system
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human
Are gluons massless?

I remember from my particle physics course that strong nuclear force has a very short range. This was explained by the fact that gluons have unlike photons a mass. Now wikipedia and other sources claim that gluon is massless. What happened? How is short range of strong nuclear force explained now?
gpt
Gluons are massless. The argument that relates the range of a force to the mass of the force-carrier particle works when the force in question is *perturbative*. This just means that the coupling constant that determines the strength of the interaction is small, so that the interactions between two interacting particles can be well-approximated by the exchange of a single force-carrying particle. If this approximation does not work, the interaction is said to be non-perturbative, and everything is much more difficult to calculate.

It turns out that whether or not an interaction is perturbative can depend on the energy (or distance scale) of the interaction, and in particular it turns out that the strong force is perturbative at high energies (or small distances), but it isn't at low energies (or large distances). The result is a phenomena known as *confinement*, which ensures that quarks can never separate from each other on large distances except in "colour-neutral" combinations like protons and neutrons. Colour-neutral combinations are the analogue of chargeless combinations of particles in electromagnetism. Since there is no net colour (like charge), the long distance interaction is very weak. 

The exact mechanism of confinement is not completely understood (because non-perturbative quantum field theory is very hard to calculate in) and it is an active area of research.