Loop quantum gravity

Loop quantum gravity (LQG) is a theory of quantum gravity, attempting to merge quantum mechanics and general relativity, while incorporating the standard model particles. It takes seriously the key insight from general relativity that space-time is a dynamic entity, not a fixed framework. It competes with string theory, another candidate for a theory of quantum gravity. However, unlike string theory, LQG is not a candidate for a theory of everything - the goal of which is to explain all of particle physics, unifying gravity with the other forces at the same time. In contrast to LQG, string theory (for the most part) is background-dependent (built on a fixed framework) and doesn’t account for the dynamic nature of space-time at the heart of relativity. Loop quantum gravity (LQG) is a theory of quantum gravity, attempting to merge quantum mechanics and general relativity, while incorporating the standard model particles. It takes seriously the key insight from general relativity that space-time is a dynamic entity, not a fixed framework. It competes with string theory, another candidate for a theory of quantum gravity. However, unlike string theory, LQG is not a candidate for a theory of everything - the goal of which is to explain all of particle physics, unifying gravity with the other forces at the same time. In contrast to LQG, string theory (for the most part) is background-dependent (built on a fixed framework) and doesn’t account for the dynamic nature of space-time at the heart of relativity. From the point of view of Einstein's theory, all attempts to treat gravity as another quantum force equal in importance to electromagnetism and the nuclear forces have failed. According to Einstein, gravity is not a force – it is a property of spacetime itself. Loop quantum gravity is an attempt to develop a quantum theory of gravity based directly on Einstein's geometric formulation. To do this, in LQG theory space and time are quantized, analogously to the way quantities like energy and momentum are quantized in quantum mechanics. The theory gives a physical picture of spacetime where space and time are granular and discrete directly because of quantization just like photons in the quantum theory of electromagnetism and the discrete energy levels of atoms. A minimum distance exists. Space's structure prefers an extremely fine fabric or network woven of finite loops. These networks of loops are called spin networks. The evolution of a spin network, or spin foam, has a scale on the order of a Planck length, approximately 10−35 metres, and smaller scales do not exist. Consequently, not just matter, but space itself, prefers an atomic structure. The vast areas of research developed in several directions that involve about 30 research groups worldwide. They all share the basic physical assumptions and the mathematical description of quantum space. Research follows two directions: the more traditional canonical loop quantum gravity, and the newer covariant loop quantum gravity, called spin foam theory. Physical consequences of the theory proceed in several directions. The most well-developed applies to cosmology, called loop quantum cosmology (LQC), the study of the early universe and the physics of the Big Bang. Its greatest consequence sees the evolution of the universe continuing beyond the Big Bang called the Big Bounce. In 1986, Abhay Ashtekar reformulated Einstein's general relativity in a language closer to that of the rest of fundamental physics. Shortly after, Ted Jacobson and Lee Smolin realized that the formal equation of quantum gravity, called the Wheeler–DeWitt equation, admitted solutions labelled by loops when rewritten in the new Ashtekar variables. Carlo Rovelli and Lee Smolin defined a nonperturbative and background-independent quantum theory of gravity in terms of these loop solutions. Jorge Pullin and Jerzy Lewandowski understood that the intersections of the loops are essential for the consistency of the theory, and the theory should be formulated in terms of intersecting loops, or graphs. In 1994, Rovelli and Smolin showed that the quantum operators of the theory associated to area and volume have a discrete spectrum. That is, geometry is quantized. This result defines an explicit basis of states of quantum geometry, which turned out to be labelled by Roger Penrose's spin networks, which are graphs labelled by spins. The canonical version of the dynamics was put on firm ground by Thomas Thiemann, who defined an anomaly-free Hamiltonian operator, showing the existence of a mathematically consistent background-independent theory. The covariant or spin foam version of the dynamics developed during several decades, and crystallized in 2008, from the joint work of research groups in France, Canada, UK, Poland, and Germany, leading to the definition of a family of transition amplitudes, which in the classical limit can be shown to be related to a family of truncations of general relativity. The finiteness of these amplitudes was proven in 2011. It requires the existence of a positive cosmological constant, and this is consistent with observed acceleration in the expansion of the Universe.