don't know the answer! I'm not even sure that's a theoretical impossibility (where by "theoretical" I mean "in quantum field theory"). There's another post (physics.stackexchange.com/q/7905/206691) discussing how we know that such things don't exist in our world, but that's separate from the question about whether or not they could exist in a self-consistent theoretical world. Great question! $\endgroup$
    – Chiral Anomaly
    Commented Jan 1, 2019 at 19:04

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