Great unified theory

In physics, a field theory that combines three of the four known basic physical forces , namely the strong interaction , the weak interaction and the electromagnetic force (and not gravity ) is called a large unified theory ( English Grand Unified Theory , GUT ; also Grand Unification ) ).

The GUT assumes that these basic forces were a single force at the time of the hot Big Bang . After the universe had cooled down, this single force would have split into the three named forces. According to GUT, the prerequisite for this would be that the strong interaction (also called strong nuclear force) becomes weaker at high energy , whereas the electromagnetic force and the weak interaction become stronger at high energy. In this context it is said that the electromagnetic force and the weak interaction are not asymptotically free , in contrast to quantum chromodynamics . At a certain, very high energy, according to GUT, all three forces would have the same strength and could prove to be different aspects of a single force. The great unified theory further predicts that at this energy all matter particles with spin ½ ( fermions ), for example electrons and quarks , are essentially the same.

In a narrower sense, GUT is understood to mean the union of the gauge theories of the Standard Model (SU (3) _C of quantum chromodynamics , SU (2) _I × U (1) _{Y of} the electroweak interaction) in a superordinate calibration group. Many such models, especially SU (5) and SO (10), but also exotic Lie groups up to E (8) were investigated starting with the implementation of the Standard Model in the 1970s. SU stands for the special unitary group , U for the unitary group , O for the orthogonal group and SO for the special orthogonal group . If you include supersymmetry , there are other possibilities. The fact that the sliding coupling constants of the gauge theories of the Standard Model only meet relatively precisely for supersymmetric GUTs at an energy scale is seen as an argument for supersymmetry.

The energy scale of the great standardization is 10 ²⁵ eV equal to 10 ¹⁶ GeV , where 1 GeV roughly corresponds to the proton mass. These energies cannot be achieved in earthly accelerator experiments (the Large Hadron Collider reaches 13,000 GeV equal to 13 TeV center of gravity energy in its highest expansion stages ). One possibility to test the existence of a unified force is proton decays , which are predicted by almost all unified theories. Due to the lack of evidence of such decays in detectors like Super-Kamiokande , even the simplest GUT models (minimal SU (5) theory, SO (10) without supersymmetry) could be excluded. According to the Super-Kamiokande results of 2017, the lower limit for the service life is 1.6 · 10 ³⁴ years (for comparison: age of the universe : 1.4 · 10 ¹⁰ years). The exclusion of the minimal SU (5) theory, the first proposed unified theory (by Howard Georgi , Sheldon Glashow ), happened as early as 1996. However, there are still some GUTs that still come into question, such as flipped SU (5) (developed by Dimitri Nanopoulos , Stephen Barr and others in the 1980s) or minimal supersymmetric SU (5) by Howard Georgi and Savas Dimopoulos (1981).

For a complete description of all known physical phenomena, this union would also have to include the fourth basic force, gravitation , with the general theory of relativity . Such a theory, which combines quantum physics and the theory of gravity ( quantum gravity ), is called a “ world formula ” (English “Theory of Everything”). Candidates are, for example, string theory or rather the unification of the five string theory approximations, the so-called M-theory , which also includes supergravity , and loop quantum gravity .

Fundamental interactions and their descriptions
	Strong interaction	Electromagnetic interaction	Weak interaction	Gravity
classic		Electrostatics & magnetostatics , electrodynamics		Newton's law of gravitation , general relativity
quantum theory	Quantum ( standard model )	Quantum electrodynamics	Fermi theory	Quantum gravity  ?
		Electroweak Interaction ( Standard Model )
	Big Unified Theory  ?
	World formula ("theory of everything")?
Theories at an early stage of development are grayed out.

literature

• Steven Weinberg : The dream of the unity of the universe . Goldmann, Munich 1995, ISBN 3-442-12641-X .
• Takeshi Fukuyama: Grand unified theories - current status and future prospects . AIP Press, Melville 2008, ISBN 978-0-7354-0536-3 .
• Rabindra Mohapatra : Unification and supersymmetry - the frontiers of quark-lepton physics . Springer, New York 2003, ISBN 0-387-95534-8 .
• Graham Ross : Grand unified theories , Benjamin-Cummings 1985, CRC Press 2003
• Anthony Zee : Unity of forces in the Universe , World Scientific 1982, 2 volumes (reprint volume on GUTs with an introductory lecture by Zee)

Individual evidence

1. ↑ Stephen Hawking: A Brief History of Time . dtv, Munich 2001, ISBN 3-423-33070-8 , p. 100ff.
2. ↑ U. Amaldi, W. de Boer, H. Fürstenau, Comparison of Grand Unified Theories with electroweak and strong coupling constants measured at LEP, Physics Letters Vol. 260, 1991, p. 447
3. ↑ M. Miura et al. a. (Super-Kamiokande), Phys. Rev. D, Volume 95, 2017, p. 012004, Arxiv
4. ↑ Natalie Wolchover, Grand Unification Dream Kept at Bay , Quanta Magazine, December 15, 2016