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Cutting-edge theory unites quantum mechanics and Einstein's theory of general relativity

The image depicts an experiment in which heavy particles(illustrated as the moon), cause an interference pattern (a quantum effect), while also bending spacetime. The hanging pendulums depict the measurement of spacetime. The actual experiment is typically performed using Carbon-60, one of the largest known molecules. The UCL calculation indicates that the experiment should also be performed using higher density atoms such as gold. The other two images represent the two experiments proposed by the UCL group, both of which constrain any theory where spacetime is treated classically. One is the weighing of a mass, the other is an interference experiment. (CREDIT: Isaac Young)

In a cutting-edge development that has sent shockwaves through the scientific community, researchers at University College London (UCL) have unveiled a radical theory that seeks to reconcile two pillars of modern physics – quantum mechanics and Einstein's theory of general relativity. These two theories, which have been the foundation of physics for over a century, have long been at odds with each other, and their unification has remained an elusive quest.

Today, we dive into the world of quantum gravity, a field of study that aims to bridge the gap between the quantum realm, which governs the behavior of particles at the smallest scales, and the macroscopic world, where gravity shapes the very fabric of spacetime. While the prevailing consensus has been that Einstein's theory of gravity must be modified to fit within the framework of quantum theory, a new theory, coined as a "postquantum theory of classical gravity," challenges this assumption in a thought-provoking way.


The Clash of Titans: Quantum Mechanics vs. General Relativity

Quantum mechanics and general relativity, developed by Albert Einstein in the early 20th century, have both stood the test of time and have been proven accurate in their respective domains. However, when it comes to merging these two theories into a single, comprehensive framework, the scientific community has hit a roadblock.

The weighing of a mass - an experiment proposed by the UCL group which constrain any theory where spacetime is treated classically. (CREDIT: Isaac Young)

Quantum mechanics, which beautifully describes the behavior of particles at the subatomic level, operates in a probabilistic realm characterized by wave functions and quantum states. In contrast, general relativity paints a different picture of the universe, where gravity arises from the curvature of spacetime caused by massive objects. While these theories excel in their own domains, they clash when brought together, leading to mathematical inconsistencies and contradictions.


A New Approach: Spacetime as Classical

Enter Professor Jonathan Oppenheim and his team at UCL, who have challenged the status quo with their groundbreaking theory. In two parallel papers published simultaneously, they propose a novel perspective that suggests spacetime may remain classical and unaffected by quantum mechanics. This theory, as described in a paper published in Physical Review X (PRX), refrains from modifying spacetime itself and instead modifies quantum theory.

The core tenet of this theory is that spacetime remains classical, not subject to the constraints of quantum theory. Instead, quantum theory is tweaked to account for intrinsic unpredictability mediated by spacetime. The consequence? Spacetime experiences random and violent fluctuations that exceed the expectations set by quantum theory. These fluctuations, if measured precisely enough, render the apparent weight of objects unpredictable.


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To put their theory to the test, the researchers propose a groundbreaking experiment aimed at detecting fluctuations in mass over time. For instance, consider a 1kg mass – the standard measurement used by the International Bureau of Weights and Measures in France. If the measurements of this 1kg mass exhibit fluctuations smaller than those required for mathematical consistency, it would challenge the new theory.

This experiment, which has far-reaching implications for our understanding of gravity and quantum mechanics, is not just theoretical but practical. It serves as a critical juncture in the ongoing debate between competing theories of quantum gravity. Professor Oppenheim, along with Professor Carlo Rovelli and Dr. Geoff Penington, leading proponents of quantum loop gravity and string theory, respectively, have even placed a 5000:1 odds bet on the outcome.


Five Years of Rigorous Testing

The UCL research group, led by Professor Oppenheim, has spent the past five years meticulously developing and examining their theory, scrutinizing its consequences from various angles. As Professor Oppenheim puts it, "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."

