system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do solar sails/photonic propulsion work if light is massless?
gpt
Light is massless but not momentum-less. You know that whole E = mc^2 equation? Well, that isn't the entire thing, actually. The actual equation is

E^2 = (mc^2 )^2 + (pc)^2 

where p is the momentum of the object in question. Photons have energy, and even with zero mass, they still have momentum, and so they can impart that momentum into something else.