Gravity presses or compresses matter together

Smallest attraction measured

All masses attract each other due to the force of gravity - one of the four fundamental forces of nature. The smaller the masses, the more difficult it is to measure the forces acting between them. But now physicists have set a new record with two lightweight gold spheres: Using a special gravitational balance, they were able to measure the extremely weak force of gravity between the two spheres. As the researchers report in the journal “Nature”, such experiments may even be able to test theories on quantum gravity in the future.

The gravitational balance by Markus Aspelmeyer from the Institute for Quantum Optics and Quantum Information in Vienna and his colleagues is basically similar to the famous experiment by Henry Cavendish from 1798. The structure consists of a vertically stretched fiber made of silicon dioxide. From this, the researchers hung a bar made of titanium - each with a 90 milligram gold ball at the end. If you now bring another gold ball near one end of the beam, the two masses would attract each other. This very weak gravitational force twisted the entire beam.

Gravitational balance

When the team periodically varied the distance between the two gold spheres between two and three millimeters, the bar began to vibrate. This movement could be measured extremely precisely using a laser beam. In order to avoid disruptive effects, Aspelmeyer and his colleagues carried out the experiment in a vacuum and shielded electromagnetic forces with a Faraday cage. With this gravitational balance, the researchers succeeded in determining the force of gravity between the spheres - it was only a fraction of a billionth of a millinewton. For comparison: This force corresponds to about a third of the force with which the earth attracts a red blood cell.

Aspelmeyer and his colleagues not only set a new record with their experiment. In principle, researchers could also use such gravitational balances to determine the forces between even smaller masses. This might even allow the force of gravity to be examined more closely at the quantum level. That would be fundamental to physics. Because both Isaac Newton's and Albert Einstein's theoretical models are reaching their limits. In the search for a uniform theory of so-called quantum gravity, physicists could use such experiments to test new approaches in the future.