Perhaps the most well known of Einstein’s innovations in the theory of relativity is the E=mc2formula. It relates the inertial mass m of any object to its total energy content E (c is the speed of light in free space: 299792458 ms-1).
Let us first see how he derived this formula in his seminal 1905 paper (A. Einstein, ZurElektrodynamikbewegterKörper. Ann. Physik.322 (10), 891-921 (1905). He considered an object which radiates energy L/2 in opposite directions. The energy lost in the object’s rest frame is therefore L. He then used a result of his theory to compute the amount of energy gained in another rest frame moving with speed v relative to the first Accordingly, this amount is γL=(1–v2c-2)-0.5L.