Over the 2019 Christmas holiday, a team of four physicists in Vienna conducted a groundbreaking experiment. In a silent laboratory, shielded from seismic, acoustic, and electromagnetic interference, they measured the gravitational attraction between two tiny gold spheres, each about the size of a ladybug. This marked the first time gravity had been detected at such a small scale. Their findings, published in Nature, represent a significant step in understanding this fundamental force.
Measuring Minute Forces
Gravity is typically studied at extreme scales: the immense gravity of black holes and distant stars, or the subtle forces at work here on Earth. While exploring cosmic gravity requires observing vast celestial bodies, investigating the weakest gravitational interactions demands meticulous control in a laboratory setting. The Vienna team chose the quiet Christmas period to minimize external vibrations, even accounting for reduced tram traffic in the city.
A Gold Standard for Gravity
The researchers selected gold for their “source mass” due to its density, purity, and well-understood properties. The tiny spheres were crafted by a local Viennese goldsmith. To isolate the gravitational interaction, the gold spheres were separated by a Faraday shield, blocking electromagnetic forces. One sphere was attached to a horizontal bar suspended from the ceiling, equipped with a mirror. The other sphere, the source of the gravitational field, was moved intermittently. A laser reflected off the mirror measured the minute movements of the first sphere as it responded to the gravitational pull of the second.
The field was measured by detecting the effect of one gold ball’s movement on another.The gravitational field was measured by observing the tiny movements of one gold sphere in response to the other’s motion. Image: Tobias Westphal, University of Vienna
The sensitivity of this setup was astonishing. “An object on the surface of our tiny gold planet would fall 30 billion times slower than an object falling on Earth,” explained Markus Aspelmeyer, a quantum physicist at the University of Vienna and co-author of the study. This illustrates the incredibly weak force they were measuring.
Validating Newton’s Law at the Microscopic Level
The experiment not only detected this minuscule gravitational signal but also determined a value for the gravitational constant (G) at this scale, confirming that Newton’s law of gravity holds true even for such tiny masses. Christian Rothleitner, a physicist at the Physikalisch-Technische Bundesanstalt in Germany, described the achievement as “exciting” in an accompanying perspective article.
The Quest for Quantum Gravity
This research paves the way for even more ambitious experiments. The ultimate goal is to measure gravitational fields in a quantum state, bridging the gap between general relativity and quantum mechanics. Understanding gravity at the quantum level could unlock mysteries like the nature of dark matter, which influences the universe’s mass despite being invisible.
Pushing the Boundaries of Measurement
Before venturing into the quantum realm, the team plans to experiment with even smaller, non-quantum masses. The main current limitation is environmental noise, specifically the thermal noise of the pendulum suspension. Eliminating the suspension and levitating the test mass, perhaps magnetically, would allow for smaller masses and even more precise measurements, according to co-author Hans Hepach.
Conclusion
This Vienna experiment represents a remarkable feat of precision, revealing the influence of gravity at an unprecedented scale. By carefully controlling their environment and employing ingenious techniques, the physicists have opened a new window into the universe’s weakest force, bringing us closer to understanding the fundamental laws governing our reality.
