The boundaries of science are clear, and can be demarcated by the concept of falsifiability. Or so we learn in our science classes. But with some areas of science, falsifiability is not the critical feature, and may be impossible on theoretical or empirical grounds. Worrying about falsifiability might even get in the way of interesting ideas. With us to discuss the history of problems in science and falsifiability is Sean Carroll, a leading physicist and science communicator.