I thought massless particles were mediators for long range forces such as electromagnetism and gravitation. How can the massless gluon mediate the short range strong force?
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Unlike photons, gluons carry the "charge" of the strong force (confusingly, it's called "color"). This means that, unlike photons, gluons interact with each other. The effect is that, rather than spreading out in all directions (as photons do), gluons tend to stick together and form strings. For example, two quarks (which have color) are not connected by a spread-out field, but by a string. The effect is that the force between the quarks doesn't weaken with distance, but is approximately constant. For an imperfect analogy, think of two balls connected by a rubber band.
Now consider trying to pull two quarks apart. Because the force (the tension in the "string") is roughly constant, the energy required is proportional to the separation between the quarks. (Work equals force times distance.) The tension is huge, about 50 tons (!! yes !!), so it requires an enormous amount of energy to separate the two quarks even by the size of an atomic nucleus. Thus, every object we see that is significantly larger than a nucleus is color neutral (no strings attached ;). Therefore, no long range forces - all because gluons aren't color neutral.
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