[cyberax]

Theoretically, a massless particle can move slower than light. But then it'll have zero energy and momentum, so it won't be able to interact with anything.

[MattPalmer1086]

Thanks. The equations give the answer in the end, I found this explanation the best [1].

The only way to get a zero mass is if the energy is zero, or the velocity is c.

[1] https://profoundphysics.com/why-do-photons-have-no-mass-simp...