Editing Dimension (section)

===Additional dimensions===

In physics, three dimensions of space and one of time is the accepted norm. However, there are theories that attempt to unify the four [[fundamental forces]] by introducing [[extra dimensions]]/[[hyperspace]]. Most notably, [[superstring theory]] requires 10 spacetime dimensions, and originates from a more fundamental 11-dimensional theory tentatively called [[M-theory]] which subsumes five previously distinct superstring theories. [[Supergravity theory]] also promotes 11D spacetime = 7D hyperspace + 4 common dimensions. To date, no direct experimental or observational evidence is available to support the existence of these extra dimensions. If hyperspace exists, it must be hidden from us by some physical mechanism. One well-studied possibility is that the extra dimensions may be "curled up" ([[Compactification (physics)|compactified]]) at such tiny scales as to be effectively invisible to current experiments.
[[File:Calabi-Yau.png|thumb|upright=0.7|Illustration of a Calabi–Yau manifold]]
In 1921, [[Kaluza–Klein theory]] presented 5D including an extra dimension of space. At the level of [[quantum field theory]], Kaluza–Klein theory unifies [[gravity]] with [[Gauge theory|gauge]] interactions, based on the realization that gravity propagating in small, compact extra dimensions is equivalent to gauge interactions at long distances. In particular when the geometry of the extra dimensions is trivial, it reproduces [[electromagnetism]]. However, at sufficiently high energies or short distances, this setup still suffers from the same pathologies that famously obstruct direct attempts to describe [[quantum gravity]]. Therefore, these models still require a [[UV completion]], of the kind that string theory is intended to provide. In particular, superstring theory requires six compact dimensions (6D hyperspace) forming a [[Calabi–Yau manifold]]. Thus Kaluza-Klein theory may be considered either as an incomplete description on its own, or as a subset of string theory model building.

In addition to small and curled up extra dimensions, there may be extra dimensions that instead are not apparent because the matter associated with our visible universe is localized on a {{nowrap|(3 + 1)-dimensional}} subspace. Thus, the extra dimensions need not be small and compact but may be [[large extra dimensions]]. [[D-brane]]s are dynamical extended objects of various dimensionalities predicted by string theory that could play this role. They have the property that open string excitations, which are associated with gauge interactions, are confined to the [[brane]] by their endpoints, whereas the closed strings that mediate the gravitational interaction are free to propagate into the whole spacetime, or "the bulk". This could be related to why gravity is exponentially weaker than the other forces, as it effectively dilutes itself as it propagates into a higher-dimensional volume.

Some aspects of brane physics have been applied to [[Brane cosmology|cosmology]]. For example, brane gas cosmology<ref>{{cite journal |last1=Brandenberger |first1=R. |last2=Vafa |first2=C. |title=Superstrings in the early universe |journal=Nuclear Physics B |volume=316 |issue=2 |pages=391–410 |year=1989 |doi=10.1016/0550-3213(89)90037-0 |bibcode=1989NuPhB.316..391B}}</ref><ref>Scott Watson, [http://www-astro-theory.fnal.gov/Conferences/cosmo02/poster/watson.pdf Brane Gas Cosmology]. {{webarchive|url=https://web.archive.org/web/20141027144123/http://www-astro-theory.fnal.gov/Conferences/cosmo02/poster/watson.pdf|date=2014-10-27}} (pdf).</ref> attempts to explain why there are three dimensions of space using topological and thermodynamic considerations. According to this idea it would be since three is the largest number of spatial dimensions in which strings can generically intersect. If initially there are many windings of strings around compact dimensions, space could only expand to macroscopic sizes once these windings are eliminated, which requires oppositely wound strings to find each other and annihilate. But strings can only find each other to annihilate at a meaningful rate in three dimensions, so it follows that only three dimensions of space are allowed to grow large given this kind of initial configuration.

Extra dimensions are said to be [[universal extra dimension|universal]] if all fields are equally free to propagate within them.