Current theories of quantum gravity are based on the assumption of a Lorentzian (pseudo-Riemannian) spacetime manifold. However, incredibly, this assumption proves to be a 100-year-old optical illusion, an impossible object: Light rays and light cones in a spacetime manifold confirm its Euclidean metric, because if it were Lorentzian, the length of lightlike phenomena would be reduced to zero. Starting from the observer-dependent Euclidean spacetime, the invariant Lorentzian metric may be retrieved by calculation, however, the Lorentzian metric does not in any way parameterize the spacetime manifold, but only its worldlines and fields. - These concerns have been known informally for nearly ten years (viXra:1612.0056), but they have been disregarded by official research. As a non-physicist, the author argues that the assumption of Lorentzian spacetime manifold should be reviewed by professional physicists on the basis of the objections raised, because in case of their confirmation, this could possibly pave the way to quantum gravity.