  Amplitudes of quantum transitions containing time zigzags are considered. The
discussion is carried out in the framework of the Minkowski metric and standard
quantum mechanics without adding new postulates. It is shown that the wave
function is singular at the instant of the time zigzag. Nevertheless, we argue
that time zigzags are not suppressed at the quantum level, but their
contribution to the amplitude is zero. The result is valid for a single
particle and a non-interacting scalar field.
