Gravity is an incredibly weak force. It may be the force we perceive the most, especially when we fall down the stairs, but it takes the mass of the entire Earth make that happen. Despite its weakness, it is also the force that gives the Universe its structure, provides the pressure to drive stars, and keeps galaxies together. We might be grateful that gravity isn't stronger, but it is also curious that it is so much weaker than everything else.

When scientists are faced with something they don't understand, their first response is to make more and better measurements. Past experience tells us that understanding is often a function of the number of significant figures in our measurements. The truth is that our measurements of gravity have been well and truly left behind by those on electromagnetism. So, taking techniques learned in our fight to understand quantum mechanics and electromagnetism and applying them to our measurements on gravity makes a great deal of sense. This is exactly what a group of physicists who happen to like bouncing balls have done.

One possible explanation for the apparent weakness of gravity is that its strength may be dissipated into more spatial dimensions than the three we know and love. Or, should there be a multiverse, then perhaps gravity can couple between universes while electromagnetism cannot. In these cases, gravity is weakened at the large scales that we experience and observe. But this dissipation shouldn't be appreciable at very small scales, it will not be stronger than expected from our experience at larger scales. The upshot is that if we can measure the force of gravity very accurately at very small scales, we might be able to detect this.

The big problem with measuring gravity at such tiny scales is not the magnitude of the force. Rather, the problem stems from electromagnetism. Imagine that you have an neutral atom flying over a mass—the aim being to measure the deflection of the atom's flight due to gravity. You will certainly measure a deflection, but it will most likely not be due to gravity. Instead, at some moment in time, the electrons in the atom will be more to one side than the other. As a result, the atoms in the mass will adjust their charges so that the temporary charge imbalance is minimized. This will create an electromagnetic attraction between the neutral atom and the neutral mass. The resulting deflection will have two properties: its size is completely unpredictable, and it will almost certainly be much larger than any deflection due to gravity.

Untangling that mess has been difficult, but the researchers from Austria and Germany appear to have come up with a technique to minimize the problem. Their answer is to use neutrons. They happen to have access to a very clean source of low energy neutrons, which will interact with other matter through only two forces: a magnetic spin moment, which can be carefully shielded, and gravity. To make their measurements, the researchers fired their slow-moving neutrons at a mirror and observed how they bounced.

To make the measurements more accurate, it's better to have a resonant system—something like measuring gravity using a pendulum instead of by running a ball down a slope. To create a resonator, the researchers vibrated the mirror so that neutrons that arrived at the right time would get an extra kick upwards because the mirror is moving that way when they hit the surface. Other neutrons would hit the mirror when it is falling, so their bounce height would be smaller.

The trick is to just select a portion of these neutrons for measurement. To do this, a very clever trick was applied. A second surface was placed just above the mirror—this surface doesn't reflect neutrons; it absorbs them. So the neutrons that arrive with a higher energy or at the wrong time get absorbed, allowing the researchers to only measure particular neutrons.

What this means is that, depending on the height of the second surface and the energy of the neutrons, one can predict how many neutrons should make it through the device when the mirror is moved at a particular frequency. This passes through a resonance that depends on the force of gravity.

The researchers use the language of quantum mechanics to describe how all this works. And although I think the use of quantum mechanics here is a bit artificial (all resonators have discrete resonant frequencies, independent of quantum mechanics), it still makes sense. The reason is that quantum mechanics has a robust set of tools for dealing with how resonators work, as well as for how systems behave when there are not many different resonant frequencies and the driving force isn't quite right for any of those frequencies.

You will note that I haven't really discussed their results. That is because this paper is more like a report that says, "we got it working." They can't report a more accurate value for the gravitational constant, or say that the inverse square law holds to some new boundaries. But now that they have the basic experiment set up, it is just a question of time and refinement before they can report those sorts of results. I look forward to those papers.

Nature Physics, 2011, DOI 10.1038/NPHYS1970

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