A Bet Against Quantum Gravity

Physicist Jonathan Oppenheim from University College London challenges the century-old pursuit of quantizing gravity, proposing a theory where matter remains quantum but gravity operates classically. He explains the inconsistency between general relativity and quantum mechanics and suggests that gravity's simplicity and role as the backdrop for matter's movement make it fundamentally different from other forces. Oppenheim addresses the black hole information paradox, arguing that if physics is not deterministic, then information loss is not paradoxical. He also discusses the possibility of coupling quantum systems with classical ones and presents experimental tests to determine whether gravity is fundamentally classical or quantum. The discussion includes a bet with physicist Carlo Rovelli and string theorist Geoff Pennington, with the outcome to be decided by experiments measuring gravitational field noise and the superposition of massive objects.

Takeaways

• 🔬 **Quantizing Space-Time**: The quest to quantize gravity and reconcile it with quantum theory has been a major challenge for physicists for a century.
• 🌌 **Classical Gravity Theory**: Jonathan Oppenheim proposes a theory where matter remains quantum, but gravity is treated classically, addressing the inconsistency between general relativity and quantum mechanics.
• ⚛️ **Quantum Mechanics**: All known particles, fields, forces, and phenomena in nature obey quantum theory, except for gravity, which is described by general relativity.
• 📏 **Gravity's Simplicity**: Gravity is fundamentally about the arena in which matter moves, with clocks running at different speeds in different places leading to space-time curvature.
• ⏱️ **Quantization of Time**: Attempting to quantize the speed of time leads to a frozen time, where things do not evolve, presenting a significant problem in quantizing gravity.
• 🕳️ **Black Hole Information Paradox**: The paradox arises from black holes seemingly destroying information through thermal radiation, a key issue in reconciling gravity with quantum theory.
• 🎲 **Stochasticity in Physics**: Accepting noise, randomness, and a lack of determinism in physics can alleviate the black hole information paradox.
• 🤝 **Coupling Quantum and Classical Systems**: The idea that quantum and classical systems can coexist consistently has been explored since the 1990s, offering a potential framework for reconciling gravity with quantum mechanics.
• 🔍 **Experimental Tests**: Oppenheim and his students propose experiments to detect randomness in the gravitational field, which would indirectly test a classical theory of gravity.
• 💯 **The Bet**: A bet with 5000 to 1 odds was formulated between Oppenheim and Carlo Rovelli, with Geoff Pennington joining, on whether spacetime is fundamentally quantum or classical.
• 🧠 **Theory of Coupling**: Any consistent theory that couples classical and quantum degrees of freedom must be fundamentally stochastic, with a significant amount of randomness involved.

Q & A

Why is reconciling gravity with quantum theory considered one of the great challenges of physics?

Reconciling gravity with quantum theory is a significant challenge because it involves integrating general relativity, which describes gravity and is a classical theory, with quantum mechanics, which governs the behavior of particles and forces at the smallest scales. Despite a century of effort, a successful unification of these two frameworks has remained elusive.

What is the main premise of Jonathan Oppenheim's proposed theory of gravity?

Jonathan Oppenheim's theory suggests that matter remains quantum while gravity is treated classically. This approach aims to resolve the inconsistency between general relativity, our current theory of gravity, and quantum mechanics.

How does general relativity differ from quantum theory in its description of nature?

General relativity, Einstein's theory of gravity, describes the bending of spacetime and is a classical theory. In contrast, quantum theory deals with the behavior of particles, fields, and forces at the quantum level and is probabilistic in nature. Quantum theory encompasses all known forces and particles, but it does not account for gravity.

What is the black hole information paradox, and how does it relate to quantum theory and gravity?

The black hole information paradox arises from the apparent loss of information when matter is thrown into a black hole, which contradicts the principles of quantum mechanics that require information to be conserved. This paradox is significant because it suggests a potential conflict between quantum theory and the classical description of gravity provided by general relativity.

What is the significance of the randomness in the context of a classical theory of gravity?

In a classical theory of gravity, the description of nature involves deterministic laws that govern the behavior of objects based on initial conditions. However, when considering quantum mechanics, randomness or probabilistic outcomes play a significant role in describing the behavior of particles and forces at the quantum level.

Outlines

00:00

🔬 The Challenge of Quantizing Gravity

Jonathan Oppenheim, a physicist at University College London, discusses the longstanding challenge of reconciling gravity with quantum theory. He explains that while all other forces and fields are described by quantum mechanics, gravity, as understood through Einstein's general relativity, remains a classical theory. Oppenheim and his students have proposed a theory where matter remains quantum but gravity is treated classically, addressing the inconsistency between general relativity and quantum mechanics. The summary touches on the simplicity of gravity, its relationship with spacetime, and the problems encountered when attempting to quantize the speed of time, which leads to a frozen concept of time. It also mentions the black hole information paradox and how it might be a key to reconciling gravity with quantum theory, as well as the discomfort of physicists with fundamental stochasticity or randomness in physics.

