Colin Marshall talks to Sean Carroll, theoretical cosmologist specializing in dark energy and special relativity at the California Institute of Technology and blogger at Cosmic Variance. In his new book, From Eternity to Here: The Quest for the Ultimate Theory of Time, Carroll explores possible answers to the question, “Why does time always move forward, never backward?” Addressing the issue necessitates drawing from various domains of physics, going all the way back to the origin of the universe.