In mathematical physics, spacetime algebra (STA) is a name for the Clifford algebra Cl1,3(R), or equivalently the geometric algebra G( M4). According to David Hestenes, spacetime algebra can be particularly closely associated with the geometry of special relativity and relativistic spacetime.
It is a vector space that allows not only vectors, but also bivectors (directed quantities associated with particular planes, such as areas, or rotations) or blades (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.
The spacetime algebra may be built up from an orthogonal basis of one time-like vector and three space-like vectors, , with the multiplication rule
where is the Minkowski metric with signature (+ − − −).
Thus, , , otherwise .
The basis vectors share these properties with the Dirac matrices, but no explicit matrix representation need be used in STA.
Associated with the orthogonal basis is the reciprocal basis for , satisfying the relation
These reciprocal frame vectors differ only by a sign, with , and for .
A vector may be represented in either upper or lower index coordinates with summation over , according to the Einstein notation, where the coordinates may be extracted by taking dot products with the basis vectors or their reciprocals.
The spacetime gradient, like the gradient in a Euclidean space, is defined such that the directional derivative relationship is satisfied:
This requires the definition of the gradient to be
Written out explicitly with , these partials are
|Spacetime split – examples:|
|where is the Lorentz factor|
In spacetime algebra, a spacetime split is a projection from four-dimensional space into (3+1)-dimensional 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. 
This is achieved by pre- or post-multiplication by the timelike basis vector , which serves to split a four vector into a scalar timelike and a bivector spacelike component. With we have
As these bivectors square to unity, they serve as a spatial basis. Utilizing the Pauli matrix notation, these are written . Spatial vectors in STA are denoted in boldface; then with the -spacetime split and its reverse are:
The spacetime algebra is not a division algebra, because it contains idempotent elements and nonzero zero divisors: . These can be interpreted as projectors onto the light-cone and orthogonality relations for such projectors, respectively. But in some cases 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.
where is the imaginary unit with no geometric interpretation, are the Pauli matrices (with the 'hat' notation indicating that is a matrix operator and not an element in the geometric algebra), and is the Schrödinger Hamiltonian. In the spacetime algebra the Pauli particle is described by the real Pauli–Schrödinger equation: 
where now is the unit pseudoscalar , and and are elements of the geometric algebra, with an even multi-vector; 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.
This equation is interpreted as connecting spin with the imaginary pseudoscalar.  is viewed as a Lorentz rotation which a frame of vectors into another frame of vectors by the operation ,  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 with Feynman's expression for it in the path integral formulation:
where is the classical action along the -path. 
where are the Dirac matrices. In the spacetime algebra the Dirac particle is described by the equation: 
Here, and are elements of the geometric algebra, and is the spacetime vector derivative.
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 derivation then leads to the geodesic equation,
and the covariant derivative
where is the connection associated with the gravitational potential, and 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.
- Lasenby, A.; Doran, C.; Gull, S. (1998), "Gravity, gauge theories and geometric algebra", Phil. Trans. R. Soc. Lond. A, 356 (1737): 487–582, arXiv: gr-qc/0405033, Bibcode: 1998RSPTA.356..487L, doi: 10.1098/rsta.1998.0178
- Doran, Chris; Lasenby, Anthony (2003), Geometric Algebra for Physicists, Cambridge University Press, ISBN 978-0-521-48022-2
- Hestenes, David (2015) , Space–Time Algebra (2nd ed.), Birkhäuser
- Hestenes, David; Sobczyk (1984), Clifford Algebra to Geometric Calculus, Springer Verlag, ISBN 978-90-277-1673-6
- Hestenes, David (1973), "Local observables in the Dirac theory", Journal of Mathematical Physics, 14 (7): 893–905, Bibcode: 1973JMP....14..893H, CiteSeerX 10.1.1.412.7214, doi: 10.1063/1.1666413
- Hestenes, David (1967), "Real Spinor Fields", Journal of Mathematical Physics, 8 (4): 798–808, Bibcode: 1967JMP.....8..798H, doi: 10.1063/1.1705279
- Lasenby, A.N.; Doran, C.J.L. (2002). "Geometric algebra, Dirac wavefunctions and black holes". In Bergmann, P.G.; De Sabbata, Venzo (eds.). Advances in the interplay between quantum and gravity physics. Springer. pp. 256–283, See p. 257. ISBN 978-1-4020-0593-0.
- Lasenby & Doran 2002, p. 259
- Arthur, John W. (2011). Understanding Geometric Algebra for Electromagnetic Theory. IEEE Press Series on Electromagnetic Wave Theory. Wiley. p. 180. ISBN 978-0-470-94163-8.
- See eqs. (75) and (81) in: Hestenes & Oersted Medal Lecture 2002
- See eq. (3.1) and similarly eq. (4.1), and subsequent pages, in: Hestenes, D. (2012) . "On decoupling probability from kinematics in quantum mechanics". In Fougère, P.F. (ed.). Maximum Entropy and Bayesian Methods. Springer. pp. 161–183. ISBN 978-94-009-0683-9. ( PDF)
- See also eq. (5.13) of Gull, S.; Lasenby, A.; Doran, C. (1993). "Imaginary numbers are not real – the geometric algebra of spacetime" (PDF).
- See eq. (205) in Hestenes, D. (June 2003). "Spacetime physics with geometric algebra" (PDF). American Journal of Physics. 71 (6): 691–714. Bibcode: 2003AmJPh..71..691H. doi: 10.1119/1.1571836.
- Hestenes, David (2003). "Oersted Medal Lecture 2002: Reforming the mathematical language of physics" (PDF). American Journal of Physics. 71 (2): 104. Bibcode: 2003AmJPh..71..104H. CiteSeerX 10.1.1.649.7506. doi: 10.1119/1.1522700.
- See eqs. (3.43) and (3.44) in: Doran, Chris; Lasenby, Anthony; Gull, Stephen; Somaroo, Shyamal; Challinor, Anthony (1996). Hawkes, Peter W. (ed.). Spacetime algebra and electron physics. Advances in Imaging and Electron Physics. 95. Academic Press. pp. 272–386, 292. ISBN 0-12-014737-8.
- Imaginary numbers are not real – the geometric algebra of spacetime, a tutorial introduction to the ideas of geometric algebra, by S. Gull, A. Lasenby, C. Doran
- Physical Applications of Geometric Algebra course-notes, see especially part 2.
- Cambridge University Geometric Algebra group
- Geometric Calculus research and development