so(3,1) — indefinite orthogonal real Lie algebra

The indefinite orthogonal real Lie algebra $\mathfrak{so}(3,1)$ preserves a symmetric bilinear form with $3$ positive and $1$ negative directions infinitesimally:

\mathfrak{so}(3,1)=\{X\in M_{4}(\mathbb R):X^T\eta+\eta X=0\},\qquad \eta=\operatorname{diag}(I_{3},-I_{1}).

The bracket is the matrix commutator.

Dimension and real form

Multiplication by $\eta$ identifies the defining vector space with skew-symmetric matrices, so its real dimension is $6$ . This is a vector-space count, not an assertion that compact and indefinite brackets are isomorphic. It is the six-dimensional Lorentz algebra, isomorphic to $\mathfrak{sl}(2,\mathbb C)_{\mathbb R}$ as a real Lie algebra.

Exchanging the positive and negative coordinates gives an isomorphic Lie algebra $\mathfrak{so}(1,3)$ .