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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 one of two types of angular momentum in quantum mechanics, the other being orbital angular momentum. The orbital angular momentum operator is the quantum-mechanical counterpart to the classical angular momentum of orbital revolution: it arises when a particle executes a rotating or twisting trajectory (such as when an electron orbits a nucleus). "Angular Momentum Operator Algebra", class notes by Michael Fowler A modern approach to quantum mechanics, by Townsend, p. 31 and p. 80 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. In some ways, spin is like a vector quantity; it has a definite magnitude, and it has a "direction" (but quantization makes this "direction" different from the direction of an ordinary vector). All elementary particles of a given kind have the same magnitude of spin angular momentum, which is indicated by assigning the particle a spin quantum number. The SI unit of spin is the ( N· m· s) or ( kg·m2·s−1), just as with classical angular momentum. In practice, spin is given as a dimensionless spin quantum number by dividing the spin angular momentum by the reduced Planck constant , which has the same units of angular momentum, although it should be noted that this is not the full computation of this value. Very often, the "spin quantum number" is simply called "spin" leaving its meaning as the unitless "spin quantum number" to be inferred from context. When combined with the spin-statistics theorem, the spin of electrons results in the Pauli exclusion principle, which in turn underlies the periodic table of chemical elements. Wolfgang Pauli was the first to propose the concept of spin, but he did not name it. In 1925, Ralph Kronig, George Uhlenbeck and Samuel Goudsmit at Leiden University suggested an erroneous physical interpretation of particles spinning around their own axis. The mathematical theory was worked out in depth by Pauli in 1927. When Paul Dirac derived his relativistic quantum mechanics in 1928, electron spin was an essential part of it.

Quantum number

As the name suggests, spin was originally conceived as the rotation of a particle around some axis. This picture is correct so far as spin obeys the same mathematical laws as quantized angular momenta do. On the other hand, spin has some peculiar properties that distinguish it from orbital angular momenta: The conventional definition of the spin quantum number, , is }}, where can be any non-negative integer. Hence the allowed values of are 0, , 1, , 2, etc. The value of for an elementary particle depends only on the type of particle, and cannot be altered in any known way (in contrast to the spin direction described below). The spin angular momentum, , of any physical system is quantized. The allowed values of are S = \hbar \, \sqrt{s (s+1)}=\frac{h}{4\pi} \, \sqrt{n(n+2)}, where is the Planck constant and }} is the reduced Planck constant. In contrast, orbital angular momentum can only take on integer values of ; i.e., even-numbered values of .

Fermions and bosons

Those particles with half-integer spins, such as , , , are known as fermions, while those particles with integer spins, such as 0, 1, 2, are known as bosons. The two families of particles obey different rules and broadly have different roles in the world around us. A key distinction between the two families is that fermions obey the Pauli exclusion principle; that is, there cannot be two identical fermions simultaneously having the same quantum numbers (meaning, roughly, having the same position, velocity and spin direction). In contrast, bosons obey the rules of Bose–Einstein statistics and have no such restriction, so they may "bunch together" even if in identical states. Also, composite particles can have spins different from their component particles. For example, a helium atom in the ground state has spin 0 and behaves like a boson, even though the quarks and electrons which make it up are all fermions. This has profound consequences:
  • Quarks and leptons (including electrons and neutrinos), which make up what is classically known as matter, are all fermions with spin {{sfrac. The common idea that "matter takes up space" actually comes from the Pauli exclusion principle acting on these particles to prevent the fermions that make up matter from being in the same quantum state. Further compaction would require electrons to occupy the same energy states, and therefore a kind of pressure (sometimes known as degeneracy pressure of electrons) acts to resist the fermions being overly close.
Elementary fermions with other spins (, , etc.) are not known to exist. Elementary bosons with other spins (0, 2, 3 etc.) were not historically known to exist, although they have received considerable theoretical treatment and are well established within their respective mainstream theories. In particular, theoreticians have proposed the graviton (predicted to exist by some quantum gravity theories) with spin 2, and the Higgs boson (explaining electroweak symmetry breaking) with spin 0. Since 2013, the Higgs boson with spin 0 has been considered proven to exist. Information about Higgs Boson in CERN's official website. It is the first scalar elementary particle (spin 0) known to exist in nature.

