# Kaluza-Klein theory

In physics, Kaluza-Klein theory (or KK theory, for short) is a model which sought to unify the two fundamental forces of gravitation and electromagnetism. The theory was first published in 1921 and was discovered by the mathematician Theodor Kaluza who extended general relativity to a five-dimensional spacetime. The resulting equations can be separated out into further sets of equations, one of which is equivalent to Einstein field equations, another set equivalent to Maxwell's equations for the electromagnetic field and the final part an extra scalar field now termed the "radion".

## Overview

A splitting of five-dimensional spacetime into the Einstein equations and Maxwell equations in four dimensions was first discovered by Gunnar Nordström in 1914, in the context of his theory of gravity, but subsequently forgotten. In 1926, Oskar Klein proposed that the fourth spatial dimension is curled up in a circle of very small radius, so that a particle moving a short distance along that axis would return to where it began. The distance a particle can travel before reaching its initial position is said to be the size of the dimension. This extra dimension is a compact set, and the phenomenon of having a space-time with compact dimensions is referred to as compactification.

In modern geometry, the extra fifth dimension can be understood to be the circle group U(1), as electromagnetism can essentially be formulated as a gauge theory on a fiber bundle, the circle bundle, with gauge group U(1). Once this geometrical interpretation is understood, it is relatively straightforward to replace U(1) by a general Lie group. Such generalizations are often called Yang-Mills theories. If a distinction is drawn, then it is that Yang-Mills theories occur on a flat space-time, whereas Kaluza-Klein treats the more general case of curved spacetime. The base space of Kaluza-Klein theory need not be four-dimensional space-time; it can be any (pseudo-)Riemannian manifold, or even a supersymmetric manifold or orbifold.

As an approach to the unification of the forces, it is straightforward to apply the Kaluza-Klein theory in an attempt to unify gravity with the strong and electroweak forces by using the symmetry group of the Standard Model, SU(3) × SU(2) × U(1). However, a naive attempt to convert this interesting geometrical construction into an bona-fide model of reality flounders on a number of issues, including the fact that the fermions must be introduced in an artificial way (in nonsupersymmetric models). A less problematic approach to the unification of the forces is taken by modern string theory and M-theory. None-the-less, KK remains an important touchstone in theoretical physics and is often embedded in more sophisticated theories. It is studied in its own right as an object of geometric interest in K-theory.

Even in the absence of a completely satisfying theoretical physics framework, the idea of exploring extra, compactified, dimensions is of considerable interest in the experimental physics and astrophysics communities. A variety of predictions, with real experimental consequences, can be made (in the case of large extra dimensions/warped models). For example, on the simplest of principles, one might expect to have standing waves in the extra compactified dimension(s). If an extra dimension is of radius R, the energy of such a standing wave would (naively) be $\displaystyle E=nhc/R$ with n an integer, h being Planck's constant and c the speed of light. This set of possible energy values often called the Kaluza-Klein tower.

Examples of experimental pursuits include work by the CDF collaboration, which has re-analyzed particle collider data for the signature of effects associated with large extra dimensions/warped models.

Brandenberger and Vafa have speculated that in the early universe, cosmic inflation causes three of the space dimensions to expand to cosmological size while the remaining dimensions of space remained microscopic.

## Space-Time-Matter theory

One particular variant of Kaluza-Klein theory is Space-Time-Matter theory or induced matter theory, chiefly promogulated by Paul Wesson. In this version of the theory, it is noted that solutions to the equation

$\displaystyle R_{AB}=0$

with $\displaystyle R_{AB}$ the five-dimensional Ricci curvature, may be re-expressed so that in four dimensions, these solutions satisfy Einstein's equations

$\displaystyle G_{\mu\nu} = 8\pi T_{\mu\nu}$

with the precise form of the $\displaystyle T_{\mu\nu}$ following from the Ricci-flat condition on the five-dimensional space. Since the energy-momentum tensor $\displaystyle T_{\mu\nu}$ is normally understood to be due to concentrations of matter in four-dimensional space, the above result is interpreted as saying that four-dimensional matter is induced from geometry in five-dimensional space.

In particular, the soliton solutions of $\displaystyle R_{AB}=0$ can be shown to contain the Robertson-Walker metric in both matter-dominated (early universe) and radiation-dominated (present universe) forms. The general equations can be shown to be sufficiently consistent with classical tests of general relativity to be acceptable on physical principles, while still leaving considerable freedom to also provide interesting cosmological models.

## Geometric interpretation

The Kaluza-Klein theory is striking because it has a particularly elegant presentation in terms of geometry. In a certain sense, it looks just like ordinary gravity in free space, except that it is phrased in five dimensions instead of four.

