# What is Kaluza-klein theory: Definition and 15 Discussions

In physics, Kaluza–Klein theory (KK theory) is a classical unified field theory of gravitation and electromagnetism built around the idea of a fifth dimension beyond the common 4D of space and time and considered an important precursor to string theory. Gunnar Nordström had an earlier, similar idea. But in that case, a fifth component was added to the electromagnetic vector potential, representing the Newtonian gravitational potential, and writing the Maxwell equations in five dimensions.The five-dimensional (5D) theory developed in three steps. The original hypothesis came from Theodor Kaluza, who sent his results to Einstein in 1919, and published them in 1921. Kaluza presented a purely classical extension of general relativity to 5D, with a metric tensor of 15 components. 10 components are identified with the 4D spacetime metric, four components with the electromagnetic vector potential, and one component with an unidentified scalar field sometimes called the "radion" or the "dilaton". Correspondingly, the 5D Einstein equations yield the 4D Einstein field equations, the Maxwell equations for the electromagnetic field, and an equation for the scalar field. Kaluza also introduced the "cylinder condition" hypothesis, that no component of the five-dimensional metric depends on the fifth dimension. Without this assumption, terms are introduced that involve derivatives of the fields with respect to the fifth coordinate. This extra degree of freedom is such that the field equations of fully variable 5D relativity grow enormous in complexity. Standard 4D physics seems to manifest the cylinder condition, and the corresponding simpler mathematics.
In 1926, Oskar Klein gave Kaluza's classical five-dimensional theory a quantum interpretation, to accord with the then-recent discoveries of Heisenberg and Schrödinger. Klein introduced the hypothesis that the fifth dimension was curled up and microscopic, to explain the cylinder condition. Klein suggested that the geometry of the extra fifth dimension could take the form of a circle, with the radius of 10−30 cm. Klein also made a contribution to the classical theory by providing a properly normalized 5D metric. Work continued on the Kaluza field theory during the 1930s by Einstein and colleagues at Princeton.
In the 1940s the classical theory was completed, and the full field equations including the scalar field were obtained by three independent research groups: Thiry, working in France on his dissertation under Lichnerowicz; Jordan, Ludwig, and Müller in Germany, with critical input from Pauli and Fierz; and Scherrer working alone in Switzerland. Jordan's work led to the scalar–tensor theory of Brans–Dicke; Brans and Dicke were apparently unaware of Thiry or Scherrer. The full Kaluza equations under the cylinder condition are quite complex, and most English-language reviews as well as the English translations of Thiry contain some errors. The curvature tensors for the complete Kaluza equations were evaluated using tensor algebra software in 2015, verifying results of Ferrari and Coquereaux & Esposito-Farese. The 5D covariant form of the energy-momentum source terms is treated by Williams.

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1. ### I Kaluza-Klein Theory and 5D Einstein Equations

Others are telling me the Einstein Field Equations can work in other dimensions other than 4D (3D space + 1D time). How true is it? So I'd like to ask for clarifications. I googled about it and found one reference for example: Kaluza–Klein theory - Wikipedia I assume the Einstein equations is...
2. G

### A Metric Ansatz For Unifying All Forces In 11D?

The ansatz for the 5D metric is $$G_{\mu \nu}= g_{\mu \nu}+ \phi A_{\mu} A_{\nu},$$ $$G_{5\nu} = \phi A_{\nu},$$ $$G_{55} = \phi.$$ This information was extremely enlightening for me, but what's the analogous...
3. ### I Can Muons or Tau Particles Be Explained by Kaluza-Klein Towers?

- Does anyone know , why for instance a muon or a tau , can't be considered as Kaluza-Klein tower(s) ? M.
4. ### A Does String Theory incorporate Kaluza Klein? How?

From time to time, I point to string theoretists that they should have considered more seriously to use Kaluza-Klein theory and they invariably answer me "we do", and move forward. So I am starting to thing that perhaps I am wrong and I have missed some developing of the theory the the XXIth...
5. ### B Can Kaluza-Klein theory accommodate magnetic charge?

Can Kaluza-Klein theory accommodate magnetic charge? If so is there a simple geometric difference between electric and magnetic charge in such a theory? Thanks!
6. ### B Kaluza-Klein Theory in 2D Space-Time: Exploring Implications

Should one be able to take the results of 5D Kaluza-Klein theory and turn it into a theory that works with two less space dimensions? Thanks!
7. ### B Energy Stored in Charged Capacitor: Kaluza-Klein Theory

In terms of Kaluza-Klein theory how does one view the stored electromagnetic energy between the plates of a charged capacitor. Thanks!
8. ### Mathematica Kaluza-Klein Reduction Via Mathematica

Does any know how to perform Kaluza-Klein reduction via mathematica? The task becomes very tedious if the job is done manually when successive reductions are done.
9. ### How Did Clifford's Theories Influence Modern Physics and String Theory?

http://en.wikipedia.org/wiki/Kaluza–Klein_theory ##http://link.springer.com/article/10.1007/BF01390677 (original german paper, Ich kan nicht Deutsche) http://www.scientificexploration.org/journal/jse_21_3_beichler.pdf (this author makes some interesting arguments) Also, a lot (if not all...
10. ### Kaluza-Klein Theory Q&A: Inducing EM Field & Ricci Tensor

I know this is technically not orthodox GR, but it is closely related. I was reading about Kaluza-Klein theory, but I don't completely understand it. Under what circumstances would could a gravitational field induce an electromagnetic field under this theory. Also, how is the Ricci tensor different?
11. ### Connections/geometry in Kaluza-Klein theory

In Kaluza-Klein theory one introduce an extra fifth spatial dimension, to the usual four-dimensional manifold ##M^4## in General Relativity called space-time. This extra dimension has a symmetry; i.e. it is spanned by a Killing vector and it's taken to be compact. One views this extra dimension...
12. ### Mathematical formulation of Kaluza-Klein theory

Good morning, what is the mathematical formalism used for a rigorous formulation of the Kaluza-Klein theory? Is there any reference? Thanks!
13. ### Gravitation and electromagnetism is unified by Kaluza-Klein theory

Recently I read that gravitation and electromagnetism is unified by Kaluza-Klein theory. Can someone tell me in depth how they are unified
14. ### Kaluza-Klein Theory: Exploring Its Merits & Drawbacks

I'd never heard of Kaluza-Klein theory before today, and from what I've read I think that's rather odd. Taking it from a purely non-quantum, relativistic unification of gravity and electromagnetism, what's wrong with it? I know it has been subsumed into other, more sophisticated theories, but on...
15. ### Kaluza-Klein Theory and spatial curvature

In Kaluza-Klein theory, the gauge symmetries for all the fundamental forces are mapped onto the higher spatial dimensions. So the internal symmetries are now externalised. Does this imply that you can extend the analogy with gravity further: so for example, if the 5th dimension contains...