Do there exist many worlds, or is there but a single
world? This is one of the most noble and exalted
questions in the study of Nature.
—Saint Albertus Magnus (c. 1206–1280)
This is a book about possibilities. It is about the possibility that, within a decade or two, robotic or human explorers will drill into the Martian surface and discover microscopic life in subterranean pockets of liquid water. It is about the possibility of landing spaceborne submarines on Jupiter’s moon Europa, where they might melt their way through miles of ice and observe life swimming in a volcanically heated ocean. It is about the possibility of strange, cold-adapted life forms on Saturn’s moon Titan, a world on which we have already landed a robotic emissary, despite its being located nearly a billion miles away. It is about the possibility of SETI researchers detecting an unmistakable signal coming to us from a civilization that has grown up around a faraway star. It is about the possibility that we may already be surrounded by a galactic civilization, populated by beings who surpassed our own current level of development millions or even billions of years ago. Most of all, it is about the possibilities that await us, if and when we learn that we are not alone in the universe.
It doesn’t take long to begin to appreciate these and other possibilities, but you have to be in the right frame of mind. If you’re reading at night and you happen to live in a place with clear, dark skies, take a moment to put the book down and go out and look at the stars. If you live in a city or it is cloudy or daytime, close your eyes and picture yourself at a favorite vacation spot on a perfect night. Personally, I like the mountain lakes not far from my home in Colorado, where the stars sometimes shine so brightly that I can make out the constellations by their reflections in the still water.