In 1905, Albert Einstein showed in his Special Theory of Relativity that space is intimately connected to time via the cosmic speed limit of light and so, strictly speaking, we live in a Universe with four dimensions of space-time. For everyday purposes however, we think of the Universe in three dimensions of space (north-south, east-west, up-down) and one dimension of time (past-future). In that case, a fifth dimension would be an extra dimension of space.
Such a dimension was proposed independently by physicists Oskar Klein and Theodor Kaluza in the 1920s. They were inspired by Einstein’s theory of gravity, which showed that mass warped four-dimensional space-time.
Since we’re unable to perceive these four dimensions, we attribute motion in the presence of a massive body, such as a planet, not to this curvature but to a ‘force’ of gravity. Could the other force known at the time (the electromagnetic force) be explained by the curvature of an extra dimension of space?
Kaluza and Klein found it could. But since the electromagnetic force was 1,040 times stronger than gravity, the curvature of the extra dimension had to be so great that it was rolled up much smaller than an atom and would be impossible to notice. When a particle such as an electron travelled through space, invisible to us, it would be going round and round the fifth dimension, like a hamster in a wheel.
Kaluza and Klein’s five-dimensional theory was dealt a serious blow by the discovery of two more fundamental forces that operated in the realm of the atomic nucleus: the strong and weak nuclear forces.
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 tiny ‘strings’ of mass-energy. To mimic all four forces, the strings vibrate in 10-dimensional space-time, with six space dimensions rolled up far smaller than an atom.
String theory gave rise to the idea that our Universe might be a three-dimensional island, or ‘brane’, floating in 10-dimensional space-time. This raised the intriguing possibility of explaining why gravity is so extraordinarily weak compared with the other three fundamental forces. While the forces are pinned to the brane, goes the idea, gravity leaks out into the six extra space dimensions, enormously diluting its strength on the brane.
There is a way to have a bigger fifth dimension, which is curved in such a way that we don’t see it, and this was suggested by the physicists Lisa Randall and Raman Sundrum in 1999. An extra space dimension might even explain one of the great cosmic mysteries: the identity of ‘dark matter’, the invisible stuff that appears to outweigh the visible stars and galaxies by a factor of six.
In 2021, a group of physicists from Johannes Gutenberg University in Mainz, Germany, proposed that the gravity of hitherto unknown particles propagating in a hidden fifth dimension could manifest itself in our four-dimensional Universe as the extra gravity we currently attribute to dark matter.
Though an exciting possibility, it’s worth pointing out that there’s no shortage of possible candidates for dark matter, including subatomic particles known as axions, black holes and reverse-time matter from the future!
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Albert Einstein's groundbreaking work in 1905 with his Special Theory of Relativity laid the foundation for our comprehension of space and time. The integration of these concepts into a four-dimensional space-time framework became a cornerstone of modern physics. For everyday cognition, we simplify this model into three spatial dimensions and one temporal dimension.
The idea of an additional spatial dimension emerged in the 1920s, courtesy of physicists Oskar Klein and Theodor Kaluza. They drew inspiration from Einstein's gravitational theory, proposing that electromagnetism could be explained by the curvature of a fifth dimension. This additional dimension, however, had to be compactified, rolled up into a tiny scale beyond our perceptual reach, to account for the observed strength disparity between electromagnetic and gravitational forces.
The Kaluza-Klein theory faced setbacks with the discovery of the strong and weak nuclear forces, diverting attention from extra dimensions. String theory, a revolutionary concept that views particles not as points but as vibrating strings in 10-dimensional space-time, revived the idea of additional dimensions.
String theory posits that our three-dimensional universe may exist as a 'brane' floating in higher-dimensional space. This framework offers a potential explanation for the apparent weakness of gravity compared to other fundamental forces, attributing this discrepancy to the leakage of gravity into extra dimensions.
In 1999, physicists Lisa Randall and Raman Sundrum introduced the idea of a larger fifth dimension, curved in a way imperceptible to us. This concept holds promise for addressing cosmic mysteries such as the nature of dark matter, which outweighs visible matter in the universe.
Fast forward to 2021, where physicists from Johannes Gutenberg University proposed that gravity from undiscovered particles in a hidden fifth dimension could contribute to the elusive dark matter. While this proposal adds an exciting dimension to our understanding, it's essential to acknowledge the plethora of alternative dark matter candidates, including axions, black holes, and exotic matter from the future.
In conclusion, the journey through the realms of extra dimensions, string theory, and potential connections to dark matter showcases the evolving landscape of theoretical physics, where imagination and empirical exploration coalesce to deepen our grasp of the fundamental nature of the universe.
