Minkowski Space
\(\eta(u_1,u_2) = u_1 \cdot u_2 = c^2 t_1 t_2 - x_1 x_2 - y_1 y_2 - z_1 z_2\)
\(\lVert u \rVert = \sqrt{ \eta(u, u) } = \sqrt{c^2 t_1 t_2 - x_1 x_2 - y_1 y_2 - z_1 z_2}\)
\(x^2 + y^2 + z^2 + (ict)^2 = C\)
\(\eta(u_1,u_2) = u_1 \cdot u_2 = c^2 t_1 t_2 - x_1 x_2 - y_1 y_2 - z_1 z_2\)
\(\lVert u \rVert = \sqrt{ \eta(u, u) } = \sqrt{c^2 t_1 t_2 - x_1 x_2 - y_1 y_2 - z_1 z_2}\)
\(x^2 + y^2 + z^2 + (ict)^2 = C\)