Clifford algebra - Citizendia

In mathematics, Clifford algebras are a type of associative algebra. Mathematics is the body of Knowledge and Academic discipline that studies such concepts as Quantity, Structure, Space and In Mathematics, an associative algebra is a Vector space (or more generally a module) which also allows the multiplication of vectors in a distributive They can be thought of as one of the possible generalizations of the complex numbers and quaternions. Complex plane In Mathematics, the complex numbers are an extension of the Real numbers obtained by adjoining an Imaginary unit, denoted Quaternions, in Mathematics, are a non-commutative extension of Complex numbers They were first described by the Irish Mathematician The theory of Clifford algebras is intimately connected with the theory of quadratic forms and orthogonal transformations. In Mathematics, a quadratic form is a Homogeneous polynomial of degree two in a number of variables In Mathematics, the orthogonal group of degree n over a field F (written as O( n, F) is the group of n Clifford algebras have important applications in a variety of fields including geometry and theoretical physics. Geometry ( Greek γεωμετρία; geo = earth metria = measure is a part of Mathematics concerned with questions of size shape and relative position Theoretical physics employs Mathematical models and Abstractions of Physics in an attempt to explain experimental data taken of the natural world They are named for the English geometer William Kingdon Clifford. William Kingdon Clifford FRS ( May 4, 1845 &ndash March 3, 1879) was an English Mathematician and

Some familiarity with the basics of multilinear algebra will be useful in reading this article. In Mathematics, multilinear algebra extends the methods of Linear algebra.

Introduction and basic properties

Specifically, a Clifford algebra is a unital associative algebra which contains and is generated by a vector space V equipped with a quadratic form Q. In Mathematics, an algebra is unital (some authors say unitary) if it contains a multiplicative Identity element (or unit) i In Mathematics, a vector space (or linear space) is a collection of objects (called vectors) that informally speaking may be scaled and added In Mathematics, a quadratic form is a Homogeneous polynomial of degree two in a number of variables The Clifford algebra Cℓ(V,Q) is the "freest" algebra generated by V subject to the condition^[1]

$v^2 = Q(v)\ \mbox{ for all } v\in V.$

If the characteristic of the ground field K is not 2, then one can rewrite this fundamental identity in the form

$uv + vu = 2\lang u, v\rang \mbox{ for all }u,v \in V,$

where <u, v> = ½(Q(u + v) − Q(u) − Q(v)) is the symmetric bilinear form associated to Q. In Mathematics, the characteristic of a ring R, often denoted char( R) is defined to be the smallest number of times one must add the ring's In Abstract algebra, a field is an Algebraic structure in which the operations of Addition, Subtraction, Multiplication and division In Linear algebra, a symmetric matrix is a Square matrix, A, that is equal to its Transpose A = A^{T} In Mathematics, a bilinear form on a Vector space V is a Bilinear mapping V × V → F, where This idea of "freest" or "most general" algebra subject to this identity can be formally expressed through the notion of a universal property (see below). In various branches of Mathematics, certain constructions are frequently defined or characterised by an abstract property which requires the existence of a unique Morphism

Quadratic forms and Clifford algebras in characteristic 2 form an exceptional case. In Mathematics, the characteristic of a ring R, often denoted char( R) is defined to be the smallest number of times one must add the ring's In particular, if char K = 2 it is not true that a quadratic form is determined by its symmetric bilinear form, or that every quadratic form admits an orthogonal basis. Many of the statements in this article include the condition that the characteristic is not 2, and are false if this condition is removed.

As quantization of exterior algebra

Clifford algebras are closely related to exterior algebras. In fact, if Q = 0 then the Clifford algebra Cℓ(V,Q) is just the exterior algebra Λ(V). For nonzero Q there exists a canonical linear isomorphism between Λ(V) and Cℓ(V,Q) whenever the ground field K does not have characteristic two. That is, they are naturally isomorphic as vector spaces, but with different multiplications (in the case of characteristic two, they are still isomorphic as vector spaces, just not naturally). In Category theory, a branch of Mathematics, a natural transformation provides a way of transforming one Functor into another while respecting the internal Clifford multiplication is strictly richer than the exterior product since it makes use of the extra information provided by Q. More precisely, they may be thought of as quantizations (cf. quantization (physics), Quantum group) of the exterior algebra, in the same way that the Weyl algebra is a quantization of the symmetric algebra. In Physics, quantization is a procedure for constructing a Quantum field theory starting from a classical field theory. In Mathematics and Theoretical physics, quantum groups are certain Noncommutative algebras that first appeared in the theory of Quantum integrable systems In Abstract algebra, the Weyl algebra is the ring of Differential operators with Polynomial coefficients (in one variable In Mathematics, the symmetric algebra S ( V) (also denoted Sym ( V) on a Vector space V over a field

Universal property and construction

Let V be a vector space over a field K, and let Q : V → K be a quadratic form on V. In Mathematics, a vector space (or linear space) is a collection of objects (called vectors) that informally speaking may be scaled and added In Abstract algebra, a field is an Algebraic structure in which the operations of Addition, Subtraction, Multiplication and division In Mathematics, a quadratic form is a Homogeneous polynomial of degree two in a number of variables In most cases of interest the field K is either R, C or a finite field.

