Is $E=mc^2$ true for all frame of references?
Which one is correct? $$E=mc^2$$ or $$E^2=(mc^2)^2+(pc)^2$$
I mostly seen $$E=mc^2$$ from my childhood, and when I was learning problem solving in classical mechanics I had seen $$E^2=(mc^2)^2+(pc)^2$$ I know than if a frame at rest then momentum is $0$ hence $$E=mc^2$$ is true. But if a frame is not at rest then $E =mc^2$ true for that frame?