- #26

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That's the definition of a massless particle. For the four-momentum you have ##p_{\mu} p^{\mu}=m^2 c^2##. The speed is ##v=\beta c## withPeroK said:

"Moreover, it can be shown that anything massless must travel at this invariant speed c."

Could you provide a reference that shows this?

$$\beta=\frac{|\vec{p}|}{p^0}=\frac{|\vec{p}|}{\sqrt{m^2c^2 +\vec{p}^2}}.$$

For ##m=0## you get ##\beta=1##, i.e., ##v=c##.

Note that in the real world there's no massless point-like object. In a very delicate sense you can consider "light beams" as trajectories of fictitious massless particles, but as I said that's a very delicate issue, and one should not (!!!) call these massless particles "photons" since photons are not describable at all in the sense of classical point particles, but that's another story.