Spin-statistics theorem

The fact that particles with half-integer spin (fermions) obey Fermi–Dirac statistics and the Pauli Exclusion Principle, and particles with integer spin (bosons) obey Bose–Einstein statistics, occupy "symmetric states", and thus can share quantum states, is known as the spin-statistics theorem. The theorem relies on both quantum mechanics and the theory of special relativity, and this connection between spin and statistics has been called "one of the most important applications of the special relativity theory".

Magnetic moments

. The neutron has a negative magnetic moment. While the spin of the neutron is upward in this diagram, the magnetic field lines at the center of the dipole are downward.]] Particles with spin can possess a magnetic dipole moment, just like a rotating electrically charged body in classical electrodynamics. These magnetic moments can be experimentally observed in several ways, e.g. by the deflection of particles by inhomogeneous magnetic fields in a Stern–Gerlach experiment, or by measuring the magnetic fields generated by the particles themselves. The intrinsic magnetic moment of a spin , mass , and spin angular momentum , isPhysics of Atoms and Molecules, B.H. Bransden, C.J.Joachain, Longman, 1983, \boldsymbol{\mu} = \frac{g_s q}{2m} \mathbf{S} where the dimensionless quantity is called the spin {{mvar. For exclusively orbital rotations it would be 1 (assuming that the mass and the charge occupy spheres of equal radius). The electron, being a charged elementary particle, possesses a nonzero magnetic moment. One of the triumphs of the theory of quantum electrodynamics is its accurate prediction of the electron , with the digits in parentheses denoting measurement uncertainty in the last two digits at one standard deviation. "After some years, it was discovered that this value }} was not exactly 1, but slightly more—something like 1.00116. This correction was worked out for the first time in 1948 by Schwinger as divided by 2 pi is the square root of the fine-structure constant, and was due to an alternative way the electron can go from place to place: instead of going directly from one point to another, the electron goes along for a while and suddenly emits a photon; then (horrors!) it absorbs its own photon." Composite particles also possess magnetic moments associated with their spin. In particular, the neutron possesses a non-zero magnetic moment despite being electrically neutral. This fact was an early indication that the neutron is not an elementary particle. In fact, it is made up of quarks, which are electrically charged particles. The magnetic moment of the neutron comes from the spins of the individual quarks and their orbital motions. Neutrinos are both elementary and electrically neutral. The minimally extended Standard Model that takes into account non-zero neutrino masses predicts neutrino magnetic moments of: \mu_{\nu}\approx 3\times 10^{-19}\mu_\mathrm{B}\frac{m_{\nu}}{\text{eV}} where the are the neutrino magnetic moments, are the neutrino masses, and is the Bohr magneton. New physics above the electroweak scale could, however, lead to significantly higher neutrino magnetic moments. It can be shown in a model independent way that neutrino magnetic moments larger than about 10−14  are unnatural, because they would also lead to large radiative contributions to the neutrino mass. Since the neutrino masses cannot exceed about 1 eV, these radiative corrections must then be assumed to be fine tuned to cancel out to a large degree. U = e^{-\frac{i}{\hbar} \boldsymbol{\theta} \cdot \mathbf{S}}, where \boldsymbol{\theta} = \theta \hat{\boldsymbol{\theta}}, and is the vector of spin operators. A generic rotation in 3-dimensional space can be built by compounding operators of this type using Euler angles: \mathcal{R}(\alpha,\beta,\gamma) = e^{-i\alpha S_z}e^{-i\beta S_y}e^{-i\gamma S_z} An irreducible representation of this group of operators is furnished by the Wigner D-matrix: D^s_{m'm}(\alpha,\beta,\gamma) \equiv \langle sm' | \mathcal{R}(\alpha,\beta,\gamma)| sm \rangle = e^{-im'\alpha} d^s_{m'm}(\beta)e^{-i m\gamma}, where d^s_{m'm}(\beta)= \langle sm' |e^{-i\beta s_y} | sm \rangle is Wigner's small d-matrix. Note that for 2π}} and β 0}}; i.e., a full rotation about the -axis, the Wigner D-matrix elements become D^s_{m'm}(0,0,2\pi) = d^s_{m'm}(0) e^{-i m 2 \pi} = \delta_{m'm} (-1)^{2m}. Recalling that a generic spin state can be written as a superposition of states with definite , we see that if is an integer, the values of are all integers, and this matrix corresponds to the identity operator. However, if is a half-integer, the values of are also all half-integers, giving −1}} for all , and hence upon rotation by 2 the state picks up a minus sign. This fact is a crucial element of the proof of the spin-statistics theorem.

