Page:Relativity (1931).djvu/130

 ence. We call the magnitude $$ds$$ the “distance” apart of the two events or four-dimensional points.

Thus, if we choose as time-variable the imaginary variable $$\sqrt{-1}ct$$ instead of the real quantity $$t$$, we can regard the space-time continuum—in accordance with the special theory of relativity—as a “Euclidean” four-dimensional continuum, a result which follows from the considerations of the preceding section.