In 1905 Einstein suggested that the new source of energy was none other than matter itself. Einstein’s 1905 relativity paper, “On the Electrodynamics of Moving Bodies”, was one of three he published that year, at age 26, during his spare time; he was at the time working as a patent clerk in Zurich. Another was a paper explaining Brownian motion in terms of kinetic theory (at a time when some people still doubted the existence of atoms), and the third proposed the existence of photons, thus laying the foundations for quantum theory.
The theory of relativity is essentially about experiencing two objects that move relative to one another, hence the name: theory of relativity. The effects of relativity occur only at high speeds, at velocities comparable to the speed of light: time increases, scales shorten, and the mass of the moving object increases.
He reasoned that if the expenditure of energy needed to accelerate an object resulted in an increase in the mass of an object, then a decrease in velocity must produce a decrease in the mass of an object. The exact mathematical relationship between the mass of an object and the energy it contained flowed directly from the equations of the special theory, and was expressed in the famous formula: E=mc2 that is, that the energy of a body is proportional to the mass of the body multiplied by the square of the speed of light.
E = mc2, which means that energy is "equivalent" to the mass. Energy can be converted to mass, and vice versa. This is the so-called "mass defect".
In 1908 physics and chemistry joined hands when Max Planck took note of Einstein’s equation and suggested that the phenomenon of radioactivity could be explained as the direct transformation of matter into energy.
Albert Einstein Theory of Relativity
