The mirror would have to be perfect, reflecting not just everyday radiation but more exotic forms such as gravitational waves — ripples in the fabric of spacetime caused by the acceleration of large masses. (newscientist.com)
The major reason is that Einstein's principle of equivalence, which correlates and consolidates the concepts of inertial and gravitational mass and which in turn introduces the curvature of spacetime in the general theory, has no analogue in Whitehead's theory.9 But perhaps more importantly for the purposes of this essay, Einstein's entire approach in special relativity presumes that the nature of causality in the universe has a direct bearing upon spatio — temporal structure. (religion-online.org)
Not all of the light rays (or photons) produced by matter falling into a black hole are trapped by the event horizon, a region of spacetime from which nothing can escape. (sciencedaily.com)