Physics has achieved extraordinary things — it can explain the behavior of matter from subatomic particles to galaxy clusters, trace the history of the universe back to fractions of a second after the Big Bang, and predict experimental outcomes with stunning precision. And yet, says theoretical physicist Sean Carroll, the deepest questions remain stubbornly, fascinatingly open.

In this lecture delivered at the Darwin College Lecture Series at Cambridge, Carroll takes listeners on a tour of the frontiers of modern physics — not the textbook version, but the genuinely unsettled, contested, and mysterious one. Three big themes structure the talk: the nature of quantum mechanics, the nature of space, and the nature of time.

Along the way, Carroll unpacks why physicists still don't agree on what quantum mechanics actually means — not just how to use it. He explores the many-worlds interpretation, the puzzle of quantum entanglement, and why the measurement problem is far from solved. He then turns to space and time: why does time have a direction at all? What does entropy really tell us about the arrow of time, the origin of complexity, and the fact that we — conscious beings — exist right now, at what Carroll calls "the fun Friday night of cosmic history"?