The speed of light is measured, postulated, and used everywhere in physics, yet standard theory offers no mechanism for why this particular speed exists or why nothing exceeds it.
This paper derives c mechanically as the reorganisation speed of a physical substrate, from its stiffness and density, and explains why photons of every energy and gravitational waves all propagate at exactly this speed, as confirmed by the GW170817 multi-messenger observation to one part in ten to the fifteenth.