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 that has been 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 behavior of gravity. 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 very well in most contexts. However, when we get to quantum scales, general relativity breaks down 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 had not incorporated the curvature of space-time. He states 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 stumbles a bit in an astrophysical environment.
But what about really, very small gravitational interactions? They have usually been really difficult to measure, because it is very difficult to separate them from the effects of the Earth’s gravity and other perturbations. Most gravity tests on smaller scales have involved masses of at least one kilogram (2.2 pounds).
Now, we have shrunk considerably. To achieve this, a team of scientists led by Tobias Westphal of the Austrian Academy of Sciences in Austria turned to the eighteenth century for inspiration: that is, the first experiment to measure gravity between two masses and give the first precise values for the gravitational constant.
This was designed by Henry Cavendish, an English scientist who discovered how to effectively undo the gravity of the Earth. He created a torsion balance, fixing lead weights at each end of a horizontally suspended bar.
The attraction between the weights caused the rod to rotate, rotating the wire on which the rod was suspended, which allowed Cavendish to measure gravity as a function of the degree of rotation of the wire. The setup was known as the Cavendish experiment.
Westphal and colleagues modified the Cavendish experiment for their small-scale gravitational attraction tests. Its masses were small golden spheres, each weighing only 1 millimeter in radius and weighing 92 milligrams.
At these scales, the equipment needed to take into account various sources of disturbances. Two gold spheres were joined to a horizontal glass bar at a distance of 40 millimeters. One of the spheres was the test mass, the other the counterweight; a third sphere, the source mass, moved near the test mass to create a gravitational interaction.
A Faraday shield was used to block the electromagnetic interaction of the spheres and the experiment was conducted in a vacuum chamber to prevent acoustic and seismic interference.
(Westphal et al., Nature, 2021)
A laser was bounced off a mirror in the center of the rod to a detector. When the rod was rotated, the movement of the laser over the detector indicated how much gravitational force was being exerted, and the movement of the source mass allowed the equipment to accurately map the gravitational field generated by the two masses.
The researchers found that even at these small scales, Newton’s universal law of gravitation still stands firm. From their measurements, they were even able to calculate the gravitational or Newton’s (G) constant, deriving a value of 9% from the internationally recommended value. According to them, this discrepancy may be completely covered by the uncertainties of their experiment, which was not designed to measure G.
In total, its result shows that even smaller measures can be taken in the future. This could help scientists investigate the quantum regime and potentially provide information on dark matter, dark energy, string theory, and scalar fields.
“Our experiment provides a viable way to enter and explore a gravitational physics regime that involves gravity precision testing with masses of microscopic sources isolated at or below Planck’s mass,” they wrote in their paper.
“This opens up possibilities as a different approach to determining the Newtonian constant, which to date remains the least well-determined of the fundamental constants. scales considerably smaller than possible today “.
The research has been published in Nature.