Physicists measure the tiniest gravitational force ever

Two tiny gold spheres respond to each other's gravitational pull.
Two tiny gold spheres respond to each other's gravitational pull. (Image credit: Tobias Westphal/University of Vienna)

Physicists have measured the smallest gravitational field ever recorded, in an experiment that could help in the search for a unified theory of physics.

Of the four fundamental forces known to physics — the weak and strong interactions, the electromagnetic force and the gravitational force — only gravity remains unintegrated into the playbook of physics called the Standard Model, which describes how the zoo of subatomic particles behaves. Gravity is instead described by Einstein's general theory of relativity, but as this breaks down at the quantum scale, our best picture of the universe is left divided in two. 

As a result, physics still can’t describe how gravity works at subatomic scales, leaving physicists scratching their heads when it comes to understanding the singularities that lie in the centers of black holes, or why gravity is so much weaker in strength than all of the other forces.

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But a new experiment that measured the miniscule gravitational attraction between two tiny gold spheres, each spanning just 2 millimeters across, could be the first of many to provide clues to how gravity operates at these scales.

"This was a proof-of-concept experiment to create a sensor capable of measuring very small accelerations and to establish methods that would allow us to detect even smaller gravitational forces," study co-author Jeremias Pfaff, a doctoral student at the University of Vienna, told Live Science. "Long-term, we'd like to answer what the gravitational field of a quantum object in a superposition looks like, but there is a lot to be done on the way there," said Pfaff, referring to the gravity experienced by a subatomic particle that is in two quantum states at once.

To get a peek at how gravity works at small scales, the researchers used a tiny version of a torsion balance — a device first devised by English scientist Henry Cavendish in 1798 to measure the density of the Earth, and from it the strength of the gravitational constant called G. 

A torsion balance is a horizontal bar suspended from its center by a wire with two masses, in this case gold spheres, attached to each end. This means that if a tiny force is applied along the horizontal axis of the bar, the wire will twist and scientists can measure the applied force  based on how much the bar has rotated. By bringing a third gold sphere into close proximity with one of those attached to the end of the bar, the researchers were able to measure the force of gravity between it and the attached sphere. 

The force the researchers were looking for was tiny. At about 9 × 10^minus 14 newtons, it would be the force that one-third of a human blood cell would experience in Earth's gravitational field. So the experiment needed to be incredibly sensitive, and the researchers had to minimize exposure to outside noise, make sure that no stray charges built up on the apparatus, and find a way to spot the desired signal. 

"The urban environment is also far from ideal," said Pfaff. "It was stunning to see that we are not only sensitive to small earthquakes, but also to the local tram and single buses. We were even able to see the Vienna city marathon in our data."

They got rid of any stray charges by flooding the area surrounding the apparatus with ionized nitrogen before placing it in a vacuum. They also made the tiny gravitational signal they were searching for stand out more by moving the two spheres closer and farther apart ever-so-slowly. 

In much the same way that a blinking light is more noticeable than a constant one, the growing and shrinking gravitational force between the spheres was much easier to pick out than if they were stationary. This allowed the researchers to find the strength of the gravitational force between the two spheres, and also find their own measurement for the gravitational constant. 

So far, at the scale they were measuring, gravity followed the same predictable rules it does at larger scales. The physicists are now hoping to make their experiment even more sensitive so they can pick up smaller signals from masses at least 1,000 times lighter and at shorter distances. This could provide important clues to a theory that explains gravity at both small and large scales, along with insights into other mysteries like the existence of dark matter, a mysterious form of matter that emits no light yet exerts gravitational pull.

At smaller scales, the researchers could begin to detect completely new ways that matter interacts through gravity — ways that follow the much more bizarre rules of the quantum world. If they do, physics might finally start to bridge the gap between our big and small pictures of the universe.

"Expanding our knowledge on this elusive force might help us gather hints to find a more fundamental understanding of our physical reality," Pfaff said.

Originally published on Live Science.

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Ben Turner
Live Science Staff Writer

Ben Turner is a U.K. based staff writer at Live Science. He covers physics and astronomy, among other topics like weird animals and climate change. He graduated from University College London with a degree in particle physics before training as a journalist. When he's not writing, Ben enjoys reading literature, playing the guitar and embarrassing himself with chess.