I've previously learned that massive particles cannot achieve the speed of light.
But recently I read that, concerning the gels that refract and bounce light within around enough that it can travel at worldly speeds, and by extension how electromagnetism propagates through matter, that pure photons are thought of as interacting with the massive objects, gaining mass and moving at a speed less than the speed of light as a result.
I'm not sure if this is just how the paradigm is set up to frame the phenomena, or if this paradigm carries over seamlessly into other accepted theories elsewhere in science, but this made me think of the possibility of the exclusivity of mass and velocity applies not only to particles with speeds at c being unable to have mass, but also the converse, particles without mass being unable to have speeds less than c.
My question about the properties of all particles, with syntactical brevity:
Mass exclusive-or speed of light is true?