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# Spacetime algebra

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### Spacetime algebra

In mathematical physics, spacetime algebra (STA) is a name for the Clifford algebra C1,3(R), or equivalently the geometric algebra G4 = G(M4), which can be particularly closely associated with the geometry of special relativity and relativistic spacetime.

It is a vector space allowing not just vectors, but also bivectors (directed quantities associated with particular planes, such as areas, or rotations) or multivectors (quantities associated with particular hyper-volumes) to be combined, as well as rotated, reflected, or Lorentz boosted. It is also the natural parent algebra of spinors in special relativity. These properties allow many of the most important equations in physics to be expressed in particularly simple forms, and can be very helpful towards a more geometric understanding of their meanings.

## Structure

The spacetime algebra is built up from combinations of one time-like basis vector \gamma_0 and three orthogonal space-like vectors, \{\gamma_1, \gamma_2, \gamma_3\}, under the multiplication rule

\gamma_\mu \gamma_\nu + \gamma_\nu \gamma_\mu = 2 \eta_{\mu \nu}

where \eta_{\mu \nu} \, is the Minkowski metric with signature (+ − − −)

Thus \gamma_0^2 = {+1}, \gamma_1^2 = \gamma_2^2 = \gamma_3^2 = {-1}, otherwise \displaystyle \gamma_\mu \gamma_\nu = - \gamma_\nu \gamma_\mu.

The basis vectors \gamma_k share these properties with the Dirac matrices, but no explicit matrix representation is utilized in STA.

This generates a basis of one scalar \{1\}, four vectors \{\gamma_0, \gamma_1, \gamma_2, \gamma_3\}, six bivectors \{\gamma_0\gamma_1, \, \gamma_0\gamma_2,\, \gamma_0\gamma_3, \, \gamma_1\gamma_2, \, \gamma_2\gamma_3, \, \gamma_3\gamma_1\}, four pseudovectors \{i\gamma_0, i\gamma_1, i\gamma_2, i\gamma_3\} and one pseudoscalar \{i\}, where i=\gamma_0 \gamma_1 \gamma_2 \gamma_3.

## Reciprocal frame

Associated with the orthogonal basis \{\gamma_\mu\} is the reciprocal basis \{\gamma^\mu = \frac{1}\} for all \mu =0,...,3, satisfying the relation

\gamma_\mu \cdot \gamma^\nu = {\delta_\mu}^\nu .

These reciprocal frame vectors differ only by a sign, with \gamma^0 = \gamma_0, and \gamma^k = -\gamma_k for k =1,...,3.

A vector may be represented in either upper or lower index coordinates a = a^\mu \gamma_\mu = a_\mu \gamma^\mu with summation over \mu =0,...,3, according to the Einstein notation, where the coordinates may be extracted by taking dot products with the basis vectors or their reciprocals.

\begin{align}a \cdot \gamma^\nu &= a^\nu \\ a \cdot \gamma_\nu &= a_\nu\end{align}

The spacetime gradient, like the gradient in a Euclidean space, is defined such that the directional derivative relationship is satisfied:

a \cdot \nabla F(x)= \lim_{\tau \rightarrow 0} \frac{F(x + a\tau) - F(x)}{\tau}

This requires the definition of the gradient to be

\nabla = \gamma^\mu \frac{\partial}{\partial x^\mu} = \gamma^\mu \partial_\mu .

Written out explicitly with x = ct \gamma_0 + x^k \gamma_k, these partials are

\partial_0 = \frac{1}{c} \frac{\partial}{\partial t}, \quad \partial_k = \frac{\partial}{\partial {x^k}}

## Spacetime split

 Spacetime split – examples: x \gamma_0 = x^0 + \mathbf{x} p \gamma_0 = E + \mathbf{p}[1] v \gamma_0 = \gamma (1 + \mathbf{v})[1] with γ the Lorentz factor \nabla\gamma_0 = \partial_t - \nabla[2]

In spacetime algebra, a spacetime split is a projection from 4D space into (3+1)D space with a chosen reference frame by means of the following two operations:

• a collapse of the chosen time axis, yielding a 3D space spanned by bivectors, and
• a projection of the 4D space onto the chosen time axis, yielding a 1D space of scalars.[3]

