Currently the only way for us to get around in space is to use rocket fuel and chemical reactions to push the rocket around. This is very expensive and current rockets are 95% fuel by weight. However NASA is developing an alternative that uses the the sun to push the rocket around. These solar sails are made with large ultra-thin mirrors that use the light of the sun to push the sail around. These sails have to be reflective because when the light hits the said it has to be reflected. This they key because thanks to DeBroglie, we know that light has momentum and as the light changes direction it exerts an impulse on the said which accelerates the sail. Granted this force is tiny compared to rocket engines, but in space the the sail will accelerate as long as it sees sunlight which means it can go much farther without fuel on board.