If massless objects ALWAYS travel at the speed of light and gluons are massless, how are they trapped within hadrons without a need for event horizon?

Question

From what I heard, unless we have some kind of other influence, all things and light move at the straight lines in spacetime. If they have a mass, then they can never reach the speed of light, but all the massless ones are constantly traveling at the speed of light.

Now, with photons it's easy to understand, that is: curvature of spacetime around the giant mass can be so big that they are eventually getting "trapped" within certain area, being unable to run away. HOWEVER, the curvature so big causes an "event horizon", i.e. we have a black hole now.

So, I recently learned from some youtube videos that gluons are massless, but only today I noticed this paradox:

1. Gluons are massless

2. They are massless, thus they should travel at the speed of light. That's what I heard many times, that massless things always travel at the speed of light.

3. Participating in strong interactions between the quarks, they are "trapped" within a very tiny region of spacetime that we call "hadron". We don't see some of the gluons running away or spilling outside of proton, resulting in breaking the "trap" and, for example, making the photon to just decay, right? So they are truly trapped there in some way, even though they should move at the speed of light.

4. Now, to make the light trapped within a small region of spacetime, we need curvature so big that it causes an event horizon, so now we have a Black Hole

5. So, is there a connection between the hadrons and black holes? Are they like a mini black holes, trapping gluons inside? If not, then how are they trapping something that travels at the speed of light without a curvature so big that it causes an event horizon?

Are some of my premises above wrong? Maybe some of the information I learned from science videos on youtube is false or I misunderstood it?

Where is the flaw in my reasoning above?

Or maybe gluons actually have some mass?

Answer 1

That quarks and gluons are trapped inside hadrons is called color confinement. As far as I know: it is unrelated to black holes but I don't read Phys Rev D... it's possible it has been suggested somewhere. (My PhD advisor caught me reading said journal in the library and laughs, "What you reading that for...it's physics porn". True story).

Color confinement is very technical, but the simple explanation is the QCD (aka: the color force) increases with range (where the range is roughly the size of a hadron). For quarks: when they become sufficiently separated, it's energetically favorable to produce a $q\bar q$ pair, with each new particle binding to a preexisting quark (or antiquark), thereby producing two regions of confinement. Since gluons carry color (that is, they interact with each other), the same principle applies to a gluon trying to escape confinement.

Note that this is very different from photons, which do not interact with each other (to 1st order, or below the Schwinger Limit $~10^{18}\, {\rm V/m}$). Nevertheless, the phenomenon of pair production in strong fields (say near a nucleus with $Z \approx 1/\alpha \approx 137$, which can be assembled briefly in heavy ion collisions) may occur. This is called "sparking of the vacuum".

Back to gluons. Gluons attempting to escape hadron has been observed in 3-jet events.(See figure).

In this figure, an escaping gluon has dressed itself with quarks in a process called hadronization. That there are 3 is unique to gluon-gluon coupling, and was considered a major validation of QCD.

Also: above the Hagedorn temperature $1.7 \times 10^{12}\,$K), it's believed confinement breaks down and hadrons phase transition to quark matter.

Answer 2

Now, to make the light trapped within a small region of spacetime, we need curvature so big that it causes an event horizon, so now we have a Black Hole

Photons do not carry an electric charge. So photons cannot create photons (directly). As a result, photons can travel off into space and will continue to do so until they hit something that does have a charge.

Toy model follows:

So one might imagine standing in front of a wood stove and the electrons in the atoms of the steel are relaxing back to ground state and giving off IR photons which travel through space until they hit you, at which point they excite electrons in your skin and those relax thermally and warm you up. The photons in question "disappear" as part of this reaction.

Gluons do have a color charge. That means gluons will react with other gluons, and even form glueballs (at least in theory), in a highly non-linear and complex fashion.

So, unlike a photon which is free to fly off into space, gluons will react with each other and the quarks that most likely caused them to come into existence. When they do try to fly off, the resulting increase in energy becomes so great that entire new particle pairs are created and the gluons are "used up" in that interaction (sort of). Those particles then react with the other quarks and that's what's holding the nucleus together, the nuclear force.

So the gluons are "bound" due to these many interactions. There's simply no analog of these with photons. There's nothing to do with gravity and event horizons in either case, particles appear and disappear all the time.