η μ ν = ( − 1 0 0 0 0 1 0 0 0 0 1 0 0 0 0 1 ) = d i a g ( − 1 , 1 , 1 , 1 ) {\displaystyle {\eta _{\mu \nu }}=\left({\begin{array}{*{20}{c}}{-1}&0&0&0\\0&1&0&0\\0&0&1&0\\0&0&0&1\\\end{array}}\right)={\mathop {\rm {diag}} \nolimits }(-1,1,1,1)}
