Editing Conservation of energy (section)

===Mass–energy equivalence===
{{Main|Mass–energy equivalence}}
{{More citations needed section|date=November 2015}}
Matter is composed of atoms and what makes up atoms. Matter has [[rest mass|''intrinsic'' or ''rest'' mass]]. In the limited range of recognized experience of the nineteenth century, it was found that such rest mass is conserved. Einstein's 1905 theory of [[special relativity]] showed that rest mass corresponds to an equivalent amount of ''rest energy''. This means that ''rest mass'' can be converted to or from equivalent amounts of (non-material) forms of energy, for example, kinetic energy, potential energy, and electromagnetic [[radiant energy]]. When this happens, as recognized in twentieth-century experience, rest mass is not conserved, unlike the [[mass in special relativity|''total'' mass]] or ''total'' energy. All forms of energy contribute to the total mass and total energy.

For example, an [[electron]] and a [[positron]] each have rest mass.  They can perish together, converting their combined rest energy into [[photon]]s which have electromagnetic radiant energy but no rest mass.  If this occurs within an isolated system that does not release the photons or their energy into the external surroundings, then neither the total ''mass'' nor the total ''energy'' of the system will change.  The produced electromagnetic radiant energy contributes just as much to the inertia (and to any weight) of the system as did the rest mass of the electron and positron before their demise. Likewise, non-material forms of energy can perish into matter, which has rest mass.

Thus, conservation of energy (''total'', including material or ''rest'' energy) and [[conservation of mass]] (''total'', not just ''rest'') are one (equivalent) law. In the 18th century, these had appeared as two seemingly-distinct laws.