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Physicists who disproved '5th force' win $3 million 'Breakthrough' prize

Left: A close-up image shows a torsion balance used to precisely measure the pull of gravity. Right: The Eöt-Wash Group poses with a torsion balance. From right to left they are Jens Gundlach, Eric Adelberger and Blayne Heckel.
Left: A close-up image shows a torsion balance used to precisely measure the pull of gravity. Right: The Eöt-Wash Group poses with a torsion balance. From right to left they are Jens Gundlach, Eric Adelberger and Blayne Heckel.
(Image: © Breakthrough)

Three physicists won a $3 million Breakthrough prize for proving there is no fifth force (that we know of). And it all started with a series of table-top experiments using cheap equipment.

Eric Adelberger, Jens Gundlach and Blayne Heckel together lead the "Eöt-Wash Group," which is devoted to precise tests of physical laws. They take their name from the early-1900s physicist Loránd Eötvös and the University of Washington, where they work. These Eöt-Wash researchers got their start in the mid-1980s, using a device known as a "torsion balance" to disprove claims of an undiscovered fifth force in physics. Since then, they've used more elaborate versions of the same device to test the true strength of gravity, detect the tug of dark matter in the Milky Way and search for theoretical physical effects like extra dimensions and "axion wind."

The group's $3 million prize is one of seven awarded this year in the life sciences, physics and mathematics. They won "for precision fundamental measurements that test our understanding of gravity, probe the nature of dark energy and establish limits on couplings to dark matter," according to Breakthrough.

Each year's winners are chosen by past recipients in a secret process, and each winning person or group receives more than three times as much money as winners of the Nobel Prizes. A group of tech billionaires — Sergey Brin, Anne Wojcicki, Mark Zuckerberg, Priscilla Chan, Yuri Milner, Julia Milner, Jack Ma and Pony Ma — supply the funds.

Related: The 18 biggest unsolved mysteries in physics

Adelberger, Gundlach and Heckel's win is remarkable in part because their group has not detected any previously-unseen phenomena, built any giant experiments, or developed any remarkable new theories. Instead, they precisely measured physical effects that scientists already knew about, and they tested claims made by other researchers with unusual rigor. Some of their most important results have falsified scientific theories, rather than proving them right.

"That's actually not what we're out for, falsifying," Gundlach told Live Science. "We actually are interested in new physics."

Adelberger somewhat disagreed.

"It's a little more complicated than that," he told Live Science. "Physics right now is in a kind of crisis. You've got two things that work extremely well: Einstein's gravity and quantum theories. … Both of them have been tested very carefully, both of them work wonderfully. But they're completely inconsistent. So there's something really big we're missing. So it's important to go back and ask 'How well do we understand the things that we think we understand?'"

That's led the team to develop torsion balances that measure gravitational effects to unheard-of levels of precision.

A torsion balance is a simple device: Weights hang from a fiber such that Earth's gravity pulls them straight down. If the only other forces acting on the weights also pull straight down, they won't move at all. But if any forces pull them at even a slight angle, they'll rotate, and the fiber will twist. It's possible to measure even very subtle twisting of a torsion balance's fiber and detect extraordinarily tiny effects.

Related: 6 weird facts about gravity

The trio built their first torsion balance in the mid-1980s after hearing a talk by another physicist, Ephraim Fischbach of Purdue University. Fischbach claimed that the four fundamental forces in physics (gravity, electromagnetism and the weak and strong nuclear forces) had a fifth companion. This fifth force, he argued, was a bit like a faint, short-range gravity, pulling masses together at distances of up to about 650 feet (200 meters).

Mostly you wouldn't notice this force, because in space most objects are much farther apart than that. And on Earth, this supposed fifth force would still mostly pull you in the same direction as gravity. At any given time, most of the mass near you is probably directly below you.

Fischbach made his argument based in large part on data from an early-20th-century torsion balance experiment by Eötvös (whose name would become part of the Eöt-Wash Group's). 

