This week, two conversations from the archives about parts of the world that are imperceptible to us, verging on almost unthinkable. We start with a moment of uncertainty in physics. Inspired by an essay written by physicist and novelist Alan Lightman, called The Accidental Universe (https://zpr.io/4965dUdNqtpQ), taken from a book of the same name. Former Radiolab co-host Robert Krulwich pays a visit to Brian Greene to ask if the latest developments in theoretical physics spell a crisis for science. He finds that we've reached the limit of what we can see and test, and we’re left with mathematical equations that can't be verified by experiments or observation. Then, come along as we kick rocks. And end up tumbling down a philosophical rabbit hole where the solid things around us might not be solid at all. We talk to Jim Holt, author of Why Does the World Exist? (https://zpr.io/UqHpLnDx2QNx) who points out that when you start slicing and sleuthing in subatomic particle land, trying to get to the bottom of what makes matter, you mostly find empty space. Your hand, your chair, the floor, it's all made up of mostly nothing. Robert and Jim go toe-to-toe over whether the universe is made up of solid bits and pieces of stuff, or a cloudy foundation that more closely resembles thoughts and ideas. Signup for our newsletter!! It includes short essays, recommendations, and details about other ways to interact with the show. Sign up (https://radiolab.org/newsletter)! Radiolab is supported by listeners like you. Support Radiolab by becoming a member of The Lab (https://members.radiolab.org/) today. Follow our show on Instagram, Twitter and Facebook @radiolab, and share your thoughts with us by emailing radiolab@wnyc.org. Leadership support for Radiolab’s science programming is provided by the Simons Foundation and the John Templeton Foundation. Foundational support for Radiolab was provided by the Alfred P. Sloan Foundation.