Lie groups andLie algebras
Classical groups General linear GL(n) Special linear SL(n) Orthogonal O(n) Special orthogonal SO(n) Unitary U(n) Special unitary SU(n) Symplectic Sp(n)
Simple Lie groups Classical An Bn Cn Dn Exceptional G2 F4 E6 E7 E8
Other Lie groups Circle Lorentz Poincaré Conformal group Diffeomorphism Loop Euclidean
Lie algebras Lie group–Lie algebra correspondence Exponential map Adjoint representation Killing form Index Simple Lie algebra Loop algebra Affine Lie algebra
Semisimple Lie algebra Dynkin diagrams Cartan subalgebra Root system Weyl group Real form Complexification Split Lie algebra Compact Lie algebra
Representation theory Lie group representation Lie algebra representation Representation theory of semisimple Lie algebras Representations of classical Lie groups Theorem of the highest weight Borel–Weil–Bott theorem
Lie groups inphysics Particle physics and representation theory Lorentz group representations Poincaré group representations Galilean group representations
Scientists Sophus Lie Henri Poincaré Wilhelm Killing Élie Cartan Hermann Weyl Claude Chevalley Harish-Chandra Armand Borel
Glossary Table of Lie groups
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There is a natural connection betweenparticle physics andrepresentation theory, as first noted in the 1930s byEugene Wigner.^[1] It links the properties ofelementary particles to the structure ofLie groups andLie algebras. According to this connection, the differentquantum states of an elementary particle give rise to anirreducible representation of thePoincaré group. Moreover, the properties of the various particles, including theirspectra, can be related to representations of Lie algebras, corresponding to "approximate symmetries" of the universe.

Inquantum mechanics, any particular one-particle state is represented as avector in aHilbert space${\displaystyle \mathcal{H}}$. To help understand what types of particles can exist, it is important to classify the possibilities for${\displaystyle \mathcal{H}}$ allowed bysymmetries, and their properties. Let${\displaystyle \mathcal{H}}$ be a Hilbert space describing a particular quantum system and let${\displaystyle G}$ be a group of symmetries of the quantum system. In a relativistic quantum system, for example,${\displaystyle G}$ might be thePoincaré group, while for the hydrogen atom,${\displaystyle G}$ might be therotation group SO(3). The particle state is more precisely characterized by the associatedprojective Hilbert space${\displaystyle \mathrm{P}\mathcal{H}}$, also calledray space, since two vectors that differ by a nonzero scalar factor correspond to the same physicalquantum state represented by aray in Hilbert space, which is anequivalence class in${\displaystyle \mathcal{H}}$ and, under the natural projection map${\displaystyle \mathcal{H}\to \mathrm{P}\mathcal{H}}$, an element of${\displaystyle \mathrm{P}\mathcal{H}}$.

By definition of a symmetry of a quantum system, there is agroup action on${\displaystyle \mathrm{P}\mathcal{H}}$. For each${\displaystyle g\in G}$, there is a corresponding transformation${\displaystyle V(g)}$ of${\displaystyle \mathrm{P}\mathcal{H}}$. More specifically, if${\displaystyle g}$ is some symmetry of the system (say, rotation about the x-axis by 12°), then the corresponding transformation${\displaystyle V(g)}$ of${\displaystyle \mathrm{P}\mathcal{H}}$ is a map on ray space. For example, when rotating astationary (zero momentum) spin-5 particle about its center,${\displaystyle g}$ is a rotation in 3D space (an element of${\displaystyle \mathrm{S}\mathrm{O}(3)}$), while${\displaystyle V(g)}$ is an operator whose domain and range are each the space of possible quantum states of this particle, in this example the projective space${\displaystyle \mathrm{P}\mathcal{H}}$ associated with an 11-dimensional complex Hilbert space${\displaystyle \mathcal{H}}$.

