Hidden Dimensions to the Universe: Kaluza and Klein introduced the idea.

The article reviews early proposals by Theodor Kaluza (1919) and Oskar Klein (1926) that added a compact extra spatial dimension to link gravity and electromagnetism and explains the idea of dimensions curled up at about the Planck scale; it also sets up a series that will ask whether larger extra dimensions might be possible.

Physicists have long taken the possibility of extra spatial dimensions seriously enough to test it in theory and experiment. Theodor Kaluza in 1919 showed that adding one extra spatial dimension to general relativity can produce equations that include electromagnetism alongside gravity. In 1926 Oskar Klein proposed that any extra dimension could be compactified — curled up so tightly that it is effectively invisible at everyday scales. This piece introduces that history and frames a follow-up series asking whether extra dimensions could be larger than the Planck scale and what that would imply. Key points: - In 1919 Theodor Kaluza proposed adding an extra spatial dimension to unify gravity and electromagnetism in a single set of equations. - In 1926 Oskar Klein suggested extra dimensions could be compactified, curled up so small that they are not observed in daily life. - The compactification scale discussed is extremely small, on the order of the Planck length (around 10^-33 centimeters). - Kaluza–Klein ideas later reappeared in string theory, which typically requires many compact extra dimensions (often cited as 10 or 11 total dimensions). - The article distinguishes those tiny, Planck-scale dimensions from the subject of the series, which will consider the possibility of larger extra dimensions and their implications. Summary: Kaluza and Klein offered a simple conceptual route to unify forces by adding a compact extra dimension, and their ideas have influenced decades of theoretical work. The question of whether extra dimensions could be larger than the Planck scale remains open, and subsequent pieces in the series will explore what such larger dimensions might do and how they might be tested.