I've been Googling to find out how Einstein worked out his famous equation. Some texts (that may or may not be factual) suggest it was an educated guess and not even expressed as the equation initially. Wiki http://en.wikipedia.org/wiki/E=mc2#Einstein:_mass.E2.80.93energy_equivalence" that Planck was critical of the initial formula. Others go into a more dry discourse on historical moments that led to it. Does anyone have a link to something reliably factual that leads you along the trail of his discovery in a way that a layman could understand? One aspect that I've often found difficult to follow is how light speed came to be connected with it. From the outside, the connection isn't readily apparent. I read here somewhere that it has some connection to F=ma.