Each map${\displaystyle V(g)}$ preserves, by definition of symmetry, theray product on${\displaystyle \mathrm{P}\mathcal{H}}$ induced by the inner product on${\displaystyle \mathcal{H}}$; according toWigner's theorem, this transformation of${\displaystyle \mathrm{P}\mathcal{H}}$ comes from a unitary or anti-unitary transformation${\displaystyle U(g)}$ of${\displaystyle \mathcal{H}}$. Note, however, that the${\displaystyle U(g)}$ associated to a given${\displaystyle V(g)}$ is not unique, but only uniqueup to a phase factor. The composition of the operators${\displaystyle U(g)}$ should, therefore, reflect the composition law in${\displaystyle G}$, but only up to a phase factor:

${\displaystyle U(gh)=e^{i\theta }U(g)U(h)}$,

where${\displaystyle \theta }$ will depend on${\displaystyle g}$ and${\displaystyle h}$. Thus, the map sending${\displaystyle g}$ to${\displaystyle U(g)}$ is aprojective unitary representation of${\displaystyle G}$, or possibly a mixture of unitary and anti-unitary, if${\displaystyle G}$ is disconnected. In practice, anti-unitary operators are always associated withtime-reversal symmetry.

It is important physically that in general${\displaystyle U(\cdot )}$ does not have to be an ordinary representation of${\displaystyle G}$; it may not be possible to choose the phase factors in the definition of${\displaystyle U(g)}$ to eliminate the phase factors in their composition law. An electron, for example, is a spin-one-half particle; its Hilbert space consists of wave functions on${\displaystyle {\mathbb{R}}^3}$ with values in a two-dimensional spinor space. The action of${\displaystyle \mathrm{S}\mathrm{O}(3)}$ on the spinor space is only projective: It does not come from an ordinary representation of${\displaystyle \mathrm{S}\mathrm{O}(3)}$. There is, however, an associated ordinary representation of the universal cover${\displaystyle \mathrm{S}\mathrm{U}(2)}$ of${\displaystyle \mathrm{S}\mathrm{O}(3)}$ on spinor space.^[2]

For many interesting classes of groups${\displaystyle G}$,Bargmann's theorem tells us that every projective unitary representation of${\displaystyle G}$ comes from an ordinary representation of the universal cover${\displaystyle \tilde{G}}$ of${\displaystyle G}$. Actually, if${\displaystyle \mathcal{H}}$ is finite dimensional, then regardless of the group${\displaystyle G}$, every projective unitary representation of${\displaystyle G}$ comes from an ordinary unitary representation of${\displaystyle \tilde{G}}$.^[3] If${\displaystyle \mathcal{H}}$ is infinite dimensional, then to obtain the desired conclusion, some algebraic assumptions must be made on${\displaystyle G}$ (see below). In this setting the result is atheorem of Bargmann.^[4] Fortunately, in the crucial case of the Poincaré group, Bargmann's theorem applies.^[5] (SeeWigner's classification of the representations of the universal cover of the Poincaré group.)

The requirement referred to above is that the Lie algebra${\displaystyle \mathfrak{g}}$ does not admit a nontrivial one-dimensional central extension. This is the case if and only if thesecond cohomology group of${\displaystyle \mathfrak{g}}$ is trivial. In this case, it may still be true that the group admits a central extension by adiscrete group. But extensions of${\displaystyle G}$ by discrete groups are covers of${\displaystyle G}$. For instance, the universal cover${\displaystyle \tilde{G}}$ is related to${\displaystyle G}$ through the quotient${\displaystyle G\approx \tilde{G}/\mathrm{\Gamma }}$ with the central subgroup${\displaystyle \mathrm{\Gamma }}$ being the center of${\displaystyle \tilde{G}}$ itself, isomorphic to thefundamental group of the covered group.

Thus, in favorable cases, the quantum system will carry a unitary representation of the universal cover${\displaystyle \tilde{G}}$ of the symmetry group${\displaystyle G}$. This is desirable because${\displaystyle \mathcal{H}}$ is much easier to work with than the non-vector space${\displaystyle \mathrm{P}\mathcal{H}}$. If the representations of${\displaystyle \tilde{G}}$ can be classified, much more information about the possibilities and properties of${\displaystyle \mathcal{H}}$ are available.

