Geometric algebra

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A geometric algebra is a multilinear algebra with a geometric interpretation. (The term is also used in a more general sense to describe the study and application of these algebras: Geometric algebra is the study of geometric algebras.) Informally, a geometric algebra is a Clifford algebra that includes a geometric product.

Geometric algebra is useful in physics problems that involve rotations, phases or imaginary numbers. Proponents of geometric algebra argue it provides a more compact and intuitive description of classical and quantum mechanics, electromagnetic theory and relativity. Current applications of geometric algebra include computer vision, biomechanics and robotics, and spaceflight dynamics.

1 The geometric product
2 Inverting a vector
3 The contraction rule
4 Inner and outer product
5 Applications of geometric algebra
6 History
7 See also
8 References
9 External links

[edit] The geometric product

A geometric algebra $\mathcal{G}_n(\mathcal{V}_n)$ is an algebra constructed over a vector space $\mathcal V_n$ in which a geometric product is defined. The elements of geometric algebra are multivectors. The geometric product has the following properties, for all multivectors $\mathbf{A}, \mathbf{B}, \mathbf{C}$ :

Closure
Distributivity over the addition of multivectors:
- $\mathbf{A}(\mathbf{B} + \mathbf{C}) = \mathbf{A}\mathbf{B} + \mathbf{A}\mathbf{C}$
- $(\mathbf{A} + \mathbf{B})\mathbf{C} = \mathbf{A}\mathbf{C} + \mathbf{B}\mathbf{C}$
Associativity
Unit (scalar) element:
- $1 \, \mathbf A = \mathbf A$
Tensor contraction: for any "vector" (a grade-one element) $\mathbf{a}, \mathbf{a}^2$ is a scalar (real number)
Commutativity of the product by a scalar:
- $\lambda \mathbf A = \mathbf A \lambda$

Properties (1) and (2) are among those needed for an algebra over a field. (3) and (4) mean that a geometric algebra is an associative, unital algebra.

The distinctive point of this formulation is the natural correspondence between geometric entities and the elements of the associative algebra. This comes from the fact that the geometric product is defined in terms of the dot product and the wedge product of vectors as

$\mathbf a \, \mathbf b = \mathbf a \cdot \mathbf b + \mathbf a \wedge \mathbf b$

The original vector space $\mathcal V$ is constructed over the real numbers as scalars. From now on, a vector is something in $\mathcal V$ itself. Vectors will be represented by boldface, small case letters.

The definition and the associativity of geometric product entails the concept of the inverse of a vector (or division by vector). Thus, one can easily set and solve vector algebra equations that otherwise would be cumbersome to handle. In addition, one gains a geometric meaning that would be difficult to retrieve, for instance, by using matrices. Although not all the elements of the algebra are invertible, the inversion concept can be extended to multivectors. Geometric algebra allows one to deal with subspaces directly, and manipulate them too. Furthermore, geometric algebra is a coordinate-free formalism.

Geometric objects like $\mathbf a \wedge \mathbf b$ are called bivectors. A bivector can be pictured as a plane segment (a parallelogram, a circle etc.) endowed with orientation. One bivector represents all planar segments with the same magnitude and direction, no matter where they are in the space that contains them. However, once either the vector $\mathbf a$ or $\mathbf b$ is meant to depart from some preferred point (e.g. in problems of Physics), the oriented plane $B=\mathbf a \wedge \mathbf b$ is determined unambiguously.

The outer product (the exterior product, or the wedge product) $\wedge$ is defined such that the graded algebra (exterior algebra of Hermann Grassmann) $\wedge^n\mathcal{V}_n$ of multivectors is generated. Multivectors are thus the direct sum of grade k elements (k-vectors), where k ranges from 0 (scalars) to n, the dimension of the original vector space $\mathcal V$ . Multivectors are represented here by boldface caps. Note that scalars and vectors become special cases of multivectors ("0-vectors" and "1-vectors", respectively).

[edit] Inverting a vector

As a meaningful result one can consider a fixed non-zero vector $\mathbf v$ , from a point chosen as the origin, in the usual Euclidean space, $\mathbb{R}^3$ . The set of all vectors $\mathbf x$ such that $\mathbf x \wedge \mathbf v = B$ , $B$ denoting a given bivector containing $\mathbf v$ , determines a line $l$ parallel to $\mathbf v$ . Since $B$ is a directed area, $l$ is uniquely determined with respect to the chosen origin. The set of all vectors $\mathbf x$ such that $\mathbf x \cdot \mathbf v = s$ , $s$ denoting a given (real) scalar, determines a plane P orthogonal to $\mathbf v$ . Again, P is uniquely determined with respect to the chosen origin. The two information pieces, $B$ and $s$ , can be set independently of one another. Now, what is (if any) the vector $\mathbf y$ that satisfies the system { $\mathbf y \wedge \mathbf v = B$ , $\mathbf y \cdot \mathbf v = s$ } ? Geometrically, the answer is plain: it is the vector that departs from the origin and arrives at the intersection of $l$ and P. By geometric algebra, even the algebraic answer is simple: $\mathbf y \mathbf v = s + B => \mathbf y = (s + B)/ \mathbf v = (s + B) \mathbf v$ ^-1, where the inverse of a non-zero vector is expressed by $\mathbf z$ ^-1 $= \mathbf z /(\mathbf z \cdot \mathbf z )$ . Note that the division by a vector transforms the multivector $s + B$ into the sum of two vectors. Note also that the structure of the solution does not depend on the chosen origin.