Lorentz transformations

We could try the same approach to determine the behavior of spin under general Lorentz transformations, but we would immediately discover a major obstacle. Unlike SO(3), the group of Lorentz transformations SO(3,1) is non-compact and therefore does not have any faithful, unitary, finite-dimensional representations. In case of spin- particles, it is possible to find a construction that includes both a finite-dimensional representation and a scalar product that is preserved by this representation. We associate a 4-component Dirac spinor with each particle. These spinors transform under Lorentz transformations according to the law \psi' = \exp{\left(\tfrac{1}{8} \omega_{\mu\nu} \gamma_{\nu}\right)} \psi where are gamma matrices and is an antisymmetric 4 × 4 matrix parametrizing the transformation. It can be shown that the scalar product \langle\psi|\phi\rangle = \bar{\psi}\phi = \psi^{\dagger}\gamma_0\phi is preserved. It is not, however, positive definite, so the representation is not unitary.

Measurement of spin along the -, -, or -axes

Each of the ( Hermitian) Pauli matrices has two eigenvalues, +1 and −1. The corresponding normalized eigenvectors are: \begin{array}{lclc} \psi_{x+}=\displaystyle\frac{1}{\sqrt{2}}\!\!\!\!\! & \begin{pmatrix}{1}\\{1}\end{pmatrix}, & \psi_{x-}=\displaystyle\frac{1}{\sqrt{2}}\!\!\!\!\! & \begin{pmatrix}{-1}\\{1}\end{pmatrix}, \\ \psi_{y+}=\displaystyle\frac{1}{\sqrt{2}}\!\!\!\!\! & \begin{pmatrix}{1}\\{i}\end{pmatrix}, & \psi_{y-}=\displaystyle\frac{1}{\sqrt{2}}\!\!\!\!\! & \begin{pmatrix}{1}\\{-i}\end{pmatrix}, \\ \psi_{z+}= & \begin{pmatrix}{1}\\{0}\end{pmatrix}, & \psi_{z-}= & \begin{pmatrix}{0}\\{1}\end{pmatrix}. \end{array} By the postulates of quantum mechanics, an experiment designed to measure the electron spin on the -, -, or -axis can only yield an eigenvalue of the corresponding spin operator (, or ) on that axis, i.e. }} or }}. The quantum state of a particle (with respect to spin), can be represented by a two component spinor: \psi = \begin{pmatrix} {a+bi}\\{c+di}\end{pmatrix}. When the spin of this particle is measured with respect to a given axis (in this example, the -axis), the probability that its spin will be measured as }} is just \left\vert \langle \psi_{x+} \vert \psi \rangle \right\vert ^2. Correspondingly, the probability that its spin will be measured as }} is just \left\vert \langle \psi_{x-} \vert \psi \rangle \right\vert ^2. Following the measurement, the spin state of the particle will collapse into the corresponding eigenstate. As a result, if the particle's spin along a given axis has been measured to have a given eigenvalue, all measurements will yield the same eigenvalue (since \left\vert \langle \psi_{x+} \vert \psi_{x+} \rangle \right\vert ^2 = 1 , etc), provided that no measurements of the spin are made along other axes.

