Apparently, I had some misunderstandings about what solar sails do. I had thought that they collected solar wind, which is a collection of sub-light particles streaming from the sun. In fact, they are propelled by light itself. I don't really get how that works, since I understood photons to have no mass, thus being unable to exert any force, but this is hardly my field of specialty. But assuming that it works, I don't get why it would be a good vehicle for interstellar travel. As Mallon writes,
Once a sailing vehicle leaves the solar system, it will run out of gas—that is, sunlight—and space-based lasers, put into orbit around the planets, would have to keep beaming the necessary juice over vast distances.
Solar sail proponent Louis Friedman suggests a mission that could carry a 1-ton payload to Alpha Centauri in under 20 years. Isn't this kinda impractical? Beaming lasers into deep space for 20 years? Aren't there other forms of propulsion that could do roughly the same job?