New theory claims to unite Einstein’s gravity with quantum mechanics

A radical theory that systematically unifies gravity and quantum mechanics while preserving Einstein’s classic concept of space-time has been announced in two papers published simultaneously by physicists at UCL (University College London).

Modern physics is based on two pillars: on the one hand quantum theory, which governs the smallest particles in the universe, and on the other hand Einstein’s theory of general relativity, which explains gravity by the curvature of space-time. But these two theories are at odds with each other and a reconciliation has remained elusive for more than a century.

The prevailing hypothesis is that Einstein’s theory of gravity must be modified, or “quantized”, in order to accommodate quantum theory. This is the approach of two main candidates for a quantum theory of gravity, string theory and loop quantum gravity.

But a new theory, developed by Professor Jonathan Oppenheim (UCL Physics & Astronomy) and presented in an article published in Physical examination challenges this consensus and takes an alternative approach by suggesting that spacetime might be classical, that is, not governed by quantum theory at all.

Instead of altering space-time, the theory – dubbed “postquantum theory of classical gravity” – modifies quantum theory and predicts an intrinsic breakdown in predictability mediated by space-time itself. This results in random, violent fluctuations in spacetime that are larger than quantum theory envisioned, making the apparent weight of objects unpredictable if measured accurately enough.

A second article, published simultaneously in Natural communications and led by Professor Oppenheim’s former doctoral student. students, examines some of the implications of the theory and suggests an experiment to test it: measuring a mass very precisely to see if its weight seems to fluctuate over time.

For example, the International Bureau of Weights and Measures in France regularly weighs a mass of 1 kg, which used to be the standard of 1 kg. If the fluctuations in measurements of this 1 kg mass are less than those required for mathematical consistency, the theory can be ruled out.

The outcome of the experiment, or other emerging evidence that would confirm the quantum versus classical nature of space-time, is the subject of a 5,000:1 bet between Professor Oppenheim, the Professor Carlo Rovelli and Dr Geoff Penington, leading proponents of the quantum loop. gravity and string theory respectively.

Over the past five years, the UCL research group has tested the theory and explored its implications.

Professor Oppenheim said: “Quantum theory and Einstein’s theory of general relativity are mathematically incompatible with each other, so it is important to understand how this contradiction is resolved. Should spacetime be quantized, or should we modify quantum theory, or is it something else entirely? Now that we have a consistent fundamental theory in which spacetime is not quantized, it’s anyone’s guess. »

Co-author Zach Weller-Davies, who as a Ph.D. A UCL student helped develop the experimental proposal and made key contributions to the theory itself, said: “This discovery challenges our understanding of the fundamental nature of gravity, but also offers clues to probe its potential quantum nature.

“We have shown that if spacetime does not have a quantum nature, then there must be random fluctuations in the curvature of spacetime that have a particular signature that can be verified experimentally.

“In quantum gravity as in classical gravity, space-time must undergo violent and random fluctuations all around us, but on a scale that we have not yet been able to detect. But if space-time is Classically, the fluctuations must be larger than a certain scale, and this scale can be determined by another experiment where we test how long we can put a heavy atom in superposition by being in two different places.

Co-authors Dr. Carlo Sparaciari and Dr. Barbara Šoda, whose analytical and numerical calculations helped guide the project, expressed hope that these experiments could determine whether pursuing a quantum theory of gravity is the right approach.

Dr Šoda (formerly UCL Physics & Astronomy, now at the Perimeter Institute for Theoretical Physics, Canada) said: “Because gravity manifests itself through the curvature of space and time, we can think about the question of know whether the rate at which time passes has a quantum nature, or a classical nature.

“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.”

Dr Sparaciari (UCL Physics & Astronomy) said: “Although the experimental concept is simple, the weighing of the object must be carried out with extreme precision.

“But what I find exciting is that, starting from very general assumptions, we can prove a clear relationship between two measurable quantities: the magnitude of space-time fluctuations and the length of time that objects like atoms or apples can be placed in quantum superposition of two different locations. We can then experimentally determine these two quantities.”

Weller-Davies added: “A delicate interaction must exist if quantum particles such as atoms are capable of bending classical spacetime. There must be a fundamental trade-off between the wave nature of atoms and the magnitude of random space-time fluctuations. “.

The proposal aimed at testing whether space-time is classical by looking for random mass fluctuations is complementary to another experimental proposal which aims to verify the quantum nature of space-time by looking for what is called “l ‘gravity-mediated entanglement’.

Professor Sougato Bose (UCL Physics & Astronomy), who was not involved in today’s announcement but was among those who first proposed the entanglement experiment, said: “Experiments to test the nature of space-time will require a large-scale effort, but they “are of central importance from the point of view of understanding the fundamental laws of nature. I believe that these experiments are at hand: These things are hard to predict, but maybe we’ll know the answer in the next 20 years.”

Postquantum theory has implications beyond gravity. The famous and problematic “measurement postulate” of quantum theory is not necessary, since quantum superpositions are necessarily localized by their interaction with classical space-time.

This theory was motivated by Professor Oppenheim’s attempt to solve the information problem about black holes. According to standard quantum theory, an object entering a black hole should be radiated somehow, because the information cannot be destroyed, but this violates general relativity, which says you can never know objects that cross the event horizon of the black hole. The new theory allows for the destruction of information, due to a fundamental breakdown in predictability.