Solar sails absorb light (sunlight) and the transfer of heat exerts pressure on them. This can be understood by studying radiometers. There are at least three types of radiometers (Nichols vacuum, Crookes imperfect vacuum, and MEMS electrostatic charges, magnetic moments, surface tension, and viscosity). Don't confuse MEMs nanotechnology with molecular nanotechnology devices that use surface chemistry. Assuming that the solar sails, in question, exist in the vacuum of space, they operate on the principles of a Nichols radiometer (which uses the principle of dispersed heat, rather than absorption of photons). The common myth is that photons turn a Nichols radiometer. If this was so, it would turn the opposite direction, since photons would be absorbed by the dark side and reflected off the light side of the metal. Instead, the Nichols and Crookes radiometer work by the transfer of heat. They turn in opposite directions due to the amount of vacuum.//////PHOTONS HAVE MASS, BUT NO REST MASS. Photons can't stand still, and must travel at the speed of light. Photons also have momentum.