New research suggests that classical gravity—the type described by Einstein’s theory of general relativity—might have the ability to entangle matter, even if gravity itself isn’t fundamentally quantum. This challenges our understanding of how gravity interacts with the quantum world and raises fascinating questions about the nature of reality at its most fundamental level.
For decades, physicists have sought a unified theory that seamlessly blends two pillars of modern physics: quantum mechanics, which governs the bizarre behavior of particles at the atomic and subatomic scales, and general relativity, which describes the large-scale structure of the universe and the force of gravity as the curvature of spacetime.
A key sticking point in this quest is the idea of quantum gravity. This hypothetical framework proposes that gravity, like other fundamental forces, exists in discrete packets called gravitons. Unlike photons (the quanta of light), gravitons have never been observed directly, and their existence remains theoretical. Some physicists even question whether gravity needs to be quantized at all.
Adding to the complexity, a thought experiment proposed by physicist Richard Feynman in 1957 has long served as a potential touchstone for detecting quantum gravity. Feynman imagined placing an object—say, an apple—into a peculiar state called quantum superposition, where it exists in multiple places simultaneously until observed. Introducing a second object and observing whether its gravitational interaction with the “superposed” apple persists even after the first apple’s superposition collapses would, according to Feynman, indicate the presence of quantum gravity at work.
This entanglement—the spooky connection between two particles regardless of distance—has been explained through the exchange of virtual gravitons in modern interpretations of Feynman’s experiment. However, physicists Joseph Aziz and Richard Howl from Royal Holloway, University of London, have now presented a new twist. They argue that even without quantum gravity, classical gravity could potentially entangle matter through a different mechanism.
Their idea hinges on virtual particles, temporary fluctuations that pop in and out of existence according to the rules of quantum field theory. Aziz and Howl propose that these virtual particles could mediate the entanglement between objects, acting as intermediaries even if the gravitational field itself remains classical. Think of it like two people whispering secrets to each other through a third person who relays the messages—even though the original senders aren’t directly connected, they become entangled due to this intermediary.
This “quasi-entanglement” wouldn’t be as strong as entanglement mediated by quantum gravitons, and its effects would likely be weaker than those predicted for genuine quantum gravity. Observing these subtle correlations between particles could potentially distinguish the two scenarios in future experiments.
While the idea of classical gravity causing entanglement might seem counterintuitive, Aziz and Howl emphasize that it doesn’t necessarily rule out quantum gravity. It simply suggests another possible avenue for exploring the complex interplay between gravity and the quantum world.
The experimental verification of this concept remains a significant challenge, requiring incredibly precise control over systems to minimize external influences that could disrupt delicate quantum states (a phenomenon known as decoherence).
Nonetheless, Aziz and Howl’s work opens new avenues for research and pushes the boundaries of our understanding of gravity. Their findings highlight the potential for unexpected connections between seemingly disparate realms of physics and emphasize the vast unknowns that still lie at the heart of the universe. The search for quantum gravity continues, fueled by groundbreaking ideas like this one, which force us to reexamine the very nature of reality itself.
