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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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migrated from Mar 22 '13 at 18:56

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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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Since gluons interact strongly with each other, the effective propogating states aren't massless gluons anymore, but massive lumps. – Siva Mar 22 '13 at 19:54
That is not correct. Interaction is not the same as mass. Colections of gluons might propagate as a massive object, but that's not the same as the mass of an individual gluon. – xxx_yyy Mar 23 '13 at 15:11
"effective propagating states" – Siva Mar 24 '13 at 2:24
You have to define that term. If, by it, you mean that the objects that propagate (ie the particles that we observe) are colorless collections of colored objects, I agree. The gluons themselves remain massless, however.. – xxx_yyy Mar 24 '13 at 20:34
This answer provides a more technical framing to the string/rubber band picture that I've heard described elsewhere. Curious: I think of gravity and electric force as emitting virtual force carriers (photons) in all directions, but this means that the number of carriers grows over time unless they are caught by another charged particle and terminate. Is there a finite (not growing) number of gluons in the universe? This would have to be true for massive force carriers b/c of energy conservation, but it's unclear for gluons. – Alan Rominger Jun 20 '13 at 18:05

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