MASS & ENERGY > Implications In classical physics, mass was generally identified with the ‘quantity of matter’ and this quantity was supposed to be conserved in any physical interaction. In Special Relativity, mass no longer measures the quantity of matter, but rather becomes an abstract measure of inertia. The two classical principles of mass conservation and energy conservation no longer hold. They are replaced by the principle of conservation of mass-energy, a very general one, holding in any physical situation, regardless of the kind of interaction involved. This conceptual revolution occurred at a time when the inadequacy of the classical notion of material particle was also becoming apparent (see complementarity and indiscernibility). The efforts to provide a coherent theoretical framework for these discoveries culminated in the creation of Quantum Field Theory, the synthesis of quantum theory and Special Relativity.

When material particles undergo relativistic collisions (that is, collisions involving velocities close to c), they can be converted totally to electromagnetic energy and vice versa. More generally, any alteration of the energy of a body (for example by heating), entails a corresponding change in its mass.

In ordinary situations, the presence of the factor c² in eq. (1) makes this change too tiny to be detected. However, this is not always the case. In the explosion of an atomic bomb, the huge amount of energy released by a few tens of kilos of plutonium is equivalent to a mass amount as large as 1 gram.

The equivalence of mass and energy plays a central role in the explanation of many processes occurring on a cosmological scale, as well as in that of the sub-nuclear phenomena of high energy physics, in which particles appear to be continuously created and annihilated. The design of the particle accelerators used to study these processes must take into account the severe constraints imposed by eq. (2).

The practical implications of eq. (1) are far-reaching. Following the discovery that even the lightest massive particle provides a huge energy reservoir, it took only a few decades to build reactors in which nuclear energy could be produced.