Mass-energy relation:

The mass-energy equivalence is the concept that the mass of a body is the measure of its energy content. In this concept the total internal energy E of the body at rest is equal to the product of its rest mass m and a appropriate conversion factor to transform from the units of mass to units of energy. The equivalence can be described by the equation

E = mc²

Where energy is represented by E, m is mass, and c is the speed of the light in a vacuum. The formula is consistent dimensionally and doesn't depend on specific system of measurement units. For instance, in many systems of natural units, the speed of light is set on 1, and the formula becomes the identity E = mc² thus the term mass-energy equivalence.

The equation indicates that the energy always exhibits mass in which ever form the energy takes. Mass-energy equivalence means that mass conservation becomes a restatement, or requirement, of the law of energy conservation, which is the first law of thermodynamics. Mass-energy equivalence does not imply that mass can be "converted" to energy, and indeed implies the opposite as well. Neither mass nor energy can be destroyed, but only moved from 1 location to another. Matter, when seen as certain types of particles, can be created and destroyed, but the precursors and products of such reactions retain both the original mass and energy; both of them remain unchanged throughout the process. E = mc² has at times it been used as an explanation for the origin of energy in nuclear processes, but mass-energy equivalence does not explain the origin of the energies. Instead, this relationship hardly indicates that the large amounts of energy released in such type of reactions can exhibit enough mass that the mass-loss can be measured, when the released energy has been removed from the system.