A Clifford algebra Cℓ(V,Q) is a unital associative algebra over K together with a linear map i : V → Cℓ(V,Q) satisfying i(v)² = Q(v)1 for all v ∈ V, defined by the following universal property: Given any associative algebra A over K and any linear map j : V → A such that

j(v)² = Q(v)1 for all v ∈ V

(where 1 denotes the multiplicative identity of A), there is a unique algebra homomorphism f : Cℓ(V,Q) → A such that the following diagram commutes (i. In Mathematics, an algebra is unital (some authors say unitary) if it contains a multiplicative Identity element (or unit) i In Mathematics, an associative algebra is a Vector space (or more generally a module) which also allows the multiplication of vectors in a distributive In Mathematics, a linear map (also called a linear transformation, or linear operator) is a function between two Vector spaces that In various branches of Mathematics, certain constructions are frequently defined or characterised by an abstract property which requires the existence of a unique Morphism A homomorphism between two algebras over a field K, A and B, is a map FA\rightarrow B such that for all k In mathematics and especially in Category theory a commutative diagram is a Diagram of objects also known as vertices, and Morphisms also e. such that f o i = j):

Working with a symmetric bilinear form <·,·> instead of Q (in characteristic not 2), the requirement on j is

j(v)j(w) + j(w)j(v) = 2<v, w> for all v, w ∈ V. In Mathematics, a bilinear form on a Vector space V is a Bilinear mapping V × V → F, where

A Clifford algebra as described above always exists and can be constructed as follows: start with the most general algebra that contains V, namely the tensor algebra T(V), and then enforce the fundamental identity by taking a suitable quotient. In Mathematics, the tensor algebra of a Vector space V, denoted T ( V) or T &bull( V) is the algebra In Mathematics a quotient ring, also known as factor ring or residue class ring, is a construction in Ring theory, quite similar to the In our case we want to take the two-sided ideal I_Q in T(V) generated by all elements of the form

$v\otimes v - Q(v)1$ for all $v\in V$

and define Cℓ(V,Q) as the quotient

Cℓ(V,Q) = T(V)/I_{Q. In Ring theory, a branch of Abstract algebra, an ideal is a special Subset of a ring.}

It is then straightforward to show that Cℓ(V,Q) contains V and satisfies the above universal property, so that Cℓ is unique up to a unique isomorphism; thus one speaks of "the" Clifford algebra Cℓ(V, Q). It also follows from this construction that i is injective. One usually drops the i and considers V as a linear subspace of Cℓ(V,Q). The concept of a linear subspace (or vector subspace) is important in Linear algebra and related fields of Mathematics.

The universal characterization of the Clifford algebra shows that the construction of Cℓ(V,Q) is functorial in nature. Namely, Cℓ can be considered as a functor from the category of vector spaces with quadratic forms (whose morphisms are linear maps preserving the quadratic form) to the category of associative algebras. In Category theory, a branch of Mathematics, a functor is a special type of mapping between categories In Mathematics, a category is a fundamental and abstract way to describe mathematical entities and their relationships In Mathematics, a morphism is an abstraction derived from structure-preserving mappings between two Mathematical structures The study of morphisms and The universal property guarantees that linear maps between vector spaces (preserving the quadratic form) extend uniquely to algebra homomorphisms between the associated Clifford algebras. A homomorphism between two algebras over a field K, A and B, is a map FA\rightarrow B such that for all k

Basis and dimension

If the dimension of V is n and {e₁,…,e_n} is a basis of V, then the set

$\{e_{i_1}e_{i_2}\cdots e_{i_k} \mid 1\le i_1 < i_2 < \cdots < i_k \le n\mbox{ and } 0\le k\le n\}$

is a basis for Cℓ(V,Q). In Mathematics, the dimension of a Vector space V is the cardinality (i Basis vector redirects here For basis vector in the context of crystals see Crystal structure. The empty product (k = 0) is defined as the multiplicative identity element. In Mathematics, an identity element (or neutral element) is a special type of element of a set with respect to a Binary operation on that For each value of k there are n choose k basis elements, so the total dimension of the Clifford algebra is

$\dim C\ell(V,Q) = \sum_{k=0}^n\begin{pmatrix}n\\ k\end{pmatrix} = 2^n.$

Since V comes equipped with a quadratic form, there is a set of privileged bases for V: the orthogonal ones. In Mathematics, the binomial coefficient \tbinom nk is the Coefficient of the x k term in the Polynomial In Mathematics, two Vectors are orthogonal if they are Perpendicular, i An orthogonal basis is one such that

$\langle e_i, e_j \rangle = 0 \qquad i\neq j. \,$

where <·,·> is the symmetric bilinear form associated to Q. In Mathematics, an orthonormal basis of an Inner product space V (i The fundamental Clifford identity implies that for an orthogonal basis

$e_ie_j = -e_je_i \qquad i\neq j. \,$

This makes manipulation of orthogonal basis vectors quite simple. Given a product $e_{i_1}e_{i_2}\cdots e_{i_k}$ of distinct orthogonal basis vectors, one can put them into standard order by including an overall sign corresponding to the number of flips needed to correctly order them (i. e. the signature of the ordering permutation). In Mathematics, the Permutations of a Finite set (ie the bijective mappings from the set to itself fall into two classes of equal size the even In several fields of Mathematics the term permutation is used with different but closely related meanings

If the characteristic is not 2 then an orthogonal basis for V exists, and one can easily extend the quadratic form on V to a quadratic form on all of Cℓ(V,Q) by requiring that distinct elements $e_{i_1}e_{i_2}\cdots e_{i_k}$ are orthogonal to one another whenever the {e_i}'s are orthogonal. Additionally, one sets

$Q(e_{i_1}e_{i_2}\cdots e_{i_k}) = Q(e_{i_1})Q(e_{i_2})\cdots Q(e_{i_k})$ .

The quadratic form on a scalar is just Q(λ) = λ². Thus, orthogonal bases for V extend to orthogonal bases for Cℓ(V,Q). The quadratic form defined in this way is actually independent of the orthogonal basis chosen (a basis-independent formulation will be given later).

