Editing Fundamental interaction (section)

=== Beyond the Standard Model ===
{{main|Physics beyond the Standard Model}}
{{See also|Elementary particle#Beyond the Standard Model}}

The fundamental forces may become unified into a single force at very high energies and on a minuscule scale, the [[Planck scale]].<ref>{{Cite news|url=http://www.symmetrymagazine.org/article/the-planck-scale|date=2016-05-16 |last= Shivni | first= Rashmi  | title=The Planck scale|work=symmetry magazine|publisher= Fermilab/SLAC |access-date=2018-10-30|language=en}}</ref> [[Particle accelerator]]s cannot produce the enormous energies required to experimentally probe this regime. The weak and electromagnetic forces have already been unified with the [[electroweak interaction|electroweak theory]] of [[Sheldon Glashow]], [[Abdus Salam]], and [[Steven Weinberg]], for which they received the 1979 Nobel Prize in physics.<ref>{{Cite web|url=https://www.nobelprize.org/prizes/physics/1979/glashow/auto-biography/|title=The Nobel Prize in Physics 1979|website=NobelPrize.org|language=en-US|access-date=2018-10-30}}</ref><ref>{{Cite web|url=https://www.nobelprize.org/prizes/physics/1979/salam/biographical/|title=The Nobel Prize in Physics 1979|website=NobelPrize.org|language=en-US|access-date=2018-10-30}}</ref><ref>{{Cite web|url=https://www.nobelprize.org/prizes/physics/1979/weinberg/auto-biography/|title=The Nobel Prize in Physics 1979|website=NobelPrize.org|language=en-US|access-date=2018-10-30}}</ref> 
Numerous theoretical efforts have been made to systematize the existing four fundamental interactions on the model of electroweak unification.

[[Grand Unified Theories]] (GUTs) are proposals to show that the three fundamental interactions described by the Standard Model are all different manifestations of a single interaction with [[symmetry (physics)|symmetries]] that break down and create separate interactions below some extremely high level of energy. GUTs are also expected to predict some of the relationships between constants of nature that the Standard Model treats as unrelated, as well as predicting [[gauge coupling unification]] for the relative strengths of the electromagnetic, weak, and strong forces.

A so-called [[theory of everything]], which would integrate GUTs with a quantum gravity theory face a greater barrier, because no quantum gravity theories, which include [[string theory]], [[loop quantum gravity]], and [[twistor theory]], have secured wide acceptance. Some theories look for a graviton to complete the Standard Model list of force-carrying particles, while others, like loop quantum gravity, emphasize the possibility that time-space itself may have a quantum aspect to it.

Some theories beyond the Standard Model include a hypothetical [[fifth force]], and the search for such a force is an ongoing line of experimental physics research. In [[supersymmetric]] theories, some particles acquire their masses only through supersymmetry breaking effects and these particles, known as [[Moduli (physics)|moduli]], can mediate new forces. Another reason to look for new forces is the discovery that the [[expansion of the universe]] is accelerating (also known as [[dark energy]]), giving rise to a need to explain a nonzero [[cosmological constant]], and possibly to other modifications of [[general relativity]]. Fifth forces have also been suggested to explain phenomena such as [[Charge parity|CP]] violations, [[dark matter]], and [[dark flow]].