<P> The conservation of both relativistic and invariant mass applies even to systems of particles created by pair production, where energy for new particles may come from kinetic energy of other particles, or from one or more photons as part of a system that includes other particles besides a photon . Again, neither the relativistic nor the invariant mass of totally closed (that is, isolated) systems changes when new particles are created . However, different inertial observers will disagree on the value of this conserved mass, if it is the relativistic mass (i.e., relativistic mass is conserved but not invariant). However, all observers agree on the value of the conserved mass if the mass being measured is the invariant mass (i.e., invariant mass is both conserved and invariant). </P> <P> The mass - energy equivalence formula gives a different prediction in non-isolated systems, since if energy is allowed to escape a system, both relativistic mass and invariant mass will escape also . In this case, the mass - energy equivalence formula predicts that the change in mass of a system is associated with the change in its energy due to energy being added or subtracted: Δ m = Δ E / c 2 . (\ displaystyle \ Delta m = \ Delta E / c ^ (2).) This form involving changes was the form in which this famous equation was originally presented by Einstein . In this sense, mass changes in any system are explained simply if the mass of the energy added or removed from the system, are taken into account . </P> <P> The formula implies that bound systems have an invariant mass (rest mass for the system) less than the sum of their parts, if the binding energy has been allowed to escape the system after the system has been bound . This may happen by converting system potential energy into some other kind of active energy, such as kinetic energy or photons, which easily escape a bound system . The difference in system masses, called a mass defect, is a measure of the binding energy in bound systems--in other words, the energy needed to break the system apart . The greater the mass defect, the larger the binding energy . The binding energy (which itself has mass) must be released (as light or heat) when the parts combine to form the bound system, and this is the reason the mass of the bound system decreases when the energy leaves the system . The total invariant mass is actually conserved, when the mass of the binding energy that has escaped, is taken into account . </P> <P> In general relativity, the total invariant mass of photons in an expanding volume of space will decrease, due to the red shift of such an expansions . The conservation of both mass and energy therefore depends on various corrections made to energy in the theory, due to the changing gravitational potential energy of such systems . </P>

Who wrote the law of conservation of mass