Examples: real and complex Clifford algebras

Main article: geometric algebra

The most important Clifford algebras are those over real and complex vector spaces equipped with nondegenerate quadratic forms. In Mathematical physics, a geometric algebra is a Multilinear algebra described technically as a Clifford algebra over a real vector space equipped In Mathematics, the real numbers may be described informally in several different ways Complex plane In Mathematics, the complex numbers are an extension of the Real numbers obtained by adjoining an Imaginary unit, denoted In Mathematics, specifically Linear algebra, a degenerate Bilinear form f(xy on a Vector space V is one such that

Every nondegenerate quadratic form on a finite-dimensional real vector space is equivalent to the standard diagonal form:

$Q(v) = v_1^2 + \cdots + v_p^2 - v_{p+1}^2 - \cdots - v_{p+q}^2$

where n = p + q is the dimension of the vector space. The pair of integers (p, q) is called the signature of the quadratic form. The signature of a Metric tensor (or more generally a nondegenerate Symmetric bilinear form, thought of as Quadratic form) is the number of positive The real vector space with this quadratic form is often denoted R^p,q. The Clifford algebra on R^p,q is denoted Cℓ_p,q(R). The symbol Cℓ_n(R) means either Cℓ_n,0(R) or Cℓ_0,n(R) depending on whether the author prefers positive definite or negative definite spaces.

A standard orthonormal basis {e_i} for R^p,q consists of n = p + q mutually orthogonal vectors, p of which have norm +1 and q of which have norm −1. In Mathematics, an orthonormal basis of an Inner product space V (i The algebra Cℓ_p,q(R) will therefore have p vectors which square to +1 and q vectors which square to −1.

Note that Cℓ_0,0(R) is naturally isomorphic to R since there are no nonzero vectors. Cℓ_0,1(R) is a two-dimensional algebra generated by a single vector e₁ which squares to −1, and therefore is isomorphic to C, the field of complex numbers. Complex plane In Mathematics, the complex numbers are an extension of the Real numbers obtained by adjoining an Imaginary unit, denoted The algebra Cℓ_0,2(R) is a four-dimensional algebra spanned by {1, e₁, e₂, e₁e₂}. The latter three elements square to −1 and all anticommute, and so the algebra is isomorphic to the quaternions H. Quaternions, in Mathematics, are a non-commutative extension of Complex numbers They were first described by the Irish Mathematician The next algebra in the sequence is Cℓ_0,3(R) is an 8-dimensional algebra isomorphic to the direct sum H ⊕ H called split-biquaternions. The symbol \oplus \! denotes direct sum it is also the astrological and astronomical symbol for Earth, and a symbol for the Exclusive disjunction In Mathematics, a split-biquaternion is a member of the Clifford algebra C �( R)

One can also study Clifford algebras on complex vector spaces. Every nondegenerate quadratic form on a complex vector space is equivalent to the standard diagonal form

$Q(z) = z_1^2 + z_2^2 + \cdots + z_n^2$

where n = dim V, so there is essentially only one Clifford algebra in each dimension. We will denote the Clifford algebra on Cⁿ with the standard quadratic form by Cℓ_n(C). One can show that the algebra Cℓ_n(C) may be obtained as the complexification of the algebra Cℓ_p,q(R) where n = p + q:

$C\ell_n(\mathbb{C}) \cong C\ell_{p,q}(\mathbb{R})\otimes\mathbb{C} \cong C\ell(\mathbb{C}^{p+q},Q\otimes\mathbb{C})$ . In Mathematics, the complexification of a Real vector space V is a vector space V C over the Complex number

Here Q is the real quadratic form of signature (p,q). Note that the complexification does not depend on the signature. The first few cases are not hard to compute. One finds that

Cℓ₀(C) = C

Cℓ₁(C) = C ⊕ C

Cℓ₂(C) = M₂(C)

where M₂(C) denotes the algebra of 2×2 matrices over C.

It turns out that every one of the algebras Cℓ_p,q(R) and Cℓ_n(C) is isomorphic to a matrix algebra over R, C, or H or to a direct sum of two such algebras. For a complete classification of these algebras see classification of Clifford algebras. In Mathematics, in particular in the theory of Nondegenerate Quadratic forms on real and complex Vector spaces the Finite-dimensional

Properties

Relation to the exterior algebra

Given a vector space V one can construct the exterior algebra Λ(V), whose definition is independent of any quadratic form on V. It turns out that if F does not have characteristic 2 then there is a natural isomorphism between Λ(V) and Cℓ(V,Q) considered as vector spaces (and there exists an isomorphism in characteristic two, which may not be natural). In Category theory, a branch of Mathematics, a natural transformation provides a way of transforming one Functor into another while respecting the internal This is an algebra isomorphism if and only if Q = 0. One can thus consider the Clifford algebra Cℓ(V,Q) as an enrichment (or more precisely, a quantization, cf. the Introduction) of the exterior algebra on V with a multiplication that depends on Q (one can still define the exterior product independent of Q).

The easiest way to establish the isomorphism is to choose an orthogonal basis {e_i} for V and extend it to an orthogonal basis for Cℓ(V,Q) as described above. The map Cℓ(V,Q) → Λ(V) is determined by

$e_{i_1}e_{i_2}\cdots e_{i_k} \mapsto e_{i_1}\wedge e_{i_2}\wedge \cdots \wedge e_{i_k}.$

Note that this only works if the basis {e_i} is orthogonal. One can show that this map is independent of the choice of orthogonal basis and so gives a natural isomorphism.

If the characteristic of K is 0, one can also establish the isomorphism by antisymmetrizing. In Mathematics, the characteristic of a ring R, often denoted char( R) is defined to be the smallest number of times one must add the ring's Define functions f_k : V × … × V → Cℓ(V,Q) by

$f_k(v_1, \cdots, v_k) = \frac{1}{k!}\sum_{\sigma\in S_k}{\rm sgn}(\sigma)\, v_{\sigma(1)}\cdots v_{\sigma(k)}$

where the sum is taken over the symmetric group on k elements. In Mathematics, the symmetric group on a set X, denoted by S X or Sym( X) is the group whose underlying Since f_k is alternating it induces a unique linear map Λ^k(V) → Cℓ(V,Q). The direct sum of these maps gives a linear map between Λ(V) and Cℓ(V,Q). The symbol \oplus \! denotes direct sum it is also the astrological and astronomical symbol for Earth, and a symbol for the Exclusive disjunction This map can be shown to be a linear isomorphism, and it is natural.

