12-08-2023

Earth.com staff writer

In a groundbreaking announcement, physicists from University College London (UCL) have presented a radical theory that unifies the realms of gravity and quantum mechanics while preserving the classical concept of spacetime, as outlined by Einstein.

This innovative approach, detailed in two simultaneously published papers, challenges over a century of scientific consensus and proposes a revolutionary perspective on the fundamental nature of our universe.

Modern physics rests on two contradictory pillars: quantum theory, which rules the microscopic world, and Einstein’s theory of general relativity, explaining gravity through spacetime curvature. These theories, despite their individual successes, have remained irreconcilable, creating a significant rift in our understanding of the universe.

Traditionally, scientists have believed that a quantum version of Einstein’s theory of gravity was necessary. This belief fueled the development of string theory and loop quantum gravity. However, the new theory from UCL takes a divergent path.

Professor Jonathan Oppenheim of UCL Physics & Astronomy, the lead proponent of this theory, argues for a “postquantum theory of classical gravity.” This radical idea, as elaborated in his paper in *Physical Review X (PRX)*, suggests that spacetime may remain classical and not subject to quantum mechanics.

Instead of altering spacetime, this theory revises quantum theory itself, predicting unpredictable and significant fluctuations in spacetime. These fluctuations, larger than those anticipated by quantum theory, could render the weight of objects uncertain at precise measurements.

A second paper in *Nature Communications*, led by Professor Oppenheim’s former PhD students, proposes an experiment to validate this theory. The experiment involves measuring a mass (like the 1kg standard previously used by the International Bureau of Weights and Measures in France) with extreme precision to detect potential weight fluctuations over time.

Professor Oppenheim, Professor Carlo Rovelli, and Dr. Geoff Penington — leading proponents of quantum loop gravity and string theory, respectively — have placed a bet with 5000:1 odds on the outcome of the experiment, or any other evidence that might emerge, which would confirm the quantum versus classical nature of spacetime.

For the past five years, the UCL research team has been rigorously examining this theory and its implications. Professor Oppenheim notes the importance of resolving the contradiction between quantum theory and general relativity.

Oppenheim stated, “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, a co-author and key contributor to the theory, highlights that this discovery not only challenges our understanding of gravity but also provides a method to probe its potential quantum nature. “If spacetime doesn’t have a quantum nature, then there must be random fluctuations in the curvature of spacetime with a particular signature that can be verified experimentally,” he explains.

“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,” Weller-Davies continued. “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.”

Co-authors Dr. Carlo Sparaciari and Dr. Barbara Šoda emphasize the significance of these experiments in determining the correct approach to understanding gravity.

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 elucidated, “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.”

This postquantum theory extends its influence beyond understanding gravity. It negates the need for the problematic “measurement postulate” in quantum theory. Quantum superpositions would naturally localize due to their interactions with classical spacetime.

Originating from Professor Oppenheim’s efforts to solve the black hole information problem, this theory allows for the possibility of information destruction, contradicting standard quantum theory but aligning with general relativity’s predictions about black holes.

This announcement marks a potential paradigm shift in physics. As Professor Sougato Bose of UCL Physics & Astronomy, not involved in this specific announcement but a pioneer in related research, remarks, “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.”

Indeed, these efforts could lead to a unified understanding of gravity and quantum mechanics, resolving one of the most profound dilemmas in modern physics. The implications of this theory, if proven correct, are vast, potentially reshaping our understanding of the universe at its most fundamental level.

As mentioned above, Einstein’s theory of relativity, a cornerstone of modern physics, revolutionized our understanding of space, time, and gravity. This theory comes in two parts: Special Relativity and General Relativity.

Albert Einstein introduced Special Relativity in 1905. This theory fundamentally changed our perception of space and time. It asserts two key principles:

The Laws of Physics are the Same for All Non-accelerating Observers: No matter how fast an observer is moving, they will measure the same speed of light and observe the same laws of physics.

The Speed of Light is Constant: The speed of light in a vacuum is the same for all observers, regardless of their relative motion or the motion of the light source.

