The small gravitational field between two 90 milligram spheres of gold has just been measured for the first time.
This makes it officially the smallest gravitational field ever successfully measured – an achievement that could open the door to investigating gravitational interactions in the quantum realm.
There is a big problem with the math we use to describe the universe; especially the way gravity behaves. Unlike the other three fundamental forces in the universe – weak, strong and electromagnetic – gravity cannot be described with the Standard Model of physics.
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 move to the quantum scale, the general theory of relativity falls apart and quantum mechanics takes over. Reconciling the two models has proven very difficult so far.
General relativity replaces an earlier model, Newton’s law of universal gravity, which did not include the curvature of spacetime. It states that the force of attraction between two objects is directly proportional to the product of their mass 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 really, really small gravitational interactions? These were usually very difficult to measure because of the difficulty of decoupling them from the effects of Earth’s gravity and other disturbances. Most smaller scale gravity tests involve masses of at least one kilogram (2.2 pounds).
Now we have become considerably smaller. To achieve this, a team of scientists led by Tobias Westphal of the Austrian Academy of Sciences in Austria actually turned to the 18th century for inspiration: namely, the very first experiment to measure the gravity between two masses and get the first accurate values. for the gravitational constant.
This was designed by Henry Cavendish, an English scientist who discovered how to effectively nullify Earth’s gravity. He created a torsional balance and attached lead weights to each end of a horizontally suspended bar.
The pull between the weights caused the rod to twist, twisting the wire the rod was hung from, allowing Cavendish to measure gravity based on how much the wire was twisted. The setup became known as the Cavendish experiment.
Westphal and colleagues adapted the Cavendish Experiment for their small-scale tests of gravity. Their mass consisted of tiny gold spheres, each only 1 millimeter in radius and 92 milligrams in weight.
On this scale, the team had to take into account a number of sources of disturbance. Two golden spheres were mounted on a horizontal glass rod at a distance of 40 millimeters. One of the spheres was the test mass, the other the counterweight; a third sphere, the source mass, was moved near the test mass to create a gravitational interaction.
A Faraday shield was used to prevent the spheres from interacting electromagnetically, and the experiment was conducted in a vacuum chamber to avoid acoustic and seismic interference.
(Westphal et al., Nature, 2021)
A laser was reflected from a mirror in the center of the rod to a detector. As the rod rotated, the movement of the laser on the detector indicated how much gravity was being exerted – and by moving the source mass, the team was able to accurately map the gravitational field generated by the two masses.
The researchers found that, even on this small scale, Newton’s universal law of gravity is still in effect. In fact, based on their measurements, they were able to calculate the gravity, or Newton’s, constant (G) and derive a value as low as 9 percent of the internationally recommended value. This discrepancy, they said, could be completely covered by the uncertainties in their experiment, which was not intended to make G.
All in all, their result shows that even smaller measurements can be carried out in the future. This could help scientists investigate the quantum regime and potentially provide insights into dark matter, dark energy, string theory, and scalar fields.
“Our experiment provides a viable avenue to enter and explore a regime of gravitational physics that involves precision testing of gravity with isolated microscopic source masses at or below Planck’s mass,” they wrote in their paper.
This opens up possibilities, such as a different approach to determine Newton’s constant, which is so far the least well-defined of the fundamental constants. possibly today. “
The research is published in Nature