Why do Massless Particles travel at the Speed of Light?
The Short Answer: “Massless” is a loose term. It should really be “zero rest mass”.
And that means: if you find a reference frame in which the particle is at rest then it will have zero mass.
But if something has zero mass it doesn’t exist. So you’ll never find such a reference frame. So the particle is always moving.
The exact speed at which it moves (the speed of light) is determined by the “permittivity” of the vacuum. If we had a Quantum theory of the vacuum (i.e. spacetime) the source of this permittivity would be very clear. But we do not yet have such a theory.
Content written and posted by Ken Abbott email@example.com