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E = mc2, equation in German-born physicist Albert Einstein’s theory of special relativity that expresses the fact that mass and energy are the same physical entity and can be changed into each other. In the equation, the increased relativistic mass (m) of a body times the speed of light squared (c2) is equal to the kinetic energy (E) of that body.
In physical theories prior to that of special relativity, mass and energy were viewed as distinct entities. Furthermore, the energy of a body at rest could be assigned an arbitrary value. In special relativity, however, the energy of a body at rest is determined to be mc2. Thus, each body of rest mass m possesses mc2 of “rest energy,” which potentially is available for conversion to other forms of energy. The mass-energy relation, moreover, implies that, if energy is released from the body as a result of such a conversion, then the rest mass of the body will decrease. Such a conversion of rest energy to other forms of energy occurs in ordinary chemical reactions, but much larger conversions occur in nuclear reactions. This is particularly true in the case of nuclear fusion reactions that transform hydrogen to helium, in which 0.7 percent of the original rest energy of the hydrogen is converted to other forms of energy. Stars like the Sun shine from the energy released from the rest energy of hydrogen atoms that are fused to form helium.
Quantum field theory, body of physical principles combining the elements of quantum mechanics with those of relativity to explain the behaviour of subatomic particles and their interactions via a variety of force fields. Two examples of modern quantum field theories are quantum electrodynamics, describing the interaction of electrically charged particles and the electromagnetic force, and quantum chromodynamics, representing the interactions of quarks and the strong force. Designed to account for particle-physics phenomena such as high-energy collisions in which subatomic particles may be created or destroyed, quantum field theories have also found applications in other branches of physics.
The prototype of quantum field theories is quantum electrodynamics (QED), which provides a comprehensive mathematical framework for predicting and understanding the effects of electromagnetism on electrically charged matter at all energy levels. Electric and magnetic forces are regarded as arising from the emission and absorption of exchange particles called photons. These can be represented as disturbances of electromagnetic fields, much as ripples on a lake are disturbances of the water. Under suitable conditions, photons may become entirely free of charged particles; they are then detectable as light and as other forms of electromagnetic radiation. Similarly, particles such as electrons are themselves regarded as disturbances of their own quantized fields. Numerical predictions based on QED agree with experimental data to within one part in 10 million in some cases.
There is a widespread conviction among physicists that other forces in nature—the weak force responsible for radioactive beta decay; the strong force, which binds together the constituents of atomic nuclei; and perhaps also the gravitational force—can be described by theories similar to QED. These theories are known collectively as gauge theories. Each of the forces is mediated by its own set of exchange particles, and differences between the forces are reflected in the properties of these particles. For example, electromagnetic and gravitational forces operate over long distances, and their exchange particles—the well-studied photon and the as-yet-undetected graviton, respectively—have no mass.
Mass, in physics, quantitative measure of inertia, a fundamental property of all matter. It is, in effect, the resistance that a body of matter offers to a change in its speed or position upon the application of a force. The greater the mass of a body, the smaller the change produced by an applied force. The unit of mass in the International System of Units (SI) is the kilogram, which is defined in terms of Planck’s constant, which is defined as equal to 6.62607015 × 10−34 joule second. One joule is equal to one kilogram times metre squared per second squared. With the second and the metre already defined in terms of other physical constants, the kilogram is determined by accurate measurements of Planck’s constant. (Until 2019 the kilogram was defined by a platinum-iridium cylinder called the International Prototype Kilogram kept at the International Bureau of Weights and Measures in Sèvres, France.) In the English system of measurement, the unit of mass is the slug, a mass whose weight at sea level is 32.17 pounds.