The question whether measurable quantities of a quantum object have definite values prior to the actual measurement is a fundamental issue ever since quantum theory has been introduced in the early 20th century. Leggett-Garg inequalities (LGIs) study temporal correlations of a single system, which are derived under the assumption of so called macro-realism. There it is assumed, that any (macroscopic) object, which may occur in two (or more) macroscopically distinct states, is at any given time in a definite one of those states. The predictions of quantum mechanics stand in stark contradiction with these realistic theories, which manifest in violations of LGIs. We report on an experiment that demonstrates the violation of an LGI with neutrons, where the final measured value of the Leggett–Garg correlator K = 1.120 ± 0.026, obtained in a neutron interferometric experiment, is clearly above the limit K = 1 predicted by macro-realistic theories. E. Kreuzgruber et al. Phys. Rev. Lett. 132, 260201 (2024), öffnet eine externe URL in einem neuen Fenster (24. Jun 2024)