This is achieved by pre or post multiplication by the timelike basis vector \gamma_0, which serves to split a four vector into a scalar timelike and a bivector spacelike component. With x = x^\mu \gamma_\mu we have

\begin{align}x \gamma_0 &= x^0 + x^k \gamma_k \gamma_0 \\ \gamma_0 x &= x^0 - x^k \gamma_k \gamma_0 \end{align}

As these bivectors \gamma_k \gamma_0 square to unity, they serve as a spatial basis. Utilizing the Pauli matrix notation, these are written \sigma_k = \gamma_k \gamma_0. Spatial vectors in STA are denoted in boldface; then with \mathbf{x} = x^k \sigma_k the \gamma_0-spacetime split x \gamma_0 and its reverse \gamma_0 x are:

\begin{align}x \gamma_0 &= x^0 + x^k \sigma_k = x^0 + \mathbf{x} \\ \gamma_0 x &= x^0 - x^k \sigma_k = x^0 - \mathbf{x} \end{align}

## Multivector division

The spacetime algebra is not a division algebra, because it contains idempotent elements \tfrac{1}{2}(1 \pm \gamma_0\gamma_i) and zero divisors: (1 + \gamma_0\gamma_i)(1 - \gamma_0\gamma_i) = 0\,\!. These can be interpreted as projectors onto the light-cone and orthogonality relations for such projectors, respectively. But in general it is possible to divide one multivector quantity by another, and make sense of the result: so, for example, a directed area divided by a vector in the same plane gives another vector, orthogonal to the first.

## Spacetime algebra description of non-relativistic physics

### Non-relativistic quantum mechanics

Spacetime algebra allows to describe the Pauli particle in terms of a real theory in place of a matrix theory. The matrix theory description of the Pauli particle is:[4]

i \hbar \, \partial_t \Psi = H_S \Psi - \frac{e \hbar}{2mc} \, \hat\sigma \cdot \mathbf{B} \Psi

where i is the imaginary unit with no geometric interpretation, \hat\sigma_i are the Pauli matrices (with the ‘hat’ notation indicating that \hat\sigma is a matrix operator and not an element in the geometric algebra), and H_S is the Schrödinger Hamiltonian. In the spacetime algebra the Pauli particle is described by the real Pauli–Schrödinger equation:[4]

\partial_t \psi \, i \sigma_3 \, \hbar = H_S \psi - \frac{e \hbar}{2mc} \, \mathbf{B} \psi \sigma_3

where now i is the unit pseudoscalar i = \sigma_1 \sigma_2 \sigma_3, and \psi and \sigma_3 are elements of the geometric algebra, with \psi an even multi-vector; H_S is again the Schrödinger Hamiltonian. Hestenes refers to this as the real Pauli–Schrödinger theory to emphasize that this theory reduces to the Schrödinger theory if the term that includes the magnetic field is dropped.

## Spacetime algebra description of relativistic physics

### Relativistic quantum mechanics

The relativistic quantum wavefunction is sometimes expressed as a spinor field, i.e.

\psi = e^{\frac{1}{2} ( \mu + \beta i + \phi )}

where ϕ is a bivector, and[5][6]

\psi = R (\rho e^{i \beta})^\frac{1}{2}

where according to its derivation by David Hestenes, \psi = \psi(x) is an even multivector-valued function on spacetime, R = R(x) is a unimodular spinor (or “rotor”[7]), and \rho = \rho(x) and \beta = \beta(x) are scalar-valued functions.[5]

This equation is interpreted as connecting spin with the imaginary pseudoscalar.[8] R is viewed as a Lorentz rotation which a frame of vectors \gamma_\muinto another frame of vectors e_\mu by the operation e_\mu = R \gamma_\mu \tilde{R},[7] where the tilde symbol indicates the reverse (the reverse is often also denoted by the dagger symbol, see also Rotations in geometric algebra).

This has been extended to provide a framework for locally varying vector- and scalar-valued observables and support for the Zitterbewegung interpretation of quantum mechanics originally proposed by Schrödinger.

Hestenes has compared his expression for \psi with Feynman's expression for it in the path integral formulation:

\psi = e^{i \Phi_\lambda / \hbar}

where \Phi_\lambda is the classical action along the \lambda-path.[5]

Spacetime algebra allows to describe the Dirac particle in terms of a real theory in place of a matrix theory. The matrix theory description of the Dirac particle is:[9]

\hat \gamma^\mu (\mathbf{j} \partial_\mu - e \mathbf{A}_\mu) |\psi\rangle = m |\psi\rangle

where \hat\gamma are the Dirac matrices. In the spacetime algebra the Dirac particle is described by the equation:[9]

\nabla \psi \, i \sigma_3 - \mathbf{A} \psi = m \psi \gamma_0

Here, \psi and \sigma_3 are elements of the geometric algebra, and \nabla = \gamma^\mu \partial_\mu is the spacetime vector derivative.

