Flavour (particle physics) In particle physics, flavour or flavor refers to a species of an elementary particle. The Standard Model counts six flavours of quarks and six flavours of leptons. They are conventionally parameterized with flavour quantum numbers that are assigned to all subatomic particles, including composite ones. In atomic physics the principal quantum number of an electron specifies the electron shell in which it resides, which determines the energy level of the whole atom. If there are two or more particles which have identical interactions, then they may be interchanged without affecting the physics. In quantum chromodynamics, flavour is a global symmetry. Isospin, less ambiguously known as "isobaric spin", which has value I3 = 1/2 for the up quark and I3 = −1/2 for the down quark.Strangeness (S): Defined as S = −(ns − ns̅), where ns represents the number of strange quarks (s) and ns̅ represents the number of strange antiquarks (s). Flavour quantum numbers are additive. The particle data group.
Elementary particle In particle physics, an elementary particle or fundamental particle is a particle whose substructure is unknown, thus it is unknown whether it is composed of other particles. Known elementary particles include the fundamental fermions (quarks, leptons, antiquarks, and antileptons), which generally are "matter particles" and "antimatter particles", as well as the fundamental bosons (gauge bosons and Higgs boson), which generally are "force particles" that mediate interactions among fermions. A particle containing two or more elementary particles is a composite particle. Everyday matter is composed of atoms, once presumed to be matter's elementary particles—atom meaning "indivisible" in Greek—although the atom's existence remained controversial until about 1910, as some leading physicists regarded molecules as mathematical illusions, and matter as ultimately composed of energy. Soon, subatomic constituents of the atom were identified. Overview Main article: Standard Model
Field (physics) The magnitude and direction of a two-dimensional electric field surrounding two equally charged (repelling) particles. Brightness represents magnitude and hue represents direction. Oppositely charged (attracting) particles. A field may be thought of as extending throughout the whole of space. Defining the field as "numbers in space" shouldn't detract from the idea that it has physical reality. "The fact that the electromagnetic field can possess momentum and energy makes it very real... a particle makes a field, and a field acts on another particle, and the field has such familiar properties as energy content and momentum, just as particles can have". To Isaac Newton his law of universal gravitation simply expressed the gravitational force that acted between any pair of massive objects. The development of the independent concept of a field truly began in the nineteenth century with the development of the theory of electromagnetism. where
Graviton Theory The three other known forces of nature are mediated by elementary particles: electromagnetism by the photon, the strong interaction by the gluons, and the weak interaction by the W and Z bosons. The hypothesis is that the gravitational interaction is likewise mediated by an – as yet undiscovered – elementary particle, dubbed as the graviton. Gravitons and renormalization When describing graviton interactions, the classical theory (i.e., the tree diagrams) and semiclassical corrections (one-loop diagrams) behave normally, but Feynman diagrams with two (or more) loops lead to ultraviolet divergences; that is, infinite results that cannot be removed because the quantized general relativity is not renormalizable, unlike quantum electrodynamics. Comparison with other forces Unlike the force carriers of the other forces, gravitation plays a special role in general relativity in defining the spacetime in which events take place. Gravitons in speculative theories
Color confinement The color force favors confinement because at a certain range it is more energetically favorable to create a quark-antiquark pair than to continue to elongate the color flux tube. This is analoguous to the behavior of an elongated rubber-band. An animation of color confinement. Color confinement, often simply called confinement, is the phenomenon that color charged particles (such as quarks) cannot be isolated singularly, and therefore cannot be directly observed. Quarks, by default, clump together to form groups, or hadrons. Origin The reasons for quark confinement are somewhat complicated; no analytic proof exists that quantum chromodynamics should be confining. Models exhibiting confinement Besides QCD in 4D, another model which exhibits confinement is the Schwinger model. Compact Abelian gauge theories also exhibit confinement in 2 and 3 spacetime dimensions. Confinement has recently been found in elementary excitations of magnetic systems called spinons. Quarks
Spin (physics) In quantum mechanics and particle physics, spin is an intrinsic form of angular momentum carried by elementary particles, composite particles (hadrons), and atomic nuclei. Spin is a solely quantum-mechanical phenomenon; it does not have a counterpart in classical mechanics (despite the term spin being reminiscent of classical phenomena such as a planet spinning on its axis). Spin is one of two types of angular momentum in quantum mechanics, the other being orbital angular momentum. Orbital angular momentum is the quantum-mechanical counterpart to the classical notion of angular momentum: it arises when a particle executes a rotating or twisting trajectory (such as when an electron orbits a nucleus). The existence of spin angular momentum is inferred from experiments, such as the Stern–Gerlach experiment, in which particles are observed to possess angular momentum that cannot be accounted for by orbital angular momentum alone. where h is the Planck constant.
Physical law A physical law or scientific law, according to the Oxford English dictionary, "is a theoretical principle deduced from particular facts, applicable to a defined group or class of phenomena, and expressible by the statement that a particular phenomenon always occurs if certain conditions be present." Physical laws are typically conclusions based on repeated scientific experiments and observations over many years and which have become accepted universally within the scientific community. The distinction between natural law in the political-legal sense and law of nature or physical law in the scientific sense is a modern one, both concepts being equally derived from physis, the Greek word (translated into Latin as natura) for nature. Description Several general properties of physical laws have been identified (see Davies (1992) and Feynman (1965) as noted, although each of the characterizations are not necessarily original to them). Examples Laws as definitions
Tests of general relativity The very strong gravitational fields that must be present close to black holes, especially those supermassive black holes which are thought to power active galactic nuclei and the more active quasars, belong to a field of intense active research. Observations of these quasars and active galactic nuclei are difficult, and interpretation of the observations is heavily dependent upon astrophysical models other than general relativity or competing fundamental theories of gravitation, but they are qualitatively consistent with the black hole concept as modelled in general relativity. As a consequence of the equivalence principle, Lorentz invariance holds locally in freely falling reference frames. Classical tests Albert Einstein proposed three tests of general relativity, subsequently called the classical tests of general relativity, in 1916: The chief attraction of the theory lies in its logical completeness. Perihelion precession of Mercury Deflection of light by the Sun
Quantum gravity Quantum gravity (QG) is a field of theoretical physics that seeks to describe the force of gravity according to the principles of quantum mechanics. Although a quantum theory of gravity is needed in order to reconcile general relativity with the principles of quantum mechanics, difficulties arise when one attempts to apply the usual prescriptions of quantum field theory to the force of gravity. From a technical point of view, the problem is that the theory one gets in this way is not renormalizable and therefore cannot be used to make meaningful physical predictions. As a result, theorists have taken up more radical approaches to the problem of quantum gravity, the most popular approaches being string theory and loop quantum gravity. Strictly speaking, the aim of quantum gravity is only to describe the quantum behavior of the gravitational field and should not be confused with the objective of unifying all fundamental interactions into a single mathematical framework.
Antimatter In particle physics, antimatter is material composed of antiparticles, which have the same mass as particles of ordinary matter but have opposite charge and other particle properties such as lepton and baryon number. Encounters between particles and antiparticles lead to the annihilation of both, giving rise to varying proportions of high-energy photons (gamma rays), neutrinos, and lower-mass particle–antiparticle pairs. Setting aside the mass of any product neutrinos, which represent released energy which generally continues to be unavailable, the end result of annihilation is a release of energy available to do work, proportional to the total matter and antimatter mass, in accord with the mass-energy equivalence equation, E=mc2. Antiparticles bind with each other to form antimatter just as ordinary particles bind to form normal matter. For example, a positron (the antiparticle of the electron) and an antiproton can form an antihydrogen atom. History of the concept Notation Positrons