It's the reverse. Light and gravity travel at c because they (at least light) are mediated by particles / systems that have no mass. Mass is what slows the spe

It's the reverse. Light and gravity travel at c because they (at least light) are mediated by particles / systems that have no mass. Mass is what slows the speed of causality for certain particles. For example, while the photons I emit may travel at C, the massive particles that make up 'me' cannot.