Here's a shorter article for your review. Any errors? ---- I was wondering, how did Albert Einstein figure this out? So I got an English translation of the original article, which is entitled "Does the Inertia of a Body Depend On It's Energy Content?" This article is only a page long, and as you can see from that question mark Albert wasn't sure about this proposal. Also, the famous formula does not appear in the article, though here is E=L/c^2, which is quite close. There is nothing about converting particles to energy, but the germ of the idea is there. The paper was a small part of Einstein's intuition that energy and matter had much in common. It had been discovered that light could cause an electron to move. So light had momentum, just like matter did. If light could carry momentum, maybe it could carry inertia. So something that released energy would get lighter and something that absorbed energy maybe would grow heavier. However the effect was so slight it would be quite a challenge to measure, so it was hard to be certain it was real. Later on the seventh Duke of Broglie (a small wooded region that was the home of Claude Monet) guessed that matter had a wavelength just like energy did, and was proven to be correct. That's when the matter/energy equivalence program really got going. Once it seemed that matter and energy really were different aspects of the same thing, there needed to be a word that included both. Today when physicists talk about "mass," that may be what they mean. By the way, the question of "Does the Inertia of a Body Depend On It's Energy Content?" remains unanswered. It has always been observed that it does, everyone believes that it does, and a mechanism has been proposed and widely accepted, but recent experimental tests of that mechanism have been negative. That's what all that news about not being able to find the "Higgs boson" is about. So we think the answer is yes, but we don't know how it works.