The hidden physics complicating interstellar lightsails

If we're to reach another star, chemical propulsion will not get us there in any reasonable time frame. We're going to need a different propulsion technology, and one of the most promising seems to be a solar sail. These giant reflective surfaces form the basis of many interstellar mission concepts. Combined with giant lasers pushing them, they can be accelerated to speeds unreachable by any other current technologies.