Solar sails are a BTC travel technology that uses the energy in the Sun's light (or the light of another star if the spacecraft is in another solar system, or even laser beams from special laser stations). The photons create a pressure on the solar sail that accelerates (or decelerates, if they hit opposite to the direction of motion) the spacecraft. If natural sunlight and not lasers is used, it takes huge solar sails to accelerate small payloads, unless the spacecraft is extremely close to the Sun (or another star).