Every time astronomers learn to exploit a new signal from space, knowledge of the universe dramatically deepens. Light, seen through telescopes, reveals that our galaxy is not alone. Microwaves hint at the Big Bang. X-rays suggest the tumult near black holes. Francis Halzen’s discovery of high-energy cosmic neutrinos shifts the paradigm again, potentially offering clues to the greatest remaining mysteries. What is dark matter? How did the universe begin? Is there a theory of everything? Yet Halzen, a University of Wisconsin physicist, focuses on the search itself: “I love to learn. Just understanding things that you thought you could never understand, that is the great pleasure of doing physics.”