The show “3 Body Problem” premiered on March 21 and quickly became one of Netflix’s most-watched titles. It is an adventure story about a group of scientists contending with an extraterrestrial threat. But despite its science fiction trappings, the show is often based in real — and complex — scientific concepts, whether string theory or nanomaterials. In this episode, Dennis Overbye, The Times’s cosmic affairs correspondent, breaks down some of the more brain-bending science behind “3 Body Problem.”