Can Scientists Really Detect a Single Graviton? Why a New U.S. Experiment Has Sparked Excitement and Doubt

The claim is audacious: scientists in the U.S. say they are building the world’s first experiment explicitly designed to detect an individual graviton — the hypothetical quantum particle of gravity. The idea has attracted $1.3 million in funding but also deep scepticism from physicists, reviving a decades-old debate about whether gravitons can ever be detected, and if doing so would actually prove that gravity is quantum in nature.

What exactly is the new experiment?

The proposal comes from researchers at Stevens Institute of Technology working with collaborators at Yale University. Their plan is to build an ultra-sensitive detector using a cylindrical resonator filled with superfluid helium — a state of matter that behaves as a single quantum object when cooled close to absolute zero.

The idea is to cool this cylinder to its quantum ground state, eliminating thermal noise entirely. In this near-perfect silence, the detector would “listen” for the faintest possible disturbance. If a powerful gravitational wave — say, from merging black holes — passes through, theory suggests it could deposit exactly one quantum of energy into the helium. That energy would appear as a single mechanical vibration, or phonon, which lasers monitoring the cylinder could detect.

Project co-leader Igor Pikovski has said the three-year effort is unlikely to catch single gravitons immediately, but aims to build a working prototype that future iterations could refine.

Why do gravitons matter so much?

The graviton is the missing piece in one of physics’ deepest puzzles. In modern physics, forces are carried by particles: photons mediate electromagnetism, while other particles transmit the strong and weak nuclear forces. Gravity, however, is described by Albert Einstein’s general relativity as the bending of spacetime — not by particle exchange.

Physicists suspect that gravity too may have a quantum carrier: the graviton. If confirmed, it would help bridge the long-standing divide between general relativity, which governs stars and galaxies, and quantum mechanics, which governs atoms and particles. In that sense, detecting a graviton would be a step — however small — toward a unified “theory of everything”.

Gravitational waves, detected since 2015 by observatories such as LIGO, are often described as ripples in spacetime. But just as light waves are made of photons, gravitational waves might consist of vast numbers of gravitons acting together. No experiment has ever isolated even one.

Why has detecting a graviton been considered impossible?

The problem is gravity’s weakness. Compared to electromagnetism, gravity is about 10³⁶ times weaker. A fridge magnet can overcome the gravitational pull of the entire Earth on a paperclip — a favourite illustration among physicists.

Because of this weakness, gravitons, if they exist, interact extraordinarily feebly with matter. In a landmark 2006 analysis, physicists Tony Rothman and Stephen Boughn calculated that a detector capable of reliably registering a single graviton would need to be absurdly massive — roughly the mass of Jupiter — and placed near an intense source like a neutron star. Even then, it might detect one graviton every decade.

Worse, shielding such a detector from background particles like neutrinos would require so much mass that it would collapse into a black hole. The conclusion was grim: any detector big enough to catch a graviton cannot physically exist.

How does the Stevens–Yale idea try to bypass this problem?

The new proposal challenges the assumptions behind those pessimistic calculations. Instead of trying to “stop” a graviton like a bullet hitting a wall, the researchers invoke a resonance effect — closer to how an opera singer can shatter a wine glass by matching its natural frequency.

In this picture, the graviton would not interact with a single atom but with the entire superfluid helium cylinder acting as a coherent quantum system. A single graviton could then excite one collective vibration — one phonon — in the fluid. This gravito-phononic effect, described in theoretical papers in 2022 and 2024, is meant to amplify an otherwise impossibly weak interaction.

The second ingredient is a new generation of quantum sensors, capable in principle of counting individual phonons in macroscopic objects — a feat unimaginable a few decades ago.

Why are some physicists unconvinced?

Not everyone is persuaded that such an experiment would actually demonstrate the existence of gravitons. Theoretical physicist Daniel Carney of Berkeley National Laboratory has argued that even if the detector “clicks”, the signal could be explained entirely using classical gravity interacting with a quantum detector.

The crux of the objection is subtle. A detector that can only produce discrete outcomes — click or no click — does not automatically imply that the incoming signal itself was quantised. A smooth, classical gravitational wave could still trigger discrete responses in a quantum system. In other words, a single “ding” does not prove that gravity arrived in indivisible packets.

This mirrors earlier debates in physics: early demonstrations of the photoelectric effect suggested photons existed, but stronger evidence came later from experiments that ruled out all semi-classical explanations.

What do the experiment’s proponents say?

Pikovski and his collaborators acknowledge this limitation. Their primary goal, they say, is detection rather than definitive proof that gravity is quantum in all respects. If the detector absorbs a precise quantum of energy from a gravitational wave, energy conservation alone allows researchers to label that quantum a graviton.

More demanding tests — capable of excluding every possible classical alternative — would require future experiments with more complex signatures. As Pikovski has noted, physics rarely advances through a single “bulletproof” experiment; evidence accumulates gradually.

So what is really at stake?

At one level, the project is a high-risk, high-reward experiment pushing quantum sensing to its limits. Even if it never detects a graviton, the technologies involved — ultra-cold superfluids, quantum-limited measurements, phonon detection — could have wide applications.

At another level, it touches a philosophical fault line in modern physics: whether gravity must be quantised at all, or whether a hybrid description — classical gravity coupled to quantum matter — might suffice. That question remains open.

For now, the claim of building a graviton detector sits in a grey zone between bold innovation and speculative ambition. Whether the experiment rings the first true “ding” of quantum gravity, or merely sharpens our understanding of what cannot be done, is something only time — and data — will decide.