Physicists from UCL have proposed a new theory that reconciles quantum mechanics and Einstein’s theory of general relativity, suggesting that spacetime may not be governed by quantum theory. The theory, developed by Professor Jonathan Oppenheim, modifies quantum theory and predicts unpredictable fluctuations in spacetime. The theory’s implications are being tested through an experiment that measures a mass very precisely to see if its weight fluctuates over time. The results could determine whether the pursuit of a quantum theory of gravity is the right approach. The theory also has implications for the black hole information problem.
Unifying Gravity and Quantum Mechanics: A New Theory
A new theory that unifies gravity and quantum mechanics while preserving Einstein’s classical concept of spacetime has been proposed by physicists at UCL. The foundation of modern physics is based on two pillars: quantum theory, which governs the smallest particles in the universe, and Einstein’s theory of general relativity, which explains gravity through the bending of spacetime. However, these two theories contradict each other and a reconciliation has been elusive for over a century.
The prevailing assumption has been that Einstein’s theory of gravity must be modified, or “quantised”, to fit within quantum theory. This is the approach of two main candidates for a quantum theory of gravity, string theory and loop quantum gravity. However, the new theory, developed by Professor Jonathan Oppenheim (UCL Physics & Astronomy), challenges this consensus and suggests that spacetime may be classical – that is, not governed by quantum theory at all.
Instead of modifying spacetime, the theory – termed a “postquantum theory of classical gravity” – modifies quantum theory and predicts an intrinsic breakdown in predictability that is mediated by spacetime itself. This results in random and violent fluctuations in spacetime that are larger than envisaged under quantum theory, making the apparent weight of objects unpredictable if measured precisely enough.
Testing the New Theory: An Experimental Proposal
A second paper, published simultaneously in Nature Communications and led by Professor Oppenheim’s former PhD students, looks at some of the consequences of the theory, and proposes an experiment to test it: to measure a mass very precisely to see if its weight appears to fluctify over time.
For example, the International Bureau of Weights and Measures in France routinely weigh a 1kg mass which used to be the 1kg standard. If the fluctuations in measurements of this 1kg mass are smaller than required for mathematical consistency, the theory can be ruled out. The outcome of the experiment, or other evidence emerging which would confirm the quantum vs classical nature of spacetime, is the subject of a 5000:1 odds bet between Professor Oppenheim and Professor Carlo Rovelli and Dr Geoff Penington – proponents of quantum loop gravity and string theory respectively.
The Implications of the Postquantum Theory
The postquantum theory has implications beyond gravity. The infamous and problematic “measurement postulate” of quantum theory is not needed, since quantum superpositions necessarily localise through their interaction with classical spacetime.
The theory was motivated by Professor Oppenheim’s attempt to resolve the black hole information problem. According to standard quantum theory, an object going into a black hole should be radiated back out in some way as information cannot be destroyed, but this violates general relativity, which says you can never know about objects that cross the black hole’s event horizon. The new theory allows for information to be destroyed, due to a fundamental breakdown in predictability.
The Quantum vs Classical Nature of Spacetime
The question of whether spacetime has a quantum or classical nature is central to this new theory. Dr Barbara Šoda (formerly UCL Physics & Astronomy, now at the Perimeter Institute of Theoretical Physics, Canada) said: “Because gravity is made manifest through the bending of space and time, we can think of the question in terms of whether the rate at which time flows has a quantum nature, or 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 Carlo Sparaciari (UCL Physics & Astronomy) added: “While the experimental concept is simple, the weighing of the object needs to 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 scale of the spacetime fluctuations, and how long objects like atoms or apples can be put in quantum superposition of two different locations. We can then determine these two quantities experimentally.”
The Future of Spacetime Research
Professor Sougato Bose (UCL Physics & Astronomy), who was not involved with the announcement today, but was among those to first propose the entanglement experiment, said: “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. I believe these experiments are within reach – these things are difficult to predict, but perhaps we’ll know the answer within the next 20 years.”
The research group at UCL has been stress-testing the theory and exploring its consequences for the past five years. The proposal to test whether spacetime is classical by looking for random fluctuations in mass is complementary to another experimental proposal which aims to verify the quantum nature of spacetime by looking for something called “gravitationally mediated entanglement.”
Professor Jonathan Oppenheim said: “Quantum theory and Einstein’s theory of general relativity are mathematically incompatible with each other, so it’s important to understand how this contradiction is resolved. Should spacetime be quantised, or should we modify quantum theory, or is it something else entirely? Now that we have a consistent fundamental theory in which spacetime does not get quantised, it’s anybody’s guess.”
Zach Weller-Davies said: “This discovery challenges our understanding of the fundamental nature of gravity but also offers avenues to probe its potential quantum nature. We have shown that if spacetime doesn’t have a quantum nature, then there must be random fluctuations in the curvature of spacetime which have a particular signature that can be verified experimentally. In both quantum gravity and classical gravity, spacetime must be undergoing violent and random fluctuations all around us, but on a scale which we haven’t yet been able to detect. But if spacetime is classical, the fluctuations have to 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 of being in two different locations.”
Dr Šoda said: “Because gravity is made manifest through the bending of space and time, we can think of the question in terms of whether the rate at which time flows has a quantum nature, or 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 said: “While the experimental concept is simple, the weighing of the object needs to 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 scale of the spacetime fluctuations, and how long objects like atoms or apples can be put in quantum superposition of two different locations. We can then determine these two quantities experimentally.”
Professor Sougato Bose said: “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. I believe these experiments are within reach – these things are difficult to predict, but perhaps we’ll know the answer within the next 20 years.”
Summary
A new theory proposed by UCL physicists challenges the long-standing assumption that Einstein’s theory of gravity must be modified to fit within quantum theory, instead suggesting that spacetime may not be governed by quantum theory at all. The theory, known as a “postquantum theory of classical gravity”, predicts unpredictable and violent fluctuations in spacetime, and proposes an experiment to test this by measuring a mass very precisely to see if its weight appears to fluctuate over time.
- UCL physicists have proposed a new theory that unifies gravity and quantum mechanics, challenging the long-standing contradiction between quantum theory and Einstein’s theory of general relativity.
- The theory, developed by Professor Jonathan Oppenheim, suggests that spacetime may be classical, not governed by quantum theory, contradicting the prevailing assumption that Einstein’s theory of gravity must be modified to fit within quantum theory.
- The new theory, called a “postquantum theory of classical gravity”, modifies quantum theory and predicts an intrinsic breakdown in predictability, mediated by spacetime itself. This could result in random and violent fluctuations in spacetime.
- An experiment has been proposed to test the theory by measuring a mass very precisely to see if its weight appears to fluctify over time.
- The theory also has implications beyond gravity, potentially resolving the black hole information problem. It allows for information to be destroyed, due to a fundamental breakdown in predictability.
- The theory was motivated by Professor Oppenheim’s attempt to resolve the black hole information problem, which contradicts standard quantum theory and general relativity.
- The UCL research group has been testing the theory and exploring its consequences for the past five years.