Animation showing a laser (red box mounted on grey support) firing photons (purple) at a space craft to accelerate it. The photons are reflected from the craft's giant reflector. As the photons bounce back, they transfer momentum to the space ship, which speeds up. Although photons are massless, they do carry energy at the speed of light and this gives them momentum. At the end of the animation, the laser beam has diverged enough that some photos miss the reflector.