When manmade probes finally reach other stars, they will not be powered by rockets. Instead, they may be riding on a gossamer-thin sail that is being blasted by a giant laser beam. Harry Atwater, Howard Hughes Professor of Applied Physics and Materials Science, is a project leader of the Breakthrough Starshot Program,which seeks to make these probes a reality. In a new paper published on May 7 in Nature Materials, Atwater explores some of the major challenges the project will face in its bid to make humanity an interstellar species. We recently sat down with him to talk about the program.