Jonathan Oppenheim, a physicist at University College London, is developing hybrid theories that could unify classical gravity and quantum mechanics. (CREDIT: Philipp Ammon)

Their journey has been marked by relentless exploration of the fundamental nature of gravity and the cosmos itself, probing the boundaries of our knowledge and challenging preconceived notions.


Beyond the Weight of Gravity: Implications of the Postquantum Theory

While the focus of the postquantum theory is on reconciling quantum mechanics and general relativity, its implications extend far beyond the realm of gravity. One notable consequence is the elimination of the notorious "measurement postulate" in quantum theory. This postulate, which has long perplexed physicists, posits that measurements collapse quantum superpositions into definite states. In the new theory, quantum superpositions naturally localize through their interaction with classical spacetime, obviating the need for this postulate.

Professor Oppenheim's journey to this groundbreaking theory was motivated by his attempt to unravel the mysteries of the black hole information paradox. According to standard quantum theory, information cannot be destroyed. Therefore, an object entering a black hole should somehow radiate information back out. However, this concept directly contradicts general relativity, which posits that once an object crosses a black hole's event horizon, it becomes inaccessible.

The postquantum theory offers a unique perspective, suggesting that the fundamental breakdown in predictability inherent to spacetime allows for information to be destroyed, resolving this long-standing paradox.


The proposal to test whether spacetime remains classical by detecting random fluctuations in mass is just one part of the puzzle.

Another experimental proposal aims to verify the quantum nature of spacetime through a phenomenon called "gravitationally mediated entanglement." These experiments, though challenging, hold immense promise in advancing our understanding of the fundamental laws of nature.

Professor Sougato Bose, an expert in the field who was not involved in the recent UCL announcement but had previously proposed the entanglement experiment, emphasized the importance of these endeavors, stating, "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."


At the heart of this theory lies a delicate interplay between quantum particles, such as atoms, and the fluctuations in classical spacetime. These fluctuations, if the theory holds, must occur on a scale yet to be detected but should be large enough to impact quantum particles' behavior.

The proposed experiments seek to find this elusive balance, shedding light on whether spacetime remains classical or succumbs to quantum mechanics at microscopic scales.

In the words of Professor Oppenheim, "Now that we have a consistent fundamental theory in which spacetime does not get quantized, it's anybody's guess." The journey has just begun, and the future of physics has never looked more intriguing.


Background information

Quantum mechanics background: All the matter in the universe obeys the laws of quantum theory, but we only really observe quantum behavior at the scale of atoms and molecules.

The prediction of the semiclassical Einstein equation is depicted in the right panel, compared with what we expect to occur based on the statistical interpretation of the density matrix in the left panel. (CREDIT: PRX)

Quantum theory tells us that particles obey Heisenberg’s uncertainty principle, and we can never know their position or velocity at the same time. In fact, they don’t even have a definite position or velocity until we measure them. Particles like electrons can behave more like waves and act almost as if they can be in many places at once (more precisely, physicists describe particles as being in a “superposition” of different locations).


Quantum theory governs everything from semiconductors which are ubiquitous in computer chips, to lasers, to superconductivity to radioactive decay. In contrast, we say that a system behaves classically if it has definite underlying properties. A cat appears to behave classically – it is either dead or alive, not both, nor in a superposition of being dead and alive.

Why do cats behave classically, and small particles quantumly? We don’t know, but the postquantum theory doesn’t require the measurement postulate, because the classicality of spacetime infects quantum systems and causes them to localize.

Gravity background: Newton’s theory of gravity, gave way to Einstein’s theory of general relativity (GR), which holds that gravity is not a force in the usual sense. Instead, heavy objects such as the sun, bend the fabric of spacetime in such a way that causes the earth to revolve around it.


Spacetime is just a mathematical object consisting of the three dimensions of space, and time considered as a fourth dimension. General relativity predicted the formation of black holes and the big bang. It holds that time flows at different rates at different points in space, and the GPS in your smartphone needs to account for this in order to properly determine your location.

For more science and technology stories check out our New Discoveries section at The Brighter Side of News.


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