### A new formulation of General Relativity

Lasenby, Doran, and Gull of Cambridge University have proposed a new formulation of gravity, termed gauge theory gravity (GTG), wherein spacetime algebra is used to induce curvature on Minkowski space while admitting a gauge symmetry under "arbitrary smooth remapping of events onto spacetime" (Lasenby, et al.); a nontrivial proof then leads to the geodesic equation,

\frac{d}{d \tau} R = \frac{1}{2} (\Omega - \omega) R

and the covariant derivative

D_\tau = \partial_\tau + \frac{1}{2} \omega ,

where ω is the connexion associated with the gravitational potential, and Ω is an external interaction such as an electromagnetic field.

The theory shows some promise for the treatment of black holes, as its form of the Schwarzschild solution does not break down at singularities; most of the results of general relativity have been mathematically reproduced, and the relativistic formulation of classical electrodynamics has been extended to quantum mechanics and the Dirac equation.

## References

• A. Lasenby, C. Doran, & S. Gull, “Gravity, gauge theories and geometric algebra,” Phil. Trans. R. Lond. A 356: 487–582 (1998).
• Chris Doran and Anthony Lasenby (2003). Geometric Algebra for Physicists, Cambridge Univ. Press. ISBN 0-521-48022-1
• David Hestenes (1966). Space-Time Algebra, Gordon & Breach.
• David Hestenes and Sobczyk, G. (1984). Clifford Algebra to Geometric Calculus, Springer Verlag ISBN 90-277-1673-0
• David Hestenes (1973). "Local observables in the Dirac theory", J. Math. Phys. Vol. 14, No. 7.
• David Hestenes (1967). "Real Spinor Fields", Journal of Mathematical Physics, 8 No. 4, (1967), 798–808.
1. ^ a b A. N. Lasenby, C. J. L. Doran: Geometric algebra, Dirac wavefunctions and black holes. In: Peter Gabriel Bergmann, Venzo De Sabbata (eds.): Advances in the interplay between quantum and gravity physics, Springer, 2002, ISBN 978-1-4020-0593-0, pp. 256-283, p. 257
2. ^ A. N. Lasenby, C. J. L. Doran: Geometric algebra, Dirac wavefunctions and black holes. In: Peter Gabriel Bergmann, Venzo De Sabbata (eds.): Advances in the interplay between quantum and gravity physics, Springer, 2002, ISBN 978-1-4020-0593-0, pp. 256-283, p. 259
3. ^ John W. Arthur: Understanding Geometric Algebra for Electromagnetic Theory (IEEE Press Series on Electromagnetic Wave Theory), Wiley, 2011, ISBN 978-0-470-94163-8, p. 180
4. ^ a b See eqs. (75) and (81) in: D. Hestenes: Oersted Medal Lecture
5. ^ a b c See eq. (3.1) and similarly eq. (4.1), and subsequent pages, in: D. Hestenes: On decoupling probability from kinematics in quantum mechanics, In: P.F. Fougère (ed.): Maximum Entropy and Bayesian Methods, Kluwer Academic Publishers, 1990, pp. 161–183 (PDF)
6. ^ See also eq. (5.13) of S. Gull, A. Lasenby, C. Doran: Imaginary numbers are not real – the geometric algebra of spacetime, 1993
7. ^ a b See eq. (205) in: D. Hestenes: Spacetime physics with geometric algebra, American Journal of Physics, vol. 71, no. 6, June 2003, pp. 691 ff., DOI 10.1119/1.1571836 (abstract, full text)
8. ^ D. Hestenes, Oersted Medal Lecture 2002: Reforming the mathematical language of physics, DOI 10.1119/1.1522700 (abstract, full text)
9. ^ a b See eqs. (3.43) and (3.44) in: Chris Doran, Anthony Lasenby, Stephen Gull, Shyamal Somaroo, Anthony Challinor: Spacetime algebra and electron physics, in: Peter W. Hawkes (ed.): Advances in Imaging and Electron Physics, Vol. 95, Academic Press, 1996, ISBN 0-12-014737-8, p. 272–386, p. 292