At large scales, the effects of gravity are easy enough to see: think falling apples, or the movement of planets around the sun. At the atomic level, however, the force is extremely weak, making its quantum effects difficult to measure. But observations detailed in a report published today in the journal Nature finally confirm what quantum rules predict: namely, that elementary particles under the influence of gravity move from one energy state to another by making, well, quantum leaps.

To study the quantum effects of gravity, Valery V. Nesvizhevsky of the Institute Laue-Langevin in Grenoble, France, and colleagues devised a clever experiment. They took a beam of ultracold neutrons and let them fly between a reflecting mirror below and a neutron absorber above. By altering the height of the absorber, they could control the vertical component of a particle's velocity as it traversed a parabolic path through the trap. In the classical realm, neutrons of any vertical velocity can travel all the way through the trap to a detector at the far end. Raising the absorber would simply allow a greater number to do so. But at the quantum level, gravity should prevent any neutron from existing in this trapand therefore reaching the detector at the other enduntil its vertical velocity corresponds exactly to the energy of the first quantum state. Experimental results bore this out.

"Our experimental observations of the neutron quantum states in the Earth's gravitational field provide another demonstration of the universality of the quantum properties of matter," the authors write, "but at this stage we have only shown a phenomenon that was expectedalthough not easy to prove." Extending the research, they say, will require improvements to the source of ultracold neutrons. Still, as Thomas Bowles of Los Alamos National Laboratory notes in a commentary accompanying the report, the work "could provide physicists with a new probe of the fundamental properties of matter."