Quantum gravity

cube of theoretical physics

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Quantum gravity

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Point&string

Calabi-Yau

Spin network

Quantum gravity is a field of theoretical physics that seeks to describe gravity according to the principles of quantum mechanics, and where quantum effects cannot be ignored, such as near compact astrophysical objects like black holes, or at the Planck scale (the scale at which quantum effects dominate the physics of the universe). The current understanding of gravity is based on Albert Einstein's general theory of relativity, which provides a description of gravity as the curvature of spacetime caused by mass and energy. However, general relativity is a classical theory and does not include the quantum effects that are present in all other fundamental interactions. Therefore, a quantum theory of gravity is needed to reconcile general relativity with the principles of quantum mechanics.

Overview

Quantum gravity attempts to answer how gravity works at the quantum level, an unsolved problem in physics. Various approaches to quantum gravity, such as string theory, loop quantum gravity, and causal dynamical triangulation, have been proposed, but none have been empirically confirmed. The main challenge in formulating a theory of quantum gravity is finding a model that is consistent with both general relativity and quantum mechanics, as these two fundamental theories are mathematically incompatible in certain situations.

Approaches

String Theory

String theory posits that the fundamental constituents of the universe are one-dimensional "strings" rather than point particles. These strings can vibrate at different frequencies, and the vibrational modes correspond to different particles. String theory is a leading candidate for a theory of quantum gravity because it naturally incorporates gravity by positing a specific type of string, the graviton, which mediates gravitational interactions.

Loop Quantum Gravity

Loop quantum gravity (LQG) is another major approach to the problem of quantum gravity. LQG suggests that spacetime itself has a discrete structure at the Planck scale. This theory attempts to quantize spacetime directly, as opposed to quantizing the fields within spacetime as in quantum field theory. LQG predicts that space is made of tiny loops of quantum fields that create a fabric of discrete spacetime.

Causal Dynamical Triangulation

Causal dynamical triangulation (CDT) is a newer approach that combines aspects of quantum mechanics and general relativity. It models spacetime as a series of discrete slices that evolve over time, which are then pieced together to form a spacetime geometry. This approach is notable for its use of computer simulations to explore the properties of quantum spacetime.

Challenges

One of the biggest challenges in quantum gravity is the lack of experimental evidence. The energies required to probe the Planck scale directly are currently beyond the reach of our most powerful particle accelerators. Additionally, the mathematical difficulties in combining the principles of general relativity and quantum mechanics are significant.

Implications

A successful theory of quantum gravity would not only unify general relativity and quantum mechanics but could also lead to a deeper understanding of the universe. It could provide insights into the behavior of the early universe, the nature of black holes, and the fundamental structure of spacetime.

See also

• Quantum field theory
• General relativity
• Unified field theory
• Hawking radiation