String Theory Quantum Cosmology And Quantum Gravity PDF Download

Quantum gravity is a field of theoretical physics that aims to describe the gravitational force according to the principles of quantum mechanics. Key Concepts General relativity, developed by Albert Einstein, describes gravity as the curvature of spacetime caused by mass and energy. It works extremely well at large scales, such as in predicting planetary orbits and the behavior of black holes. On the other hand, quantum mechanics deals with the fundamental behavior of particles at the smallest scales, such as atoms and subatomic particles. It incorporates principles like wave-particle duality, quantization, and uncertainty. The Need for Quantum Gravity Despite their success, general relativity and quantum mechanics are fundamentally incompatible. General relativity treats gravity as a smooth, continuous field, while quantum mechanics relies on quantized fields and inherently probabilistic events. When attempting to describe situations where both quantum effects and strong gravitational effects are significant, such as in the vicinity of black holes or during the Big Bang, the theories conflict. Quantum gravity aims to unify these two frameworks. Approaches to Quantum Gravity Several approaches have been proposed to formulate a theory of quantum gravity. String theory proposes that fundamental particles are not point-like but instead one-dimensional "strings" that vibrate at different frequencies. This theory inherently includes gravity and attempts to unify all fundamental forces. Loop quantum gravity (LQG) focuses on quantizing spacetime itself, positing that spacetime has a discrete structure at the smallest scales and attempts to describe the quantum properties of spacetime directly, without relying on a background. Causal dynamical triangulations (CDT) is a method of simulating quantum spacetime by breaking it down into discrete, simplicial building blocks, like triangles or tetrahedra, and studying how these blocks combine to form spacetime. Asymptotic safety aims to define a quantum theory of gravity that remains consistent and finite at all energy scales by identifying a fixed point in the theory's parameter space where the theory remains predictive and well-behaved.