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The Pauli exclusion principle is the "quantum mechanical principle which states that two or more "identical "fermions (particles with halfinteger "spin) cannot occupy the same "quantum state within a "quantum system simultaneously. In the case of "electrons in atoms, it can be stated as follows: it is impossible for two electrons of a polyelectron atom to have the same values of the four "quantum numbers: n, the "principal quantum number, ℓ, the "angular momentum quantum number, m_{ℓ}, the "magnetic quantum number, and m_{s}, the "spin quantum number. For example, if two electrons reside in the same "orbital, and if their n, ℓ, and m_{ℓ} values are the same, then their m_{s} must be different, and thus the electrons must have opposite halfinteger spin projections of 1/2 and −1/2. This principle was formulated by Austrian physicist "Wolfgang Pauli in 1925 for electrons, and later extended to all fermions with his "spin–statistics theorem of 1940.
Particles with an integer spin, or "bosons, are not subject to the Pauli exclusion principle: any number of identical bosons can occupy the same quantum state, as with, for instance, photons produced by a "laser and "Bose–Einstein condensate.
A more rigorous statement is that with respect to exchange of two identical particles the total "wave function is "antisymmetric for fermions, and symmetric for bosons. This means that if the space and spin coordinates of two identical particles are interchanged the wave function changes its sign for fermions, and does not change for bosons.
The Pauli exclusion principle describes the behavior of all "fermions (particles with "halfinteger "spin"), while "bosons (particles with "integer spin") are subject to other principles. Fermions include "elementary particles such as "quarks, "electrons and "neutrinos. Additionally, baryons such as "protons and "neutrons ("subatomic particles composed from three quarks) and some "atoms (such as "helium3) are fermions, and are therefore described by the Pauli exclusion principle as well. Atoms can have different overall "spin", which determines whether they are fermions or bosons — for example "helium3 has spin 1/2 and is therefore a fermion, in contrast to "helium4 which has spin 0 and is a boson.^{[1]}^{:123–125} As such, the Pauli exclusion principle underpins many properties of everyday matter, from its largescale stability, to the "chemical behavior of atoms.
"Halfinteger spin" means that the intrinsic "angular momentum value of fermions is (reduced "Planck's constant) times a "halfinteger (1/2, 3/2, 5/2, etc.). In the theory of "quantum mechanics fermions are described by "antisymmetric states. In contrast, particles with integer spin (called bosons) have symmetric wave functions; unlike fermions they may share the same quantum states. Bosons include the "photon, the "Cooper pairs which are responsible for "superconductivity, and the "W and Z bosons. (Fermions take their name from the "Fermi–Dirac statistical distribution that they obey, and bosons from their "Bose–Einstein distribution).
In the early 20th century it became evident that atoms and molecules with even numbers of electrons are more "chemically stable than those with odd numbers of electrons. In the 1916 article "The Atom and the Molecule" by "Gilbert N. Lewis, for example, the third of his six postulates of chemical behavior states that the atom tends to hold an even number of electrons in any given shell, and especially to hold eight electrons which are normally arranged symmetrically at the eight corners of a cube (see: "cubical atom).^{[2]} In 1919 chemist "Irving Langmuir suggested that the "periodic table could be explained if the electrons in an atom were connected or clustered in some manner. Groups of electrons were thought to occupy a set of "electron shells around the nucleus.^{[3]} In 1922, "Niels Bohr updated his model of the atom by assuming that certain numbers of electrons (for example 2, 8 and 18) corresponded to stable "closed shells".^{[4]}^{:203}
Pauli looked for an explanation for these numbers, which were at first only "empirical. At the same time he was trying to explain experimental results of the "Zeeman effect in atomic "spectroscopy and in "ferromagnetism. He found an essential clue in a 1924 paper by "Edmund C. Stoner, which pointed out that, for a given value of the "principal quantum number (n), the number of energy levels of a single electron in the "alkali metal spectra in an external magnetic field, where all "degenerate energy levels are separated, is equal to the number of electrons in the closed shell of the "noble gases for the same value of n. This led Pauli to realize that the complicated numbers of electrons in closed shells can be reduced to the simple rule of one electron per state, if the electron states are defined using four quantum numbers. For this purpose he introduced a new twovalued quantum number, identified by "Samuel Goudsmit and "George Uhlenbeck as "electron spin.^{[5]}^{[6]}
The Pauli exclusion principle with a singlevalued manyparticle wavefunction is equivalent to requiring the wavefunction to be antisymmetric. An antisymmetric twoparticle state is represented as a "sum of states in which one particle is in state and the other in state , and is given by:
and antisymmetry under exchange means that A(x,y) = −A(y,x). This implies A(x,y) = 0 when x = y, which is Pauli exclusion. It is true in any basis since local changes of basis keep antisymmetric matrices antisymmetric.
Conversely, if the diagonal quantities A(x,x) are zero in every basis, then the wavefunction component
is necessarily antisymmetric. To prove it, consider the matrix element
This is zero, because the two particles have zero probability to both be in the superposition state . But this is equal to
The first and last terms are diagonal elements and are zero, and the whole sum is equal to zero. So the wavefunction matrix elements obey:
or
According to the "spin–statistics theorem, particles with integer spin occupy symmetric quantum states, and particles with halfinteger spin occupy antisymmetric states; furthermore, only integer or halfinteger values of spin are allowed by the principles of quantum mechanics. In relativistic "quantum field theory, the Pauli principle follows from applying a "rotation operator in "imaginary time to particles of halfinteger spin.
