Over the Christmas season of 2019, four physicists hovered over two minuscule gold orbs, each about the size of a ladybug, in a Vienna laboratory. It was silent, in all the ways you can imagine: audibly, seismically, even electromagnetically. It had to be, as the researchers were trying to detect the influence of one of the sphere’s gravity on the other.
Detect they did, in a first for gravitational probings at this scale. One of the golden balls (the “source mass”) was recorded wobbling the other sphere, ever so slightly. The team’s results were published today in Nature.
“If you take our little gold planet, an object on the surface of the planet would actually fall down with a velocity that is 30 billion times slower compared to 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 magnitude we are talking about.”
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Interrogations of gravity, one of nature’s fundamental forces and perhaps its most perceptible, tend to happen on the most massive and miniature of scales. Querying large gravities deals with masses far away — examinations of black holes and neutron stars flung far across the cosmos. But better understanding the slightest exertions of the force happens here on Earth, where researchers can control the environment of their experiments with infinitely more ease than in the intractable sprawl of space.
For Aspelmeyer’s team, that control meant muffling variables that could mess with the team’s results, from a researcher drifting too close to the gold orbs while testing to the traffic outside. The physicists intentionally conducted the experiments over the holidays, when fewer trams would be running outside and the normal jostle of Viennese business would be slowed as people stayed home with their families.
“You need to play some tricks,” Aspelmeyer said, “to distinguish the acceleration from the source mass against the accelerations of all the other masses.”
Gold was chosen for the source mass because it is heavy, dense, can be pretty pure, and physicists can pretty easily understand all the properties of the mass. Much like you would with a new piece of jewellery, they bought the gold meant for fundamental physics research at a local goldsmith in Vienna, who crafted them specifically to scale.
In the experiment, the little golden beads were separated by a small Faraday shield, to prevent any electromagnetic interference. One bead was attached to a horizontal bar hanging 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 on the receiving end of this minute force field were recorded in the movements of the laser, which were recorded with precision.
“The detection of such a minuscule gravitational signal is itself an exciting result, but the authors went even further by determining a value for G from their experiment,” said Christian Rothleitner, an unaffiliated physicist at 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 source masses as small as these.”
This isn’t the end of the line for itsy-bitsy gravitational inquiries. Eventually, physicists’ hope 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 minute the measurements of the fields, the closer researchers get to answering the big questions, like why dark matter is invisible but still contributes to the universe’s mass.
Well before such small-scale experimentation takes place, the team will work with smaller, non-quantum masses.
“The main limiting factor at the moment is still environmental noise, which does not 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 suspension of the pendulum. Thus eliminating the suspension and levitating the test mass (for example, magnetically) would allow for smaller masses.”
The gravitational tinkering has revealed a newly small scale to the universe’s weakest force. To detect it required a very controlled lab environment, and diligent maths. The next time you’re in Vienna, just remember to shush. Physicists are working.