Is anybody out there? Probably. Most stars have planets; we've discovered more than 6,000 exoplanets thus far, and the most basic statistics point toward the existence of countless potentially habitable worlds in the Universe. But when we have looked for any scrap of evidence for alien civilizations, we have found nothing so far. The question is why? There are the usual ideas: life actually doesn't find a way; Earth is being kept in a cosmic zoo; civilizations destroy themselves as soon as they