An ongoing endeavor in physics for at least the past three centuries has been the attempt to unify the disparate phenomena of nature within a single theoretical framework and a single set of descriptive equations. Isaac Newton was perhaps the first to succeed in part during the 17th century when he united astronomy and physics by showing that the elliptical orbits of the planets discovered by Johannes Kepler could be fully explained by Newton’s own laws of motion and a universal gravitational force. In the 19th century, James Clerk Maxwell offered a second great unification by demonstrating that electricity and magnetism were different manifestations of a single underlying electromagnetic force. In the 20th century, physicists continued to discover new primordial particles and forces and labored to unite them as well, with mixed success. The challenge became all the harder with the dual rise of Albert Einstein’s theory of general relativity and the theory of quantum mechanics, both of which have been robustly verified by experiments even though they seem irreconcilable. The quest to unify all these concepts and observations within a single framework will no doubt remain one of the greatest challenges in physics for years to come. See also: Electromagnetism; Gravitation; Gravity; Physics; Quantum mechanics; Relativity
Today, physicists’ most complete theory for explaining nature is the standard model, which has been extremely successful in describing all the known fundamental particles constituting matter. In the standard model, the atomic particles classified as hadrons (such as protons, neutrons, and mesons) consist of different combinations of more fundamental particles called quarks. (Electrons, muons, and their like fall into a separate family of particles called leptons.) The four interactions affecting matter are electromagnetism, the weak nuclear force (which governs the radioactive decay of atomic nuclei), the strong nuclear force (which bundles quarks into hadrons and also holds together baryonic hadrons in atomic nuclei), and gravity. The detection of the long-sought Higgs boson particle in 2012 at the Large Hadron Collider was taken as a powerful confirmation of the standard model. See also: Baryon; Detection of the Higgs boson; Elementary particle; Gluons; Hadron; Higgs boson; Higgs boson detection at the LHC; Lepton; Matter (physics); Quarks; Standard model; Strong nuclear interactions; Weak nuclear interactions
Nevertheless, the standard model was and is incomplete. It originally treated the four fundamental forces as distinct, but under supremely high energy conditions, such as during the origin of the universe, all the forces should obey the same equation. Moreover, although electromagnetism and the two nuclear forces can be described in terms of particles obeying quantum mechanics, relativistic gravitation cannot. See also: Quantum field theory; Universe
Starting with Einstein, physicists sought for decades to devise a unified field theory that would present all four fundamental forces as specialized cases of a single underlying field equation. As yet, no unified field theory has broad support among physicists, though progress has been made toward unifying some of the forces.
In 1968 Sheldon Glashow, Abdus Salam, and Steven Weinberg developed an electroweak theory that united electromagnetism and the weak nuclear force. Observations supporting it emerged in the 1970s and early 1980s. (Glashow, Salam, and Weinberg received a Nobel prize for electroweak theory in 1979, and the physicists Carlo Rubbia and Simon van der Meer shared one in 1984 for their experimental validation of it.) Several models for unifying the electroweak and strong nuclear force have also been proposed, and physicists refer to them as grand unified theories (GUTs) because they would bring together three out of the four forces. The GUT candidates remain untested and unconfirmed, however, because the energies required to investigate them far exceed the capacities of even the most powerful particle colliders. See also: Electroweak interaction; Grand unification theories; Quantum electrodynamics
Gravity remains the hardest of the forces to unify because a convincing model for quantum gravity has yet to emerge. In technical terms, the obstacle is that the quantum field theories for the electroweak and strong nuclear forces allow the use of mathematical techniques called renormalization to avoid problematic infinities arising in the calculations. The field equations for gravity are not renormalizable. See also: Renormalization
Currently, many physicists are developing ambitious concepts often called theories of everything, which would not only unify gravity with the other forces but would also, ideally, explain why fundamental physical constants (such as the gravitational constant, G) have the values they do. Among the approaches considered promising by many physicists is a class of ideas often labeled string theory, in which the ultimate building blocks of matter are not particles but rather extremely tiny one-dimensional strings highly coiled through more than four dimensions. (Some well-developed concepts related to string theory go by the name of superstring theory and M-theory.) A criticism of the string theory approach, however, is that experimental observations may never be able to distinguish which of a nearly infinite number of variations on it is correct. See also: M-theory; Quantum gravitation; Strong-interaction theories based on gauge/gravity duality; Superstring theory; Supersymmetry
Loop quantum gravity is a rival to string theory with many advocates: It proposes that space itself has a discrete granularity on the order of the Planck scale (about 10–35 meters). Formulations of loop quantum gravity still have difficulty predicting how smooth space emerges at larger scales, however. Moreover, even if loop quantum gravity does reconcile relativity and quantum mechanics, it would not directly help with the unification of gravity, electromagnetism, and the two nuclear interactions as manifestations of a single underlying force. See also: The quest for loop quantum gravity