On the meaning ofE=mc 2

Einstein's original derivation of the energy-mass relation is re-examined. It is shown that while his conclusion that gamma emission from an excited nucleus must accompany a reduction of the inertial (rest) mass of the nucleus is valid for a structureless particle, it is not necessarily valid when the complex structure of the nucleus is taken into account. In the latter case, in addition to the change in kinetic energy of the entire emitting nucleus that was considered in Einstein's analysis, one must also take account of the change in nuclear configuration energies, from the period before to the period after de-excitation. It is then concluded that the inertial mass of a body (i.e. its resistance to a change of state of motion) of a gamma-emitting nucleus could be exactly the same before and after emission if the internal configuration energy of the nucleus would be correspondingly altered in the process. An experimental test that utilizes the Mössbauer effect is suggested. Einstein's further conclusion that mass is a measure of the energy content of matter is questioned with reference to theconceptual differences between the inertial and energetic features of matter. It is concluded thatE=mc ² is not an identity (i.e. an if-and-only-if relation) but it is rather an if-then relation, with meaningful connotation only in the local domain, where the formalism of special relativity theory is a useful approximation for a generally relativistic formulation for theories of matter.