How can Gluon travel at speed of light when it is bound inside a proton or neutron?
It doesn't travel for a long distance. But that view is way too classical. Gluons on hadrons do not exist as real particles
How can Gluon travel at speed of light when it is bound inside a proton or neutron?
It doesn't travel for a long distance. But that view is way too classical. Gluons on hadrons do not exist as real particles, so it is meaningless to ask about their speed.
8 New Are you saying that Gluon is a virtual particle?
The gluons in hadrons: sure.
By specifying the gluons in hadrons, does it mean there are gluons that are NOT in hadrons..?
I think he meant they "aren't anywhere specifically" but when they are in hadrons we know they are there somewhere.
They can be in a QCD plasma, or part of hard parton showers before hadronization occurs. In both cases, their properties are not so far away from real particles.
Quarks and gluons have never ever been seen as asymptotic free particles. The reason is a property of QCD, called confinement, which is a non-perturbative phenomenon and thus not rigorously understood.
What's understood within perturbative QCD is asymptotic freedom, which is related to confinement but not the same. Asymptotic freedom says that in QCD the running coupling constant becomes small at large renormalization scales, i.e., for scattering process with large energy transfers, and thus perturbative QCD is only valid in that case.
At low energies the effective degrees of freedom and asymptotic states of QCD are not quarks and gluons but rather hadrons, i.e., bound states of quarks and gluons. The most convincing evidence for this picture of confinement is from lattice-QCD calculations, which are nowadays quite accurate in obtaining the mass spectrum of hadrons.
A way to study something close to free quarks and gluons is to create a socalled Quark Gluon Plasma (QGP) in ultrarelativistic heavy-ion collisions. There you smash atomic nuclei at very high energies together. Then a hot "blob" of strongly interacting matter is formed. Now one expects from the asymptotic freedom of QCD that for a hot and/or dense enough blob of matter that the hadrons melt into quarks and gluons.
However, also this picture is highly oversimplified. First of all one can perform lattice-QCD calculations also for many-body systems in thermal equilibrium, but one is restricted to the case of low net-baryon numbers (small baryo-chemical potentials), i.e., a situation reached on the largest energies at the Relativistic Heavy Ion Collider (RHIC) in Brookhaven, Upton, NY or at the Large Hadron Collider (LHC) at CERN, Geneva. Lattice-QCD predicts that the transition from a hot and dense hadron resonance gas to a QGP) (deconfinement transition) is a cross-over transition in a temperature range of about 150-160 MeV.
A lot of heavy-ion phenomenology can be understood by describing the hot and dense fireball of strongly interacting matter created in relativistic heavy-ion collisions in terms of relativistic hydrodynamics, and even with ideal relativistic hydrodynamics. This implies that the fireball comes (a) in a astonishingly short "formation time" of about 0.5-1 fm/c to local thermal equilibrium and (b) then evolves according to (nearly) ideal hydrodynamics. Comparing the hydro simulations of the hot and dense fireball with the measured particle spectra, leads to the conclusion that indeed energy densities and temperatures are reached which are well above the critical temperature range. This also implies that the plasma is still strongly coupled. In fact it is the most "sloshy" liquid ever observed on earth. The viscosity over entropy-density ratio is close to the lower bound i.e., ##\eta/s \simeq 1/(4 \pi)##.
This comparison leads to the picture that the hot and dense fireball expands as a kind of liquid drop, thereby becoming colder and more and more dilute. This implies that at one point the transition from the deconfined QGP phase to a hot hadron resonance gas occurs. Analyzing the particle abundancies leads to the conclusion that the thermal freezeout, i.e., the point at which the inelastic interactions within the fireball cease, occurs close to this transition, i.e., the particle abundancies are described by a (grand-)canonical ensemble at a temperature of about 160 MeV (at RHIC and LHC). Then the fireball evolution goes on in chemical off-equilibrium but there are still elastic interactions keeping the matter in local thermal equilibrium until also these cease at a temperature of about 100 MeV, and then the hadrons run freely to the detector.
The main chalenge of research in Heavy-Ion Collisions is thus to find signatures for this picture of the evolving QCD-matter fireb