Modern physics is based on two pillars. One is quantum theory, which governs the smallest particles in the universe. The other is Einstein’s general theory of relativity, which explains gravity through the curvature of spacetime. But these two theories contradict each other, and reconciling them has been difficult for over a century.

The prevailing hypothesis has been that Einstein’s theory of gravity must be modified or “quantized” to fit into quantum theory. This is the approach of the two leading candidates for a theory of quantum gravity, string theory and loop quantum gravity.

But a new theory developed by Professor Jonathan Oppenheim challenges this consensus. Instead of modifying spacetime, the theory – called the “Post-Quantum Theory of Classical Gravity” – modifies quantum theory. It predicts an intrinsic breakdown of predictability mediated by spacetime itself. This leads to random and violent fluctuations in spacetime that are greater than expected in quantum theory. As a result, the apparent weight of objects would become unpredictable if measured with enough precision.

A second paper proposes an experiment to test the theory: precisely measuring the mass of an object over time to see if its weight appears to fluctuate. For example, the International Bureau of Weights and Measures routinely weighs a 1 kg mass that used to be the standard kilogram. If fluctuations in measurements of this mass are smaller than required by the theory, the theory could be ruled out.

Professor Oppenheim and other leading supporters of loop quantum gravity and string theory have placed a 5,000-1 bet on the outcome of this experiment or other tests that determine the quantum or classical nature of spacetime.

All matter obeys quantum theory, but we only see truly quantum behavior at the atomic and molecular scale. Particles obey Heisenberg’s uncertainty principle and don’t have defined positions or velocities. They can behave like waves and exist in many places at once.

Quantum theory governs everything from semiconductors to lasers to radioactive decay. But we say a system behaves classically if it has definite underlying properties, like a cat being alive or dead.

Einstein’s general relativity replaced Newton’s theory of gravity. It holds that gravity is not a force but arises from massive objects like the sun curving spacetime in a way that causes the Earth to move.

Over the past five years, Professor Oppenheim’s research group has been testing the theory and exploring its consequences. If borne out, it could resolve the incompatibility between quantum theory and general relativity and help unify our understanding of physics. Experiments in the next 20 years may determine if spacetime has a classical or quantum nature.


University College London | Jonathan Oppenheim, A Postquantum Theory of Classical Gravity?. Physical Review X, 13, 041040.

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