The quest to unify the universe’s fundamental forces has driven physicists to design groundbreaking quantum gravity experiments. For a century, the scientific community has struggled to reconcile Albert Einstein’s general theory of relativity, which describes gravity as the curving of spacetime, with quantum mechanics, which governs the subatomic realm. Now, researchers are proposing novel tabletop setups and ultra-cold detectors to determine if gravity possesses quantum properties.
These new quantum gravity experiments aim to isolate elusive phenomena like gravitationally induced entanglement and the theoretical graviton. By pushing the limits of quantum sensing and interference, scientists hope to answer whether gravity is fundamentally quantized or if a classical description suffices. The latest research spans multiple international teams, sparking both excitement and deep theoretical debates.
A Tabletop Test for Repulsive Gravity
Physicists Pablo L. Saldanha of the Federal University of Minas Gerais, alongside Chiara Marletto and Vlatko Vedral of the University of Oxford, have proposed a tabletop experiment to test the quantum nature of gravity. Their scheme relies on quantum interference between two gravitational pulls, which could produce an effective repulsive momentum shift. While this does not mean gravity becomes an anti-gravity force, the average momentum transfer to a probe particle could be negative, causing it to move as if pushed away.
Unlike previous concepts that required placing two massive objects into spatial superposition, this new proposal requires only one source mass to be placed in a superposition. This mass would weigh roughly one hundred-trillionth of a kilogram, while a smaller probe mass remains in an ordinary state. Using weak-value amplification and post-selection, the tiny effect could be amplified to produce an anomalous momentum shift of about 0.2 percent of the probe’s intrinsic momentum uncertainty. Researchers state that observing this effect lacks a classical explanation and provides evidence that gravity is quantum.
The proposal also highlights practical challenges. Competing forces, such as electromagnetic interactions or Casimir-Polder forces, could mask the incredibly weak gravitational signal. Isolating the genuine gravitational effect requires unprecedented shielding and experimental design. Furthermore, relying on weak-value amplification means that most measurement attempts will not meet the strict post-selection criteria, heavily reducing the number of successful runs.
The Quest to Detect a Single Graviton
Meanwhile, researchers at the Stevens Institute of Technology and Yale University have secured $1.3 million in funding to build a detector explicitly designed to isolate a single graviton, the hypothetical quantum particle of gravity. Led by project co-leader Igor Pikovski, the team plans to use a cylindrical resonator filled with superfluid helium cooled near absolute zero to eliminate thermal noise.
If a powerful gravitational wave passes through, it could deposit exactly one quantum of energy into the helium, creating a single mechanical vibration known as a phonon. This challenges a 2006 analysis by Tony Rothman and Stephen Boughn, which calculated that a reliable graviton detector would need to be the mass of Jupiter. Instead of stopping a graviton, the new detector relies on a resonance effect within the superfluid helium.
In modern physics, electromagnetism, as well as the strong and weak nuclear forces, are mediated by particles. Gravity remains the outlier, described by Einstein as the bending of spacetime rather than particle exchange. Isolating a graviton would help bridge the divide between general relativity and quantum mechanics.
However, the project faces skepticism. Theoretical physicist Daniel Carney of the Berkeley National Laboratory argues that a discrete response in the detector could still be explained by classical gravity interacting with a quantum system. Pikovski acknowledges this limitation, noting that the primary goal is initial detection rather than definitive proof that gravity is quantized in all aspects.
The Entanglement Debate
The conversation surrounding quantum gravity experiments extends into how gravitational fields interact with matter. A study published in the journal Nature by Joseph Aziz and Richard Howl of Royal Holloway, University of London, suggests that gravitational fields can enable matter to become quantum entangled even if the gravitational field itself is classical.
Building on a 1957 thought experiment by Richard Feynman, Aziz and Howl demonstrated that virtual-matter processes could generate entanglement indirectly. Feynman originally theorized that if a mass was placed into a quantum superposition of two locations and its gravitational field also entered superposition, it would indicate that gravity is quantum. Modern interpretations describe this coupling as quantum entanglement, a phenomenon Einstein referred to as “spooky action at a distance.”
Aziz and Howl argue that classical gravity could potentially entangle matter. This conclusion has drawn pushback from other experts. Marletto disputes the interpretation, stating that no classical theory of gravity can mediate entanglement through local means. She argues the study merely demonstrates a classical theory with direct, non-local interactions between quantum probes.
Despite the disagreement, Howl notes that the predicted classical-gravity entangling effects are incredibly small. Consequently, if future physical experiments successfully observe strong entanglement, scientists can be confident that gravity is indeed quantized.
