Are there 11 dimensions or 12 dimensions in string theory?

HoloGPT: String theory is a theoretical framework in which the point-like particles of particle physics are replaced by one-dimensional strings. The number of dimensions can be confusing because there are different versions of string theory. The two most prominent are:

Bosonic String Theory: The simplest version, but not a complete theory of the universe. It has 26 spacetime dimensions (25 spatial + 1 time). No longer considered viable for describing our universe, but a useful starting point.

Superstring Theory: More advanced, incorporating supersymmetry, which relates bosons and fermions. It can be divided into five versions, but they all require ten spacetime dimensions (9 spatial + 1 time). These consist of the familiar four dimensions of spacetime plus six additional compactified dimensions hidden or curled up at extremely small scales — often described as “Calabi-Yau manifolds.”

There are also more speculative versions such as M-theory, which suggests an 11-dimensional spacetime. M-theory unifies the different superstring theories and introduces an eleventh dimension that plays a key role but is not necessarily an extra spatial dimension in the same way as the others. So, depending on the version, you can have either 10 or 11 dimensions in spacetime.

Explain a compactified spatial dimension.

HoloGPT: A compactified spatial dimension is one that is hidden or “curled up” at extremely small scales, making it effectively imperceptible in our everyday macroscopic world. Consider an analogy: a drinking straw. From a distance it appears one-dimensional — a line with no width. But zoom in and it has two dimensions: length and circumference. The circumference is “compactified,” wrapped around and hidden within the small cross-section of the straw.

In string theory and related theories, it’s proposed that our universe may have extra spatial dimensions beyond the familiar three, compactified in a similar way — curled into tiny shapes at scales far smaller than subatomic particles. They are introduced for the unification of forces, the resolution of theoretical issues in quantum gravity, and mathematical consistency, and are typically represented using constructs like Calabi-Yau manifolds or tori.