Scientists have managed to measure the smallest gravitational field on record, a breakthrough that could open new avenues of research into fundamental cosmic mysteries such as the existence of dark matter, the possibility of extra dimensions, and the quantum nature of gravity.
A team of researchers based in Vienna, Austria, demonstrated gravitational coupling between two gold spheres that measured about two millimeters across, about the size of sand grains, which represents “the smallest single object whose gravitational field has been measured,” according to a paper published on Wednesday in Nature.
Hans Hepach and Jeremias Pfaff, PhD students at the University of Vienna who co-authored the new study, said the team was “very excited” and “relieved at the same time” that this novel experiment was successful, given the high precision required to conduct the test.
“We were very confident that we could build a sensor for these small forces, but it remains technologically challenging to actually do it,” Hepach and Pfaff said in an email. “It is fascinating that it is possible to isolate gravity, especially if we consider that our experiment was sensitive enough to detect the 1st finisher of the Vienna City Marathon crossing the finishing line about two kilometers away.”
You might mistake this for a figurative description of the experiment’s sensitivity, but the team actually did pick up the festivities that erupted when the winner passed the finish line. Hepach and Pfaff explained that the team continuously monitored environmental disruptions with their instruments, and were able to detect increased noise as the athletes ran by their building and when the race was won.
“There were no earthquakes recorded at that time, so we concluded our signal must stem from the celebrations,” they said.
Gravity is one of four fundamental forces known to science, along with electromagnetism and the strong and weak nuclear forces. But though it is perhaps the most familiar force in our daily lives, gravity is the scientific oddball of the force quartet. The Standard Model of particle physics, a highly corroborated theory that classifies elementary particles and their interactions, can explain the other three forces, but it cannot account for gravity.
Einstein’s theory of general relativity, in contrast, does explain gravitational forces on large scales, but this theory does not cohere with the laws of quantum mechanics that exist in the tiny realms of atoms and particles. Uniting general relativity with quantum mechanics, and thus describing “quantum gravity” (gravity that exists at quantum scales) is one of the major quests of modern physics.
For this reason, scientists are eager to measure gravitational attractions on ever-smaller scales, but that is no easy task. Gravity is the weakest of all the fundamental forces, which means that the disruptions of seismic, electromagnetic, and even other gravitational sources can overwhelm the minute signal of a gravitational field.
To get around this problem, Hepach, Pfaff, and study co-leads Thomas Westphal and Markus Aspelmeyer—all of whom belong to the University of Vienna’s Aspelmeyer Group—meticulously found ways to filter out external interference with the gravitational coupling between a grounded source mass and a moving test mass, both of weighed about 90 milligrams.
The experimental setup. Image: Tobias Westphal, University of Vienna
For instance, the electromagnetic disruption was muted by connecting the source mass to a vacuum chamber, and reducing the charge of the test mass using ionized nitrogen, a technique inspired by highly sensitive gravitational wave detectors. A shield of gold-plated aluminum placed between the masses further suppressed electrostatic noise.
The team also took their measurements “during the seismically quiet Christmas season,” according to the study, which reduced anthropogenic interference with the gravitational field.
“Designing, building and testing our apparatus took time so it was certainly a lucky coincidence that we were ready to measure right before the holidays,” Hepach and Pfaff noted.
“However, at this point we had already realized how sensitive we are to the urban environment,” they added. “For example, we could trace the nightly bus schedule and could even distinguish those going to the city center from those going the other way just by looking at our measurement data. We therefore made sure we would be able to benefit from the holiday season.”
Fortunately, all of these precautions paid off. When the test mass was moved closer to the source mass, the team was able to isolate the gravitational field between the gold spheres, which was equal to an extremely tiny fraction of a Newton, a unit of force.
The new milestone is thrilling on its own merits, but perhaps the bigger takeaway of the study is its implications for future research into small-scale gravitational fields. Eventually, scientists hope to be able to study gravity at quantum scales to resolve tantalizing questions about its behavior in this realm, though this “remains a completely different experimental challenge” from the new milliscale test, the team noted in the study.
“From the experimental side, it is a good idea to include measurement techniques developed for gravitational wave detectors for example,” Hepach and Pfaff said. “All experiments involved in detection and measurement of gravitational coupling suffer from the same problem, namely isolating the object under investigation from all other influences.”
“Therefore advances in measurement techniques, quantum measurements and microfabrication will profit all experiments aimed at tackling these open questions,” they said.
If advances along these lines continue to be made, experiments may eventually constrain theories about dark matter, a non-luminous substance that scientists think is far more common in the universe than the ordinary matter that makes up stars and planets. Dark matter was proposed as a way to explain the weird gravitational forces that appear to be influencing stars and galaxies on large scales, and a better understanding of the fundamentals of gravity could help to resolve its nature, or shed light on whether it even exists at all.
This experimental technique could also reveal new insights about other exotic theories in cosmology and physics. For instance, dark energy, an unknown energy source that appears to be accelerating the expansion of the universe, and string theory, a proposal that the universe contains extra dimensions that go beyond the four that human beings can sense, both overlap with the unsolved mysteries of gravity. These are only a few of the exciting new directions that could eventually spin off or build on this team’s new experimental technique.
“We are already looking into constraining theories of dark energy and modified gravity with our current data set,” Hepach and Pfaff said. “But expanding our research to smaller masses as well as smaller distances will put us in a position to put even tighter constraints on deviations from the known laws of gravitation.”
“Most of all we are motivated by the question of how the gravitational field of a quantum system looks like,” they concluded. “Although gravity is omnipresent, our understanding of the underlying force is incomplete and finding clues as to how gravity works at the quantum level might help us answer these questions.”