Do all massless particles (e.g. photon, graviton, gluon) necessarily have the same speed $c$?

Question

I suppose there was a discussion already on speed-of-gravity-and-speed-of-light.

But I silly wonder whether all the massless mediators of four fundamental forces, i.e.

Graviton: $g_{\mu\nu}$ (gravity)

Photon $\gamma$: $A_\mu$ (electromagnetism)

Gluons: $A_\mu^a$ (strong interactions)

Necessarily travel at the same speed? Is there a no-go theorem or theoretical proof to say that it is impossible to have these three mediators have different speeds?

Or does QCD confinement makes the story of gluons any different from gravitons and photons?

[ps. excluded massive $Z^{0}$ and $W^{\pm}$ bosons (weak interactions)]

Another way to say this: Speed of photon, graviton, gluon all equal to $c$? or Whether all massless particles necessarily have the same speed?

Note add: Notice however, in condensed matter systems, there can be emergent gauge fields and emergent massless particles (Dirac or Weyl cones), but their speeds need not be the same, unless there is some emergent symmetry...

Answer 1

If basic symmetry and homogeneity assumptions about the Universe hold, then yes, all massless real particles (see Anna V's answer for virtual particles must travel at a universal constant $c$, the speed of a massless particle, in all frames of reference.

Given these basic symmetry and homogeneity assumptions, one can derive the possible co-ordinate transformations for the relativity of inertial frames: see the section "From Group Postulates" on the Wikipedia Page "Lorentz Transformation". (Also see my summary here). Galilean relativity is consistent with these assumptions, but not uniquely so: the other possibility is that there is some speed $c$ characterizing relativity such that $c$ is the same when measured from all frames of reference. Time dilation, Lorentz-Fitzgerald contraction and the impossibility of accelerating a massive particle to $c$ are all simple consequences of these other possible relativities.

So now it becomes an experimental question as to which relativity holds: Galilean or Lorentz transformation? And the experiment is answered by testing how speeds transform between inertial frames. Otherwise put, the experimental question is are there any speeds that are the same for all inertial observers?. The question is not about measuring the values of any speed, but rather, how they transform. Now of course we know the answer: the Michelson Morley experiment found such a speed, the speed of light. So there are two conclusions here: (1) Relativity of inertial frames is Lorentzian, not Galilean (which can be thought of as a Lorentz transformation with infinite $c$) and (2) light is a massless particle, because light is observed to go at this speed that transforms in this special way.

Notice that at the outset of this argument we mention nothing about particles or any particular physical phenomenon (even though special relativity's historical roots were in light). It follows that, if $c$ is experimentally observed to be finite (i.e. Galilean relativity does not hold), then the specially invariant speed is unique: it can only be reached by massless particles and there can't be more than one such $c$ - the Lorentz laws are what they are and are the only ones consistent with our initial symmetry and homogeneity assumptions. So if we observed two different speeds transforming like $c$, this would falsify our basic symmetry and homogeneity assumptions about the World. No experiment gives us grounds for doing that.

This is why all massless particles have the same speed $c$.

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Update: Experimental Results

As is now common knowledge, the gravitational wave event GW170817 and gamma ray burst GRB170817A give strong experimental evidence of the equality of the speeds of light and gravitation. As discussed in:

Gravitational Waves and Gamma-Rays from a Binary Neutron Star Merger: GW170817 and GRB 170817A

the 1.7 second time delay between the gravitational wave arrival and the gamma ray burst, together with conservative assumptions about other sources of delay, yields an experimental bound on the fractional difference between the speed of light and of gravitation:

$$\frac{v_g-v_{em}}{c} \leq 3\times 10^{-15}$$

an impressive experimental bound indeed. Within the next 10 years, we probably shall see several such events, and thus this experimental bound will tighten further (unless something really theoretically unforeseen happens!).

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Mass From Confined Massless Particles

Incidentally, if we confine massless particles, e.g. put light into a perfectly reflecting box, the box's inertia increases by $E/c^2$, where $E$ is the energy content. This is the mechanism for most of your body's mass: massless gluons are confined and are accelerating backwards and forwards all the time, so they have inertia just as the confined light in a box did. Likewise, an electron can be thought of as comprising two massless particles, tethered together by a coupling term that is the mass of the electron. The Dirac and Maxwell equations can be written in the same form: the left and right hand circularly polarized components of light are uncoupled and therefore travel at $c$, but the massless left and right hand circular components of the electron are tethered together. This begets the phenomenon of the Zitterbewegung - whereby an electron can be construed as observable at any instant in time as traveling at $c$, but it swiftly oscillates back and forth between left and right hand states and is thus confined in one place. Therefore it takes on mass, just as the "tethered" light in the box does.

Answer 2

Another way to say this: Speed of photon, graviton, gluon all equal to c? or Whether all massless particles necessarily have the same speed?

You must not have been introduced to the concept of a virtual particle:

In physics, a virtual particle is a transient fluctuation that exhibits many of the characteristics of an ordinary particle, but that exists for a limited time. The concept of virtual particles arises in perturbation theory of quantum field theory where interactions between ordinary particles are described in terms of exchanges of virtual particles. Any process involving virtual particles admits a schematic representation known as a Feynman diagram, in which virtual particles are represented by internal lines.

A virtual particle is an internal line in a Feynman diagram which represents the propagator mathematics that has to be substituted to get the integral necessary for computing measurable quantities . Virtual particles have the quantum numbers of their homonymous ( having the same name) particles except not the mass. The mass is off shell.

So it is a general rule that massless particles travel at the velocity of light, but only when in external lines in Feynman diagrams. This is true for photons, and we thought it was true for neutrinos but were proven wrong with neutrino oscillations.

Gluons on the other hand we only find within a nucleus and these are by definition internal lines in Feynman diagrams and therefore are not constrained to have a mass of 0, even though in the theory they are supposed to. In the asymptotically free case, at very high energies they should display a mass of zero.

Answer 3

It is not hard to imagine a toy universe in which different fundamental forces propagate at different speeds. However, a necessary consequence of that would be violations of lorentzian symmetry, and the ability to triangulate a preferred rest frame.

Although I don't see a theoretical reason why these speeds need be the same (I might be missing something though, perhaps some stability arguments require it), empirically there isn't much room for different speeds.

Answer 4

I thought the same thing for a long time. I wondered why gluons don't fly out of the nucleus at the speed of $c$. The difference is that photons don't interact with other photons and gravitons don't interact with other gravitons. They can move around and pass through each other. On the other hand, gluons do interact with each other.

In fact, gluons form chains/flux tubes which is part of why quarks are confined. Gluons do travel at $c$ but not for very far before they interact with other quarks or gluons, which keeps them from moving any appreciable distance.