 Everyone agrees that according to special relativity one can measure spatial intervals in units of time. We can do this because of the postulate of special relativity that states that the speed of light has the same value in all inertial frames. If we perform what amounts to a substitution of variables and take our spatial dimensions to be \(x_n^* = x_n /c\), where \(c\) is the speed of light and \(n = 1, 2, 3\), then we select units in which \(c = 1\).