Loop quantum gravity is one of several concepts under development by theoretical physicists to provide a description of gravity in terms of quantum mechanics.
For decades, physicists have sought to reconcile the two major pillars of modern physics theory: general relativity and quantum mechanics. General relativity, postulated by Albert Einstein, advanced beyond Isaac Newton’s gravitational dynamics by stating that gravitational masses distort the spacetime around them and that the resulting curvature in spacetime is synonymous with gravity. Because gravity is the weakest of the four fundamental forces of nature—the others being electromagnetism and the weak and strong nuclear interactions that predominate inside the atomic nucleus—general relativity is extremely useful for describing the gravitational effects associated with massive objects, including planets, stars, and galaxies. Quantum mechanics, in contrast, describes objects and their properties in terms of probabilistic wave functions. It excels at describing the submicroscopic universe where unusual quantum effects associated with wavelike behaviors (such as positional uncertainty, diffraction, and tunneling) become highly significant. See also: Gravity; Gravitation; Quantum field theory; Quantum gravitation; Quantum mechanics; Relativity
The problem is that the theoretical frameworks for relativity and quantum mechanics are different and irreconcilable. In mathematical terms, the obstacle is that when physicists apply the quantum field theories for electromagnetism and the two nuclear interactions, they can use a technique called renormalization to avoid problematic infinities arising in the calculations. The equations for relativistic gravity, however, are not renormalizable. Physicists would prefer a single comprehensive framework that could encompass all scales of mass and size accurately: a theory of quantum gravity. See also: Electromagnetism; Renormalization; Strong nuclear interactions; Weak nuclear interactions
Loop quantum gravity addresses this problem by positing that space itself has a discrete granularity on the order of the Planck scale (about 10–35 meters). The granularity is defined in terms of minutely looping gravitational fields interwoven into a so-called spin network or spin foam. A conceptual advantage of loop quantum gravity is that unlike some competing ideas, such as string theories (also called superstring theories, which are specialized cases of broader M-theory), it does not invoke the existence of additional, as-yet-unseen spatial dimensions. On the other hand, direct observation of Planck-scale granularity is also essentially impossible, so testing of the hypothesis is difficult and may eventually depend on indirect evidence from astronomical observations and gravity wave detectors. Moreover, out of multiple possible variations of loop quantum gravity, no single formulation is yet favored by a consensus of theorists, and all of the variations have had difficulties with predicting how the phenomena of general relativity should emerge at large scales. Even if loop quantum gravity does reconcile relativity and quantum mechanics, it would not help with the unification of gravity, electromagnetism, and the weak and strong nuclear interactions as manifestations of a single underlying force. See also: M-theory; Superstring theory; Unification theories and a theory of everything