In string theory, which is a theoretical framework that attempts to describe the fundamental particles and forces in the universe, there are multiple dimensions beyond the three spatial dimensions (length, width, and height) that we are familiar with. These additional dimensions are compactified or curled up, meaning they are tiny and not directly observable in our everyday experience.
In the original formulation of string theory, there were ten dimensions, with nine spatial dimensions and one dimension of time. However, a significant development in the late 20th century called "string dualities" revealed that different formulations of string theory are actually different descriptions of the same underlying theory. This led to the realization that string theory is part of a more encompassing framework known as M-theory, which incorporates 11 dimensions.
In M-theory, the fundamental objects are not just one-dimensional strings but also two-dimensional membranes (referred to as "branes"). These branes can have different dimensions, and depending on the particular configuration, the dimensions can be compactified in various ways.
To answer your question about the fastest dimension in string theory, it's important to note that the concept of speed or velocity as we understand it in our everyday experience does not directly apply to these extra dimensions. In string theory, the behavior of particles and their interactions are described by the vibrations of strings or the dynamics of branes.
However, within the framework of string theory, there is a notion of an "energy scale" associated with each dimension. The energy scale determines the size and properties of that dimension. Generally, dimensions with higher energy scales are considered to be "larger" and dimensions with lower energy scales are considered to be "smaller" or more compact.
In some formulations of string theory, it is suggested that the dimension with the highest energy scale (corresponding to the largest or least compact dimension) may play a special role. This dimension is sometimes referred to as the "longitudinal" dimension, and it is associated with the motion of strings or branes along their length.
However, it's important to note that the concept of speed in the context of string theory is more intricate and involves the dynamics of the theory as a whole, rather than individual dimensions. The idea of a "fastest" dimension, in the sense of a dimension with the highest speed, is not a well-defined concept within the framework of string theory.