Eötvös was testing Einstein's equivalence principle, the idea that two objects dropped at the same time would fall toward a source of gravity at the same rate no matter their mass, no matter how fast the room they're in is moving or where it is in the universe. Eötvös, like countless later experimenters, found that Einstein's equivalence principle was correct Adelberger said. But Fischbach, pouring through the data decades later, thought he'd found a hint of something else, a signature of this fifth force.

"It was a pretty compelling argument," Gundlach said.

"And if it were true, it would have been a big deal," Adelberger said.

(Fischbach's argument had problems, he said, which the trio uncovered even before they ran their own experiment. One of them: Eötvös himself was a large man, and his own gravity may have pulled his older torsion balance sideways, simulating a fifth force.)

Other researchers had seen Fischbach's claim, and some of them seemed to be detecting a fifth force. 

That's because their experiments weren't designed well enough, Adelberger said. "Fischbach kept telling us about all these other researchers who were getting positive results, and he said 'You're in the minority here.' And I said, 'You don't vote on physics.'"

Previous experiments involved a ball floating in water, which is problematic because "a ball floating in water is subject to all kinds of forces," Adelberger said.

People flew all over the world looking for ideal test sites for a fifth force, cliffs next to flat land where the sideways pull of a short-range fifth force would be most exaggerated. The Eöt-Wash Group considered flying to Hawaii before realizing it was more practical to just use a hillside in Seattle.

Related: The 11 biggest unanswered questions about dark matter

"There were other people who were using torsion balances," Gundlach said. "But we made a bunch of little innovations that made the torsion balance so much better."

One of the biggest: Placing the device on a turntable to cancel out external forces acting on the torsion balance. The first turntable they used was a lazy susan, the sort found in some kitchens and dining rooms.

"I remember everyone was laughing at it," Gundlach said.

Other physicists thought the wobbling of the turntable would overwhelm the faint effects they were hunting. But in the end, with some refining, it worked.

"We just did a lot of lovely, clever things and there was no doubt that this was convincing," Adelberger said.

There was no fifth force.

Over time, the team's torsion balances have become more refined, requiring precise engineering. They rely heavily on workers in the University of Washington's machine shop —— one of the few left in the country attached to a physics department —— to constantly update and test their balances before experiments. It's important to get a torsion balance calibrated just right, Heckel said, because once an experiment begins it can run for days, months or years. And all that time is wasted if the machining and calibrating isn't perfect. Any unexpected wobbles or forces beyond the experiment that don't get cancelled out can spoil a batch of data.

The trio's techniques have been used in all sorts of experiments that require very stable measurement devices. Technology they developed to counteract seismic rumbling now helps keep the laser beams of gravitational-wave detectors stable — contributing to the Nobel Prize-winning first detection of gravitational waves in 2016 and a whole new field of astronomy.

Their newer, more advanced torsion balances hunt much fainter effects than the disproved fifth force. Heckel designed a torsion balance that detects the subtle force of electrons whirling circles in a metal disk . It was designed to hunt the subtle pressure of "axion wind," a possible effect of dark matter passing through Seattle. The balance never detected the wind, but did put new limits on how dark matter particles couls look and behave.

The researchers have also built a torsion balance sensitive to the gravitational pull of the Milky Way. Because the mass of visible star systems in the Milky Way is well-known, they were able to cancel it out of their experiment. That left just the effect of the Milky Way's dark matter on the torsion balance, which they could directly measure. Their measurement showed no effect of Modified Newtonian Dynamics (MOND), a theory that rejects dark matter and claims that more complicated theories of gravity explain its apparent effects. (Unlike the fifth-force experiment, this result didn't convince MOND theorists to give up their claims, Edelberger said. MOND researchers are a tough crowd to persuade.)

And using yet another torsion balance they measured the force of gravity to unprecedented precision, canceling out other effects to come up with an extraordinarily precise number for the gravitational constant —— a number that governs equations using gravity.

Originally published on Live Science.