Quantum theory is counterintuitive to the point where the physicist sometimes seems to be battling insanity. We are asked to believe that a single quantum behaves like a particle in going through one hole instead of another but simultaneously behaves like a wave in interfering with a nonexistent copy of itself, if another hole is opened through which that nonexistent copy could have traveled (if it had existed). ¶ It gets worse, to the point where some physicists resort to a vast number of parallel but mutually unreachable worlds that proliferate to accommodate every alternative quantum event. Other physicists, equally desperate, suggest that quantum events are determined retrospectively by our decision to examine their consequences. Quantum theory strikes us as so weird, so defiant of common sense, that even the great physicist Richard Feynman was moved to remark, “I think I can safely say that nobody understands quantum mechanics.” Yet the many predictions by which quantum theory has been tested stand up, with an accuracy so stupendous that Feynman compared it to measuring the distance between New York and Los Angeles accurately to the width of one human hair. On the basis of these stunningly successful predictions, quantum theory, or some version of it, seems to be as true as anything we know.