An example of an alternative conception:
It is often claimed that the law of conservation of energy, once considered almost 'sacrosanct' by scientists [conservation of energy is like a sacred commandment], was shown by Einstein to only be approximate. The famous equation, E=mc2, is said to refer to the conversion of mass into energy. The equation applies to all kinds of processes (chemical reactions, lifting a weight, boiling water), but because c (the speed of light in a vacuum) is large (c.300 000 000 ms-1), and so c2 even larger, the 'mass defect' only becomes of relevance in procersses involving mssive energy changes – such as nuclear reactions.
For example, if a uranium atom decays, the decay products are found to collectively have slighly less mass than the original atom – as long as their 'rest masses' are measured. A helium atom can be considered as a bound combination of 2 protons, 2 neutrons and 2 electrons. – but has a mass very slightly less than the sum of the rest masses of 2 isolated protons, 2 isolated neutrons and 2 isolated electrons (the difference is called the mass defect). The alternative conception is that the difference in mass in these cases is due to some mass being converted to energy when the uranium atom decays or the helium atom is formed.
Einstein, however claims that both mass and energy were conserved in such processes – as long as one considered everything inolved. For Einstein, mass-energy equivalence (the '=' in 'E=mc2') means that mass and energy are always associated, so that a mass defect is a measure not of a change in overall mass, but an indication of some mass (and associated energy) leaving the system being considered.
Energy emitted out when an atom decays, or when a new atom forms by fusion, in effect carries some mass away.
Read: Conceptions of mass defect
Read: 'The missing mass of the electron – Annihilating mass in communicating science'
Read 'How much damage can eight neutrons do? Scientific literacy and desk accessories in science fiction'
