Page 218 - Neutrons for Sciences and Society

Neutrons for Science
demonstrate for the first time the quantum states of matter in
a gravitational field. The weakness of this field compared to a magnetic field makes observation difficult. As an uncharged but heavy particle the neutron is the best contender to show these states. V.V. Nesvizhevsky and co-workers116 used ultra-cold neutrons with a velocity less than 5-10 ms-1 produced by a device developed by Steyerl117. In this apparatus neutrons from the vertical beam from the cold source are scattered from the surface of the blades of a turbine which is moving in the same direction as the incident neutrons and thus are strongly slowed down as a ball in a tennis drop shot. The idea was developed in 1966, with
a first realisation at the Munich reactor in 1975, then the turbine was installed at the ILL in 1985. The experiment of Nesvizhevsky et al. made these very slow neutron bounce on the surface of a mirror and measured their jump heights. Macroscopic quantified jump heights were observed thus demonstrating the existence of neutron quantum states with discrete energies in a gravitational field. This was the founding experiment of a new gravitational spectroscopy capable of measuring extremely low energies.
The experiment above which tests the generality of quantum physics is not the only one in the field of fundamental physics which has been carried out with the help of neutron beams. We know that at the beginning of the twentieth century Einstein and Bohr were in opposition debating whether quantum physics was a complete theory, or whether there were hidden parameters waiting to be found. Einstein was for the former. Experimental tests were proposed and performed using photons which disproved the
116 V.V. Nesvizhevsky et al.(2002) Nature, 415, p297-299, DOI 10.1038/415297a
117 A. Steyerl and S.S.Malik (1989) Nucl. Instrum. Methods Phys. Res. A, 284, p200-207, DOI 10.1016/0168- 9002(89)90282-9
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