Researchers at MIT have made a groundbreaking advancement in one of the most challenging questions in modern physics: Is gravity a quantum force? By using advanced laser technology to cool a tiny mirror to near absolute zero, they have opened new experimental avenues to investigate the relationship between quantum mechanics and gravity. This unique combination of cutting-edge cooling techniques and classical physics tools may finally allow scientists to determine if gravity operates under the same principles as other quantum forces, a mystery that has eluded physicists for decades.
One of the most profound questions in contemporary physics is whether gravity can be described by quantum theory. Unlike other fundamental forces such as the electromagnetic, weak nuclear, and strong nuclear forces, gravity remains a puzzle. While scientists have successfully formulated quantum theories for these forces, a consistent quantum theory of gravity has yet to emerge, highlighting a significant gap in our understanding of the universe.
“Theoretical physicists have proposed various scenarios, ranging from gravity being purely classical to being fully quantum,” explains Dongchel Shin, a PhD candidate in the MIT Department of Mechanical Engineering (MechE). “However, the debate remains unresolved as we have yet to find a clear method to test gravity’s quantum characteristics in the laboratory.” Shin emphasizes that achieving this requires creating mechanical systems that are large enough to experience gravity but also quiet enough to demonstrate quantum effects.
In a recent study, Shin, who is also a MathWorks Fellow, and his team made significant progress by employing lasers to cool a tiny mechanical device known as a torsional oscillator. Their open-access paper, titled “Active laser cooling of a centimeter-scale torsional oscillator,” published in Optica, details how this innovative method could potentially reveal whether gravity adheres to quantum rules.
Historically, lasers have been utilized to cool atomic gases since the 1980s and have recently been applied to manipulate nanoscale mechanical systems. However, this marks the first instance of laser cooling being applied to a torsional oscillator—a device that has been pivotal in experiments aimed at uncovering the true nature of gravity. “Torsion pendulums have served as classical tools in gravity research since Henry Cavendish’s experiment in 1798,” Shin elaborates. “They have been instrumental in measuring Newton’s gravitational constant, G, testing the inverse-square law, and searching for new gravitational phenomena.”
By harnessing lasers to eliminate nearly all thermal motion from atoms, scientists have developed ultracold atomic gases at micro- and nanokelvin temperatures, powering the world’s most precise timekeeping devices—optical lattice clocks. “Traditionally, gravitational physics and atomic physics have evolved separately,” Shin notes. “Our research aims to merge these two fields. By applying laser cooling techniques for atoms to a centimeter-scale torsional oscillator, we are bridging classical and quantum realms.” This hybrid platform paves the way for a new class of experiments that may finally allow researchers to test whether gravity can be described by quantum theory.
The recent paper demonstrates the successful laser cooling of a centimeter-scale torsional oscillator from room temperature to an astonishing 10 millikelvins (1/1,000th of a kelvin) using a mirrored optical lever. “An optical lever is a powerful measurement technique where a laser shines onto a mirror, and even a slight tilt of the mirror results in a noticeable shift of the reflected beam on a detector,” Shin explains. “This magnifies small angular movements into measurable signals.” Despite the simplicity of the concept, the team encountered practical challenges, such as the laser beam's jitter caused by air currents and vibrations, which could falsely indicate motion of the mirror.
To address this issue, the researchers employed a mirrored optical lever approach, using a second mirrored laser beam to negate unwanted jitter. “One beam interacts with the torsional oscillator, while the other reflects off a corner-cube mirror to cancel out jitter without affecting the oscillator’s motion,” Shin describes. “When combined at the detector, the real signal from the oscillator is preserved while canceling out the false motion from the laser jitter.”
This innovative method reduced noise by a factor of a thousand, enabling the researchers to detect motion with remarkable precision—nearly ten times better than the oscillator’s own quantum zero-point fluctuations. “This level of sensitivity allowed us to cool the system down to just 10 millikelvins using laser light,” Shin states. He emphasizes that this work is merely the beginning. “While we’ve achieved quantum-limited precision below the oscillator's zero-point motion, reaching the actual quantum ground state is our next goal,” he adds. “To achieve this, we will need to enhance the optical interaction, potentially using an optical cavity to amplify angular signals or employing optical trapping strategies. These improvements could facilitate experiments where two oscillators interact solely through gravity, allowing us to directly test whether gravity is indeed quantum.”
The paper’s co-authors include Vivishek Sudhir, an assistant professor of mechanical engineering and the Class of 1957 Career Development Professor, and PhD candidate Dylan Fife. Additional contributors are Tina Heyward and Rajesh Menon from the Department of Electrical and Computer Engineering at the University of Utah. Both Shin and Fife are members of Sudhir’s lab, the Quantum and Precision Measurements Group.
Shin reflects on the broad scope of challenges the research team is addressing. “Studying the quantum aspects of gravity experimentally requires not only a deep understanding of physics—relativity and quantum mechanics—but also practical expertise in system design, nanofabrication, optics, control, and electronics,” he notes. “My background in mechanical engineering, which encompasses both theoretical and practical dimensions of physical systems, has been invaluable in navigating and contributing effectively across these diverse domains.” He finds it incredibly rewarding to see how this interdisciplinary training can tackle one of science's most fundamental questions.
For further details, refer to the paper titled “Active laser cooling of a centimeter-scale torsional oscillator” by Dong-Chel Shin et al., published on April 19, 2025, in Optica. DOI: 10.1364/OPTICA.548098.
