Is gravity quantum?

One of the most profound open questions in modern physics is: “Is gravity quantum?” The other fundamental forces — electromagnetic, weak, and strong — have all been successfully described, but no complete and consistent quantum theory of gravity yet exists.  “Theoretical physicists have proposed many possible scenarios, from gravity being inherently classical to fully quantum, but the debate remains unresolved because we’ve never had a clear way to test gravity’s quantum nature in the lab,” says Dongchel Shin, a PhD candidate in the MIT Department of Mechanical Engineering (MechE). “The key to answering this lies in preparing mechanical systems that are massive enough to feel gravity, yet quiet enough — quantum enough — to reveal how gravity interacts with them.”Shin, who is also a MathWorks Fellow, researches quantum and precision metrology platforms that probe fundamental physics and are designed to pave the way for future industrial technology. He is the lead author of a new paper that demonstrates laser cooling of a centimeter-long torsional oscillator. The open-access paper, “Active laser cooling of a centimeter-scale torsional oscillator,” was recently published in the journal Optica. Lasers have been routinely employed to cool down atomic gases since the 1980s, and have been used in the linear motion of nanoscale mechanical oscillators since around 2010. The new paper presents the first time this technique has been extended to torsional oscillators, which are key to a worldwide effort to study gravity using these systems.“Torsion pendulums have been classical tools for gravity research since [Henry] Cavendish’s famous experiment in 1798. They’ve been used to measure Newton’s gravitational constant, G, test the inverse-square law, and search for new gravitational phenomena,” explains Shin.By using lasers to remove nearly all thermal motion from atoms, in recent decades scientists have created ultracold atomic gases at micro- and nanokelvin temperatures. These systems now power the world’s most precise clocks — optical lattice clocks — with timekeeping precision so high that they would gain or lose less than a second over the age of the universe.“Historically, these two technologies developed separately — one in gravitational physics, the other in atomic and optical physics,” says Shin. “In our work, we bring them together. By applying laser cooling techniques originally developed for atoms to a centimeter-scale torsional oscillator, we try to bridge the classical and quantum worlds. This hybrid platform enables a new class of experiments — ones that could finally let us test whether gravity needs to be described by quantum theory.”The new paper demonstrates laser cooling of a centimeter-scale torsional oscillator from room temperature to a temperature of 10 millikelvins (1/1,000th of a kelvin) using a mirrored optical lever.“An optical lever is a simple but powerful measurement technique: You shine a laser onto a mirror, and even a tiny tilt of the mirror causes the reflected beam to shift noticeably on a detector. This magnifies small angular motions into easily measurable signals,” explains Shin, noting that while the premise is simple, the team faced challenges in practice. “The laser beam itself can jitter slightly due to air currents, vibrations, or imperfections in the optics. These jitters can falsely appear as motion of the mirror, limiting our ability to measure true physical signals.”To overcome this, the team used the mirrored optical lever approach, which employs a second, mirrored version of the laser beam to cancel out the unwanted jitter.“One beam interacts with the torsional oscillator, while the other reflects off a corner-cube mirror, reversing any jitter without picking up the oscillator’s motion,” Shin says. “When the two beams are combined at the detector, the real signal from the oscillator is preserved, and the false motion from [the] laser jitter is canceled.”This approach reduced noise by a factor of a thousand, which allowed the researchers to detect motion with extreme precision, nearly 10 times better than the oscillator’s own quantum zero-point fluctuations. “That level of sensitivity made it possible for us to cool the system down to just 10 milli-kelvins using laser light,” Shin says.Shin says this work is just the beginning. “While we’ve achieved quantum-limited precision below the zero-point motion of the oscillator, reaching the actual quantum ground state remains our next goal,” he says. “To do that, we’ll need to further strengthen the optical interaction — using an optical cavity that amplifies angular signals, or optical trapping strategies. These improvements could open the door to experiments where two such oscillators interact only through gravity, allowing us to directly test whether gravity is quantum or not.”The paper’s other authors from the Department of Mechanical Engineering include Vivishek Sudhir, assistant professor of mechanical engineering and the Class of 1957 Career Development Professor, and PhD candidate Dylan Fife. Additional authors are Tina Heyward and Rajesh Menon of the Department of Electrical and Computer Engineering at the University of Utah. Shin and Fife are both members of Sudhir’s lab, the Quantum and Precision Measurements Group.Shin says one thing he’s come to appreciate through this work is the breadth of the challenge the team is tackling. “Studying quantum aspects of gravity experimentally doesn’t just require deep understanding of physics — relativity, quantum mechanics — but also demands hands-on expertise in system design, nanofabrication, optics, control, and electronics,” he says.“Having a background in mechanical engineering, which spans both the theoretical and practical aspects of physical systems, gave me the right perspective to navigate and contribute meaningfully across these diverse domains,” says Shin. “It’s been incredibly rewarding to see how this broad training can help tackle one of the most fundamental questions in science.”