Tobias Westphal / University of Vienna
Gravity may feel like one of the most familiar forces, but it’s actually among those we understand least. We know our current model of gravity is inconsistent with quantum mechanics. It also fails to account for the phenomena we’ve termed dark matter and dark energy. Unfortunately, studying gravity is extremely challenging because it’s far and away the weakest of the forces. To get around this issue for the detection of gravitational waves, we’ve had to build two immense observatories, far enough apart so that the noise affecting one wouldn’t be picked up at the other.
The gravitational waves we’ve detected come from utterly massive objects like neutron stars and black holes. Now, researchers in Vienna have announced progress toward detecting the gravitational force generated by tiny objects—in this case, spheres of gold only two millimeters across and weighing less than a tenth of a gram. Their work provides the first measurement of gravity at these scales, and the researchers are pretty sure they can go smaller.
It’s so noisy
The work in question involves a fairly typical device for these sorts of experiments. It involves a solid bar with a gold ball attached to each end. The bar is suspended at its center point, which allows it to rotate freely around the horizontal plane. There’s also a mirror placed at its center point, which is used to reflect a laser.
If a mass is brought near one of the gold balls, it will exert a gravitational force that pulls the ball toward it. The ensuing rotation will cause the mirror to rotate with it, changing where the laser ends up reflected to. This creates an extremely sensitive measure of the gravitational attraction generated by the mass. Or it would if environmental noise didn’t swamp everything out.
The catalog of noise sources that the authors have to account for is mind-boggling. To begin with, the researchers estimate that the gravitational force they’re looking to measure could also be generated by a person walking within three meters of the experimental device or a Vienna tram traveling within 50 meters of it. In the end, they performed the experiment at night over the winter holidays in order to cut down on stray sources of gravitational interference, which had the added effect of cutting down on local seismic noise.
The whole experiment was performed inside a vacuum, and they found rubber feet that remain soft in a vacuum to cushion the structure that holds the suspended metal bar.
Before pulling a vacuum on the experiment, the researchers flooded the apparatus with ionized nitrogen to get rid of any stray charges. And, just in case, they put a Faraday shield between the two gold balls in order to block any electrostatic attraction.
While all of this kept the noise in the experiment extremely low, the signal of the attraction between two 90 milligram gold spheres is also going to be extremely low. So, rather than simply measuring the pull, the researchers moved the sphere in a regular pattern, setting up a steady back-and-forth resonant attraction. The frequency of this resonance was carefully chosen to be very different from the natural resonances of the pendulum that the bar forms.
Tiny forces
The behavior of the whole setup is monitored by a video camera that 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 researches estimate their system is capable of picking up accelerations as small as 2 x 10-11 meters/second2, although it would take about a half-day of monitoring to do so.
Overall, the gravitational force here ended up being about 9 x 10-14 Newtons. The researchers also use their results to derive the gravitational constant. While this ends up being off by quite a bit (9 percent), it’s still within the uncertainties of their experimental measurement.
The result is an impressive technical achievement. But the researchers think that 90 mg is actually on the heavy side of the objects that might be measured this way. And, as things get lighter, there are some dramatically odd things that could potentially be tested.
For example, as mentioned above, our theory of gravity is incompatible with quantum mechanics. But we’ve managed to get ever-larger systems to behave as quantum objects. If we get these measurements sensitive enough, then it might be possible to measure the gravitational attraction of an object that is in a quantum superposition between two locations. In other words, there is no way to tell where exactly it is, while at the same time the gravitational force it exerts depends on where it is.
Other potential tests include some variants of string theory, modified Newtonian Dynamics (MOND, a hypothetical and unpopular replacement for dark matter), and some explanations for dark energy. But all of these will be utterly dependent upon this experimental setup working on masses that are far, far smaller than the milligram scale. So as a first step, it’s going to be important for the researchers behind this work to show that they have at least some of the promised ability to scale down.
Nature, 2021. DOI: 10.1038/s41586-021-03250-7 (About DOIs).