In 1950, the great physicist Enrico Fermi posed a question that people have been puzzling over ever since: Where is everybody? The universe has been around for billions of years, so why haven't we seen any signs of alien civilizations? This episode features physicist Stephen Webb, who describes some of the potential solutions to the puzzle. Stephen and Julia also discuss questions such as: What evidence have we gotten so far that helps us answer the Fermi problem? How do we estimate how rare/difficult it is for human-level intelligence to evolve? And why does it matter what the answer to Fermi's question is?