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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.