A more sophisticated way to view the relationship is to construct a filtration on Cℓ(V,Q). In Mathematics, a filtration is an Indexed set Si of Subobjects of a given Algebraic structure S, with the index Recall that the tensor algebra T(V) has a natural filtration: F⁰ ⊂ F¹ ⊂ F² ⊂ … where F^k contains sums of tensors with rank ≤ k. In Mathematics, the tensor algebra of a Vector space V, denoted T ( V) or T &bull( V) is the algebra Projecting this down to the Clifford algebra gives a filtration on Cℓ(V,Q). The associated graded algebra

$Gr_F C\ell(V,Q) = \bigoplus_k F^k/F^{k-1}$

is naturally isomorphic to the exterior algebra Λ(V). In Mathematics, in particular Abstract algebra, a graded algebra is an Algebra over a field (or Commutative ring) with an extra piece of structure Since the associated graded algebra of a filtered algebra is always isomorphic to the filtered algebra as filtered vector spaces (by choosing complements of F^k in F^k+1 for all k), this provides an isomorphism (although not a natural one) in any characteristic, even two.

Grading

In the following, assume that the characteristic is not 2. ^[2]

Clifford algebras are Z₂-graded algebra (also known as superalgebras). In Mathematics, in particular Abstract algebra, a graded algebra is an Algebra over a field (or Commutative ring) with an extra piece of structure In Mathematics and Theoretical physics, a superalgebra is a Z 2- Graded algebra. Indeed, the linear map on V defined by $v \mapsto -v$ preserves the quadratic form Q and so by the universal property of Clifford algebras extends to an algebra automorphism

α : Cℓ(V,Q) → Cℓ(V,Q). In Mathematics, an automorphism is an Isomorphism from a Mathematical object to itself

Since α is an involution (i. e. it squares to the identity) one can decompose Cℓ(V,Q) into positive and negative eigenspaces

$C\ell(V,Q) = C\ell^0(V,Q) \oplus C\ell^1(V,Q)$

where Cℓⁱ(V,Q) = {x ∈ Cℓ(V,Q) | α(x) = (−1)ⁱx}. This article is about the Identity Map software design pattern Since α is an automorphism it follows that

$C\ell^{\,i}(V,Q)C\ell^{\,j}(V,Q) = C\ell^{\,i+j}(V,Q)$

where the superscripts are read modulo 2. This gives Cℓ(V,Q) the structure of a Z₂-graded algebra. In Mathematics, in particular Abstract algebra, a graded algebra is an Algebra over a field (or Commutative ring) with an extra piece of structure The subspace Cℓ⁰(V,Q) forms a subalgebra of Cℓ(V,Q), called the even subalgebra. In Algebra (mathematics, the word "algebra" usually means a Vector space or module equipped with an additional bilinear operation The subspace Cℓ¹(V,Q) is called the odd part of Cℓ(V,Q) (it is not a subalgebra). The Z₂-grading plays an important role in the analysis and application of Clifford algebras. The automorphism α is called the main involution or grade involution.

Remark. In characteristic not 2 the underlying vector space of Cℓ(V,Q) inherits a Z-grading from the canonical isomorphism with the underlying vector space of the exterior algebra Λ(V). It is important to note, however, that this is a vector space grading only. That is, Clifford multiplication does not respect the Z-grading, only the Z₂-grading: for instance if $Q(v)\neq 0$ , then $v\in C\ell^1(V,Q)$ , but $v^2\in C\ell^0(V,Q)$ , not in $C\ell^2(V,Q)$ . Happily, the gradings are related in the natural way: Z₂ = Z/2Z. Further, the Clifford algebra is Z-filtered: $C\ell^{\leq i}(V,Q) \cdot C\ell^{\leq j}(V,Q) \subset C\ell^{\leq i+j}(V,Q)$ . In Mathematics, a filtered algebra is a generalization of the notion of a Graded algebra. The degree of a Clifford number usually refers to the degree in the Z-grading. Elements which are pure in the Z₂-grading are simply said to be even or odd.

The even subalgebra Cℓ⁰(V,Q) of a Clifford algebra is itself a Clifford algebra^[3]. If V is the orthogonal direct sum of a vector a of norm Q(a) and a subspace U, then Cℓ⁰(V,Q) is isomorphic to Cℓ(U,−Q(a)Q), where −Q(a)Q is the form Q restricted to U and multiplied by −Q(a). In particular over the reals this implies that

$C\ell_{p,q}^0(\mathbb{R}) \cong C\ell_{p,q-1}(\mathbb{R})$ for q > 0, and

$C\ell_{p,q}^0(\mathbb{R}) \cong C\ell_{q,p-1}(\mathbb{R})$ for p > 0.

In the negative-definite case this gives an inclusion Cℓ_0,n−1(R) ⊂ Cℓ_{0, n}(R) which extends the sequence

R ⊂ C ⊂ H ⊂ H⊕H ⊂ …

Likewise, in the complex case, one can show that the even subalgebra of Cℓ_n(C) is isomorphic to Cℓ_n−1(C).

