Somewhere in the Milky Way, if the arithmetic is kind, there may be a committee much like ours arguing about funding. Or there may be nobody at all. The remarkable thing is that we can now say, with real precision, exactly which pieces of ignorance stand between us and the answer.

In 1961 a young radio astronomer needed an agenda for a three-day meeting about a subject most of his colleagues considered slightly disreputable. He broke the impossible question into seven smaller ones and wrote them on a blackboard. He was not predicting anything. He was managing a discussion. The line of reasoning escaped the room anyway, and it has been loose in the world ever since, turning up in textbooks, films, policy debates, and arguments between strangers who have had one drink too many.

This book takes that famous chain of reasoning apart and looks at every link. It follows the men who assembled it, the telescope they pointed at two ordinary stars, the false alarm that briefly convinced them they had succeeded, and the six decades of magnificent, expensive, informative silence that followed. It explains what has genuinely been settled since, what has quietly gotten worse, and why one item at the end of the chain turns an astronomy problem into a question about human nature that no telescope will ever resolve.

There is no mathematics here. There are, instead, dolphins, a Nobel Prize delivered by telegram in the middle of the meeting, a message fired at a distant star cluster that will arrive long after every language now spoken has been forgotten, two great biologists disagreeing about whether intelligence was ever a good bet, and a sky that has said nothing at all.

The silence is real. It is also evidence. What it is evidence of depends entirely on what you believe about seven numbers, and that, it turns out, is the most revealing thing about you that anyone could possibly measure.