Photons carry a small amount of momentum. This means that an object that absorbs or deflects a beam of light experiences a small force. With ordinary light sources, this radiation pressure is too small to be significant. But it is important on a cosmic scale (helping prevent gravitational collapse inside stars), and, more modestly, in the cell biology lab, where an intense focused laser beam can exert large enough forces to push small objects around inside a cell.

– molecular biology of the cell p. 575