I read photons have energy but no mass, so how can massless photons produce a force upon striking a solar sail?
EDWARD J. DUGAN III, CAMAS, WASHINGTON
Photons have no mass, but they do have energy, which is proportional to their frequency. We are familiar with this type of energy at various frequencies microwave ovens are one example, visible light is another. The fact that light has energy is obvious (you can feel the heat).