UCL physicists have unveiled a groundbreaking theory that harmonizes gravity and quantum mechanics while upholding Einstein's classical spacetime concept. The clash between quantum theory, governing tiny particles, and Einstein's general relativity, explaining gravity through spacetime bending, has long perplexed physicists.

Modern physics rests on two pillars: quantum theory and general relativity. Quantum theory rules the smallest particles, while general relativity explains gravity by bending spacetime. However, these pillars contradict each other, defying a unifying solution for over a century.

The prevailing assumption has been that Einstein's gravity theory must be modified or “quantized” to fit within quantum theory. This approach is evident in leading candidates like string theory and loop quantum gravity. However, a new theory from Professor Jonathan Oppenheim challenges this consensus, proposing that spacetime itself may be classical, not governed by quantum theory.

Published in Physical Review X, the theory, labeled a “postquantum theory of classical gravity,” modifies quantum theory rather than spacetime. It predicts intrinsic unpredictability mediated by spacetime, leading to random and violent fluctuations larger than those envisioned by quantum theory. These fluctuations render the apparent weight of objects unpredictable under precise measurement.

Simultaneously, a paper in Nature Communications led by Oppenheim's former Ph.D. students explores the theory's consequences and proposes an experiment to test it—precisely measuring a mass over time to observe potential weight fluctuations.

For example, the International Bureau of Weights and Measures in France regularly weighs a 1kg mass, the standard until recently. If fluctuations in these measurements are smaller than mathematically consistent, the theory could be invalidated.

The outcome of the experiment, or any evidence confirming the quantum vs. classical nature of spacetime, is subject to a 5000:1 odds bet between Oppenheim and proponents of quantum loop gravity and string theory.

Over the past five years, the UCL research group has rigorously tested the theory, exploring its consequences. Professor Oppenheim emphasizes the importance of resolving the mathematical incompatibility between quantum theory and general relativity. With a consistent fundamental theory where spacetime isn't quantized, the resolution remains an open question.

Co-author Zach Weller-Davies, a Ph.D. student at UCL involved in theory development and the experimental proposal, notes that this discovery challenges our understanding of gravity's fundamental nature. The theory proposes that if spacetime lacks a quantum nature, there must be random fluctuations in its curvature, experimentally verifiable.

The theory suggests that spacetime undergoes violent and random fluctuations, both in quantum and classical gravity, on a scale undetectable thus far. If spacetime is classical, these fluctuations must surpass a certain scale, determinable by another experiment testing the duration a heavy atom can exist in superposition at different locations.

Co-authors Dr. Carlo Sparaciari and Dr. Barbara Šoda, contributing analytical and numerical calculations, express hope that these experiments will determine the suitability of pursuing a quantum theory of gravity.

Dr. Šoda, now at the Perimeter Institute of Theoretical Physics, Canada, emphasized the question of whether the rate at which time flows possesses a quantum or classical nature, considering gravity's manifestation through spacetime bending. Comparing it to the simplicity of testing the constant weight of a mass versus its fluctuation, she highlighted the experimental simplicity of investigating the quantum nature of time flow.

Dr. Sparaciari of UCL Physics & Astronomy noted the need for extreme precision in weighing objects for the experiment. He found excitement in establishing a clear relationship between measurable quantities—the scale of spacetime fluctuations and the duration objects like atoms can exist in quantum superposition. These quantities, derived from general assumptions, can be determined experimentally.

Weller-Davies added insight into the delicate interplay required for quantum particles to bend classical spacetime, emphasizing a fundamental trade-off between the wave nature of atoms and the magnitude of random spacetime fluctuations.

The proposal to test spacetime's classical nature by detecting random mass fluctuations complements another experimental approach seeking to verify spacetime's quantum nature through “gravitationally mediated entanglement.” Professor Sougato Bose, not involved in the recent announcement but a pioneer in proposing the entanglement experiment, emphasized the large-scale effort required for such experiments, foreseeing potential answers within the next two decades.

Beyond its impact on gravity, the postquantum theory has broader implications. It eliminates the need for the problematic “measurement postulate” in quantum theory, as quantum superpositions naturally localize through interaction with classical spacetime.

Motivated by the black hole information problem, Professor Oppenheim developed this theory. In standard quantum theory, objects entering a black hole should be radiated back out, challenging general relativity's assertion that information about objects beyond the event horizon is unknowable. The new theory allows for information destruction, introducing a fundamental breakdown in predictability.

This innovative theory not only tackles the longstanding quantum-gravity conflict but also opens avenues for reevaluating foundational aspects of quantum theory. The potential resolution of the black hole information problem highlights the theory's broader implications in reshaping our understanding of fundamental laws governing the universe.

Source: University College London