Hi Ivars!
Sounds like ... ghost-hunting
! Or, in physics, .... "negative" energy, or particles having "imaginary" mass (tachyons). Or, in mathematics, imaginary straight-lines being perpendicular to ... themselves. Or, always in math, varieties with an infinite number of dimensions (Hilbert?). Or, very modern subject indeed, physical bodies with a "fractal extension" (Mandelbrot?), like the length of the coast of Brittany, which is not ... a length, otherwise it will be ... un-determinate 
.
GFR
Sounds like ... ghost-hunting
! Or, in physics, .... "negative" energy, or particles having "imaginary" mass (tachyons). Or, in mathematics, imaginary straight-lines being perpendicular to ... themselves. Or, always in math, varieties with an infinite number of dimensions (Hilbert?). Or, very modern subject indeed, physical bodies with a "fractal extension" (Mandelbrot?), like the length of the coast of Brittany, which is not ... a length, otherwise it will be ... un-determinate .GFR