Algebra of physical space

In APS, the spacetime position is represented as the paravector

${\displaystyle x=x^{0}+x^{1}\mathbf {e} _{1}+x^{2}\mathbf {e} _{2}+x^{3}\mathbf {e} _{3},}$

where the time is given by the scalar part x⁰ = t, and e₁, e₂, e₃ are the standard basis for position space. Throughout, units such that c = 1 are used, called natural units. In the Pauli matrix representation, the unit basis vectors are replaced by the Pauli matrices and the scalar part by the identity matrix. This means that the Pauli matrix representation of the space-time position is

${\displaystyle x\rightarrow {\begin{pmatrix}x^{0}+x^{3}&&x^{1}-ix^{2}\\x^{1}+ix^{2}&&x^{0}-x^{3}\end{pmatrix}}}$

The restricted Lorentz transformations that preserve the direction of time and include rotations and boosts can be performed by an exponentiation of the spacetime rotation biparavector W

${\displaystyle L=e^{{\frac {1}{2}}W}.}$

In the matrix representation the Lorentz rotor is seen to form an instance of the SL(2,C) group (special linear group of degree 2 over the complex numbers), which is the double cover of the Lorentz group. The unimodularity of the Lorentz rotor is translated in the following condition in terms of the product of the Lorentz rotor with its Clifford conjugation

${\displaystyle L{\bar {L}}={\bar {L}}L=1.}$

This Lorentz rotor can be always decomposed in two factors, one Hermitian B = B^†, and the other unitary R^† = R⁻¹, such that

${\displaystyle L=BR.}$

The unitary element R is called a rotor because this encodes rotations, and the Hermitian element B encodes boosts.

The four-velocity, also called proper velocity, is defined as the derivative of the spacetime position paravector with respect to proper time τ:

${\displaystyle u={\frac {dx}{d\tau }}={\frac {dx^{0}}{d\tau }}+{\frac {d}{d\tau }}(x^{1}\mathbf {e} _{1}+x^{2}\mathbf {e} _{2}+x^{3}\mathbf {e} _{3})={\frac {dx^{0}}{d\tau }}\left[1+{\frac {d}{dx^{0}}}(x^{1}\mathbf {e} _{1}+x^{2}\mathbf {e} _{2}+x^{3}\mathbf {e} _{3})\right].}$

This expression can be brought to a more compact form by defining the ordinary velocity as

${\displaystyle \mathbf {v} ={\frac {d}{dx^{0}}}(x^{1}\mathbf {e} _{1}+x^{2}\mathbf {e} _{2}+x^{3}\mathbf {e} _{3}),}$

and recalling the definition of the gamma factor:

${\displaystyle \gamma (\mathbf {v} )={\frac {1}{\sqrt {1-{\frac {|\mathbf {v} |^{2}}{c^{2}}}}}},}$

so that the proper velocity is more compactly:

${\displaystyle u=\gamma (\mathbf {v} )(1+\mathbf {v} ).}$

The proper velocity is a positive unimodular paravector, which implies the following condition in terms of the Clifford conjugation

${\displaystyle u{\bar {u}}=1.}$

The proper velocity transforms under the action of the Lorentz rotor L as

${\displaystyle u\rightarrow u^{\prime }=LuL^{\dagger }.}$

The four-momentum in APS can be obtained by multiplying the proper velocity with the mass as

${\displaystyle p=mu,}$

with the mass shell condition translated into

${\displaystyle {\bar {p}}p=m^{2}.}$

The electromagnetic field is represented as a bi-paravector F:

${\displaystyle F=\mathbf {E} +i\mathbf {B} ,}$

with the Hermitian part representing the electric field E and the anti-Hermitian part representing the magnetic field B. In the standard Pauli matrix representation, the electromagnetic field is:

${\displaystyle F\rightarrow {\begin{pmatrix}E_{3}&E_{1}-iE_{2}\\E_{1}+iE_{2}&-E_{3}\end{pmatrix}}+i{\begin{pmatrix}B_{3}&B_{1}-iB_{2}\\B_{1}+iB_{2}&-B_{3}\end{pmatrix}}\,.}$

The source of the field F is the electromagnetic four-current:

${\displaystyle j=\rho +\mathbf {j} \,,}$

where the scalar part equals the electric charge density ρ, and the vector part the electric current density j. Introducing the electromagnetic potential paravector defined as:

${\displaystyle A=\phi +\mathbf {A} \,,}$

in which the scalar part equals the electric potential ϕ, and the vector part the magnetic potential A. The electromagnetic field is then also:

${\displaystyle F=\partial {\bar {A}}.}$

The field can be split into electric

${\displaystyle E=\langle \partial {\bar {A}}\rangle _{V}}$

and magnetic

${\displaystyle B=i\langle \partial {\bar {A}}\rangle _{BV}}$

components. Where

${\displaystyle \partial =\partial _{t}+\mathbf {e} _{1}\,\partial _{x}+\mathbf {e} _{2}\,\partial _{y}+\mathbf {e} _{3}\,\partial _{z}}$

and F is invariant under a gauge transformation of the form

${\displaystyle A\rightarrow A+\partial \chi \,,}$

where ${\displaystyle \chi }$ is a scalar field.

The electromagnetic field is covariant under Lorentz transformations according to the law

${\displaystyle F\rightarrow F^{\prime }=LF{\bar {L}}\,.}$

The Maxwell equations can be expressed in a single equation:

${\displaystyle {\bar {\partial }}F={\frac {1}{\epsilon }}{\bar {j}}\,,}$

where the overbar represents the Clifford conjugation.

The Lorentz force equation takes the form

${\displaystyle {\frac {dp}{d\tau }}=e\langle Fu\rangle _{R}\,.}$

The electromagnetic Lagrangian is

${\displaystyle L={\frac {1}{2}}\langle FF\rangle _{S}-\langle A{\bar {j}}\rangle _{S}\,,}$

which is a real scalar invariant.

• Baylis, William (2002). Electrodynamics: A Modern Geometric Approach (2nd ed.). ISBN 0-8176-4025-8.
• Baylis, William, ed. (1999) [1996]. Clifford (Geometric) Algebras: with applications to physics, mathematics, and engineering. Springer. ISBN 978-0-8176-3868-9.
• Doran, Chris; Lasenby, Anthony (2007) [2003]. Geometric Algebra for Physicists. Cambridge University Press. ISBN 978-1-139-64314-6.
• Hestenes, David (1999). New Foundations for Classical Mechanics (2nd ed.). Kluwer. ISBN 0-7923-5514-8.

• Baylis, W E (2004). "Relativity in introductory physics". Canadian Journal of Physics. 82 (11): 853–873. arXiv:physics/0406158. Bibcode:2004CaJPh..82..853B. doi:10.1139/p04-058. S2CID 35027499.
• Baylis, W E; Jones, G (7 January 1989). "The Pauli algebra approach to special relativity". Journal of Physics A: Mathematical and General. 22 (1): 1–15. Bibcode:1989JPhA...22....1B. doi:10.1088/0305-4470/22/1/008.
• Baylis, W. E. (1 March 1992). "Classical eigenspinors and the Dirac equation". Physical Review A. 45 (7): 4293–4302. Bibcode:1992PhRvA..45.4293B. doi:10.1103/physreva.45.4293. PMID 9907503.
• Baylis, W. E.; Yao, Y. (1 July 1999). "Relativistic dynamics of charges in electromagnetic fields: An eigenspinor approach". Physical Review A. 60 (2): 785–795. Bibcode:1999PhRvA..60..785B. doi:10.1103/physreva.60.785.