In physics, the algebra of physical space (APS) is the use of the Clifford or geometric algebra Cl3,0(R) of the three-dimensional Euclidean space as a model for (3+1)-dimensional spacetime, representing a point in spacetime via a paravector (3-dimensional vector plus a 1-dimensional scalar). The Clifford algebra Cl3,0(R) has a faithful representation, generated by Pauli matrices, on the spin representation C2; further, Cl3,0(R) is isomorphic to the even subalgebra Cl(R) of the Clifford algebra Cl3,1(R). APS can be used to construct a compact, unified and geometrical formalism for both classical and quantum mechanics. APS should not be confused with spacetime algebra (STA), which concerns the Clifford algebra Cl1,3(R) of the four-dimensional Minkowski spacetime. Special relativity Spacetime position paravector In APS, the spacetime position is represented as the paravector where the time is given by the scalar part , and e1, e2, e3 are the standard basis for position space. Throughout, units such that 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 Lorentz transformations and rotors 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 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 This Lorentz rotor can be always decomposed in two factors, one Hermitian , and the other unitary , such that The unitary element R is called a rotor because this encodes rotations, and the Hermitian element B encodes boosts. Four-velocity paravector The four-velocity, also called proper velocity, is defined as the derivative of the spacetime position paravector with respect to proper time τ: This expression can be brought to a more compact form by defining the ordinary velocity as and recalling the definition of the gamma factor: so that the proper velocity is more compactly: The proper velocity is a positive unimodular paravector, which implies the following condition in terms of the Clifford conjugation The proper