How do you build a real-world machine to test the most abstract of theories? Janna Levin talks with Rai Weiss, one of the original designers of LIGO, the four-kilometer-long instrument that has now twice detected the distant reverberations of two black holes crashing into one another. Janna Levin is a theoretical physicist — she works with pen and paper to turn the elegant rules of the universe into theory. Rainer Weiss, or Rai, as he’s known, is an experimental physicist — he thinks about how to find and measure something that may or may not exist outside of theory. Weiss...