In one dimension, bosons, as well as fermions, can obey the exclusion principle. A onedimensional Bose gas with deltafunction repulsive interactions of infinite strength is equivalent to a gas of free fermions. The reason for this is that, in one dimension, exchange of particles requires that they pass through each other; for infinitely strong repulsion this cannot happen. This model is described by a quantum "nonlinear Schrödinger equation. In momentum space the exclusion principle is valid also for finite repulsion in a Bose gas with deltafunction interactions,^{[7]} as well as for "interacting spins and "Hubbard model in one dimension, and for other models solvable by "Bethe ansatz. The "ground state in models solvable by Bethe ansatz is a "Fermi sphere.
The Pauli exclusion principle helps explain a wide variety of physical phenomena. One particularly important consequence of the principle is the elaborate "electron shell structure of "atoms and the way atoms share electrons, explaining the variety of chemical elements and their chemical combinations. An "electrically neutral atom contains bound "electrons equal in number to the protons in the "nucleus. Electrons, being fermions, cannot occupy the same quantum state as other electrons, so electrons have to "stack" within an atom, i.e. have different spins while at the same electron orbital as described below.
An example is the neutral "helium atom, which has two bound electrons, both of which can occupy the lowestenergy ("1s) states by acquiring opposite spin; as spin is part of the quantum state of the electron, the two electrons are in different quantum states and do not violate the Pauli principle. However, the spin can take only two different values ("eigenvalues). In a "lithium atom, with three bound electrons, the third electron cannot reside in a 1s state, and must occupy one of the higherenergy 2s states instead. Similarly, successively larger elements must have shells of successively higher energy. The chemical properties of an element largely depend on the number of electrons in the outermost shell; atoms with different numbers of occupied electron shells but the same number of electrons in the outermost shell have similar properties, which gives rise to the "periodic table of the elements.^{[8]}^{:214–218}
In "conductors and "semiconductors, there are very large numbers of "molecular orbitals which effectively form a continuous "band structure of "energy levels. In strong conductors ("metals) electrons are so "degenerate that they cannot even contribute much to the "thermal capacity of a metal.^{[9]}^{:133–147} Many mechanical, electrical, magnetic, optical and chemical properties of solids are the direct consequence of Pauli exclusion.
The stability of the electrons in an atom itself is unrelated to the exclusion principle, but is described by the quantum theory of the atom. The underlying idea is that close approach of an electron to the nucleus of the atom necessarily increases its kinetic energy, an application of the "uncertainty principle of Heisenberg.^{[10]} However, stability of large systems with many electrons and many "nucleons is a different matter, and requires the Pauli exclusion principle.^{[11]}
It has been shown that the Pauli exclusion principle is responsible for the fact that ordinary bulk matter is stable and occupies volume. This suggestion was first made in 1931 by "Paul Ehrenfest, who pointed out that the electrons of each atom cannot all fall into the lowestenergy orbital and must occupy successively larger shells. Atoms therefore occupy a volume and cannot be squeezed too closely together.^{[12]}
A more rigorous proof was provided in 1967 by "Freeman Dyson and Andrew Lenard, who considered the balance of attractive (electron–nuclear) and repulsive (electron–electron and nuclear–nuclear) forces and showed that ordinary matter would collapse and occupy a much smaller volume without the Pauli principle.^{[13]}^{[14]}
The consequence of the Pauli principle here is that electrons of the same spin are kept apart by a repulsive "exchange interaction, which is a shortrange effect, acting simultaneously with the longrange electrostatic or "Coulombic force. This effect is partly responsible for the everyday observation in the macroscopic world that two solid objects cannot be in the same place at the same time.
"Freeman Dyson and Andrew Lenard did not consider the extreme magnetic or gravitational forces that occur in some "astronomical objects. In 1995 "Elliott Lieb and coworkers showed that the Pauli principle still leads to stability in intense magnetic fields such as in "neutron stars, although at a much higher density than in ordinary matter.^{[15]} It is a consequence of "general relativity that, in sufficiently intense gravitational fields, matter collapses to form a "black hole.
Astronomy provides a spectacular demonstration of the effect of the Pauli principle, in the form of "white dwarf and "neutron stars. In both bodies, atomic structure is disrupted by extreme pressure, but the stars are held in "hydrostatic equilibrium by "degeneracy pressure, also known as Fermi pressure. This exotic form of matter is known as "degenerate matter. The immense gravitational force of a star's mass is normally held in equilibrium by "thermal pressure caused by heat produced in "thermonuclear fusion in the star's core. In white dwarfs, which do not undergo nuclear fusion, an opposing force to gravity is provided by "electron degeneracy pressure. In "neutron stars, subject to even stronger gravitational forces, electrons have merged with "protons to form "neutrons. Neutrons are capable of producing an even higher degeneracy pressure, "neutron degeneracy pressure, albeit over a shorter range. This can stabilize neutron stars from further collapse, but at a smaller size and higher "density than a white dwarf. Neutrons are the most "rigid" objects known; their "Young modulus (or more accurately, "bulk modulus) is 20 orders of magnitude larger than that of "diamond. However, even this enormous rigidity can be overcome by the "gravitational field of a massive star or by the pressure of a "supernova, leading to the formation of a "black hole.^{[16]}^{:286–287}