The mystery of mass-energy

J W Warren
1976 Physics Education  
The mass-energy relationship was announced by Einstein in 1905 in a short paper in Annalen der Physik entitled '1st die Tragheit eines Korpers von seinem Energiegehalt abhangig? ('Does the inertia of a body depend upon its energy content ?') In this paper Einstein assumed results which he had already published concerning energy transfer by electromagnetic radiation and the variation of the mass of a body with motion. He considered the transfer of energy by radiation from a body as seen by two
more » ... dy as seen by two observers in uniform relative motion. As an essential axiom Einstein assumed that the law of conservation of energy must apply. Momentum conservation is implicit in the analysis, but not explicit. It was deduced that 'if a body gives off the energy L in the form of radiation its mass diminishes by L/cz'. Einstein then generalized the argument to all forms of' energy transfer and hence concluded that: 'The mass of a body is a measure of its energy content'. He also asserted that 'radiation conveys inertia between the emitting and absorbing bodies'. The arguments of this pioneering article referred explicitly to inertial mass. The identity of inertial and gravitational mass was asserted by Einstein in a later paper (Einstein 1911) where he stated 'energy must therefore possess a gravitational mass which is equal to its inertia mass'. Clearly, since the mass of any body is a measure of its energy content, and similarly where energy is transferred there is a corresponding transfer of mass, then the law of energy conservation inevitably entails the conservation of mass. Einstein refers to 'that highly satisfactory result of the theory of relativity by which the law of conservation of mass is merged in the law of conservation of energy'. In another work (Einstein 1920) he writes 'The law of the conservation of mass of a system becomes
doi:10.1088/0031-9120/11/1/006 fatcat:gaoj5fvxnnfr7nctloenswchti