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Gluons are massless so move at the speed of light. Gluons also are supposed to exchange gluons themselves. Wouldn't this require the gluons which are exchanged between gluons to move even faster than light?

Qmechanic
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Anton
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1 Answers1

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There have been good comments to this question, which should be in an answer, because comments have a way of disappearing, so here they are:

For any two particles traveling at the speed of light, unless they're traveling in exactly the same direction, the speed required to intercept one from the other is less than c. – @probably_someone

and another:

Gluons are massless and move at the speed of light. The difference between a photon and gluon is a gluon has a charge of the QCD field, called color. As a result they strongly interact with each other as well as quarks. As a result they are scattered around in a tiny region, sometimes called the QCD bag. At high energy the coupling is reduced and the tend to behave more freely. The gluon then behaves more like a photon moving at the speed of light. Further, gluons most certainly do not move at speeds faster than light. – @Lawrence B. Crowell

And the last,

Your confusion seems to arise due to the misconception that if an object (such as a rocket) moving at the speed of light shoots something out in the direction of its motion then that something must travel faster than light. This is not true. See the relativistic velocity addition formula . @Prahar

The above hold generally in set ups where the velocities are high and special relativity has to be used to calculate numbers. I will address gluons in particular:

Gluons are defined within a mathematical model that describes elementary particle interactions, and exist in the table of particles of the standard model. .

The construction of the standard model is such, that one cannot measure a free gluon the way one can measure a free photon because of the structure of the quantum chromodynamic force, thus there is only the mathematical definition that keeps them within a confined volume generated by the QCD forces.

Gluons are always virtual, which means that the four vector describing them in the feynman diagrams is off mass shell, thus they have a variable mass under the integral which will finally give a prediction for a measurable quantity. Thus it is not meaningful to ask about the off mass shell kinematics within hadrons, which is the only place where gluons have an existence.

anna v
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  • If you're planning to add the comments into your answer, it might be a good idea to just go ahead and do it. At some point I'll be deleting those comments. – David Z Apr 20 '18 at 05:16
  • @DavidZ I will give them a day to think about it ? I will alert them – anna v Apr 20 '18 at 05:50
  • Nice work, Anna, especially the last paragraph. Classical notions of particles are rather useless when applied to off-shell gauge bosons. It's bad enough doing it with virtual photons, but they're pretty tame compared to gluons. ;) – PM 2Ring Apr 20 '18 at 06:35
  • @Prahar: While the addition of two velocities < c certainly always is < c; this argument doesn’t hold if both the velocities to be added are equal to c. – Anton Apr 20 '18 at 22:38
  • @Lawrence: Do you mean that gluons at low energy move at a speed < c -so have mass in that state -and act more like quarks themselves? – Anton Apr 20 '18 at 22:38
  • If at the speed of light the state of a particle -a massless gluon- is completely frozen in time so its energy cannot change as long as it moves at c, then how can it emit or absorb energy, another gluon? – Anton Apr 20 '18 at 22:44
  • @anton, take the pi0 , it has a velocity less than c. It decays to two gamma each with velocity c. The kinematics works, with lorenttz transforamtiions both forward and backward. frozen in time has no meaning in an ensamble of particles.Massless particles cannot emit or absorb, they can only interact with a coupling constant and change fourvectors. In the Big Bang situation before symmetry breaking all particles are massless and interact with each other. – anna v Apr 21 '18 at 03:26
  • @ Anton: We know that photons can move slower than c in a medium. One might almost think of it as similar. There though the medium is made of gluons in a complicated set of self interactions. There are so called glue-balls, which are wads of gluons interacting with each other where the glue-ball is massive. This is all related to the mass-gap problem. – Lawrence B. Crowell Apr 21 '18 at 12:32
  • If gluons move slower than c in some medium, then shouldn’t the same should hold for the gluons they emit and absorb?

    While I’m not really satisfied with these answers (thanks anyway, everybody!), I suppose they have to suffice.

    I do have another unanswered question you might help me with: https://physics.stackexchange.com/questions/399862/a-particle-must-have-energy-to-interact-with-the-higgs-field-as-energy-is-a-sou

     – Anton Apr 22 '18 at 23:17
  • Gluons are a mathematical construct of a quantum mechanical particle that can never be free but always virtual . The construct explains the real ( not virtual ) measurements of strong interactions in experiments. The always exist inside an integral, see the link, and their four vectors may be interpreted as going below or above c so as to create the mass needed in the integral, for an interval of dt dE or dx dp. which is always integrated between the limits of energy available. – anna v Apr 23 '18 at 03:56
  • @LawrenceB.Crowell On that note, see Origins of Mass by Frank Wilczek "Superconductivity is a prototype for spontaneous symmetry breaking and for mass-generation, since photons acquire mass inside superconductors. A conceptually similar but more intricate form [...] in the context of the electroweak standard model, gives us a successful, economical account of W and Z boson masses. It also allows a phenomenologically successful, though profligate, accommodation of quark and lepton masses." Also see http://frankwilczek.com/core.html – PM 2Ring Apr 23 '18 at 12:00
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    @PM 2Ring. Interestingly enough the Higgs mechanism involves a quartic scalar field theory. This has a degenerate vacuum state which permits Goldstone particles to be absorbed by the $W^{\pm}$ and $Z$ particles. A nonabelian gauge field is self interacting and results in a quartic term, though the field is vector. QCD in a strong limit is strongly self interacting and off shell gluon fields form a sort of self-energy condition. Most of the mass of a hadron is due to this mass-gap. – Lawrence B. Crowell Apr 25 '18 at 00:23