In its most widespread modification, string theory claims that the universe exists in ten dimensions, but six of them we cannot perceive. What might these extra dimensions look like?
When someone says "other dimensions", most often you think about such things as parallel universes - alternative realities that exist parallel to ours, in which the world is arranged somewhat or completely differently. However, the reality of measurements and the role in the structure of the universe are very different from such a popular understanding.
In a nutshell: dimensions are different facets of what we perceive as reality. We are well aware of the three spatial dimensions that we encounter and in which we live every day. They determine the length, height and depth of all objects in the Universe (and correspond to the x, y, z coordinate axes).
However, some scientists believe that, in addition to the three visible dimensions, there may be others. According to the basics of string theory, the universe exists in ten different dimensions. Recently we published a material on how these additional, not perceived by us, dimensions can be twisted, compactified, - you can read it at this link. Thus, these different aspects determine the fundamental forces of nature and all elementary particles in the universe.
Let's start in order. The first dimension, as we have already noted, determines the length (x-axis). It is convenient to describe a one-dimensional object by a straight line that exists only within the framework of the concept of length and has no other distinctive features. If you add a second dimension to it, the y-axis, or height, you get a two-dimensional object (for example, a square).
The third dimension characterizes the depth (z-axis) - it gives all objects the concept of area and cross-section. The ideal example would be a cube: it exists in three dimensions - it has length, height and depth, and therefore volume.
The fourth dimension is time, and this can already be called the classical, generally accepted understanding of it. It is an inseparable part of the space-time continuum. It determines the properties of all known matter at any given time. Along with three other dimensions, in order to determine the position of an object in the Universe, it is necessary to know its position in time. So, these four dimensions define our reality - the Universe, to which we are accustomed and which we understand to one degree or another.
In addition to the dimensions described above, there are seven more that are not so obvious, but can still be perceived by direct impact on the universe and reality as we know it. Other, additional dimensions are associated with deeper possibilities. Physicists are faced with serious questions when trying to explain their interactions with four "basic" dimensions.
According to superstring theory, the concept of possible worlds arises in the fifth and sixth dimensions. If we could perceive the fifth dimension, we would see a world that is somewhat different from what we are used to. We would be able to measure the similarities and differences between possible worlds and ours.
In the sixth dimension, we would see the plane of possible worlds, where we could compare and determine the location of all possible universes that began under the same conditions as ours (that is, the Big Bang).Theoretically, if we managed to master the fifth and sixth dimensions, we could move into the past or into different variations of the future.
In the seventh dimension, we would have access to possible worlds that originated under different initial conditions. Whereas in the fifth and sixth dimensions the initial conditions were the same and the consequences were different, in this dimension everything is different from the very beginning of time. The eighth dimension also opens access to the plane of such possible universes, each of which began under different conditions. These universes branch out endlessly, which is why they are called infinities.
In the ninth dimension, we have the opportunity to compare the histories of all possible universes that originated under all possible laws of physics and initial conditions. Finally, in the tenth dimension, we find ourselves at the point where everything possible and imaginable is open. Beyond that, such limited beings as we are unable to imagine anything, which makes this dimension a natural limitation of what we can comprehend in this regard.
The existence of these additional six dimensions, which we cannot perceive, is necessary for string theory: they naturally follow from mathematical calculations and models of the theory, and therefore describe the Universe within the framework of this theory. The fact that we perceive only four dimensions of space-time can be explained by one of two mechanisms: either the extra dimensions are compactified on a very small scale, or we live in a three-dimensional submanifold - a kind of brane that limits all known particles, excluding gravity (theory bran).
If the extra dimensions are indeed compactified, they must exist as so-called Calabi-Yau manifolds. Despite the fact that they are inaccessible for perception by our senses, in this case they would determine the formation of the Universe from the very beginning. This is why scientists believe that looking back in time with telescopes and observing light from the early universe is likely to help them see how the existence of these extra dimensions could have influenced the evolution of the cosmos.
As one of the candidates for a theory of everything, arguing that the universe consists of ten dimensions (or more, depending on which theory you are talking about), string theory tries to reconcile the Standard Model of particle physics with General Relativity (the theory of gravity). In essence, this is an attempt to explain and describe how all the known forces of the universe interact and how other possible universes can be arranged.