&gt; The underlying question is: When we say in general relativity that particles can&#x27;t travel faster than light, what is that speed measured relative to?<p>It&#x27;s based on the observation of two points of references moving a different speeds shining light and the light from each traveling at the same speed. So the speed of light is measured relative to other light. The speed of light is constant, and it is so regardless of point of reference. Flashlights pointed in opposite directions has nothing to do with the speed of light. The light is traveling at the same speed in opposite directions.