cyberax parent
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.
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...