August 1977. Astronomers detect the so-called "WOW" signal: a burst of sound from deep space, near the constellation Sagittarius. The signal is believed to be extraterrestrial in origin. The next month, the astronomers send a reply. Boy, is that a huge mistake. Because more than four decades later, the alien intelligence has sent an invasion force of exactly one photon--and it's the kind of microscopic threat we could have never seen coming... In...