### The Einstein equations

The equations governing ordinary gravity in free space can be obtained from an action, by applying the variational principle to a certain action. Let M be a (pseudo-)Riemannian manifold, which may be taken as the spacetime of general relativity. If g is the metric on this manifold, one defines the action $\displaystyle S(g)$ as

$\displaystyle S(g)=\int_M R(g) \mbox{vol}(g)\,$

where R(g) is the scalar curvature and vol(g) is the volume element. By applying the variational principle to the action

$\displaystyle \frac{\delta S(g)}{\delta g} = 0$

$\displaystyle R_{ij} - \frac{1}{2}g_{ij}R = 0$

Here, $\displaystyle R_{ij}$ is the Ricci tensor.

### The Maxwell equations

By contrast, the Maxwell equations describing electromagnetism can be understood to be the Hodge equations of a principal U(1)-bundle or circle bundle $\displaystyle \pi:P\to M$ with fiber U(1). This is, the electromagnetic field F is a harmonic 2-form in the space $\displaystyle \Omega^2(M)$ of differentiable 2-forms on the manifold M. In the absence of charges and currents, the free-field Maxwell equations are

$\displaystyle dF=0\,\quad$ and $\displaystyle \quad d*F=0\,$

where * is the Hodge star.

### The Kaluza-Klein geometry

To build the Kaluza-Klein theory, one picks an invarient metric on the circle $\displaystyle S^1$ that is the fiber of the U(1)-bundle of electromagnetism. In this discussion, an invariant metric is simply one that is invariant under rotations of the circle. Suppose this metric gives the circle a total length of $\displaystyle \Lambda$ . One then considers metrics $\displaystyle \widehat{g}$ on the bundle P that are consistent with both the fiber metric, and the metric on the underlying manifold M. The consistency conditions are:

• The projection of $\displaystyle \widehat{g}$ to the vertical subspace $\displaystyle \mbox{Vert}_pP \subset T_pP$ needs to agree with metric on the fiber over a point in the manifold M.
• The projection of $\displaystyle \widehat{g}$ to the horizontal subspace $\displaystyle \mbox{Hor}_pP \subset T_pP$ of the tangent space at point $\displaystyle p\in P$ must be isomorphic to the metric g on M at $\displaystyle \pi(p)$ .

The Kaluza-Klein action for such a metric is given by

$\displaystyle S(\widehat{g})=\int_P R(\widehat{g}) \;\mbox{vol}(\widehat{g})\,$

The scalar curvature, written in components, then expands to

$\displaystyle R(\widehat{g}) = \pi^*\left( R(g) - \frac{\Lambda^2}{2} \vert F \vert^2 \right)$

where $\displaystyle \pi^*$ is the pullback of the fiber bundle projection $\displaystyle \pi:P\to M$ . The connection A on the fiber bundle is related to the electromagnetic field strength as

$\displaystyle \pi^*F = dA$

That there always exists such a connection, even for fiber bundles of arbitrarily complex topology, is a result from homology and specifically, K-theory. Applying Fubini's theorem and integrating on the fiber, one gets

$\displaystyle S(\widehat{g})=\Lambda \int_M \left( R(g) - \frac{1}{\Lambda^2} \vert F \vert^2 \right) \;\mbox{vol}(g)$

Varying the action with respect to the component A, one regains the Maxwell equations. Applying the variational principle to the base metric g, one gets the Einstein equations

$\displaystyle R_{ij} - \frac{1}{2}g_{ij}R = \frac{1}{\Lambda^2} T_{ij}$

with the stress-energy tensor being given by

$\displaystyle T^{ij} = F^{ik}F^{jl}g_{kl} - \frac{1}{4}g^{ij} \vert F \vert^2$ ,

sometimes called the Maxwell stress tensor.

The original theory identifies $\displaystyle \Lambda$ with the fiber metric $\displaystyle g_{55}$ , and allows $\displaystyle \Lambda$ to vary from fiber to fiber. In this case, the coupling between gravity and the electromagnetic field is not constant, but has its own dynamical field, the radion.

### Commentary and generalizations

In the above, the size of the loop $\displaystyle \Lambda$ acts as a coupling constant between the gravitational field and the electromagnetic field. If the base manifold is four-dimensional, the Kaluza-Klein manifold P is five-dimensional. The fifth dimension is a compact space, and is called the compact dimension. The phenomenon of having a higher-dimensional manifold where some of the dimensions are compact is referred to as compactification.

The above development generalizes in a more-or-less straightforward fashion to general principal G-bundles for some arbitrary Lie group G taking the place of U(1). In such a case, the theory is often referred to as a Yang-Mills theory, and is sometimes taken to be synonymous. If the underlying manifold is supersymmetric, the resulting theory is a supersymmetric Yang-Mills theory.