An example in which Bargmann's theorem does not apply comes from a quantum particle moving in${\displaystyle {\mathbb{R}}^n}$. The group of translational symmetries of the associated phase space,${\displaystyle {\mathbb{R}}^{2n}}$, is the commutative group${\displaystyle {\mathbb{R}}^{2n}}$. In the usual quantum mechanical picture, the${\displaystyle {\mathbb{R}}^{2n}}$ symmetry is not implemented by a unitary representation of${\displaystyle {\mathbb{R}}^{2n}}$. After all, in the quantum setting, translations in position space and translations in momentum space do not commute. This failure to commute reflects the failure of the position and momentum operators—which are the infinitesimal generators of translations in momentum space and position space, respectively—to commute. Nevertheless, translations in position space and translations in momentum spacedo commute up to a phase factor. Thus, we have a well-defined projective representation of${\displaystyle {\mathbb{R}}^{2n}}$, but it does not come from an ordinary representation of${\displaystyle {\mathbb{R}}^{2n}}$, even though${\displaystyle {\mathbb{R}}^{2n}}$ is simply connected.

In this case, to obtain an ordinary representation, one has to pass to theHeisenberg group, which is a nontrivial one-dimensional central extension of${\displaystyle {\mathbb{R}}^{2n}}$.

The group of translations andLorentz transformations form thePoincaré group, and this group should be a symmetry of a relativistic quantum system (neglectinggeneral relativity effects, or in other words, inflat spacetime).Representations of the Poincaré group are in many cases characterized by a nonnegativemass and a half-integerspin (seeWigner's classification); this can be thought of as the reason that particles have quantized spin. (There are in fact other possible representations, such astachyons,infraparticles, etc., which in some cases do not have quantized spin or fixed mass.)

While thespacetime symmetries in the Poincaré group are particularly easy to visualize and believe, there are also other types of symmetries, calledinternal symmetries. One example iscolorSU(3), an exact symmetry corresponding to the continuous interchange of the threequark colors.

Many (but not all) symmetries or approximate symmetries formLie groups. Rather than study therepresentation theory of these Lie groups, it is often preferable to study the closely relatedrepresentation theory of the corresponding Lie algebras, which are usually simpler to compute.

Now, representations of the Lie algebra correspond to representations of theuniversal cover of the original group.^[6] In thefinite-dimensional case—and the infinite-dimensional case, provided thatBargmann's theorem applies—irreducible projective representations of the original group correspond to ordinary unitary representations of the universal cover. In those cases, computing at the Lie algebra level is appropriate. This is the case, notably, for studying the irreducible projective representations of the rotation group SO(3). These are in one-to-one correspondence with the ordinary representations of theuniversal cover SU(2) of SO(3). The representations of the SU(2) are then in one-to-one correspondence with the representations of its Lie algebra su(2), which is isomorphic to the Lie algebra so(3) of SO(3).

Thus, to summarize, the irreducible projective representations of SO(3) are in one-to-one correspondence with the irreducible ordinary representations of its Lie algebra so(3). The two-dimensional "spin 1/2" representation of the Lie algebra so(3), for example, does not correspond to an ordinary (single-valued) representation of the group SO(3). (This fact is the origin of statements to the effect that "if you rotate the wave function of an electron by 360 degrees, you get the negative of the original wave function.") Nevertheless, the spin 1/2 representation does give rise to a well-definedprojective representation of SO(3), which is all that is required physically.

Although the above symmetries are believed to be exact, other symmetries are only approximate.

As an example of what an approximate symmetry means, suppose an experimentalist lived inside an infiniteferromagnet, with magnetization in some particular direction. The experimentalist in this situation would find not one but two distinct types of electrons: one with spin along the direction of the magnetization, with a slightly lower energy (and consequently, a lower mass), and one with spin anti-aligned, with a higher mass. Our usualSO(3) rotational symmetry, which ordinarily connects the spin-up electron with the spin-down electron, has in this hypothetical case become only anapproximate symmetry, relatingdifferent types of particles to each other.