Antiautomorphisms

In addition to the automorphism α, there are two antiautomorphisms which play an important role in the analysis of Clifford algebras. In Mathematics, an antihomomorphism is a type of function defined on sets with multiplication that reverses the order of multiplication Recall that the tensor algebra T(V) comes with an antiautomorphism that reverses the order in all products:

$v_1\otimes v_2\otimes \cdots \otimes v_k \mapsto v_k\otimes \cdots \otimes v_2\otimes v_1$ . In Mathematics, the tensor algebra of a Vector space V, denoted T ( V) or T &bull( V) is the algebra

Since the ideal I_Q is invariant under this reversal, this operation descends to an antiautomorphism of Cℓ(V,Q) called the transpose or reversal operation, denoted by x^t. In informal language a transposition is a function that swaps two elements of a set The transpose is an antiautomorphism: $(x y) t = y t x t$ . The transpose operation makes no use of the Z₂-grading so we define a second antiautomorphism by composing α and the transpose. We call this operation Clifford conjugation denoted $\bar x$

$\bar x = \alpha(x^t) = \alpha(x)^t.$

Of the two antiautomorphisms, the transpose is the more fundamental. ^[4]

Note that all of these operations are involutions. One can show that they act as ±1 on elements which are pure in the Z-grading. In fact, all three operations depend only on the degree modulo 4. That is, if x is pure with degree k then

$\alpha(x) = \pm x \qquad x^t = \pm x \qquad \bar x = \pm x$

where the signs are given by the following table:

k mod 4	0	1	2	3
$\alpha(x)\,$	+	−	+	−	(−1)^k
$x^t\,$	+	+	−	−	(−1)^k(k−1)/2
$\bar x$	+	−	−	+	(−1)^k(k+1)/2

The Clifford scalar product

When the characteristic is not 2 the quadratic form Q on V can be extended to a quadratic form on all of Cℓ(V,Q) as explained earlier (which we also denoted by Q). A basis independent definition is

$2Q(x) = \lang x^t x\rang$

where <a> denotes the scalar part of a (the grade 0 part in the Z-grading). One can show that

$Q(v_1v_2\cdots v_k) = Q(v_1)Q(v_2)\cdots Q(v_k)$

where the v_i are elements of V — this identity is not true for arbitrary elements of Cℓ(V,Q).

The associated symmetric bilinear form on Cℓ(V,Q) is given by

$\lang x, y\rang = \lang x^t y\rang.$

One can check that this reduces to the original bilinear form when restricted to V. The bilinear form on all of Cℓ(V,Q) is nondegenerate if and only if it is nondegenerate on V. In Mathematics, specifically Linear algebra, a degenerate Bilinear form f(xy on a Vector space V is one such that

It is not hard to verify that the transpose is the adjoint of left/right Clifford multiplication with respect to this inner product. In Mathematics, specifically in Functional analysis, each Linear operator on a Hilbert space has a corresponding adjoint operator. That is,

$\lang ax, y\rang = \lang x, a^t y\rang,$ and

$\lang xa, y\rang = \lang x, y a^t\rang.$

Structure of Clifford algebras

For more details on this topic, see classification of Clifford algebras. In Mathematics, in particular in the theory of Nondegenerate Quadratic forms on real and complex Vector spaces the Finite-dimensional

In this section we assume that the vector space V is finite dimensional and that the bilinear form of Q is non-singular. A central simple algebra over K is a matrix algebra over a (finite dimensional) division algebra with center K. In Ring theory and related areas of Mathematics a central simple algebra ( CSA) over a field K (also called a Brauer algebra For example, the central simple algebras over the reals are matrix algebras over either the reals or the quaternions.

If V has even dimension then Cℓ(V,Q) is a central simple algebra over K.
If V has even dimension then Cℓ⁰(V,Q) is a central simple algebra over a quadratic extension of K or a sum of two isomorphic central simple algebras over K.
If V has odd dimension then Cℓ(V,Q) is a central simple algebra over a quadratic extension of K or a sum of two isomorphic central simple algebras over K.
If V has odd dimension then Cℓ⁰(V,Q) is a central simple algebra over K.

The structure of Clifford algebras can be worked out explicitly using the following result. Suppose that U has even dimension and a non-singular bilinear form with discriminant d, and suppose that V is another vector space with a quadratic form. In Algebra, the discriminant of a Polynomial with real or complex Coefficients is a certain expression in the coefficients of the The Clifford algebra of U+V is isomorphic to the tensor product of the Clifford algebras of U and (−1)^dim(U)/2dV, which is the space V with its quadratic form multiplied by (−1)^dim(U)/2d. Over the reals, this implies in particular that

$Cl_{p+2,q}(\mathbb{R}) = M_2(\mathbb{R})\otimes Cl_{q,p}(\mathbb{R})$

$Cl_{p+1,q+1}(\mathbb{R}) = M_2(\mathbb{R})\otimes Cl_{p,q}(\mathbb{R})$

$Cl_{p,q+2}(\mathbb{R}) = \mathbb{H}\otimes Cl_{q,p}(\mathbb{R})$

These formulas can be used to find the structure of all real Clifford algebras; see the classification of Clifford algebras. In Mathematics, in particular in the theory of Nondegenerate Quadratic forms on real and complex Vector spaces the Finite-dimensional

Notably, the Morita equivalence class of a Clifford algebra (its representation theory: the equivalence class of the category of modules over it) depends only on the signature $p - q$ mod 8. In Abstract algebra, Morita equivalence is a relationship defined between rings that preserves many ring-theoretic properties This is an algebraic form of Bott periodicity. In Mathematics, the Bott periodicity theorem is a result from Homotopy theory discovered by Raoul Bott during the latter part of the 1950s which proved

The Clifford group Γ

In this section we assume that V is finite dimensional and the quadratic form Q is nondegenerate. In Mathematics, specifically Linear algebra, a degenerate Bilinear form f(xy on a Vector space V is one such that

The invertible elements of the Clifford algebra act on it by twisted conjugation: conjugation by x maps $y \mapsto x y \alpha(x)^{-1}$ .

The Clifford group Γ is defined to be the set of invertible elements x that stabilize vectors, meaning that

$x v \alpha(x)^{-1}\in V$

for all v in V.

