The next two notes describe the origins of Einstein’s most famous insight: the intimate connection between mass and energy, culminating in the legendary equation E=mc2.

Second letter to Habicht – In the summer of 1905, Einstein wrote a follow-up letter to his friend Conrad Habicht describing an additional insight he had developed after his special-relativity paper. In this second letter [L-Einstein, 1905b], he hinted at a revolutionary idea: that mass and energy were deeply connected, with mass being a form of concentrated energy [Isaacson-1].

Paper on mass–energy equivalence – Later that year, Einstein submitted a brief but profound paper titled “Does the Inertia of a Body Depend Upon Its Energy Content?” [Einstein, 1905e]. In just a few pages, he proposed that a body’s mass is directly related to its energy content by the now-iconic formula E = mc² (though the equation is presented in a slightly different form in the paper). This paper, although initially overlooked, later became one of the most famous scientific propositions in history, showing that energy and mass are interchangeable.