This segment originally aired live on December 3, 2014. An edited version was included in a best-of episode of The Brian Lehrer Show on December 31, 2015. The unedited audio can be found here.

Is time an illusion? What happens to space and time inside a black hole? And would somebody please explain the Twins Paradox? 

Janna Levin, a professor of physics and astronomy at Barnard College and the author of the novels A Madman Dreams of Turing Machines and How the Universe Got Its Spots: Diary of a Finite Time in a Finite Space, talks about time from the perspective of an astrophysicist.