&quot;When we say in general relativity that particles can&#x27;t travel faster than light, what is that speed measured relative to?&quot;<p>It&#x27;s a theory postulating that a particle can&#x27;t travel faster than the speed of light in relationship to a particle that isn&#x27;t moving.  In practice all particles are moving in relation to some other particle thus it&#x27;s not something you can visibly measure or see.  At least that is the way I always viewed it.