Special Relativity led to several groundbreaking conclusions:

Time Dilation: Time passes slower for objects moving at high speeds compared to those at rest. This effect becomes significant only at speeds close to the speed of light.

Length Contraction: Objects contract in length along the direction of motion as they approach the speed of light.

E=mc²: This famous equation relates energy (E) to mass (m) with the speed of light (c) as the constant of proportionality. It implies that energy and mass are interchangeable, laying the groundwork for nuclear energy and weapons.

Ten years later, Einstein expanded on his theory with General Relativity, which addresses gravity and acceleration:

Einstein proposed that gravity is not a force between masses but rather a result of the curvature of spacetime caused by mass and energy. Massive objects like stars and planets warp the space around them, and other objects move along these curves, which we perceive as gravitational attraction.

This principle states that the effects of gravity are indistinguishable from the effects of acceleration. For instance, being in a closed room on Earth’s surface (where gravity pulls you down) feels the same as being in a room in a spaceship that accelerates upwards.

Light Bending: It predicts that light bends in a gravitational field. Observations during solar eclipses have confirmed this, where stars’ positions near the sun appear shifted due to the sun’s gravity bending the light.

Time Dilation Due to Gravity: Clocks run slower in stronger gravitational fields. This effect, tested using precise atomic clocks at different altitudes, is integral for the accuracy of GPS systems.

Gravitational Waves: Predicted by Einstein, these ripples in spacetime, caused by massive accelerating objects (like merging black holes), were directly detected in 2015, confirming a major prediction of General Relativity.

In summary, Einstein’s theory of relativity redefined our understanding of the universe. Special Relativity showed that space and time are relative and interconnected, leading to phenomena like time dilation and mass-energy equivalence. General Relativity further advanced this by describing gravity as the curvature of spacetime, profoundly influencing cosmology and astrophysics.

As also discussed above, quantum mechanics, a fundamental theory in physics, describes the behavior of matter and energy at the atomic and subatomic levels. It emerged in the early 20th century as scientists explored phenomena that classical physics couldn’t explain. Here are some key aspects of quantum mechanics:

Quantum mechanics introduces the concept of wave-particle duality. Particles, such as electrons and photons, exhibit both particle-like and wave-like properties. For example, electrons can produce interference patterns (a wave property) in a double-slit experiment, while also showing particle characteristics in other contexts.

Werner Heisenberg formulated the Uncertainty Principle, a cornerstone of quantum mechanics. It states that it is impossible to simultaneously know the exact position and momentum of a particle. The more precisely you measure one, the less precise the measurement of the other becomes. This principle challenges the classical notion of determinism.

Quantum Superposition: Quantum particles can exist in multiple states simultaneously, as illustrated by Schrödinger’s cat thought experiment. A particle in a superposition state doesn’t have a specific position, energy, or other physical property until it’s measured.

Quantum Entanglement: Particles can become entangled, meaning the state of one particle instantly influences the state of another, regardless of the distance separating them. This phenomenon, famously described by Einstein as “spooky action at a distance,” defies classical ideas of spatial separation and information transfer.

Quantum tunneling occurs when particles pass through barriers that they shouldn’t be able to, according to classical physics. This effect is crucial in many modern technologies, such as semiconductors and superconducting devices.

Upon measurement, a quantum system ‘collapses’ from a superposition of states to a single state. This collapse is instantaneous and is at the heart of many interpretations of quantum mechanics, including the famous Copenhagen interpretation.

Quantum mechanics has led to numerous technological advancements:

Semiconductors: The foundation of modern electronics, including computers and smartphones, relies on quantum mechanics.

Quantum Computing: Quantum computers use quantum bits or qubits, which can be in superpositions of states, offering potentially exponential increases in computing power for certain problems.

Medical Imaging: Techniques like MRI and PET scans depend on principles of quantum mechanics.

In summary, quantum mechanics reveals a strange, counterintuitive world at the smallest scales, fundamentally different from our everyday experiences. Its principles have not only deepened our understanding of the universe but also driven significant technological progress.

The full study was published in the journal *Nature Communications*.

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