[ Ссылка ] ... Albert Einstein's Theory of Relativity (Chapter 4): E = mc² (Mass-Energy Equivalence)

In physics, mass-energy equivalence is the concept that the mass of a body is a measure of its energy content. In this concept the total internal energy E of a body at rest is equal to the product of its rest mass m and a suitable conversion factor to transform from units of mass to units of energy. If the body is not stationary relative to the observer then account must be made for relativistic effects where m is given by the relativistic mass and E the relativistic energy of the body.

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Albert Einstein proposed mass-energy equivalence in 1905 in one of his Annus Mirabilis papers entitled "Does the inertia of a body depend upon its energy-content?" The equivalence is described by the famous equation E = mc2, where E is energy, m is mass, and c is the speed of light in a vacuum. The formula is dimensionally consistent and does not depend on any specific system of measurement units. For example, in many systems of natural units, the speed of light is set equal to 1, and the formula becomes the identity E = m; hence the term "mass-energy equivalence".

The equation E = mc2 indicates that energy always exhibits mass in whatever form the energy takes. Mass-energy equivalence also 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 may be 'converted' to energy, and indeed implies the opposite. Modern theory holds that neither mass nor energy may be destroyed, but only moved from one location to another.

In physics, mass must be differentiated from matter, a more poorly defined idea in the physical sciences. 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 which remain unchanged (conserved) throughout the process. Letting the m in E = mc2 stand for a quantity of "matter" may lead to incorrect results, depending on which of several varying definitions of "matter" are chosen.

E = mc2 has sometimes been used as an explanation for the origin of energy in nuclear processes, but mass-energy equivalence does not explain the origin of such energies. Instead, this relationship merely indicates that the large amounts of energy released in such reactions may exhibit enough mass that the mass-loss may be measured, when the released energy (and its mass) have been removed from the system.

Einstein was not the first to propose a mass-energy relationship. However, Einstein was the first scientist to propose the E = mc2 formula and the first to interpret mass-energy equivalence as a fundamental principle that follows from the relativistic symmetries of space and time.

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