Using Neutrons To Precisely Test Newton's Law of Gravity

NotSanguine writes with this excerpt from the BBC: "The neutrons are shot between two parallel plates, one above another and separated by about 25 micrometres — half a hair's width. The upper plate absorbs neutrons, and the lower plate reflects them. As they pass through, they trace out an arc, just like a thrown ball falling due to gravity. ... The new work by the ILL team has added what is known as a piezoelectric resonator to the bottom plate; its purpose is to jiggle the bottom plate at a very particular frequency. The researchers found that as they changed the bottom plate's vibration frequency, there were distinct dips in the number of neutrons detected outside the plates — particular, well-spaced 'resonant' frequencies that the neutrons were inclined to absorb. These frequencies, then, are the gravitational quantum states of neutrons, essentially having energy bounced into them by the bottom plate, and the researchers were able for the first time to force the neutrons from one quantum state to another. The differences in the frequencies — which are proportional to energy — of each of these transitions will be an incredibly sensitive test of gravity at the microscopic scale."

Re:Neutrons

[femtobyte]

The way that the lower/upper plates "repel/attract" the neutrons is not to be due to familiar forces (e.g. electromagnetic, gravitational, weak, strong), but rather due to quantum scattering effects from the bulk of nuclei in the plate material (which can be either attractive or repulsive, depending on material composition) based on the Fermi exclusion principle (identical fermions, such as neutrons, cannot occupy the same quantum state, resulting in effective forces between them not caused by any other forces). While the statement (as is often true of science journalism for the general public) is unclear and confusing, it is somewhat true in the sense that the neutrons are not interacting through a mechanism that would show up on a list of "forces of nature".