Two articles published simultaneously by physicists at University College London (UCL) unveil a new theory that consistently combines quantum mechanics and gravity while maintaining Einstein’s traditional view of spacetime.

The two cornerstones of modern physics are Einstein’s general theory of relativity, which explains gravity through the bending of spacetime, and quantum theory, which controls the tiniest particles in the universe. However, there is a conflict between these two hypotheses, and a solution has eluded researchers for more than a century.

The general consensus has been that Einstein’s theory of gravity needs to be “quantized,” or changed, to make it compatible with quantum theory. This is the methodology of two prominent contenders for a quantum theory of gravity: loop quantum gravity and string theory.

The consensus is challenged by a new theory, however, which was created by Professor Jonathan Oppenheim of UCL Physics & Astronomy and presented in a publication in Physical Review X. This theory suggests that spacetime might be classical, meaning it might not be governed by quantum theory at all.

The hypothesis—dubbed a “postquantum theory of classical gravity”—modifies quantum theory and anticipates an innate breakdown in predictability mediated via spacetime, rather than altering spacetime. If an object’s apparent weight is measured accurately enough, this leads to larger-than-expected random and violent fluctuations in spacetime, which are not consistent with the predictions of quantum theory.

A second study, undertaken by former Ph.D. students of Professor Oppenheim and concurrently published in Nature Communications, examines some of the implications of the theory and suggests a test procedure that involves accurately measuring a mass to see if its weight appears to change over time.

For instance, a 1 kg mass—which was formerly the 1 kg standard—is regularly weighed by the International Bureau of Weights and Measures in France. The idea can be ruled out if the variations in measurements of this 1 kg mass are less than what is necessary for mathematical consistency.

Professor Oppenheim, Professor Carlo Rovelli, and Dr. Geoff Penington—leading proponents of quantum loop gravity and string theory, respectively—are betting 5000:1 on the experiment’s result or any other evidence that emerges that would support the quantum vs. classical nature of spacetime.

The UCL research team has been investigating the implications of the idea and putting it to the test for the last five years.

Professor Oppenheim stated, “It’s critical to comprehend how the mathematical incompatibilities between quantum theory and Einstein’s general theory of relativity are reconciled. Is it better to change quantum theory, quantize spacetime, or do something else entirely? It’s anybody’s guess now that we have a consistent fundamental theory that avoids quantization of spacetime.”

Co-author Zach Weller-Davies, who as a Ph.D. student at UCL helped create the experimental proposal and made major contributions to the theory itself, stated, “This discovery challenges our understanding of the fundamental nature of gravity but also offers paths to study its potential quantum character.

We have demonstrated that in the absence of a quantum component, spacetime’s curvature must exhibit random fluctuations with an experimentally verifiable trace.

“Spacetime must be undergoing violent and random fluctuations all around us in both quantum gravity and classical gravity, but on a scale that we haven’t yet been able to detect.” However, for spacetime to be considered classical, the fluctuations must exceed a specific threshold, which may be established through an additional experiment that examines the duration of time that a heavy atom can be in superposition—that is, in two distinct places at the same time.”

The effort was guided by the analytical and numerical calculations of co-authors Dr. Carlo Sparaciari and Dr. Barbara Šoda, who expressed hope that these experiments could ascertain whether pursuing a quantum theory of gravity is the appropriate course of action.

Dr. Šoda, who was previously the Physics and astronomy professor at UCL and is currently at the Perimeter Institute of Theoretical Physics in Canada, stated, “We can think of the question in terms of whether the rate at which time flows has a quantum nature, or classical nature because gravity is made manifest through the bending of space and time.

“And testing this is almost as simple as testing whether the weight of a mass is constant, or appears to fluctuate in a particular way.”

What excites me, though, is that we can establish a direct correlation between two quantifiable quantities—the size of spacetime fluctuations and the length of time that atoms or apples may be placed in a quantum superposition of two distinct locations—starting from very general assumptions. Then, we can conduct experiments to identify these two quantities.”

Weller-Davies continued, “If quantum particles like atoms can bend classical spacetime, then there must be a delicate interplay. The size of the random fluctuations in spacetime and the wave nature of atoms must be fundamentally balanced.”

Two complementary experimental proposals are put out to test whether spacetime is classical or quantum. The first seeks to confirm the quantum character of spacetime by searching for “gravitationally mediated entanglement.”

Not involved in today’s announcement, Professor Sougato Bose of UCL Physics & Astronomy stated that experiments to test the nature of spacetime “will take a large-scale effort, but they’re of huge importance from the perspective of understanding the fundamental laws of nature.” Bose was among those who first proposed the entanglement experiment. Though these things are hard to forecast, I think these studies are doable, and maybe in 20 years we’ll know the answer.”

Beyond gravity, the postquantum theory has ramifications. Quantum superpositions inevitably localize through their interaction with classical spacetime, negating the requirement for the notorious and controversial “measurement postulate” of quantum theory.

Professor Oppenheim’s attempt to find a solution to the black hole information problem served as inspiration for the theory. Since information cannot be destroyed, normal quantum theory states that an object falling into a black hole should radiate back out in some way. However, this defies general relativity, which states that objects that cross a black hole’s event horizon can never be known to exist. The new theory permits information annihilation because of a basic failure in prediction.