Something has bothered me for a long time about our model of light: that it has no mass. Just for a moment, forget what you know about light and lets call it the unknown, X, and suppose we want to model it. Here's what we know for certain: X has momentum, as seen by radiation pressure X is affected by gravity, as seen by gravitational lensing X can act as a particle and a wave So we take something with momentum that is affected by gravity and we can easily conclude that it has mass. And since it has mass, we can easily say that it must be a particle... And systems of particles can show wave behavior. So then we conclude that light is a particle with mass. How exactly did we come to the conclusion that Light has no mass?