In general, an approximate symmetry arises when there are very strong interactions that obey that symmetry, along with weaker interactions that do not. In the electron example above, the two "types" of electrons behave identically under thestrong andweak forces, but differently under theelectromagnetic force.

An example from the real world isisospin symmetry, anSU(2) group corresponding to the similarity betweenup quarks anddown quarks. This is an approximate symmetry: while up and down quarks are identical in how they interact under thestrong force, they have different masses and different electroweak interactions. Mathematically, there is an abstract two-dimensional vector space

${\displaystyle \text{up quark}\to \left(\begin{array}{c}1\\ 0\end{array}\right),\text{down quark}\to \left(\begin{array}{c}0\\ 1\end{array}\right)}$

and the laws of physics areapproximately invariant under applying a determinant-1unitary transformation to this space:^[7]

${\displaystyle \left(\begin{array}{c}x\\ y\end{array}\right)\mapsto A\left(\begin{array}{c}x\\ y\end{array}\right),\text{where }A\text{ is in }SU(2)}$

For example, would turn all up quarks in the universe into down quarks and vice versa. Some examples help clarify the possible effects of these transformations:

• When these unitary transformations are applied to aproton, it can be transformed into aneutron, or into a superposition of a proton and neutron, but not into any other particles. Therefore, the transformations move the proton around a two-dimensional space of quantum states. The proton and neutron are called an "isospin doublet", mathematically analogous to how aspin-½ particle behaves under ordinary rotation.
• When these unitary transformations are applied to any of the threepions (π⁰
  ,π⁺
  , andπ⁻
  ), it can change any of the pions into any other, but not into any non-pion particle. Therefore, the transformations move the pions around a three-dimensional space of quantum states. The pions are called an "isospin triplet", mathematically analogous to how a spin-1 particle behaves under ordinary rotation.
• These transformations have no effect at all on anelectron, because it contains neither up nor down quarks. The electron is called an isospin singlet, mathematically analogous to how a spin-0 particle behaves under ordinary rotation.

In general, particles formisospin multiplets, which correspond to irreducible representations of theLie algebra SU(2). Particles in an isospin multiplet have very similar but not identical masses, because the up and down quarks are very similar but not identical.

Isospin symmetry can be generalized toflavour symmetry, anSU(3) group corresponding to the similarity betweenup quarks,down quarks, andstrange quarks.^[7] This is, again, an approximate symmetry, violated by quark mass differences and electroweak interactions—in fact, it is a poorer approximation than isospin, because of the strange quark's noticeably higher mass.

Nevertheless, particles can indeed be neatly divided into groups that form irreducible representations of theLie algebra SU(3), as first noted byMurray Gell-Mann and independently byYuval Ne'eman.

• Charge (physics)
• Representation theory:
  • Of Lie algebras
  • Of Lie groups
• Projective representation
• Special unitary group

• Howard Georgi (2018).Lie Algebras In Particle Physics: from Isospin To Unified Theories.Taylor & Francis.ISBN 9780429499210.
• Hall, Brian C. (2013),Quantum Theory for Mathematicians, Graduate Texts in Mathematics, vol. 267, Springer,Bibcode:2013qtm..book.....H,ISBN 978-1461471158.
• Hall, Brian C. (2015),Lie Groups, Lie Algebras, and Representations: An Elementary Introduction, Graduate Texts in Mathematics, vol. 222 (2nd ed.), Springer,ISBN 978-3319134666.
• Sternberg, Shlomo (1994)Group Theory and Physics. Cambridge Univ. Press.ISBN 0-521-24870-1. Especially pp. 148–150.
• Weinberg, Steven (1995).The Quantum Theory of Fields, Volume 1: Foundations. Cambridge Univ. Press.ISBN 0-521-55001-7. Especially appendices A and B to Chapter 2.

• Baez, John C.; Huerta, John (2010). "The Algebra of Grand Unified Theories".Bull. Am. Math. Soc.47 (3):483–552.arXiv:0904.1556.doi:10.1090/S0273-0979-10-01294-2.S2CID 2941843.

• Lie algebras
• Representation theory of Lie groups
• Conservation laws
• Quantum field theory

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