E=m*c**2 or ... I'm reading the book The Ring of Truth, written in the 80s as a companion for the PBS series of the same name. The author is discussing energy and the Subject equation. He remarks that another form is E=M, and c**2 is considered a "conversion factor". Is he using E=M as a concept or an "actual" equation? Is the conversion factor malleable depending upon circumstances? I would think a concept.