During the 2019 Christmas season, four physicists hovered over two tiny golden spheres, each about the size of a ladybug, in a Viennese laboratory. It was quiet, in every way you can imagine: audible, seismic, even electromagnetic. It had to be, because the researchers were trying to detect the influence of the sphere’s gravity on the other.
Detecting that they did, in a first for gravity probes on this scale. One of the golden balls (the “source mass”) was picked up while the other ball wobbled, 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 down at a speed 30 billion times slower than how fast objects fall on Earth,” Markus Aspelmeyer, a quantum physicist at the University of Vienna and a co-author of the paper, said in a video call. “This is the size we are talking about.”
Interrogations of gravity, one of the fundamental forces of nature and perhaps the most observable, usually take place on the most massive and miniature scales. Exploring great gravitational forces involves distant masses – examining black holes and neutron stars hurled far across the cosmos. But a better understanding of the slightest effort is happening here on Earth, where researchers can master the environment of their experiments with infinitely more ease than in the stubborn sprawl of space.
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For Aspelmeyer’s team, that control meant dampening variables that could mess with the team’s results, from a researcher floating too close to the golden spheres during testing to the traffic outside. The physicists deliberately conducted the experiments during the holidays, when fewer trams would run outside and the normal hustle and bustle of Viennese business would be slowed down if people stayed at home with their families.
“You have to do a few tricks,” said Aspelmeyer, “to distinguish the acceleration of the source mass from the accelerations of all other masses.”
Gold was chosen as the source mass because it is heavy, dense, can be quite pure, and physicists can quite easily understand all the properties of the mass. Just as you would with a new piece of jewelry, they bought the gold intended for basic physics research from a local goldsmith in Vienna, who made them specially to scale.
In the experiment, the small gold beads were separated by a small Faraday shield to avoid electromagnetic interferenceA bead was attached to a horizontal beam suspended from the ceiling with a mirror on top, and the other – the mass exerting a gravitational field – was moved intermittently. A laser was aimed at the mirror and the incremental movements of the sphere on the receiving side of this minute force field were registered in the movements of the laser, which were recorded with precision.
“The detection of such a miniscule gravitational signal is an exciting result in itself, but the authors went even further by determining a value for G from their experiment,” said Christian Rothleitner, an unaffiliated physicist with the Physikalisch-Technische Bundesanstalt. in Germany, in an accompanying perspective article. “The experiment is therefore the first to show that Newton’s law of gravity holds even for such small source masses.”
This isn’t the end of the line for his y-bitsy gravity studies. In the endphysicists hope to measure gravitational fields in a quantum state, reconciling the fact that general relativity, the theory that best explains gravity, cannot be explained in terms of quantum mechanics. The longer the measurements of the fields, the closer researchers get to answer the big questions, such as why dark matter is invisible but still adds to the mass of the universe.
Well before such small-scale experiments take place, the team will be working with smaller, non-quantum masses.
“The main limiting factor at the moment is still environmental noise, which does not necessarily imply another experimental setup,” said study 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 from the pendulum suspension. Thus, the suspension is eliminated and the test mass is floating (for example magnetic) would allow for a smaller mass. “
Gravity work has revealed a new small scale for the universe’s weakest force. Detecting it required a highly controlled laboratory environment and diligent math. Next time you’re in Vienna, remember to shut up. Physicists are at work.