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*invariant*

**(1)**$$ 0 = (cdt)^{2}-dx^{2}$$

While,

**(2)**$$ 0 = (cdt)^{2}+dx^{2} $$

is not invariant?

The first expressions (1) measures the coordinates of a wavefront propagating away from the observer with the speed och light, and since c is a constant, this expression must be zero in all reference frames moving with constant velocity relative to it.

But why not use expression (2) as a way to measure distans?