Loop quantum gravity (LQG) is a theory that describes the quantum properties of gravity. It is also a theory of quantum space and quantum time, because, as discovered with general relativity, the geometry of spacetime is a manifestation of gravity. LQG is an attempt to merge and adapt standard quantum mechanics and standard general relativity. The main output of the theory is a physical picture of space where space is granular. The granularity is a direct consequence of the quantization. It has the same nature of the granularity of the photons in the quantum theory of electromagnetism or the discrete levels of the energy of the atoms. But here it is space itself which is discrete.
More precisely, space can be viewed as an extremely fine fabric or network "woven" of finite loops. These networks of loops are called spin networks. The evolution of a spin network over time, is called a spin foam. The predicted size of this structure is the Planck length, which is approximately meters. According to the theory, there is no meaning to distance at scales smaller than the Planck scale. Therefore LQG predicts that not just matter, but also space itself, has an atomic structure.
Today LQG is a vast area of research, developed in several directions, which involves about 50 research groups world wide. They all share the basic physical assumptions and the mathematical description of quantum space. The full development of the theory is pursued in two directions: the more traditional canonical loop quantum gravity and the younger covariant loop quantum gravity, or spin foam theory.
Several research directions study the physical consequences of the theory. Among these, the most developed is the application of LQG to cosmology, called Loop quantum cosmology (LQC). LQC applies LQG ideas to the study of the early universe and the physics of the Big Bang. Its most spectacular consequence is that the evolution of the universe can be continued beyond the Big Bang. The Big Bang appears thus to be replaced by a sort of cosmic Big Bounce.
Read more about Loop Quantum Gravity: History, Key Concepts, Problems and Comparisons With Alternative Approaches
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