Mumbai, March 11 Researchers at the Indian Institute of Technology Bombay have proposed a new test to measure the "quantumness" of gravity after finding that conventional "quantum entanglement-based" tests may be insufficient to reconcile gravity with quantum physics.
IIT B researchers P George Christopher and Prof S Shankaranarayanan show that gravity could still be fundamentally quantum even if it fails a widely used test called the entanglement test, according to the study.
Quantum gravity is a theoretical framework that combines the principles of quantum physics with the understanding of gravity through dynamical spacetime geometry.
The team has proposed a new diagnostic tool called dynamical fidelity susceptibility (DFS), which serves as a more sensitive probe of the true nature of the universe's most mysterious force.
For over a century, theoretical physics has relied on two highly successful but separate theories to explain the universe. Classical physics and General Relativity, developed by Albert Einstein, explain large-scale objects such as planets, stars and galaxies, describing gravity not as a force but as the bending of spacetime.
In contrast, quantum physics explains how matter behaves at the atomic and subatomic level, where particles can act like waves and exist in multiple states at the same time, the study said.
While both theories work extremely well in their own domains, they break down when these realms overlap, such as inside a black hole. Physicists believe the two theories could be reconciled if gravity itself is shown to be quantum in nature, it said.
In that case, gravity would be treated not just as the curvature of spacetime but as a force carried by hypothetical particles known as graviton, it said.
In 2017, researchers proposed the Bose-Marletto-Vedral experiment to test the quantum nature of gravity, which relied on a quantum property known as entanglement.
Quantum entanglement is a phenomenon in which pairs of particles, such as photons and electrons, become so deeply linked that their properties become interdependent and they are described as a single entity sharing a single quantum state.
According to the study, the researchers built a mathematical model with three connected oscillators - two masses at the ends and a mediator in the middle representing the gravitational field.
They found that if the mediator is light, it spreads quantum entanglement quickly between the masses. But if it is heavy, the interaction slows down and no clear entanglement appears, even though the mediator still behaves quantum mechanically with its own states and fluctuations, it said.
This suggests that if experiments fail to detect entanglement, it does not necessarily mean gravity is not quantum. It may simply indicate that the mediating particles are too heavy and slow to produce observable effects, the study said.
To address this, the team proposed DFS, a more sensitive probe of gravitons.
Here, rather than simply checking whether the masses are linked or entangled, the DFR quantifies how the system’s quantum state evolves over time in response to infinitesimal microscopic perturbations, it said.
Even when gravity is frozen and entanglement is negligible, this new measurement can still pick up the fingerprints of its quantum nature. The new method also offers experimentalists a new way to test whether gravity is indeed quantum, it said.
“In principle, one could estimate fidelity by preparing an initial quantum state of the masses, allowing them to evolve under the gravitational interaction, and then measuring the overlap between the evolved state and the original one,” explained Prof S Shankaranarayanan.
The study provides a roadmap for future quantum gravity experiments, he added.