Aristotle argued almost 2,400 years ago that a perfect vacuum could never exist. Today, the concept of nothingness figures at least implicitly into almost every theory of modern physics. In this episode closing out season 2 of “The Joy of Why,” the theoretical physicist Isabel Garcia Garcia of New York University and the Institute for Advanced Study talks with host Steven Strogatz about the impact of quantum mechanics on the definition of a “true vacuum,” the possibility of false vacuums, how the concept of vacuum energy relates to the cosmological constant, and how her studies of the vacuum could help to resolve frustrating puzzles in string theory and cosmology. “The Joy of Why” will return with new episodes in early 2024.