I was going to comment this as well. Here is a thorough explanation of the missing term: https://physics.stackexchange.com/questions/143652/is-e2-mc22pc2-or-is-

I was going to comment this as well. Here is a thorough explanation of the missing term: https://physics.stackexchange.com/questions/143652/is-e2-mc22pc2-or-is-e-mc2-the-correct-one https://physics.stackexchange.com/questions/143652/is-e2-mc2... The thing that I love about this formulation of the equation is that it directly ties it to the Pythagorean theorem: c^2 = a^2 + b^2 . It really shows you just how pretty that equation really is!