The small gravitational field between two 90-milligram gold spheres 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 probing gravitational interactions in the quantum realm.
There is a big problem with the math we use to describe the Universe; in particular, 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 theory of general relativity is the model we use to describe and predict gravitational interactions and works well in most contexts. However, when we reach quantum scales, general relativity decomposes and quantum mechanics takes over. Reconciling the two models has so far proved very difficult.
General relativity replaces an earlier model, Newton’s law of universal gravitation, which did not incorporate the curvature of space-time. It is stated that the gravitational 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 is slightly impeded in an astrophysics setting.
But what about really, really small gravitational interactions? Usually, they were really challenging to measure, because it’s so hard to separate them from the effects of Earth’s gravity and other perturbations. Most weight tests on smaller scales involved weights of at least one kilogram (2.2 pounds).
Now, I have become considerably smaller. To achieve this, a team of scientists led by Tobias Westphal of the Austrian Academy of Sciences in Austria turned to the 18th century for inspiration: namely, the first experiment that measured gravity between two masses and gave the first exact values for the gravitational constant.
It was designed by Henry Cavendish, an English scientist who learned how to effectively reverse Earth’s gravity. He created a torsional balance by attaching lead weights to each end of a horizontally suspended rod.
The attraction between the weights caused the rod to rotate, twisting the wire on which the rod was suspended, allowing Cavendish to measure gravity based on how much the wire twisted. The set-up became known as the Cavendish experiment.
Westphal and colleagues modified the Cavendish Experiment for their small-scale gravitational attraction tests. Their masses were small golden spheres, each with a radius of only 1 millimeter and a weight of 92 milligrams.
On these scales, the team had to take into account a number of sources of disturbances. Two gold spheres were attached to a horizontal glass rod at a separation of 40 millimeters. One of the spheres was the test table, the other the counterbalancing; a third sphere, the source mass, was moved near the test mass to create a gravitational interaction.
A Faraday shield was used to block the spheres from interacting electromagnetically, and the experiment was performed in a vacuum chamber to prevent acoustic and seismic interference.
(Westphal et al., Nature, 2021)
A laser was jumped from a mirror in the center of the rod to a detector. As the rod twisted, the movement of the laser on the detector indicated how much gravitational force was exerted – and the movement of the source mass allowed the team to accurately map the gravitational field generated by the two masses.
Researchers have found that even at these small scales, Newton’s law of universal gravitation remains firm. From their measurements, they even managed to calculate the gravitational constant or Newton, (G), deriving a value of only 9% of the internationally recommended value. They said that this discrepancy can be fully covered by the uncertainties in their experiment, which was not designed to measure G.
In total, their result shows that even smaller measurements can be taken in the future. This could help scientists research the quantum regime and provide information about dark matter, dark energy, string theory and scalar fields.
“Our experiment provides a viable way to enter and explore a regime of gravitational physics that involves precision tests of gravity with microscopic sources isolated at the table or under the Planck table,” they wrote in their paper.
“This opens up possibilities, such as a different approach, for determining Newton’s constant, which so far remains the least well-determined of the fundamental constants. In general, miniaturized precision experiments can test the law of gravity of the inverse square at considerable scales. smaller than possible today “.
The research was published in The nature.