05:01

🎲 The Bet on Spacetime's Quantum Nature

Following a discussion with Carlo Rovelli, a proponent of loop quantum gravity, Oppenheim formulated a bet with 5000 to 1 odds on whether spacetime is fundamentally quantum. Geoff Pennington, a leading string theorist, also joined the bet. The bet is structured to be experimentally testable, focusing on the presence of randomness in the gravitational field, which would be indicative of a classical theory of gravity. Proposed experiments aim to measure gravitational field noise and the possibility of putting a massive object into superposition. The outcomes of these experiments will determine the result of the bet and further the understanding of gravity's quantization. The summary emphasizes the importance of experimental testing of theoretical beliefs and the ongoing struggle to solve the problem of gravity's quantization.

Keywords

Quantum Gravity

Quantum gravity refers to the theoretical framework that attempts to reconcile the principles of quantum mechanics, which describes the smallest scales of energy levels of particles, with those of general relativity, which describes the dynamics of gravity at large scales. In the video, the challenge of quantizing gravity is presented as one of the great unsolved problems in physics.

General Relativity

General relativity, proposed by Albert Einstein, is a theory of gravitation that describes gravity not as a force, but as a curvature of spacetime caused by mass and energy. It is the current prevailing theory of gravity and is mentioned in the script as inconsistent with quantum mechanics.

Quantum Mechanics

Quantum mechanics is the branch of physics that deals with the behavior of particles at the atomic and subatomic levels. It is characterized by the wave-particle duality and the probabilistic nature of physical phenomena. In the video, quantum mechanics is highlighted as a framework that everything in nature obeys except for gravity.

Classical Mechanics

Classical mechanics is the collection of the laws of physics that apply to macroscopic objects under the influence of forces. It is contrasted with quantum mechanics in the video, where the speaker proposes treating gravity classically while keeping matter quantum.

Spacetime

Spacetime is a mathematical model that combines the three dimensions of space with the one dimension of time into a four-dimensional continuum. It is central to the theory of general relativity, where it is described as bending due to gravity. The video discusses the idea of quantizing spacetime, which leads to theoretical difficulties.

Black Hole Information Paradox

The black hole information paradox arises from the conflict between quantum mechanics and general relativity regarding the fate of information that enters a black hole. The paradox is mentioned in the context of Hawking radiation, which seemingly destroys the information, contradicting the principles of quantum mechanics.

Hawking Radiation

Hawking radiation is a theoretical process through which black holes can lose mass and eventually evaporate. It is named after physicist Stephen Hawking, who predicted this phenomenon. In the video, it is linked to the black hole information paradox.

Determinism

Determinism is the philosophical proposition that all events, including moral choices, are determined completely by previously existing causes. In the context of the video, determinism is questioned in relation to the black hole information paradox and the potential for fundamental randomness in physics.

Stochasticity

Stochasticity refers to the randomness or probabilistic nature of a system or process. The video suggests that if one accepts the existence of randomness in physics, the black hole information paradox may not be a paradox at all.

Loop Quantum Gravity

Loop quantum gravity is a theoretical framework attempting to quantize spacetime itself. It is mentioned in the video as one of the approaches to quantum gravity, contrasting with the speaker's proposal of treating gravity classically.

String Theory

String theory is a theoretical framework in which the point-like particles of particle physics are replaced by one-dimensional objects called strings. It is briefly mentioned in the video in the context of a bet regarding the nature of spacetime.

Gravitational Field

A gravitational field is a model used in physics to explain the gravitational force between bodies. It is used in the video to discuss experiments that could test the classical nature of gravity by measuring noise in the gravitational field.

Highlights

Physicists have been trying to quantize space-time for a century, but with no success.

Jonathan Oppenheim proposes a theory where gravity is treated classically while matter remains quantum.

General relativity, Einstein's theory of gravity, is inconsistent with quantum mechanics.

All known forces and particles in nature obey quantum theory, except for gravity.

Gravity is fundamentally different from other forces; it's about the arena in which matter moves.

Quantizing gravity equates to quantizing the speed of time, which freezes time and halts evolution.

The black hole information paradox is a key to reconciling gravity and quantum theory.

Hawking's discovery of black holes radiating suggests information destruction, contradicting a fully quantum theory of gravity.

A non-deterministic view of physics resolves the paradox by accepting fundamental randomness.

The idea of coupling quantum systems with classical systems has been explored since the 1990s.

If space-time is the classical background, it simplifies the evolving space-time and resolves the black hole information paradox.

Any consistent theory coupling classical and quantum degrees of freedom must be fundamentally stochastic.

The amount of randomness in a classical gravity theory is bounded between 10^-1 and 10^-40.

Oppenheim and Rovelli made a bet with 5000 to 1 odds on whether spacetime is fundamentally quantum.

The bet is designed to be precise and testable experimentally, focusing on the presence of randomness in the gravitational field.

Proposed experiments aim to measure noise in the gravitational field as an indirect test of a classical theory of gravity.

Experiments will measure the gravitational field noise and the possibility of a massive object in superposition.

The outcome of the bet will be determined by the results of these experiments, potentially solving a century-long struggle in physics.