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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."
Wow, my physics courses apparently forgot to mention that Newton's Law of Gravity had anything to say about the quantum states of neutrons. In fact, I was taught it's not a law; it's a falsified hypothesis.
Newton's Law of Gravity can be seen as an approximation of Einstein's theory. We have to be careful when we speak of "falsified". We haven't discovered that gravity is proportional to 1/r, or that gravity isn't attractive but repulsive. We have discovered that Einstein's models are better predictors of experimental results. We can still us Newton's models to send humans to the Moon. But Newton's model makes no sense when asking questions such as "what would happen to the Earth if the Sun suddenly disappeared. It doesn't predict the bending of light, nor does it properly describe certain orbital phenomenon.
This and no other is the root from which a tyrant springs; when first he appears as a protector - Plato (423 to 327 BC)
If you bother to read the article, you see that they are trying to see whether good old Newtonian gravity is a good approximation at extremely small length and mass scales (scales where the additional accuracy provided by general relativity is unnecessary). They're trying to see if when you make the experiment this sensitive, do you see some kind of quantum effect. The answer so far seems to be no. Yes, the neutrons behave in a quantum mechanical way. The question is, do they behave as you'd predict if Newton's/Einstein's gravity is true, or do they do something unexpected? This has nothing to do with Newton vs. Einstein.
According to the links, one plate is smooth, and the other is rough. so a neutron will glide over the smooth plate or be scattered at small angles but if it hits the rough plate it will be scattered more, on average. Why the difference? So that they have a different effect and you can tell if perturbing their course causes more to hit the smooth plate or the rough one.
The neutron's course can be perturbed by gravity. In the steady state, this means the neutron just drops in a parabolic arc following gravity, which at these length scales (microns) can be more determined by massive nearby objects (1/r^2 is huge) than by the distant center of the Earth (1/r^2 is tiny). (You might even get a setup where the top plate gravity is equal and opposite to the Earth's gravity, for objects that are close enough.)
Moving one plate nearer or farther away makes the arc change shape, changing how many neutrons are scattered for a given beam intensity and launch angle. Moving the plate in an oscillating motion at a given magnitude should give you an oscillating scattering measurement with a fairly constant magnitude. You would expect the number of neutrons scattered to be irrelevant of the frequency, when averaged over many cycles of the oscillation, if you considered gravity to be purely Newtonian (i.e., Newtonian gravity, f = GmM/r^2, is monotonic with changes in r, even when r is changing with time).
But they don't see that. They see distinct frequencies of plate oscillation that result in bumps or sharp bends in the average scattering.
That says they're seeing non-monotonic, quantized, time-dependent effects that Einsteinian gravity suggests.
Yes, we obviously need to be worried that the large team of scientists and engineers who designed and built this experiment have overlooked the most basic principles of freshman physics and mechanical design. Good thing we have the keen intelligence of Slashdot science critics to catch all these subtle flaws that would otherwise slip by the reviewers at Nature un-noticed. Should we also worry that the scientists are all part of the government conspiracy to cover up the true Time Cube four-side harmony perfection of gravity symmetry?