The conservation of energy and mass
Early in the twentieth century Albert Einstein put forward new ideas regarding the relationship between space, time, mass, and energy which have come to be known as the theory of relativity. It had long been accepted that matter could not be destroyed. This assumption was expressed in the law of conservation of matter, which states that the total quantity of matter in the universe is fixed and cannot be increased or decreased by human agency. Similarly, another law, called the law of conservation of energy, states that the total quantity of energy in the universe is also constant and can be neither created nor destroyed. While we cannot here enter into a discussion of the meaning of relativity, it may be pointed out that Einstein as simplified our picture of the universe by showing that the mass of a body is a measure of the quantity of energy contained in it. We find that, in a nuclear reactor, the sum total of the masses of the atoms produced as a result of fission is slightly less than the mass of the original uranium nuclei. The difference represents the mass of the energy liberated as heat, radiation, and kinetic energy of fission products. Thus, in the light of modern physics, we have to consider the laws of conservation of energy and mass as separate aspects of a single principle. We now take the view that the sum total of mass plus energy in the universe is fixed.