2.1 General Relativity 5 ity. We define a 4-vector in Minkowski coordinates (in a Minkowski space). This defines a threading of spacetime, which can be represented by a series of lines, corresponding to a fixed x i ( i = 1 , 2 , 3), as well as a slicing of spacetime into hypersurfaces, corresponding to a fixed x $^{0}$. The line element between two spacetime points is given by where we use natural units for which c = ℏ = k$_{B}$ =1. By defining a symmetric Minkowski Metric Tensor we can denote the line element as ds 2 = η$_{µν}$dx $^{µ}$dxν (2.2) η$_{µν}$ ≡    $^{−}$$^{1}$ 0 0 0 0 1 0 0 0 0 1 0 0 0 0 1     ds 2 = − dt 2 + dx 2 + dy 2 + dz 2 (2.1) x µ ≡    $^{x}$$^{0}$ x 1 x 2 x 3     ≡    $^{ct}$ x y z     =   $^{ct}$ x   