Tobias Westphal / University of Vienna
Gravity may feel like one of the most well-known forces, but it is actually one of the forces we least understand. We know that our current model of gravity conflicts with quantum mechanics. Nor does it take into account the phenomena we have called dark matter and dark energy. Unfortunately, studying gravity is extremely challenging, as it is by far the weakest of the forces. To get around this gravitational wave detection problem, we had to build two immense observatories far enough apart so that the sound affecting one is not picked up by the other.
The gravitational waves we detected come from extremely massive objects such as neutron stars and black holes. Now, researchers in Vienna have announced progress in detecting the gravity generated by small objects – in this case, gold spheres just two millimeters wide and less than a tenth of a gram. Their work provides the first measurement of gravity on these scales, and the researchers are pretty sure they can get smaller.
It’s so noisy
The work in question is a fairly typical device for this type of experiment. It is a sturdy bar with a golden ball at each end. The beam is suspended in the center, allowing it to rotate freely around the horizontal plane. There is also a mirror placed in the center, which is used to reflect a laser.
If a mass is brought close to one of the golden balls, it will exert a force of gravity that pulls the ball towards it. The subsequent rotation causes the mirror to rotate and change where the laser is reflected. This creates an extremely sensitive measure of the gravity generated by the mass. Or if environmental noise wouldn’t engulf everything.
The catalog of sound sources that the authors have to consider is mind-boggling. For starters, the researchers estimate that the gravity they are trying to measure could also be generated by a person walking within 3 meters of the experimental device or by a Viennese tram traveling within 50 meters of it. They ended up running the experiment at night during the winter holidays to reduce the stray sources of gravitational interference, which had the added effect of reducing local seismic noise.
The whole experiment was done in a vacuum, and they found rubber feet that stay soft in a vacuum to dampen the structure that holds the suspended metal rod.
Before the experiment was evacuated, the researchers flooded the device with ionized nitrogen to remove any stray charges. And just in case they put a Faraday shield between the two gold balls to block any electrostatic attraction.
While all of this kept the noise extremely low in the experiment, the signal from the attraction between two 90 milligram gold spheres will also be extremely low. So instead of simply measuring the pull, the researchers moved the sphere in a regular pattern, creating a steady reciprocating resonant pull. The frequency of this resonance was carefully chosen to be very different from the natural resonances of the pendulum forming the rod.
Small forces
The behavior of the entire 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 researchers estimate that their system is capable of absorbing accelerations as small as 2 x 10-11 meter / second2, although it would take about half a day of monitoring to do this.
In general, gravity here was about 9 x 10-14 Newton. The researchers also used their results to derive the gravitational constant. While this is quite off (9 percent), it is 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 could be measured in this way. And as things get lighter, there are some dramatically strange things that could potentially be tested.
For example, as mentioned above, our theory of gravity is incompatible with quantum mechanics. But we have managed to make ever larger systems behave like quantum objects. If we get these measurements sensitive enough, it may be possible to measure the gravity 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, whereas the force of gravity it exerts depends on where it is.
Other possible tests include some variations of string theory, modified Newtonian dynamics (MOND, a hypothetical and unpopular substitute for dark matter), and some explanations for dark energy. But all of these will depend entirely on this experimental setup operating on masses much, 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 some of the promised ability to shrink.
Nature, 2021. DOI: 10.1038 / s41586-021-03250-7 (over DOIs).