This formula also defines an action of the Clifford group on the vector space V that preserves the norm Q, and so gives a homomorphism from the Clifford group to the orthogonal group. The Clifford group contains all elements r of V of nonzero norm, and these act on V by the corresponding reflections that take v to v − <v,r>r/Q(r) (In characteristic 2 these are called orthogonal transvections rather than reflections. )

Many authors define the Clifford group slightly differently, by replacing the action xvα(x)⁻¹ by xvx⁻¹. This produces the same Clifford group, but the action of the Clifford group on V is changed slightly: the action of the odd elements Γ¹ of the Clifford group is multiplied by an extra factor of −1. This action used here has several minor advantages: it is consistent with the usual superalgebra sign conventions, elements of V correspond to reflections, and in odd dimensions the map from the Clifford group to the orthogonal group is onto, and the kernel is no larger than K^*. Using the action α(x)vx⁻¹ instead of xvα(x)⁻¹ makes no difference: it produces the same Clifford group with the same action on V.

The Clifford group Γ is the disjoint union of two subsets Γ⁰ and Γ¹, where Γⁱ is the subset of elements of degree i. The subset Γ⁰ is a subgroup of index 2 in Γ.

If V is a finite dimensional real vector space with positive definite (or negative definite) quadratic form then the Clifford group maps onto the orthogonal group of V with respect to the form (by the Cartan-Dieudonné theorem) and the kernel consists of the nonzero elements of the field K. In Mathematics, the Cartan-Dieudonné theorem, named after Élie Cartan and Jean Dieudonné, is a Theorem on the structure of the Automorphism This leads to exact sequences

$1 \rightarrow K^* \rightarrow \Gamma \rightarrow O_V(K) \rightarrow 1,\,$

$1 \rightarrow K^* \rightarrow \Gamma^0 \rightarrow SO_V(K) \rightarrow 1.\,$

Over other fields or with indefinite forms, the map is not in general onto, and the failure is captured by the spinor norm.

Spinor norm

For more details on this topic, see Spinor_norm#Galois_cohomology_and_orthogonal_groups. In Mathematics, the orthogonal group of degree n over a field F (written as O( n, F) is the group of n

In arbitrary characteristic, the spinor norm Q is defined on the Clifford group by

$Q(x) = x^tx\,$

It is a homomorphism from the Clifford group to the group K^* of non-zero elements of K. In Mathematics, the orthogonal group of degree n over a field F (written as O( n, F) is the group of n It coincides with the quadratic form Q of V when V is identified with a subspace of the Clifford algebra. Several authors define the spinor norm slightly differently, so that it differs from the one here by a factor of −1, 2, or −2 on Γ¹. The difference is not very important in characteristic other than 2.

The nonzero elements of K have spinor norm in the group K^*2 of squares of nonzero elements of the field K. So when V is finite dimensional and non-singular we get an induced map from the orthogonal group of V to the group K^*/K^*2, also called the spinor norm. The spinor norm of the reflection of a vector r has image Q(r) in K^*/K^*2, and this property uniquely defines it on the orthogonal group. This gives exact sequences:

$1 \to \{\pm 1\} \to \mbox{Pin}_V(K) \to O_V(K) \to K^*/K^{*2},\,$

$1 \to \{\pm 1\} \to \mbox{Spin}_V(K) \to SO_V(K) \to K^*/K^{*2}.\,$

Note that in characteristic 2 the group {±1} has just one element.

From the point of view of Galois cohomology of algebraic groups, the spinor norm is a connecting homomorphism on cohomology. In Mathematics, Galois cohomology is the study of the Group cohomology of Galois modules that is the application of Homological algebra to In Algebraic geometry, an algebraic group (or group variety) is a group that is an Algebraic variety, such that the multiplication and inverse In Mathematics, particularly Homological algebra, the snake lemma, a statement valid in every Abelian category, is the crucial tool used to construct the Writing μ₂ for the algebraic group of square roots of 1 (over a field of characteristic not 2 it is roughly the same as a two-element group with trivial Galois action), the short exact sequence

$1 \to \mu_2 \rightarrow \mbox{Pin}_V \rightarrow O_V \rightarrow 1\,$

yields a long exact sequence on cohomology, which begins

$1 \to H^0(\mu_2;K) \to H^0(\mbox{Pin}_V;K) \to H^0(O_V;K) \to H^1(\mu_2;K)\,$

The 0th Galois cohomology group of an algebraic group with coefficients in K is just the group of K-valued points: $H 0 (G; K) = G (K)$ , and $H^1(\mu_2;K) \cong K^*/K^{*2}$ , which recovers the previous sequence

$1 \to \{\pm 1\} \to \mbox{Pin}_V(K) \to O_V(K) \to K^*/K^{*2},\,$

where the spinor norm is the connecting homomorphism $H^0(O_V;K) \to H^1(\mu_2;K)$ . In Mathematics and Group theory the term multiplicative group refers to one of the following concepts depending on the context any group \scriptstyle\mathfrak

Spin and Pin groups

For more details on this topic, see Spin group, Pin group and spinor. In Mathematics the spin group Spin( n) is the double cover of the Special orthogonal group SO( n) such that there exists a Short In Mathematics and Physics, in particular in the theory of the Orthogonal groups spinors are elements of a complex vector space introduced to expand the

In this section we assume that V is finite dimensional and its bilinear form is non-singular. (If K has characteristic 2 this implies that the dimension of V is even. )

The Pin group Pin_V(K) is the subgroup of the Clifford group Γ of elements of spinor norm 1, and similarly the Spin group Spin_V(K) is the subgroup of elements of Dickson invariant 0 in Pin_V(K). In Mathematics the spin group Spin( n) is the double cover of the Special orthogonal group SO( n) such that there exists a Short In Mathematics, the orthogonal group of degree n over a field F (written as O( n, F) is the group of n When the characteristic is not 2, these are the elements of determinant 1. The Spin group usually has index 2 in the Pin group.

