Gravity has always been a reliable constant. Apples fall, planets orbit, and the universe moves in a dance guided by this force. Over a century ago, Albert Einstein transformed our understanding of gravity, framing it as a warping of space and time. But gravity, unlike other forces, remains a mystery in the quantum world.

Physicists have long believed that gravity, at its most fundamental level, is carried by tiny particles known as gravitons, just like photons carry electromagnetic energy. These particles, however, have never been observed — and some believe they might never be found.

A new study led by Professor Igor Pikovski and his team at Stevens Institute of Technology suggests that this may soon change. The researchers propose using next-generation quantum sensors to detect single gravitons. Such a feat could connect gravity with quantum mechanics and solve one of the biggest holes in the Standard Model of Physics. This experiment, they claim, is no longer a distant dream but could soon become a reality.

“This is a foundational experiment that was long thought impossible, but we think we’ve found a way to do it,” Pikovski said.

Bridging Two Worlds: Quantum Mechanics and Gravity

For decades, physicists have sought to link gravity with quantum mechanics, the theory that describes the strange behavior of particles on the smallest scales. All other fundamental forces have been successfully unified under quantum theory, but gravity has remained the exception. Gravitons, if they exist, would be the quantum building blocks of gravity, similar to how photons are the quantum carriers of light.

Detecting a graviton, however, has been an insurmountable challenge. Current technology is capable of observing gravitational waves — ripples in space-time caused by massive cosmic events like black hole collisions. Yet, these detectors fall short when it comes to detecting individual gravitons even though the number of gravitations in a typical gravitational wave is estimated to be a staggering 10³⁶ (one followed by 36 zeros).

Pikovski’s team believes they have found a solution by coupling an acoustic resonator with sophisticated quantum sensing technology. Such resonators amplify waves (usually sound waves). This setup could detect the tiny energy shifts caused by a single graviton interacting with the resonator. The concept is similar to the photoelectric effect that helped Einstein develop the quantum theory of light but with gravitational waves instead of electromagnetic waves.

How Does it Work?

An interesting twist in the proposed experiment involves rediscovering an old technology — Weber bars. Named after Joseph Weber, who pioneered their use in the 1960s to detect gravitational waves, these heavy, cylindrical bars have fallen out of favor with the advent of more sophisticated optical detectors. However, Weber bars are ideally suited for detecting single gravitons. They can absorb and emit gravitons in a way that mirrors the “stimulated emission and absorption” of photons.

To detect gravitons, the experiment involves cooling a Weber bar-like resonator to near absolute zero and observing for any changes in energy. The resonators must be cooled down as much as possible to isolate the device from external noise. Highly sensitive quantum sensors could then detect the minute changes in these vibrations.

Each discrete change, or quantum jump, could indicate the absorption or emission of a single graviton — a phenomenon they have termed the “gravito-phononic effect.” The absorption and emissions of gravitations would mirror the “stimulated emission and absorption” of photons, a concept first pioneered by Einstein.

An Approach With Promise

While the experiment remains theoretical, the researchers are optimistic. They suggest using data from LIGO could help to detect these subtle quantum events. The idea is to cross-reference gravitational wave data with signals from the proposed detector to isolate events that might indicate a single graviton.

Despite the promise of this new approach, there are significant hurdles. The required quantum sensors for such a sensitive experiment are not yet available. However, technological advancements in recent years give us hope that these sensors will be developed soon.

The detection of a single graviton would be a monumental achievement in physics, providing the first experimental evidence for the quantum theory of gravity. This would bridge a significant gap between Einstein’s theory of general relativity, which describes gravity as a curvature of spacetime, and quantum mechanics, which governs the microcosm of particles and forces.

Currently, our understanding of gravity does not align with the principles of quantum mechanics — a fundamental inconsistency in modern physics. Detecting gravitons would help solidify the idea that gravity, like light, has a quantum nature, potentially opening the door to new physics beyond our current theories.

“We’re certain this experiment would work,” says Thomas Beitel, a first-year graduate student in Pikovski’s lab. “Now that we know that gravitons can be detected, it’s added motivation to further develop the appropriate quantum-sensing technology. With some luck, one will be able to capture single gravitons soon.”

The findings appeared in the journal Nature Communications.