Measurement of spin along an arbitrary axis

The operator to measure spin along an arbitrary axis direction is easily obtained from the Pauli spin matrices. Let (ux, uy, uz)}} be an arbitrary unit vector. Then the operator for spin in this direction is simply S_u = \frac{\hbar}{2}(u_x\sigma_x + u_y\sigma_y + u_z\sigma_z). The operator has eigenvalues of }}, just like the usual spin matrices. This method of finding the operator for spin in an arbitrary direction generalizes to higher spin states, one takes the dot product of the direction with a vector of the three operators for the three -, -, -axis directions. A normalized spinor for spin- in the direction (which works for all spin states except spin down where it will give ), is: \frac{1}{\sqrt{2+2u_z}}\begin{pmatrix} 1+u_z \\ u_x+iu_y \end{pmatrix}. The above spinor is obtained in the usual way by diagonalizing the matrix and finding the eigenstates corresponding to the eigenvalues. In quantum mechanics, vectors are termed "normalized" when multiplied by a normalizing factor, which results in the vector having a length of unity.

Compatibility of spin measurements

Since the Pauli matrices do not commute, measurements of spin along the different axes are incompatible. This means that if, for example, we know the spin along the -axis, and we then measure the spin along the -axis, we have invalidated our previous knowledge of the -axis spin. This can be seen from the property of the eigenvectors (i.e. eigenstates) of the Pauli matrices that: \left\vert \langle \psi_{x\pm} \mid \psi_{y\pm} \rangle \right\vert ^ 2 = \left\vert \langle \psi_{x\pm} \mid \psi_{z\pm} \rangle \right\vert ^ 2 = \left\vert \langle \psi_{y\pm} \mid \psi_{z\pm} \rangle \right\vert ^ 2 = \tfrac{1}{2}. So when physicists measure the spin of a particle along the -axis as, for example, }}, the particle's spin state collapses into the eigenstate \mid \psi_{x+} \rangle. When we then subsequently measure the particle's spin along the -axis, the spin state will now collapse into either \mid \psi_{y+} \rangle or \mid \psi_{y-} \rangle, each with probability . Let us say, in our example, that we measure }}. When we now return to measure the particle's spin along the -axis again, the probabilities that we will measure }} or }} are each (i.e. they are \left\vert \langle \psi_{x+} \mid \psi_{y-} \rangle \right\vert ^ 2 and \left\vert \langle \psi_{x-} \mid \psi_{y-} \rangle \right\vert ^ 2 respectively). This implies that the original measurement of the spin along the x-axis is no longer valid, since the spin along the -axis will now be measured to have either eigenvalue with equal probability.

