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The Pauli exclusion principle, commonly referred to simply as the exclusion principle, is a quantum mechanical principle formulated by Wolfgang Pauli in 1925, which states that no two identical fermions may occupy the same quantum state. It is one of the most important principles in physics, primarily because the three types of particle from which ordinary matter is made - electrons, protons, and neutrons - are all subject to it. The Pauli exclusion principle underlies many of the characteristic properties of matter, from the large-scale stability of matter to the existence of the periodic table of the elements. Particles obeying the Pauli exclusion principle are called fermions. Apart from the familiar electron, proton and neutron, these include the neutrinos, the quarks (from which protons and neutrons are made), as well as atomFor alternative meanings see atom (disambiguation). An atom is a microscopic structure found in all ordinary matter around us. Atoms are composed of 3 types of subatomic particles: electrons, which have a negative charge; protons, which have a positive chs like helium-3Helium-3 is a non- radioactive and light isotope of helium. It has two protons but only one neutron in contrast to two neutrons in ordinary helium. Helium-3 is rare on Earth and sought-after for use in fusion. More abundant helium-3 is thought to exist on. All fermions possess "half-integer spinIn physics, spin is an intrinsic angular momentum associated with microscopic particles. It is a purely quantum mechanical phenomenon without any analogy in classical mechanics. Whereas classical angular momentum arises from the rotation of an extended ob", meaning that they possess an intrinsic angular momentumIn physics, angular momentum intuitively measures how much the linear momentum is directed around a certain point called the origin; the moment of momentum. Since angular momentum depends upon the origin of choice, one must be careful when discussing angu whose value is ( Planck's constantPlanck's constant denoted h is a physical constant that is used to describe the sizes of quanta. It plays a central role in the theory of quantum mechanics, and is named after Max Planck, one of the founders of quantum theory. It has a value of approximat divided by 2π) times a half-integer (1/2, 3/2, 5/2, etc.). In the theory of quantum mechanics, fermions are described by "antisymmetric states", which are explained in greater detail in the article on identical particles.
Particles that are not fermions are almost invariably bosonBosons named after Satyendra Nath Bose, are particles which form totally-symmetric composite quantum states. As a result, they obey Bose-Einstein statistics. The spin-statistics theorem states that bosons have integer spin. Bosons are also the only partics, which are particles described using "symmetric states" in quantum theory. Bosons are allowed to share quantum states, and possess integer spin. Examples of bosons include the photonFor the Japanese anime video, see Photon (anime). In physics, the photon (from Greek φοτος, meaning light is a quantum of excitation of the quantised electromagnetic field and is one of the elementary particles studied by qu and the W and Z bosons.
1 Connection to quantum state symmetry
The Pauli exclusion principle was originally formulated as an empirical principle. It was invented by Pauli in 1924 to explain experimental results in atomic spectroscopy, well before the 1925 formulation of the modern theory of quantum mechanics by Werner Heisenberg and Erwin Schrodinger. However, this does not mean that the principle is in any way approximate or unreliable; in fact, it is one of the most well-tested and commonly-accepted results in physics.
The Pauli exclusion principle can be derived starting from the assumption that a system of particles occupy antisymmetric quantum states. According to the spin-statistics theorem, particles with integer spin occupy symmetric quantum states, and particles with half-integer spin occupy antisymmetric states; furthermore, only integer or half-integer values of spin are allowed by the principles of quantum mechanics.
As discussed in the article on identical particles, an antisymmetric two-particle state in which one particle exists in state |ψ1⟩ and the other in state |ψ2⟩ is
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However, if |ψ1⟩ and |ψ2⟩ are just the same state, the above formula gives the zero ket:
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This does not represent a valid quantum state, because the state vectors representing quantum states must have norm 1. In other words, we can never find the particles in this system occupying the same quantum state.