[edit] The contraction rule

The connection between Clifford algebras and quadratic forms come from the contraction property. This rule also gives the space a metric defined by the naturally derived inner product. It is to be noted that in geometric algebra in all its generality there is no restriction whatsoever on the value of the scalar, it can very well be negative, even zero (in that case, the possibility of an inner product is ruled out if you require $\langle x, x \rangle \ge 0$ ).

The contraction rule can be put in the form:

$Q(\mathbf a) = \mathbf a^2 = \epsilon_a {\Vert \mathbf a \Vert}^2$

where $\Vert \mathbf a \Vert$ is the modulus of vector a, and $\epsilon_a=0, \, \pm1$ is called the signature of vector a. This is especially useful in the construction of a Minkowski space (the spacetime of special relativity) through $\mathbb{R}_{1,3}$ . In that context, null-vectors are called "lightlike vectors", vectors with negative signature are called "spacelike vectors" and vectors with positive signature are called "timelike vectors" (these last two denominations are exchanged when using $\mathbb{R}_{3,1}$ instead).

[edit] Inner and outer product

The usual dot product and cross product of traditional vector algebra (on $\mathbb{R}^3$ ) find their places in geometric algebra $\mathcal{G}_3$ as the inner product

$\mathbf{a}\cdot\mathbf{b} = \frac{1}{2}(\mathbf{a}\mathbf{b} + \mathbf{b}\mathbf{a})$

(which is symmetric) and the outer product

$\mathbf{a}\wedge\mathbf{b} = \frac{1}{2}(\mathbf{a}\mathbf{b} - \mathbf{b}\mathbf{a})$

with

$\mathbf{a}\times\mathbf{b} = -i(\mathbf{a}\wedge\mathbf{b})$

(which is antisymmetric). Relevant is the distinction between axial and polar vectors in vector algebra, which is natural in geometric algebra as the mere distinction between vectors and bivectors (elements of grade two). The $i$ here is the unit pseudoscalar of Euclidean 3-space, which establishes a duality between the vectors and the bivectors, and is named so because of the expected property $i 2 = - 1$ .

While the cross product can only be defined in a three-dimensional space, the inner and outer products can be generalized to any dimensional $\mathcal G_{p,q,r}$ .

Let $\mathbf{a},\, \mathbf{A}_{\langle k \rangle}$ be a vector and a homogeneous multivector of grade k, respectively. Their inner product is then

$\mathbf a \cdot \mathbf A_{\langle k \rangle} = {1 \over 2} \, \left ( \mathbf a \, \mathbf A_{\langle k \rangle} + (-1)^{k+1} \, \mathbf{A}_{\langle k \rangle} \, \mathbf{a} \right ) = (-1)^{k+1} \mathbf A_{\langle k \rangle} \cdot \mathbf{a}$

and the outer product is

$\mathbf a \wedge \mathbf A_{\langle k \rangle} = {1 \over 2} \, \left ( \mathbf a \, \mathbf A_{\langle k \rangle} - (-1)^{k+1} \, \mathbf{A}_{\langle k \rangle} \, \mathbf{a} \right ) = (-1)^{k} \mathbf A_{\langle k \rangle} \wedge \mathbf{a}$

[edit] Applications of geometric algebra

A useful example is $\mathbb{R}_{3, 1}$ , and to generate $\mathcal{G}_{3, 1}$ , an instance of geometric algebra called spacetime algebra by Hestenes. The electromagnetic field tensor, in this context, becomes just a bivector $\mathbf{E} + i\mathbf{B}$ where the imaginary unit is the volume element, giving an example of the geometric reinterpretation of the traditional "tricks".

Boosts in this Lorenzian metric space have the same expression $e^{\mathbf{\beta}}$ as rotation in Euclidean space, where $\mathbf{\beta}$ is of course the bivector generated by the time and the space directions involved, whereas in the Euclidean case it is the bivector generated by the two space directions, strengthening the "analogy" to almost identity.

[edit] History

The geometric algebra of David Hestenes et al. (1984) reinterprets Clifford algebras over the reals, and is claimed to return to the name and interpretation Clifford originally intended. Emil Artin's Geometric Algebra discusses the algebra associated with each of a number of geometries, including affine geometry, projective geometry, symplectic geometry, and orthogonal geometry.

[edit] See also

[edit] References

Baylis, W. E., ed., 1996. Clifford (Geometric) Algebra with Applications to Physics, Mathematics, and Engineering. Boston: Birkhäuser.
Baylis, W. E., 2002. Electrodynamics: A Modern Geometric Approach, 2nd ed. Birkhäuser. ISBN 0-8176-4025-8
Nicholas Bourbaki, 1980. Eléments de Mathématique. Algèbre. Chpt. 9, "Algèbres de Clifford". Paris: Hermann.
Chris Doran and Anthony Lasenby, 2003. Geometric Algebra for Physicists. Cambridge Univ. Press.
David Hestenes and Sobczyk, G., 1987. Clifford Algebra to Geometric Calculus. Sprinver Verlag.
Hestenes, D., 1999. New Foundations for Classical Mechanics, 2nd ed. Springer Verlag.
Lasenby, J., Lazenby, A. N., and Doran, C. J. L., 2000, "A Unified Mathematical Language for Physics and Engineering in the 21st Century," Philosophical Transactions of the Royal Society of London A 358: 1-18.