How did albert Einstein found out that E equals mc2?

Answer 1

The simplest derivation of this that I've seen considers an empty box far out in space, well away from the influence of any other matter or energy.

By Newton's laws of motion, the position of the center of mass of this box cannot change. But let's cause a photon to be emitted from one end of the box toward the other end; this photon has momentum, and therefore the box must recoil, moving in the opposite direction as the photon. Then the other end of the box absorbs the photon, stopping the motion of the box.

But now the box is in a slightly different position than before. Since the center of mass of the box cannot move, this must mean that the energy of the traveling photon has a mass equivalence.

Working the relevant equations leads directly to E = m * c^2

The actual route Einstein took to reach this conclusion was much different and less straightforward.

Answer 2

He created it in the year 1905