Are photons massless? They have energy and can be affected by gravity, so don't they have mass? I've heard plenty of people say that photons are massless, does this just mean that they have negligible mass? Also, if only light/photons can move at the speed of light, then wouldn't everything be light considering that, relative to the photons, they aren't moving while everything else is moving at the speed of light? I'm not super knowledgable about physics so I expect I have errors in a few of my assumptions so please correct me, I'm just curious and have been thinking about this.