Recall from the previous section that there is a homomorphism from the Clifford group onto the orthogonal group. We define the special orthogonal group to be the image of Γ⁰. In Mathematics, the orthogonal group of degree n over a field F (written as O( n, F) is the group of n If K does not have characteristic 2 this is just the group of elements of the orthogonal group of determinant 1. If K does have characteristic 2, then all elements of the orthogonal group have determinant 1, and the special orthogonal group is the set of elements of Dickson invariant 0.

There is a homomorphism from the Pin group to the orthogonal group. The image consists of the elements of spinor norm 1 ∈ K^*/K^*2. The kernel consists of the elements +1 and −1, and has order 2 unless K has characteristic 2. Similarly there is a homomorphism from the Spin group to the special orthogonal group of V.

In the common case when V is a positive or negative definite space over the reals, the spin group maps onto the special orthogonal group, and is simply connected when V has dimension at least 3. Please note, however, that this is not true in general: if V is R^p,q for p and q both at least 2 then the spin group is not simply connected. In this case the algebraic group Spin_p,q is simply connected as an algebraic group, even though its group of real valued points Spin_p,q(R) is not simply connected. This is a rather subtle point, which completely confused the authors of at least one standard book about spin groups.

Spinors

Clifford algebras Cℓ_p,q(C), with p+q=2n even, are matrix algebras which have a complex representation of dimension 2ⁿ. By restricting to the group Pin_p,q(R) we get a complex representation of the Pin group of the same dimension, called the spinor representation. If we restrict this to the spin group Spin_p,q(R) then it splits as the sum of two half spin representations (or Weyl representations) of dimension 2^n-1.

If p+q=2n+1 is odd then the Clifford algebra Cℓ_p,q(C) is a sum of two matrix algebras, each of which has a representation of dimension 2ⁿ, and these are also both representations of the Pin group Pin_p,q(R). On restriction to the spin group Spin_p,q(R) these become isomorphic, so the spin group has a complex spinor representation of dimension 2ⁿ.

More generally, spinor groups and pin groups over any field have similar representations whose exact structure depends on the structure of the corresponding Clifford algebras: whenever a Clifford algebra has a factor that is a matrix algebra over some division algebra, we get a corresponding representation of the pin and spin groups over that division algebra. In Mathematics, in particular in the theory of Nondegenerate Quadratic forms on real and complex Vector spaces the Finite-dimensional For examples over the reals see the article on spinors. In Mathematics and Physics, in particular in the theory of the Orthogonal groups spinors are elements of a complex vector space introduced to expand the

Real spinors

For more details on this topic, see spinor. In Mathematics and Physics, in particular in the theory of the Orthogonal groups spinors are elements of a complex vector space introduced to expand the

To describe the real spin representations, one must know how the spin group sits inside its Clifford algebra. The Pin group, Pin_p,q is the set of invertible elements in Cl_p,q which can be written as a product of unit vectors:

${\mathit{Pin}}_{p,q}=\{v_1v_2\dots v_r |\,\, \forall i, \|v_i\|=\pm 1\}$

Comparing with the above concrete realizations of the Clifford algebras, the Pin group corresponds to the products of arbitrarily many reflections: it is a cover of the full orthogonal group O(p,q). The Spin group consists of those elements of Pin_p,q which are products of an even number of unit vectors. In Mathematics the spin group Spin( n) is the double cover of the Special orthogonal group SO( n) such that there exists a Short Thus by the Cartan-Dieudonné theorem Spin is a cover of the group of proper rotations SO(p,q). In Mathematics, the Cartan-Dieudonné theorem, named after Élie Cartan and Jean Dieudonné, is a Theorem on the structure of the Automorphism

Let α : Cℓ → Cℓ be the automorphism which is given by -Id acting on pure vectors. Then in particular, Spin_p,q is the subgroup of Pin_p,q whose elements are fixed by α. Let

$Cl_{p,q}^0 = \{ x\in Cl_{p,q} |\, \alpha(x)=x\}$ .

(These are precisely the elements of even degree in Cℓ_p,q. ) Then the spin group lies within Cℓ⁰_p,q.

The irreducible representations of Cℓ_p,q restrict to give representations of the pin group. Conversely, since the pin group is generated by unit vectors, all of its irreducible representation are induced in this manner. Thus the two representations coincide. For the same reasons, the irreducible representations of the spin coincide with the irreducible representations of Cℓ⁰_p,q

To classify the pin representations, one need only appeal to the classification of Clifford algebras. In Mathematics, in particular in the theory of Nondegenerate Quadratic forms on real and complex Vector spaces the Finite-dimensional To find the spin representations (which are representations of the even subalgebra), one can first make use of either of the isomorphisms (see above)

Cℓ⁰_p,q ≈ Cℓ_p,q-1, for q > 0

Cℓ⁰_p,q ≈ Cℓ_q,p-1, for p > 0

and realize a spin representation in signature (p,q) as a pin representation in either signature (p,q-1) or (q,p-1).

Applications

Differential geometry

One of the principal applications of the exterior algebra is in differential geometry where it is used to define the bundle of differential forms on a smooth manifold. Differential geometry is a mathematical discipline that uses the methods of differential and integral Calculus to study problems in Geometry In Mathematics, in particular in Topology, a fiber bundle (or fibre bundle) is a space which looks locally like a Product space. In the mathematical fields of Differential geometry and Tensor calculus, differential forms are an approach to Multivariable calculus which is A differentiable manifold is a type of Manifold that is locally similar enough to Euclidean space to allow one to do Calculus. In the case of a (pseudo-)Riemannian manifold, the tangent spaces come equipped with a natural quadratic form induced by the metric. In Differential geometry, a pseudo-Riemannian manifold (also called a semi-Riemannian manifold) is a generalization of a Riemannian manifold. In Riemannian geometry, a Riemannian manifold ( M, g) (with Riemannian metric g) is a real Differentiable manifold M In Mathematics, the tangent space of a Manifold is a concept which facilitates the generalization of vectors from Affine spaces to general manifolds since In the mathematical field of Differential geometry, a metric tensor is a type of function defined on a Manifold (such as a Surface in space Thus, one can define a Clifford bundle in analogy with the exterior bundle. In Mathematics, a Clifford bundle is a Algebra bundle whose fibers have the structure of a Clifford algebra and whose Local trivializations respect In Mathematics, the exterior bundle of a Manifold M is the Subbundle of the Tensor bundle consisting of all antisymmetric covariant This has a number of important applications in Riemannian geometry. Elliptic geometry is also sometimes called Riemannian geometry.

