Grasping gravity: Former FSU graduate student’s theory predicted groundbreaking physics discovery

Here on Earth, gravity is part of everyday life. We can’t live without it, but most people never stop to think about how gravity occurs and how gravity impacts mass on a subatomic level, where it’s even harder to observe.

Scientists around the world are still working to understand “gravitons,” the elusive, theoretical particles believed to be responsible for Earth’s gravitational force. Researchers from Florida State University’s Department of Physics have theorized an analog of it, which may help in solving this universal puzzle.

That theory has now been confirmed in new research by a team from China’s Nanjing University.

Researchers from Nanjing published a new paper in the journal Nature this spring that confirmed the groundbreaking scientific prediction made five years ago by FSU physics alumna Shiuan-Fan “Sonya” Liou.

At the time, Liou was a graduate student in Professor Kun Yang’s group when she published research shedding light on gravitons in her 2019 paper, “Chiral Gravitons in Fractional Quantum Hall Liquids,” proposing a theory about the specific quantum spin of these particles. It is common practice for theoretical physicists to postulate a new phenomenon that experimental physicists later confirm.

Liou and Yang collaborated with Princeton University professor and 2016 Nobel Prize winner Duncan Haldane, and California State University Los Angeles Professor Edward Rezayi. The study built upon prototypical topological states of matter known as “fractional quantum Hall liquids,” a topic on which Haldane, Rezayi and Yang had many years of prior collaboration.

The electrons in fractional quantum Hall liquids collectively behave as if they are made of particles with fractional electric charge. These quantum fluids are known as a topological state of matter because their behavior remains the same even if you change their shape or move things around inside them.

In previous research, Haldane argued that just using the shapes and structures of a purely topological description wasn’t enough to understand how these fluids behaved.

Those descriptions overlooked an important phenomenon: When energy was added to the fluids, researchers noticed a regular oscillation within them. That oscillation had a positive or negative angular momentum, meaning some spun clockwise and others spun counterclockwise. That property is known as chirality, in which particles have a right or left “handedness,” and it was much like the description of gravitons in the quantum theory of gravity widely postulated by scientists.

Liou’s research provided for the first time a detailed numerical study of such graviton-like excitations and demonstrated that these gravitons carry a definitive chirality, or angular momentum, which is either negative 2 or positive 2, depending on different properties of the fractional quantum Hall liquid.

“I was new to this project and didn’t know the big-picture physics behind it in the very beginning,” Liou said. “I got the result quite fast, in about two weeks. Even though I initially doubted the result, a peak absorption indicating chiral gravitons, I sent it out to Professor Yang. He was thrilled about the result and asked to discuss it with me immediately after seeing the findings. I’d never seen him that excited.”

“This paper is important because it points out a very specific property of these gravitons — chirality — and we pinned down what that chirality is for different liquids and how to measure it,” Yang said.

During a collaboration meeting in 2019, the FSU-led research team tapped Aron Pinczuk, a pioneer during the 1990s of a technique called Raman scattering used to study collective excitations of fractional quantum Hall liquids. Pinczuk, who planned to use polarized light for his Raman experiment to reveal the graviton chirality, passed away unexpectedly in 2022.

As a result, a group of Pinczuk’s former students, postdocs, and collaborators from Nanjing University, Columbia University, Princeton and the University of Münster in Germany picked up and completed the project. The Nanjing University-led group successfully performed the polarized Raman scattering in several prominent fractional quantum Hall liquids, using a semiconductor, a strong magnetic field, and a laser to study quantum effects. The excitation observed through scattered light had a quantum spin matching the anticipated graviton chirality, confirming Liou’s theoretical predictions.

“This is likely the first time that a spin-2 (graviton-like) excitation has been seen in nature. These experiments open the door for an entirely new way to study the topological properties of fractional quantum Hall liquids,” Yang said.

The publication in Nature, which has garnered attention from several major news outlets, has the potential to unlock further mysteries surrounding the quantum theory of gravity.

“In the physics field, there are few theoretical predictions that can be confirmed in experiments due to the technical limitations,” Liou said. “I believed our prediction, chiral gravitons, would be found one day, but didn’t expect the results to come so soon. I’m very happy and lucky to see our work being carried out in the real world.”

To learn more about physics research at FSU, visit physics.fsu.edu.