In 1905, Albert Einstein showed in his special theory of relativity that space is closely related to time through the cosmic limit of the speed of light, and therefore, strictly speaking, we live in a universe with four dimensions of space-time.
However, for everyday purposes, we represent the universe in three dimensions of space (north-south, east-west, up-down) and one dimension of time (past-future). In this case, the fifth dimension will be an additional dimension of space.
Such a measurement was independently proposed by physicists Oscar Klein and Theodor Kaluza in the 1920s. They were inspired by Einstein's theory of gravity, which showed that mass bends four-dimensional spacetime.
Since we cannot perceive four dimensions, we attribute motion in the presence of a massive body such as a planet to the "force" of gravity.
Could another force known at that time (electromagnetic force) be explained by the curvature of the extra dimension of space? Kaluza and Klein discovered that this was possible.
But since the electromagnetic force is 1040 times stronger than gravity, the curvature of the extra dimension would have to be so great that it would be curled up into a tiny ring much smaller than an atom, and it would be impossible to notice it.
When a particle, such as an electron, travels in space invisible to us, it will revolve around the fifth dimension, like a hamster in a wheel.
The five-dimensional theory of Kaluza and Klein suffered a serious blow as a result of the discovery of two more fundamental forces acting in the region of the atomic nucleus: strong and weak nuclear interactions.
But the idea that extra dimensions explain forces was revived half a century later by proponents of "string theory," which views the fundamental building blocks of the universe not as particles, but as tiny "strings" of mass-energy. To mimic all four forces, the strings vibrate in 10-dimensional space-time, with the six dimensions coiled into a size much smaller than an atom.
String theory gave rise to the idea that our universe could be a three-dimensional island or "brane" (a hypothetical fundamental multidimensional physical object less than the dimension of the space in which it is located) floating in 10-dimensional space-time.
This opened up an intriguing opportunity to explain why gravity is so weak compared to the other three fundamental forces. While the forces are attached to the brane, the idea is that gravity seeps into six additional spatial dimensions, greatly weakening its strength on the brane.
There is a way to get a larger fifth dimension that is curved in such a way that we cannot see it, and this was proposed by physicists Lisa Randall and Raman Sundram in 1999. An extra spatial dimension could even explain one of the great cosmic mysteries: the identity of "dark matter," invisible matter that seems to outweigh visible stars and galaxies by a factor of six.
In 2021, a group of physicists from the Johannes Gutenberg University in Mainz, Germany, suggested that the gravity of hitherto unknown particles propagating in the latent fifth dimension could manifest in our four-dimensional universe as additional gravity, which we currently attribute to dark matter.
It's worth noting that there is no shortage of possible candidates for dark matter, including subatomic particles known as axions, black holes, and backward time matter from the future!