Physics

Clifford algebras have numerous important applications in physics. Physicists usually consider a Clifford algebra to be an algebra spanned by matrices γ₁,…,γ_n called Dirac matrices which have the property that

$\gamma_i\gamma_j + \gamma_j\gamma_i = 2\eta_{ij}\,$

where η is the matrix of a quadratic form of signature (p,q) — typically (1,3) when working in Minkowski space. In Mathematical physics, the gamma matrices, {γ0 γ1 γ2 γ3} also known as the Dirac matrices, form a matrix-valued In Physics and Mathematics, Minkowski space (or Minkowski spacetime) is the mathematical setting in which Einstein's theory of Special relativity These are exactly the defining relations for the Clifford algebra Cl_1,3(C) (up to an unimportant factor of 2), which by the classification of Clifford algebras is isomorphic to the algebra of 4 by 4 complex matrices. In Mathematics, in particular in the theory of Nondegenerate Quadratic forms on real and complex Vector spaces the Finite-dimensional

The Dirac matrices were first written down by Paul Dirac when he was trying to write a relativistic first-order wave equation for the electron, and give an explicit isomorphism from the Clifford algebra to the algebra of complex matrices. The electron is a fundamental Subatomic particle that was identified and assigned the negative charge in 1897 by J The result was used to define the Dirac equation. In Physics, the Dirac equation is a relativistic quantum mechanical wave equation formulated by British physicist Paul Dirac in 1928 and provides The entire Clifford algebra shows up in quantum field theory in the form of Dirac field bilinears. In quantum field theory (QFT the forces between particles are mediated by other particles In Physics, the Dirac equation is a relativistic quantum mechanical wave equation formulated by British physicist Paul Dirac in 1928 and provides

Footnotes

^ Mathematicians who work with real Clifford algebras and prefer positive definite quadratic forms (especially those working in index theory) sometimes use a different choice of sign in the fundamental Clifford identity. In Physics, the algebra of physical space (APS is the Clifford or Geometric algebra Cl3 of the three-dimensional Euclidean space, In Mathematics, in particular in the theory of Nondegenerate Quadratic forms on real and complex Vector spaces the Finite-dimensional In Mathematical physics, the gamma matrices, {γ0 γ1 γ2 γ3} also known as the Dirac matrices, form a matrix-valued In Mathematical physics, a geometric algebra is a Multilinear algebra described technically as a Clifford algebra over a real vector space equipped In Mathematics the spin group Spin( n) is the double cover of the Special orthogonal group SO( n) such that there exists a Short In Mathematics and Physics, in particular in the theory of the Orthogonal groups spinors are elements of a complex vector space introduced to expand the The name paravector is used for the sum of a scalar and a vector in any Clifford algebra (Clifford algebra is also known as Geometric algebra in the physics community In Physics, a sign convention is a choice of the signs (plus or minus of a set of quantities in a case where the choice of sign is arbitrary That is, they take v² = −Q(v). One must replace Q with −Q in going from one convention to the other.
^ Thus the group algebra K[Z/2] is semisimple and the Clifford algebra splits into eigenspaces of the main involution. In Mathematics, the group algebra is any of various constructions to assign to a Locally compact group an Operator algebra (or more generally a Banach In mathematics the term semisimple is used in a number of related ways within different subjects
^ We are still assuming that the characteristic is not 2.
^ The opposite is true when uses the alternate (−) sign convention for Clifford algebras: it is the conjugate which is more important. In general, the meanings of conjugation and transpose are interchanged when passing from one sign convention to the other. For example, in the convention used here the inverse of a vector is given by $v - 1 = v t / Q (v)$ while in the (−) convention it is given by $v^{-1} = \bar{v}/Q(v)$ .

References

Bourbaki, Nicolas (1988), Algebra, Berlin, New York: Springer-Verlag, ISBN 978-3-540-19373-9 , section XI. Nicolas Bourbaki is the collective Pseudonym under which a group of (mainly French) 20th-century Mathematicians wrote a series of books presenting an exposition Springer Science+Business Media or Springer (ˈʃpʁɪŋɐ is a worldwide Publishing company based in Germany, which publishes textbooks academic 9.
Carnahan, S. Borcherds Seminar Notes, Uncut. Week 5, "Spinors and Clifford Algebras".
Lawson, H. Blaine & Michelsohn, Marie-Louise (1989), Spin Geometry, Princeton, NJ: Princeton University Press, ISBN 978-0-691-08542-5 . The Princeton University Press is an independent publisher with close connections to Princeton University. An advanced textbook on Clifford algebras and their applications to differential geometry.
Lounesto, Pertti (2001), Clifford algebras and spinors, Cambridge: Cambridge University Press, ISBN 978-0-521-00551-7
Porteous, Ian R. Cambridge University Press (known colloquially as CUP is a Publisher given a Royal Charter by Henry VIII in 1534 (1995), Clifford algebras and the classical groups, Cambridge: Cambridge University Press, ISBN 978-0-521-55177-9

External links

Cambridge University Press (known colloquially as CUP is a Publisher given a Royal Charter by Henry VIII in 1534

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