String theory proposes the existence of extra dimensions beyond the familiar three spatial dimensions (length, width, and height) and one time dimension. These extra dimensions are a key feature of string theory and are often referred to as "compactified dimensions." The exact number of extra dimensions in string theory depends on the specific formulation and version of the theory.
In most versions of string theory, including the popular ten-dimensional superstring theory, there are six compactified spatial dimensions in addition to the three observable spatial dimensions and one time dimension. These compactified dimensions are curled up and have a small size on scales that are currently inaccessible to our observations. These extra dimensions are often visualized as being "curled" or "wrapped" into intricate shapes called Calabi-Yau manifolds.
However, in some formulations of string theory, such as M-theory, which is an extension of superstring theory, there can be a total of eleven dimensions. In M-theory, the extra dimension is considered to be an additional spatial dimension, bringing the total to ten spatial dimensions and one time dimension. The nature and properties of this 11th dimension are still the subject of ongoing research and exploration in theoretical physics.
It's important to note that the existence of these extra dimensions in string theory is a mathematical construct that helps explain certain aspects of the theory. Since these dimensions are compactified and currently beyond our experimental reach, they are not directly observed or experienced in our everyday lives.