Krista Tippett, with On Being. Today: "Mathematics, Purpose, and Truth," with physicist and novelist Janna Levin. We tend to think time is like a straight arrow, always moving forward. But Einstein called that a stubbornly persistent illusion. And Janna Levin's novel, A Madman Dreams of Turing Machines, is structured to evoke time the way physicists know it — as relative and curved, with past, present, and future in a fluid interplay.