Researchers measure the gravity exerted by a 90-milligram object

Gravity may seem like one of the most familiar forces, but it’s actually one we understand the least. We know that our current model of gravity is incompatible with quantum mechanics. It also ignores the phenomena we have called dark matter and dark energy. Unfortunately, studying gravity is extremely difficult because it is by far the weakest of the forces. To get around this problem of detecting gravitational waves, we had to build two huge observatories, far enough apart so that the noise affecting one was not picked up by the other.

The gravitational waves we have detected come from totally massive objects like neutron stars and black holes. Now researchers in Vienna have announced advances in detecting the gravitational force generated by tiny objects – in this case, gold spheres just two millimeters in diameter and weighing less than a tenth of a gram. Their work provides the first measure of gravity at these scales, and the researchers are pretty sure they can be smaller.

It’s so loud

The work in question involves a fairly typical device for this kind of experience. It is a solid bar with a gold ball attached to each end. The bar is suspended at its central point, which allows it to rotate freely around the horizontal plane. There is also a mirror placed at its central point, which is used to reflect a laser.

If a mass is brought near one of the golden balls, it will exert a gravitational force that pulls the ball towards it. The ensuing rotation will cause the mirror to spin with it, changing where the laser ends up being reflected. This creates an extremely sensitive measure of the gravitational pull generated by the mass. Or it would be if the ambient noise did not overwhelm everything.

The catalog of noise sources that authors must take into account is breathtaking. For starters, the researchers estimate that the gravitational force they are trying to measure could also be generated by a person walking within 3 meters of the experimental set-up or by a Vienna tram running within 50 meters of it. Ultimately, they performed the experiment at night during winter vacation to reduce stray sources of gravitational interference, which had the additional effect of reducing local seismic noise.

The entire experiment was performed in a vacuum, and they found rubber feet that stay soft in the vacuum to cushion the structure that holds the metal bar suspended.

Before evacuating the experiment, the researchers flooded the device with ionized nitrogen to get rid of any parasitic charge. And, just in case, they put a Faraday shield between the two golden balls in order to block any electrostatic attraction.

While all of this kept the noise of the experiment extremely low, the signal of the attraction between two 90 milligram gold spheres will also be extremely weak. So, rather than just measuring the attraction, the researchers moved the sphere in a regular pattern, creating a continuous resonant attraction. The frequency of this resonance has been carefully chosen to be very different from the natural resonances of the pendulum formed by the bar.

Tiny forces

The behavior of the entire installation is monitored by a video camera which constantly monitors the position of the two golden spheres. During the experiment, their separation varied from 2.5 millimeters to 5.8 millimeters. Overall, the researchers estimate that their system is capable of speeding up as little as 2 x 10^-11 meters / second², although it would take about half a day of monitoring to do this.

Overall the gravitational force here ended up being around 9 x 10^-14 Newtons. The researchers also used their results to derive the gravitational constant. Although this ends up being a little offbeat (9%), it remains within the uncertainties of their experimental measurement.

The result is an impressive technical achievement. But researchers believe that 90 mg is actually the heavy side of things that could be measured this way. And, as things get lighter, there are some dramatically weird things that could potentially be tested.

For example, as mentioned above, our theory of gravity is incompatible with quantum mechanics. But we have succeeded in making ever larger systems behave like quantum objects. If we get these sufficiently sensitive measurements, then it might be possible to measure the gravitational pull of an object that is in a quantum superposition between two locations. In other words, there is no way to tell where it is exactly, while at the same time the gravitational force it exerts depends on where it is.

Other potential tests include some variations of string theory, modified Newtonian dynamics (MOND, a hypothetical and unpopular dark matter replacement), and some explanation of dark energy. But all of this will totally depend on this experimental setup operating on masses well, much smaller than the milligram scale. So, as a first step, it will be important for the researchers behind this work to show that they have at least part of the capacity promised for reduction.

Nature, 2021. DOI: 10.1038 / s41586-021-03250-7 (About DOIs).