Grand Unified Theory
A long sought-after goal of modern physics is the formulation of a complete description of all interactions mediated by the four fundamental forces by a grand unified theory that would account for all phenomena within a single theoretical framework.
There are four fundamental forces that occur in nature: the strong, the electromagnetic, the weak, and the gravitational force. The strong force is aptly named because it represents the strongest force and is responsible for binding nuclear particles together. The standard model describes all of the fundamental forces except gravity, the weakest of the fundamental forces. Gravity is described by the general theory of relativity.
Although weak in comparison to the other fundamental forces, gravity acts on all particles that have energy and mass and it is a long-range force. As of 2000, there is no complete quantum theory of gravity. The postulated force carrier for gravity is the, as of yet, unobserved graviton. Because gravitational interactions depend on the mass of the gravitating system, and since it is the weakest force, it effectively only applies to the macroscopic universe.
The strong (nuclear) force is the force that holds subatomic particles together, it acts on particles such as hadrons and its effects are transmitted by gluons at a very short range, i.e., 10-15 m. The much weaker electromagnetic force describes electric and magnetic interactions between atoms and molecules. Accordingly the electromagnetic force, also a long-range force, acts on charged particles and it is carried by photons. The weak (nuclear) force acts in the nucleus on all leptons and hadrons and is associated with radioactive phenomena such as beta decay. Its particles of exchange are the weak bosons, W and Z, and its range of action is very short, i.e., ~10-17 m.
The first step towards developing a unified theoretical framework was the realization, mainly through the experiments of English physicist and chemist Michael Faraday, that electricity and magnetism were both manifestations of the same electromagnetic force. This work led to the formulation of the electromagnetic equations and theory by Scottish physicist James Clerk Maxwell and French scientist André-Marie Ampère. There were, however, experimental observations that classical electromagnetic theory could not account for, including the photoelectric effect and blackbody radiation. These phenomena were explained by the theoretical advancements of German physicist Max Planck, French physicist Louis-Victor de Broglie, and German American physicist Albert Einstein, who were able to explain these phenomena by introducing the quantum mechanical concepts of the dual nature of light and the quantization of energy and photons.
Classical electromagnetic theory evolved into quantum electrodynamics (QED), also known as the quantum field theory of the electromagnetic force. After first incorporating the contributions of English physicist Paul Dirac, advances developed independently by American physicists Richard Feynman, Julian Schwinger, and Japanese physicist Sin-Itiro Tomonaga allowed QED theory to accurately describe the interactions of photons and to provide a complete conceptual framework for interactions with fields of charged particles.
Other fundamental steps toward a still elusive unified theory included the incorporation of quantum electrodynamics into a field theory for the weak force known as electroweak theory. In this formulation, the electromagnetic and weak forces are considered different manifestations of the same force, the electroweak force. In parallel to these developments, another quantum chromodynamic theory (QCD) was refined during the 1970's to account for the manifestations of the strong force. QCD derives its name from a property of quarks and gluons termed "color."
A linkage of the electroweak and strong forces was achieved by the Yang-Mills generalization of the classical equations of electromagnetism in 1954 and the subsequent work of Gerardus 't Hooft and Martinus Veltman. In the 1970s these contributions resulted in the standard model that specifically accounted for interactions involving elementary particles such as electrons, neutrinos, tau particles, and force carriers such as photons, gluons, and W and Z bosons. In addition to the fact that the standard model does not yet incorporate gravity, physicists continue to strive to resolve several problems, including the identification of the Higgs boson and the inability to theoretically derive fractional charges for quarks. In addition, there are attempts to determine the nature of leptons and quarks as they relate to the multiplicity of elementary particles.
The need to refine the standard model has provided impetus for the development of several grand unified theories (GUTs), theoretical frameworks striving to link the electroweak interactions of quarks and leptons to the strong interactions of quarks. For the most part, they are based on the assumption that at sufficiently high energies, the strong, electromagnetic, and weak forces may be of the same magnitude and related to each other by a gauge symmetry that could occur in a high-energy regime. As energy is decreased, the symmetry breaks down and split the three forces. A major difficulty is that the high energies required to experimentally validate the theory are of the order of 1016 GeV, which is well beyond the range of present-day particle accelerators. Another approach, first aimed at lowering the number of elementary particles, resulted in what has become known as supersymmetry. Although supersymmetry theory failed to lower the number of elementary particles, it succeeded in predicting the existence of partner particles that would have the same mass in a supersymmetry regime and with breakdown energies that, even if they are still too high for the currently available accelerators, may be studied at energy levels within realistic attainment in the very near future.
Another theoretical development of the 1970s was string theory, which describes elementary particles not as points in space but as extended straight, curved, or looped strings. Several variants of superstring theory were refined in the 1990s that were actually different manifestations of the same theory.
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