The small gravitational field between two 90-milligram spheres of gold has just been measured for the first time.
This officially makes it the smallest gravitational field ever successfully measured – an achievement that could open the door to gravitational interactions in the quantum realm.
There is a big problem with the mathematics we use to describe the Universe; especially the way gravity acts. Unlike the other three fundamental forces in the Universe – weak, strong and electromagnetic – gravity cannot be described with the standard physics model.
Einstein’s general theory of relativity is the model we use to describe and predict gravitational interactions, and it works beautifully in most contexts. However, when we reach quantum scales, general relativity breaks down, and quantum mechanics takes over. The reconciliation of the two models has so far been very difficult.
General relativity replaces a previous model, Newton’s law of universal gravitation, which did not incorporate the curvature of space-time. It states that the force of attraction between two objects is directly proportional to the product of their masses and inversely proportional to the square of the distance between their centers.
Newtonian physics works well for most terrestrial applications, even if it stumbles a bit in an astrophysical environment.
But what about real, really small gravitational interactions? It was usually very difficult to measure because it is so difficult to disconnect it from the effects of the Earth’s gravity and other disturbances. Most smaller-scale gravity tests involved masses of at least one kilogram (2.2 pounds).
Now we have become significantly smaller. To achieve this, a team of scientists led by Tobias Westphal of the Austrian Academy of Sciences in Austria actually turned to inspiration after the 18th century: namely the very first experiment to measure gravity between two masses and the first accurate values for the gravitational constant.
It was designed by Henry Cavendish, an English scientist who invented how to effectively destroy the earth’s gravity. He created a torsional balance, and he attached lead weights to each end of a horizontal hanging rod.
The force of attraction between the weights caused the rod to rotate and the wire on which the rod hung rotated so that Cavendish could measure the force of gravity based on how much the wire rotated. The setup became known as the Cavendish experiment.
Westphal and colleagues adapted the Cavendish experiment on a small scale for their tests on attraction. Their masses were small golden spheres, each only 1 millimeter in radius and 92 milligrams in weight.
At these scales, the team had to account for a number of sources of disturbance. Two gold spheres were attached to a horizontal glass rod with a separation of 40 millimeters. One of the spheres was the test mass, the other the counterbalance; a third sphere, the source mass, is moved close to the test mass to create a gravitational interaction.
A Faraday shield was used to prevent the spheres of electromagnetic interaction, and the experiment was performed in a vacuum chamber to prevent acoustic and seismic interference.
(Westphal et al., Nature, 2021)
A laser was bounced from a mirror in the center of the bar to a detector. As the rod rotated, the movement of the laser on the detector indicated how much gravitational force was being exerted – and by moving the source mass, the team was able to map the gravitational field generated by the two masses exactly.
The researchers found that Newton’s universal law of gravity, even on this small scale, is still valid. From their measurements, they could even calculate the gravitational, or Newton, constant (G), and obtain only 9 percent of the internationally recommended value. This difference can, according to them, be completely covered by the uncertainties in their experiment, which are not designed to measure G.
In total, their results show that even smaller measurements can be undertaken in the future. It can help scientists investigate the quantum regime and possibly provide insights into dark matter, dark energy, string theory and scalar fields.
“Our experiment provides a viable way to enter and explore a gravitational physics that includes precision tests of gravity with isolated microscopic masses at or below the Planck mass,” they wrote in their paper.
“This opens up possibilities like another approach to determining Newton’s constant, which so far is the least well-determined of the fundamental constants. In general, miniaturized precision experiments can test the inverse-square law on much smaller scales than today . “
The research was published in Nature.