Imagine that I had the technology with which to visit my grandfather in his youth. Once there, what is to stop me from killing him before he’d had the opportunity to reproduce? But if I did succeed, who was it who had travelled backward. . .

Einstein showed that gravity is only a manifestation of mass, a curvature of the space-time continuum caused by the presense of mass. No mass, no gravity. Time is similar—it passes only in relationship to. . . stuff. Each astronomical object, each planet, has a single achievable time transfer influenced by—and that can be calculated from—net local gravitation effects. That interval depends on its own mass, its sun’s, and the galaxy’s.

Some assert that the realm of quantum mechanics is so removed from the realm of our senses we’re unequipped to judge.

Stop Hitler and what else do you alter? Millions of lives saved, sure, but billions of lives changed.