Gravity and quantum mechanics rarely mix in laboratory circles. The weakness of the gravitational interaction makes measuring its effects difficult at the quantum level. However, researchers at the Institut Laue-Langevin (ILL) in Grenoble, France, have now observed quantum effects of gravity on ultracold neutrons trapped in the Earth’s gravitational field. Their technique relies on bouncing neutrons off a reflective surface and observing quantization in the height of the bounce.
The key to the experiment was using ultracold neutrons from the ILL reactor. Neutrons do not bounce, except when they strike a surface at very grazing incidence. Instead, they are absorbed or transmitted. However, by firing neutrons with a velocity of less than 8 cm/s over a horizontal mirror, the ILL researchers were able to reduce the vertical component of the neutrons’ velocity as they fell under gravity to just 1.7 cm/s. These neutrons bounced along the mirror like flat pebbles across a pond until they were captured by a detector at the far end of the mirror. A neutron absorber could be positioned at varying heights above the mirror, allowing the researchers to identify the lowest-energy neutrons passing through the apparatus.
The ILL apparatus behaved as a neutron trap, bound from below by the mirror and from above by gravity. According to quantum mechanics, neutrons in such a trap should occupy discrete gravitational energy levels just as electrons trapped by nuclei occupy discrete electromagnetic energy levels.
This result shows that the lowest level occupied by neutrons in the trap is 1.41 x 10-12 eV. Comparison with the minimum energy for an electron in a hydrogen atom, 13.6 eV, shows why the quantization of energy in a gravitational potential has not been seen before.
The next step is to use a more intense beam and a trap mirrored on all sides to prolong the period of entrapment and thus improve the resolution of the apparatus. This will allow a precision test of the equivalence between gravitational and inertial mass.
Further reading
Nesvizhevsky et al. Nature 415 297.
