Conservation of Mass and Energy
The concept of mass–energy equivalence connects the concepts of conservation of mass and conservation of energy, which continue to hold separately in any isolated system (one that is closed to loss of any type of energy, including energy associated with loss of matter). The theory of relativity allows particles which have rest mass to be converted to other forms of mass which require motion, such as kinetic energy, heat, or light. However, the system mass remains. Kinetic energy or light can also be converted to new kinds of particles which have rest mass, but again the energy remains. Both the total mass and the total energy inside an isolated system remain constant over time, as seen by any single observer in a given inertial frame.
In other words, energy can neither be created nor destroyed, and energy, in all of its forms, has mass. Mass also can neither be created nor destroyed, and in all of its forms, has energy. According to the theory of relativity, mass and energy as commonly understood, are two names for the same thing, and neither one is changed nor transformed into the other. Rather, neither one exists without the other existing also, as a property of a system. Rather than mass being changed into energy, the view of special relativity is that rest mass has been changed to a more mobile form of mass, but remains mass. In the transformation process, neither the amount of mass nor the amount of energy changes, since both are properties which are connected to each other via a simple constant. Thus, if energy leaves a system by changing its form, it simply takes its system mass with it. This view requires that if either mass or energy disappears from a system, it will always be found that both have simply moved off to another place, where they may both be measured as an increase of both mass and energy corresponding to the loss in the first system.
Read more about this topic: E=MC^2
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