Exciting new research from two physicists in London suggests that gravitational fields might allow matter to become quantum entangled, even if the idea of quantum gravity isn’t real. Joseph Aziz and Richard Howl from Royal Holloway, University of London, are questioning our current understanding of quantum fields and gravity.
The quest for quantum gravity is a crucial challenge in physics. Researchers want to unite the physics of tiny particles with the large-scale laws that govern the universe. While quantum mechanics explains small-scale phenomena, general relativity describes how gravity works, but these two frameworks often clash.
Aziz and Howl’s findings build on a famous thought experiment by physicist Richard Feynman, proposed in 1957. This thought experiment involves placing an object, like an apple, into a state of quantum superposition, where it exists in two places at once until observed, at which point it “collapses” into one state.
When Feynman imagined this scenario, he proposed that if the first apple interacted with a second apple, it could indicate that gravity functions at the quantum level. Howl tells us, “If you could show that the gravitational field is also in superposition, that would mean gravity behaves quantum mechanically.”
In modern interpretations, such interactions might lead to quantum entanglement, where the properties of two particles become linked. Changes to one could affect the other, regardless of the distance between them. Albert Einstein famously called this “spooky action at a distance.” Yet, Aziz and Howl propose that entanglement could occur even without quantum gravity.
This raises a major contradiction. Einstein described gravity as the curvature of spacetime. However, in quantum physics, fundamental forces, like gravity, are thought to operate through particles known as gravitons. No one has ever detected a graviton, and their existence remains uncertain. Aziz and Howl’s study suggests that gravity might interact with quantum matter even if gravity doesn’t have its own quantum aspects.
They explain that traditional views required gravitational fields to be quantum to entangle matter. Instead, they present a broader perspective where gravitational interactions can still occur through “virtual” particles—particles that exist for very short periods. This might allow classical gravity to engage in quasi-entanglement with quantum matter.
Howl notes that while their work doesn’t eliminate the possibility of quantum gravity, it hints that classical gravity can still have entangling effects, albeit weaker than if gravity were fully quantum. For instance, suppose we have two particles with opposing quantum spins. In quantum gravity, knowing the state of one immediately tells you the state of the other. But under classical gravity, this certainty diminishes, turning it into probabilities.
As exciting as these findings are, performing Feynman’s experiment in reality remains speculative. Eliminating factors that could collapse superposition is a daunting task, but researchers in the UK and beyond are investigating this possibility.
Notably, there are diverse views on whether gravity is inherently quantum. For instance, Jonathan Oppenheim at University College London recently proposed a model that merges classical general relativity with quantum field theory.
Howl anticipates there will be differing opinions on his and Aziz’s findings. “I doubt everyone will see eye to eye with us!” he admits. Still, he feels hopeful that Feynman’s experiment could eventually be realized, potentially shedding light on the nature of gravity and its quantum connections.
This research, published on October 22 in Nature, might reshape our understanding of how gravity and quantum physics interact, pushing the boundaries of physics further than we ever imagined.
