The equation above is similar to the Pythagorean theorem, except with a minus sign between the ( c t ) 2 and the x 2 terms. The spacetime interval is the quantity s 2 , not s itself. The reason is that unlike distances in Euclidean geometry, intervals in Minkowski spacetime can be negative. Rather than deal with square roots of negative numbers, physicists customarily regard s 2 as a distinct symbol in itself, rather than the square of something.