String theory is a theoretical framework that attempts to unify quantum mechanics and general relativity by describing elementary particles as tiny vibrating strings. In some formulations of string theory, additional spatial dimensions beyond the familiar three (length, width, and height) are proposed.
According to certain versions of string theory, there are six extra spatial dimensions, making a total of nine dimensions in our universe. These additional dimensions are compactified, which means they are curled up and "hidden" at extremely small scales, much smaller than we can currently probe experimentally.
The specific shape and size of these extra dimensions can take various forms in different string theory models. The most commonly considered compactification scenario is known as the Calabi-Yau shape, where the extra dimensions are curled up into a six-dimensional manifold with a complex geometric structure.
The reason we don't perceive these extra dimensions in our everyday experience is due to their tiny size and the fact that they are compactified. The extra dimensions are thought to be on the order of the Planck length (about 10^(-35) meters), which is far smaller than any length scale we can currently probe with experiments.
In our macroscopic world, the effects of these extra dimensions are not directly observable because they are "curled up" and do not significantly affect the familiar three dimensions of space. The behavior of particles and forces in our observable universe is effectively described by the interactions occurring within the three large dimensions.
However, the presence of these extra dimensions can have indirect effects on the properties of particles and forces that we do observe. For example, the number of particle types and their masses, as well as the strengths of fundamental forces, can be influenced by the geometry and properties of the extra dimensions in string theory.
It is important to note that string theory is still an active area of research, and the exact details and predictions related to the extra dimensions are not yet fully resolved. The topic of extra dimensions and their role in string theory continues to be a subject of investigation, and scientists are exploring various theoretical and experimental approaches to test the predictions of these theories.