In physics, mass-energy equivalence is the relationship between mass and energy in a system's rest frame, where the two values differ only by a constant and the units of measurement.
The principle is described by the physicist Albert Einstein's famous formula: E = m c 2.
The formula defines the energy E of a particle in its rest frame as the product of mass (m) with the speed of light squared (c2). Because the speed of light is a large number in everyday units (approximately 3×108 meters per second), the formula implies that a small amount of rest mass corresponds to an enormous amount of energy, which is independent of the composition of the matter.
Rest mass, also called invariant mass, is the mass that is measured when the system is at rest. It is a fundamental physical property that is independent of momentum, even at extreme speeds approaching the speed of light.
Massless particles such as photons have zero invariant mass, but massless free particles have both momentum and energy. The equivalence principle implies that when energy is lost in chemical reactions, nuclear reactions, and other energy transformations, the system will also lose a corresponding amount of mass. The energy and mass, can be released to the environment as radiant energy, such as light, or as thermal energy.
The principle is fundamental to many fields of physics, including nuclear and particle physics.
Mass–energy equivalence arose from special relativity as a paradox described by the French polymath Henri Poincaré.
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Einstein was the first to propose the equivalence of mass and energy as a general principle and a consequence of the symmetries of space and time.
The principle first appeared in Does the inertia of a body depend upon its energy-content?, one of his Annus Mirabilis (Miraculous Year) papers, published on 21 November 1905.
The formula and its relationship to momentum, as described by the energy–momentum relation, were later developed by other physicists.
Mass–energy equivalence states that all objects having mass, or massive objects, have a corresponding intrinsic energy, even when they are stationary. In the rest frame of an object, whereby definition it is motionless and so has no momentum, the mass and energy are equivalent, and they differ only by a constant, the speed of light squared (c2).
In Newtonian mechanics, a motionless body has no kinetic energy, and it may or may not have other amounts of internal stored energy, like chemical energy or thermal energy, in addition to any potential energy it may have from its position in a field of force. These energies tend to be much smaller than the mass of the object multiplied by c2, which is on the order of 1017 joules for a mass of one kilogram (joules being measured in kg⋅m2⋅s-2, or units of mass times velocity squared).
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Due to this principle, the mass of the atoms that come out of a nuclear reaction is less than the mass of the atoms that go in, and the difference in mass shows up as heat and light with the same equivalent energy as the difference. In analysing these explosions, Einstein's formula can be used with E as the energy released and removed, and m as the change in mass.
In relativity, all the energy that moves with an object contributes to the total mass of the body, which measures how much it resists acceleration. If an isolated box of ideal mirrors could contain light, the individually massless photons would contribute to the total mass of the box, by the amount equal to their energy divided by c2.
For an observer in the rest frame, removing energy is the same as removing mass, and the formula m = E/c2 indicates how much mass is lost when energy is removed. In the same way, when any energy is added to an isolated system, the increase in the mass is equal to the added energy divided by c2.
The nuclear binding energy is the minimum energy that is required to disassemble the nucleus of an atom into its component parts. The mass of an atom is less than the sum of the masses of its constituents due to the attraction of the strong nuclear force. The difference between the two masses is called the mass defect and is related to the binding energy through Einstein's formula.
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The principle is used in modelling nuclear fission reactions, and it implies a great amount of energy can be released by the nuclear fission chain reactions used in both nuclear weapons and nuclear power.
Einstein used the centimetre gram second system of units (cgs), but the formula is independent of the system of units. In natural units, the numerical value of the speed of light is set to equal 1, and the formula expresses an equality of numerical values: E = m.
In the SI system (expressing the ratio E/m in joules per kilogram using the value of c in meters per second):
E/m = c2 = (299792458 m/s)2 = 89875517873681764 J/kg (≈ 9.0 × 1016 joules per kilogram).
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you don't understand it well enough.
Albert Einstein
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