If there are aliens out there, why haven’t we heard from them yet? It is the biggest question in the universe. Philosophers have debated the question for millennia. For some, the sheer size of the universe makes it unlikely that life formed only once. For others, the remarkable complexity of life on Earth is testament to its uniqueness. Until recently, vague philosophical answers of this kind were the best science could do. The signs of life were far too ambiguous to pin down for certain, and ou