Despite keeping us grounded and warping light that travels through space, gravity is actually quite a weak force. The smaller the mass, the less gravity appears to have any pull, until at quantum scales it appears to have no force at all.
Now, physicists in England and Europe have measured a tiny—but apparent—gravitational pull on a minuscule mass, making it the smallest mass to yet show the signs of gravity, a force that has perplexed physicists for centuries. The team’s research is published today in Science Advances.
“We have successfully measured gravitational signals at the smallest mass ever recorded, it means we are one step closer to finally realizing how it works in tandem,” said Tim Fuchs, a physicist at the University of Southampton and the study’s lead author, in a university release. “From here we will start scaling the source down using this technique until we reach the quantum world on both sides.”
Two realms of physics, quantum mechanics and Newtonian gravity, don’t appear connected. At least not yet. The quantum realm is where the theories of classical physics break down. The rules that govern our universe don’t apply to those small masses. But understanding how gravitational force manifests on the quantum scale—whether in loops of fields, in vibrational strings, or some other means—could shed light on some of the most vexing questions in physics.
“By understanding quantum gravity, we could solve some of the mysteries of our universe – like how it began, what happens inside black holes, or uniting all forces into one big theory,” Fuchs added.
To make their measurement, the team placed a 0.000015 ounce (0.43 milligram) mass, composed of three magnets and a glass bead, in a cryostat. In order to measure its gravitational force, the team levitated it in a magnetic trap made of tantalum, cooled down in the cryostat to just over absolute zero to make it superconductive. (To detect such a weak gravitational force, the researchers needed to quiet the environment as much as possible and minimize the test object’s movement).
They chilled the magnetic trap down to 4.48 kelvin (about -274° Celsius), and used a SQUID (a Superconducting QUantum Interference Device), a quantum sensor developed by of all entities the Ford Motor Company in the 1960s, to measure gravitational coupling between the test mass and 2.2-pound (1 kilogram) source masses about 3 feet away. The team measured a pull of 30 attonewtons on the test mass.
“Our new technique that uses extremely cold temperatures and devices to isolate vibration of the particle will likely prove the way forward for measuring quantum gravity,” said Hendrik Ulbright, a researcher at the University of Southampton and co-author of the study, in the same release. “Unraveling these mysteries will help us unlock more secrets about the universe’s very fabric, from the tiniest particles to the grandest cosmic structures.”
New information about gravity at its extremes has implications for what happens at the center of a black hole, the inner workings of dense objects like neutron stars, and the nature of so-called dark matter, invisible stuff whose effects are only observed gravitationally. Plenty of new insights into such exotic physics can be made by looking up, toward the interactions of the universe’s largest objects. But plenty more can be revealed by looking down, at the same phenomena played out in Earthly laboratories.
The quantum world is weird, and we’re far from understanding the nature of gravity beyond the limits of classical physics. But the recent experiment appears to have drawn a new line in the sand.