Higher spins

The spin- operator σ}} form the fundamental representation of SU(2). By taking Kronecker products of this representation with itself repeatedly, one may construct all higher irreducible representations. That is, the resulting spin operators for higher spin systems in three spatial dimensions, for arbitrarily large , can be calculated using this spin operator and ladder operators. The resulting spin matrices for spin 1 are: \begin{align} S_x &= \frac{\hbar}{\sqrt{2}} \begin{pmatrix} 0 &1 &0\\ 1 &0 &1\\ 0 &1 &0 \end{pmatrix} \, \\ S_y &= \frac{\hbar}{\sqrt{2}} \begin{pmatrix} 0 &-i &0\\ i &0 &-i\\ 0 &i &0 \end{pmatrix} \, \\ S_z &= \hbar \begin{pmatrix} 1 &0 &0\\ 0 &0 &0\\ 0 &0 &-1 \end{pmatrix} \, \end{align} for spin they are \begin{align} S_x &= \frac\hbar2 \begin{pmatrix} 0 &\sqrt{3} &0 &0\\ \sqrt{3} &0 &2 &0\\ 0 &2 &0 &\sqrt{3}\\ 0 &0 &\sqrt{3} &0 \end{pmatrix} \, \\ S_y &= \frac\hbar2 \begin{pmatrix} 0 &-i\sqrt{3} &0 &0\\ i\sqrt{3} &0 &-2i &0\\ 0 &2i &0 &-i\sqrt{3}\\ 0 &0 &i\sqrt{3} &0 \end{pmatrix} \, \\ S_z &= \frac\hbar2 \begin{pmatrix} 3 &0 &0 &0\\ 0 &1 &0 &0\\ 0 &0 &-1 &0\\ 0 &0 &0 &-3 \end{pmatrix} \, \end{align} for spin 2 they are \begin{align} S_x &= \frac\hbar2 \begin{pmatrix} 0 &2 &0 &0 &0\\ 2 &0 &\sqrt{6} &0 &0\\ 0 &\sqrt{6} &0 &\sqrt{6} &0\\ 0 &0 &\sqrt{6} &0 &2\\ 0 &0 &0 &2 &0 \end{pmatrix} \, \\ S_y &= \frac\hbar2 \begin{pmatrix} 0 &-2i &0 &0 &0\\ 2i &0 &-\sqrt{6}i&0 &0\\ 0 &\sqrt{6}i&0 &-\sqrt{6}i&0\\ 0 &0 &\sqrt{6}i&0 &-2i\\ 0 &0 &0 &2i &0 \end{pmatrix} \, \\ S_z &= \hbar \begin{pmatrix} 2 &0 &0 &0 &0\\ 0 &1 &0 &0 &0\\ 0 &0 &0 &0 &0\\ 0 &0 &0 &-1 &0\\ 0 &0 &0 &0 &-2 \end{pmatrix} \, \end{align} and for spin they are \begin{align} S_x &= \frac\hbar2 \begin{pmatrix} 0 &\sqrt{5} &0 &0 &0 &0 \\ \sqrt{5} &0 &2\sqrt{2} &0 &0 &0 \\ 0 &2\sqrt{2} &0 &3 &0 &0 \\ 0 &0 &3 &0 &2\sqrt{2} &0 \\ 0 &0 &0 &2\sqrt{2} &0 &\sqrt{5} \\ 0 &0 &0 &0 &\sqrt{5} &0 \end{pmatrix} \, \\ S_y &= \frac\hbar2 \begin{pmatrix} 0 &-i\sqrt{5} &0 &0 &0 &0 \\ i\sqrt{5} &0 &-2i\sqrt{2} &0 &0 &0 \\ 0 &2i\sqrt{2} &0 &-3i &0 &0 \\ 0 &0 &3i &0 &-2i\sqrt{2} &0 \\ 0 &0 &0 &2i\sqrt{2} &0 &-i\sqrt{5} \\ 0 &0 &0 &0 &i\sqrt{5} &0 \end{pmatrix} \, \\ S_z &= \frac\hbar2 \begin{pmatrix} 5 &0 &0 &0 &0 &0 \\ 0 &3 &0 &0 &0 &0 \\ 0 &0 &1 &0 &0 &0 \\ 0 &0 &0 &-1 &0 &0 \\ 0 &0 &0 &0 &-3 &0 \\ 0 &0 &0 &0 &0 &-5 \end{pmatrix} \,. \end{align} The generalization of these matrices for arbitrary spin is \begin{align} \left(S_x\right)_{ab} & = \frac{\hbar}{2}(\delta_{a,b+1}+\delta_{a+1,b} ) \sqrt{(s+1)(a+b-1)-ab} \, \\ \left(S_y\right)_{ab} & = \frac{i\hbar}{2}(\delta_{a,b+1}-\delta_{a+1,b} ) \sqrt{(s+1)(a+b-1)-ab} \, \quad 1 \le a, b \le 2s+1 \, \\ \left(S_z\right)_{ab} & = \hbar (s+1-a) \delta_{a,b} =\hbar (s+1-b) \delta_{a,b} \,. \end{align} Also useful in the quantum mechanics of multiparticle systems, the general Pauli group is defined to consist of all -fold tensor products of Pauli matrices. The analog formula of Euler's formula in terms of the Pauli matrices: e^{i \theta(\hat{\mathbf{n}} \cdot \boldsymbol{\sigma})} = I\cos\theta + i (\hat{\mathbf{n}} \cdot \boldsymbol{\sigma}) \sin \theta \, for higher spins is tractable, but less simple.


