Physicists measure smallest gravitational field ever detected

During the 2019 Christmas season, four physicists hovered over two tiny golden orbs, each the size of a ladybug, in a laboratory in Vienna. It was silent, in every way you can imagine: audible, seismic, even electromagnetic. It had to be, because the researchers were trying to detect the influence of one of the sphere’s gravities on the other.

Do you detect that they did, in a first for gravitational soundings on this scale. One of the gold balls (the “source mass”) was recorded by oscillating the other sphere, very slightly. The team’s results were published today in nature.

“If you take our little golden planet, an object on the planet’s surface would actually fall at a rate 30 billion times slower than the rate at which objects fall on Earth.” Markus Aspelmeyer, quantum physicist at the University of Vienna and co-author of the article, said in a video call. “This is the magnitude we’re talking about.”

Questions about gravity, one of nature’s fundamental forces and perhaps the most noticeable, tend to occur on the most massive and miniature scales. Questioning great gravities is about distant masses – examinations of black holes and neutron stars projected far across the cosmos. But better understanding of the smallest efforts of the force occurs here on Earth, where researchers can control the environment of their experiments with infinitely more ease than in the intractable spread of space.

For Aspelmeyer’s team, that control meant stifling variables that could disrupt the team’s results, from a researcher drifting too close to the gold orbs. while testing the outside traffic. Physicists intentionally conducted the experiments while on vacation, when fewer streetcars were circulating outside and the normal hustle and bustle of Viennese affairs would be slowed down as people stayed at home with their families.

“You have to play a few rounds,” said Aspelmeyer, “to distinguish the acceleration of the source mass from the accelerations of all other masses.”

Gold was chosen for the source mass because it is heavy, dense, can be quite pure, and physicists can easily understand all the properties of mass. Just like you would with a new piece of jewelry, they bought the gold for fundamental physics research from a local goldsmith in Vienna, who made them specifically to scale.

In the experiment, the small golden beads were separated by a small Faraday shield, to avoid electromagnetic interference. A pearl was attached to a horizontal bar suspended from the ceiling with a mirror on it, and the other – the mass exerting a gravitational field – was moved intermittently. A laser was pointed at the mirror, and the incremental movements of the sphere at the receiving end of this tiny force field were recorded in the movements of the laser, which were accurately recorded.

“The detection of such a tiny gravitational signal is in itself an exciting result, but the authors went even further in determining a value for G from their experience,” said Christian Rothleitner, a non-affiliated physicist at Physikalisch- Technische Bundesanstalt in Germany, in an accompanying perspective article. “The experiment is therefore the first to show that Newton’s law of gravity is valid even for source masses as small as these.”

This is not the end of the line for its gravitational surveys. Finally, the hope of physicists to measure gravitational fields in a quantum state, thus reconciling the fact that general relativity, the theory that best explains gravity, cannot be explained in terms of quantum mechanics. The more detailed the field measurements, the closer researchers get to big questions, such as why dark matter is invisible but still contributes to the mass of the universe.

Long before such small-scale experimentation takes place, the team will be working with smaller non-quantum masses.

“The main limiting factor at the moment is still ambient noise, which doesn’t necessarily mean a different experimental setup,” said co-author Hans Hepach, a physicist at the University of Vienna, in the same video call. “The fundamentally limiting factor for the current experiment is the thermal noise of the pendulum suspension. In this way remove the suspension and levitate the test mass (for example, magnetically) would allow smaller masses. “

Gravitational tinkering has revealed a new small scale to the weakest force in the universe. To detect it required a very controlled laboratory environment and diligent calculations. The next time you’re in Vienna, don’t forget to shut up. Physicists are working.