You need to upgrade your knowledge of five-dimensional theory of relativity.

“It appears that the union of gravitation and Maxwell’s theory is achieved in a completely satisfactory way by the five-dimensional theory (Kaluza-Klein).”

(Einstein to H. A. Lorentz, 16 February 1927)

“Kaluza's roundabout way of introducing the five dimensional continuum allows us to regard the gravitational and electromagnetic fields as a unitary space structure”

Einstein, A. & Bergman, P., On a Generalization of Kaluza's Theory of Electricity. In: Modern Kaluza-Klein Theories. Menlo Park: Addison-Wesley, p. 93.

T. Kaluza, Zum Unitatsproblem der Physik, Sitz. Preuss. Akad. Wiss. Phys.

Math. K1 (1921) 966

O. Klein, Quantentheorie und funfdimensionale Relativitatstheorie, Zeits.

Phys. 37 (1926) 895

In 1921, T. Kaluza showed that the gravitational and electromagnetic fields stem from a single universal tensor and such an intimate combination of the two interactions is possible in principle, with the introduction of an additional spacial dimension.

In 1926, Oscar Klein provided an explanation for Kaluza’s fifth dimension by proposing it to have a circular topology so that the coordinate y is periodic i.e., 0 ≤ y ≤ 2πR, where R is the radius of the circle S1. Thus the global space has topology R4× S1.

Kaluza-Klein compactification: although there are four space dimensions, one of the space dimensions is compact with a small radius.

Theodor Kaluza and Oscar Klein were able to recover four dimensional gravity as well as Maxwell’s equations for a vector field.

The extra space dimension somehow had collapsed down to a tiny circle "smaller than the smallest atom".

"Klein theorized that Kaluza's new dimension likely had somehow collapsed down to the "Planck length" itself -- supposedly the smallest possible size allowed by these fundamental interactions: 10-33 cm."

"Kaluza and Klein showed that this extra dimension would still have an effect on the space around us. In particular they showed that the effect of gravity in that very small fifth dimension would actually appear to us, from our larger-scale perspective, as electromagnetism."