In tables of the spin quantum number for nuclei or particles, the spin is often followed by a "+" or "−". This refers to the parity with "+" for even parity (wave function unchanged by spatial inversion) and "−" for odd parity (wave function negated by spatial inversion). For example, see the isotopes of bismuth.


Spin has important theoretical implications and practical applications. Well-established direct applications of spin include: Electron spin plays an important role in magnetism, with applications for instance in computer memories. The manipulation of nuclear spin by radiofrequency waves ( nuclear magnetic resonance) is important in chemical spectroscopy and medical imaging. Spin-orbit coupling leads to the fine structure of atomic spectra, which is used in atomic clocks and in the modern definition of the second. Precise measurements of the -factor of the electron have played an important role in the development and verification of quantum electrodynamics. Photon spin is associated with the polarization of light. An emerging application of spin is as a binary information carrier in spin transistors. The original concept, proposed in 1990, is known as Datta-Das spin transistor. Electronics based on spin transistors are referred to as spintronics. The manipulation of spin in dilute magnetic semiconductor materials, such as metal-doped ZnO or TiO2 imparts a further degree of freedom and has the potential to facilitate the fabrication of more efficient electronics. There are many indirect applications and manifestations of spin and the associated Pauli exclusion principle, starting with the periodic table of chemistry.


Spin was first discovered in the context of the emission spectrum of alkali metals. In 1924 Wolfgang Pauli introduced what he called a "two-valued quantum degree of freedom" associated with the electron in the outermost shell. This allowed him to formulate the Pauli exclusion principle, stating that no two electrons can share the same quantum state at the same time. lecturing]] The physical interpretation of Pauli's "degree of freedom" was initially unknown. Ralph Kronig, one of Landé's assistants, suggested in early 1925 that it was produced by the self-rotation of the electron. When Pauli heard about the idea, he criticized it severely, noting that the electron's hypothetical surface would have to be moving faster than the speed of light in order for it to rotate quickly enough to produce the necessary angular momentum. This would violate the theory of relativity. Largely due to Pauli's criticism, Kronig decided not to publish his idea. In the autumn of 1925, the same thought came to two Dutch physicists, George Uhlenbeck and Samuel Goudsmit at Leiden University. Under the advice of Paul Ehrenfest, they published their results. It met a favorable response, especially after Llewellyn Thomas managed to resolve a factor-of-two discrepancy between experimental results and Uhlenbeck and Goudsmit's calculations (and Kronig's unpublished results). This discrepancy was due to the orientation of the electron's tangent frame, in addition to its position. Mathematically speaking, a fiber bundle description is needed. The tangent bundle effect is additive and relativistic; that is, it vanishes if goes to infinity. It is one half of the value obtained without regard for the tangent space orientation, but with opposite sign. Thus the combined effect differs from the latter by a factor two ( Thomas precession). Despite his initial objections, Pauli formalized the theory of spin in 1927, using the modern theory of quantum mechanics invented by Schrödinger and Heisenberg. He pioneered the use of Pauli matrices as a representation of the spin operators, and introduced a two-component spinor wave-function. Pauli's theory of spin was non-relativistic. However, in 1928, Paul Dirac published the Dirac equation, which described the relativistic electron. In the Dirac equation, a four-component spinor (known as a " Dirac spinor") was used for the electron wave-function. In 1940, Pauli proved the spin-statistics theorem, which states that fermions have half-integer spin and bosons have integer spin. In retrospect, the first direct experimental evidence of the electron spin was the Stern–Gerlach experiment of 1922. However, the correct explanation of this experiment was only given in 1927.{{cite journal |author=B. Friedrich, D. Herschbach |title=Stern and Gerlach: How a Bad Cigar Helped Reorient Atomic Physics |journal= Physics Today |volume=56 |issue=12 |page=53 |year=2003 |doi=10.1063/1.1650229 |bibcode = 2003PhT....56l..53F }}

See also


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