Loop quantum gravity

Loop quantum gravity

Loop quantum gravity (LQG) is a theory of quantum gravity, merging quantum mechanics and general relativity, making it a possible candidate for a theory of everything. Its goal is to unify gravity in a common theoretical framework with the other three fundamental forces of nature, beginning with relativity and adding quantum features. It competes with string theorythat begins with quantum field theoryand adds gravity.

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 spacetimeitself. 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 electromagnetismand the discrete energy levels of atoms. Distance exists with a minimum.

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 involves 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.