What isnt usually mentioned is that this equation is a special case valid in some circumstances. The more general relation is E^2=(mc^2)^2+(pc)^2 which includes

What isnt usually mentioned is that this equation is a special case valid in some circumstances. The more general relation is E^2=(mc^2)^2+(pc)^2 which includes momentum term, and applies to photons which dont have a mass. Here's a simple video about this from Fermi Lab, https://www.youtube.com/watch?v=eOCKNH0zaho https://www.youtube.com/watch?v=eOCKNH0zaho Also interesting is how conservation laws for collisions are handled much better in relativistic setting.