Ultra Cold Neutrons (UCN) are free neutrons with kinetic energies up to several hundred nano-eV. Low energy neutrons can be reflected by material potential or magnetic fields as illustrated here. Subject to the gravitational field of the earth, they cannot rise more than a few meters.
When brought close to material surfaces, neutrons with their zero charge only experience the strong forces from individual atomic nuclei, each over a very short range. However, the matter-wave as a whole can be reflected by the material surface, just like this Indian Yoga guru is supported quite comfortably by a bed of nails.
Storage of UCN allows for the long observation times needed for precision measurement of many neutron observables. High-precision measurements, confronted with theoretical predictions, probe high-energy physics through loop effects. Ultracold neutrons (UCN), due to their very low kinetic energies, can be stored in material bottles or magnetic traps for a duration of up to hundreds of seconds. The long storage time allows for measurements of many fundamental properties of this charge-neutral hadronic system, with improved precision.
In fact, UCN experiences the four fundamental forces (strong, weak, electromagnetic, and gravity) at comparable scales: