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Even in ancient Greek society, philosopher-scientists engaged in heated debate about the origin, composition, and structure of our universe. Tracking our understanding of cosmology from then until now shows monumental shifts in thinking. So what did the Ancients think was the fundamental nature of the cosmos, and what kind of evidence did they use to support their theories? How did Copernicus provoke such a radical shift in cosmology? And what should we think about the status of scientific theories if they can be subject to such massive conceptual shifts? John and Ken ponder the cosmos with Carlo Rovelli from Aix-Marseille University, author of "Seven Brief Lessons on Physics."