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Follow the neutron. When the ceiling gets below a certain height, no more neutrons hit the detector, indicating a quantum effect.

The Puny Force Follows the Pack

Suspend a small magnet over a nail, and the magnetic force from the tiny object can overmatch the gravitational force of the entire Earth. Gravity's weakness has made it difficult for scientists to understand its properties on small scales, such as whether particles under the influence of gravity obey the laws of quantum mechanics. Now, an experiment in the 17 January issue of Nature answers this question, affirming that gravity is not such a black sheep among the forces after all.

To conduct the experiment, Valery Nesvizhevsky of the Institut Laue-Langevin in Grenoble, France, and his colleagues relied on "potential wells," which are traps caused by forces. For example, you're trapped in a gravitational potential well right now; you don't have enough energy to free yourself from the gravitational pull of Earth, so you're stuck on the planet. Scientists have long known that tiny objects caught in potential wells of the strong force (like nuclear particles in a nucleus) or the electroweak force (like the electron around an atom) can only inhabit certain energy levels: they are quantized. But gravitational attraction is so puny that it had been tough to prove whether objects under its influence were equally restricted.

To get around this problem, Nesvizhevsky and his colleagues bounced ultracold neutrons, which have very little energy, across a reflecting surface and counted them with a detector. They also put an absorber--a ceiling of sorts--above the bouncing neutrons. If the neutrons behaved classically, they could inhabit virtually any energy level, and hence no matter how high the ceiling, some would reach the detector. But the researchers found that when the ceiling was roughly 15 micrometers or less above the surface, no neutrons got through. They concluded that the energy levels of the neutrons were quantized. "This is the first experiment that proves the phenomenon exists," says Nesvizhevsky.

Such a result has long been predicted by physicists--indeed, "it would be a surprise if something like this did not happen," says Steven Chu, a physicist at Stanford. According to Nesvizhevsky, further experiments will be able to fine-tune physicists' understanding of gravity on small scales.