Quantum Gravitational States

Quantum gravitational states have been observed for the first time. An experiment with ultracold neutrons shows that their vertical motion in Earth's gravitational field come in discrete sizes. Quantum properties--such as the quantization of energies, wavelike dynamics including interference, and an irreducible uncertainty in the simultaneous measurement of position and momentum--usually emerge only at the atomic level or under special circumstances (e.g., low temperatures) wherein a particle is trapped in a potential well by a controlling force. Observing such properties in phenomena governed by the electromagnetic or the weak and strong nuclear forces is common enough, but the strength of gravity, many orders of magnitude weaker than the other forces, has not previously been strong enough to enforce the kind of confinement needed to make quantum reality manifest.

Such an effect has now been seen. Physicists at the Institute Laue-Langevin reactor in Grenoble, France employ a beam of ultracold neutrons. Moving at a pace of 8 m/sec (compared to 300 m/sec for an oxygen molecule at room temperature), the neutrons are sent on a gently parabolic trajectory through a baffle and onto a horizontal plate. Because the neutrons bounce at such a grazing angle, the plate is essentially a mirror for the neutrons, which are reflected back upwards until gravity saps their ascent; then the neutrons start falling again, eventually to be captured by a detector. In effect the neutrons are caught in a vertical potential well: gravity pulls down, while atoms in the surface of the mirror push up.

The researchers report seeing a minimum (quantum) energy of 1.4 picoelectron volts (1.4 x 10^-12 eV), which corresponds to a vertical velocity of 1.7 cm/sec. A comparison of this energy level to the minimum energy for an electron trapped inside a hydrogen atom, -13.6 eV, demonstrates why this kind of detection has not been made before. The experiment provides also preliminary evidence for higher quantized motion states as well. In the horizontal direction there is no confinement and therefore no quantum effect. [By the way, neutron-interferometry experiments, in which neutron waves are split apart, moved around separate paths, and then brought back together in order to produce an interference pattern, have been influenced by gravity, but these neutron waves were not quantum states owing to the gravitational field. By contrast, the Laue-Langevin experiment is the first to observe quantum states of matter (neutrons) in Earth's gravitational field.]

The next step is to use a more intense beam and an enclosure mirrored on all sides (the energy resolution improves the longer the neutrons spend in the device). An energy resolution as sharp as 10^-18 eV is expected, which would allow one to test such basic propositions as the equivalence principle, according to which the neutron's gravitational mass (as measured by its free fall in gravity) is the same as its inertial mass (as prescribed by Newton's second law, F=ma, where F is a generic force and a the acceleration imparted). (Nesvizhevsky et al., Nature, 17 Jan 2002.)