Minkowski Metric
The Minkowski metric, also called the Minkowski tensor or pseudo-Riemannian metric, is a tensor eta_(alphabeta) whose elements are defined by the matrix
(eta)_(alphabeta)=[-1 0 0 0; 0 1 0 0; 0 0 1 0; 0 0 0 1],
(1)
where the convention c=1 is used, and the indices alpha,beta run over 0, 1, 2, and 3, with x^0=t the time coordinate and (x^1,x^2,x^3) the space coordinates.
The Euclidean metric
(g)_(alphabeta)=[1 0 0; 0 1 0; 0 0 1],
(2)
gives the line element
ds^2 = g_(alphabeta)dx^alphadx^beta
(3)
= (dx^1)^2+(dx^2)^2+(dx^3)^2,
(4)
while the Minkowski metric gives its relativistic generalization, the proper time
dtau^2 = eta_(alphabeta)dx^alphadx^beta
(5)
= -(dx^0)^2+(dx^1)^2+(dx^2)^2+(dx^3)^2.
(6)
The Minkowski metric is fundamental in relativity theory, and arises in the definition of the Lorentz transformation as
Lambda^alpha_gammaLambda^beta_deltaeta_(alphabeta)=eta_(gammadelta),
(7)
where Lambda^alpha_beta is a Lorentz tensor. It also satisfies
eta^(betadelta)Lambda^gamma_delta=Lambda^(betagamma)
(8)
eta_(alphagamma)Lambda^(betagamma)=Lambda_alpha^beta
(9)
Lambda_alpha^beta=eta_(alphagamma)Lambda^(betagamma)=eta_(alphagamma)eta^(betadelta)Lambda^gamma_delta.
(10)
The metric of Minkowski space is diagonal with
eta_(alphaalpha)=1/(eta_(alphaalpha)),
(11)
and so satisfies
eta^(betadelta)=eta_(betadelta).
(12)
The necessary and sufficient conditions for a metric g_(munu) to be equivalent to the Minkowski metric eta_(alphabeta) are that the Riemann tensor vanishes everywhere (R^lambda_(munukappa)=0) and that at some point g^(munu) has three positive and one negative eigenvalues.
http://mathworld.wolfram.com/MinkowskiMetric.html