Unraveling the quantum nature of gravity

Gravity, the force that keeps our feet on the ground and the planets in orbit, is an integral part of our daily lives. Despite its ubiquity, the true nature of gravity remains a mystery. Scientists are still grappling with the question of whether gravity is fundamentally a geometrical phenomenon, as proposed by Einstein, or if it is governed by the laws of quantum mechanics.

In a study published in Physical Review X, researchers from Amsterdam and Ulm have proposed an innovative experiment that could shed light on this age-old question.

Ludovico Lami, a mathematical physicist at the University of Amsterdam and QuSoft, and his colleagues have designed a new approach that circumvents the challenges faced by previous experimental proposals.

Quantum-gravity conundrum

The quest to unify quantum mechanics and gravitational physics is one of the most significant challenges in modern science. Progress in this field has been hindered by the inability to conduct experiments in regimes where both quantum and gravitational effects are relevant.

As Nobel Prize laureate Roger Penrose once stated, we don’t even know if a combined theory of gravity and quantum mechanics will require a “quantization of gravity” or a “granitization of quantum mechanics.”

“The central question, initially posed by Richard Feynman in 1957, is to understand whether the gravitational field of a massive object can enter a so-called quantum superposition, where it would be in several states at the same time,” explains Lami.

“Prior to our work, the main idea to decide this question experimentally was to look for gravitationally induced entanglement – a way in which distant but related masses could share quantum information. The existence of such entanglement would falsify the hypothesis that the gravitational field is purely local and classical,” he continued.

Overcoming the delocalisation dilemma

The primary obstacle in previous experimental proposals has been the creation of distant but related massive objects, known as delocalised states.

The heaviest object for which quantum delocalisation has been observed to date is a large molecule, which is significantly lighter than the smallest source mass whose gravitational field has been detected. This discrepancy has pushed the hope of an experimental realization decades into the future.

Designing a novel experiment to beat the odds

Lami and his colleagues have proposed a potential solution to this deadlock. Their experiment aims to reveal the quantumness of gravity without generating any entanglement.

“Our team designed and investigate a class of experiments involving a system of massive ‘harmonic oscillators’ – for example, torsion pendula, essentially like the one that Cavendish used in his famous 1797 experiment to measure the strength of the gravitational force,” Lami explains.

“We establish mathematically rigorous bounds on certain experimental signals for quantumness that a local classical gravity should not be able to overcome. We have carefully analysed the experimental requirements needed to implement our proposal in an actual experiments, and find that even though some degree of technological progress is still needed, such experiments could really be within reach soon.”

Power of entanglement theory

Surprisingly, the researchers still rely on the mathematical machinery of entanglement theory in quantum information science to analyze their experiment, despite the absence of physical entanglement.

Lami clarifies, “The reason is that, although entanglement is not physically there, it is still there in spirit — in a precise mathematical sense. It is enough that entanglement could have been generated.”

New era of quantum gravity research

In summary, the study by Lami and his colleagues from Amsterdam and Ulm opens a new chapter in the quest to unravel the quantum nature of gravity.

Their innovative experimental proposal, which relies on the mathematical framework of entanglement theory without requiring physical entanglement, brings us closer to answering the fundamental question posed by Richard Feynman over six decades ago.

As technological progress continues to advance, the realization of this experiment becomes increasingly feasible, promising to shed light on one of the most profound mysteries in modern physics.

The implications of this research extend far beyond the realm of theoretical physics, as a deeper understanding of the quantum-gravity relationship could revolutionize our perception of the universe and our place within it.

The full study was published in the journal Physical Review X.

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