Editing Conservation of mass (section)

=== Modern physics ===
{{main|Mass–energy equivalence|Mass in general relativity}}

The law of conservation of mass was challenged with the advent of special relativity. In one of the [[Annus Mirabilis papers]] of [[Albert Einstein]] in 1905, he suggested an equivalence between mass and energy. This theory implied several assertions, like the idea that internal energy of a system could contribute to the mass of the whole system, or that mass could be converted into [[electromagnetic radiation]]. However, as [[Max Planck]] pointed out, a change in mass as a result of extraction or addition of chemical energy, as predicted by Einstein's theory, is so small that it could not be measured with the available instruments and could not be presented as a test of special relativity. Einstein speculated that the energies associated with newly discovered [[radioactivity]] were significant enough, compared with the mass of systems producing them, to enable their change of mass to be measured, once the energy of the reaction had been removed from the system. This later indeed proved to be possible, although it was eventually to be the first artificial [[nuclear transmutation]] reaction in 1932, demonstrated by [[Cockcroft-Walton generator|Cockcroft and Walton]], that proved the first successful test of Einstein's theory regarding mass loss with energy gain.

The law of conservation of mass and the analogous law of [[conservation of energy]] were finally generalized and unified into the principle of [[mass–energy equivalence]], described by [[Albert Einstein]]'s equation <math qid=Q35875>E = mc^2</math>. Special relativity also redefines the concept of mass and energy, which can be used interchangeably and are defined relative to the frame of reference. Several quantities had to be defined for consistency, such as the ''[[invariant mass|rest mass]]'' of a particle (mass in the rest frame of the particle) and the ''relativistic mass'' (in another frame). The latter term is usually less frequently used.

In [[general relativity]], conservation of both mass and energy is not globally conserved and its definition is more complicated.