Coordinates

A point in space-time is specified by $x^0,x^1,x^2,x^3$, in which $x^0\equiv ct,x^1\equiv x, x^2\equiv y, x^3\equiv z$. A four-vector in this notation can be written as as

$$x^\mu\equiv (x^0,x^k)\equiv (x^0,x^1,x^2,x^3) ,$$ (2.8)

where $\mu=0,1,2,3$ and $k=1,2,3$.