“retrocausality” was indeed real and had been observed in numerous experiments, including one called “the delayed-choice quantum eraser.” Chisholm described it as “a tricked-out version of the classic double-slit experiment.” The original, Langdon knew, had stunned the world by proving that light traveling through a double-slitted barrier could move either as a particle or as a wave…and, inconceivably, it seemed to “decide” which way to act each time based on whether someone chose to observe it.
