Superstring Theory Predicts a 10-Dimensional Universe
Humans tend to perceive the world as existing in three dimensions: length, width, and depth. But some physicists believe that not only does time constitute a fourth dimension, but that the mathematics of superstring theory requires the presence of ten distinct dimensions in our universe.
Physicists tend to conceive of a four-dimensional spacetime in which time is just another dimension, rather than a three-dimensional space that is modulated by time. In other words, time is a dimension that operates in the same way as height or width. As a result, different times are analogous to different locations; although we experience time as "moving forward," predominant theories contend that the past, present, and future are all equally in existence, and our consciousness is present at a specific point in time. But just as places do not cease to exist when one leaves them, the past and future are not lost or imaginary simply because we are located in the present.
A four-dimensional universe is mostly accepted by the scientific community, but many scientists claim that the equations of superstring theory do not work, meaning they fail to unify quantum mechanics and general relativity, as well as explain all the forces of the universe, unless there are ten dimensions. According to superstring theorists, the fifth and sixth dimensions consist of possible worlds that are different from ours to varying degrees, except that they all start with the same initial conditions, as in the Big Bang. In the seventh and eighth dimensions, the worlds start out with different initial conditions, but retain the same basic physical laws of the universe. The ninth dimension covers all possible worlds with all possible initial conditions and laws of physics, and the tenth, theoretically, encompasses all possible variations.
Physicists have proposed two explanations for the apparent contradiction between this 10-dimensional superstring theory and the three or four-dimensional world we experience. First, it's possible that the extra dimensions are compactified to the point that they would elude detection by present-day technology. In that case, they would be "curled up" in the form of a Calabi–Yau manifold, a six-dimensional space that is smaller than our currently observable lengths. But since strings vibrate through all dimensions, the properties of the elementary particles that we observe would be affected by the "curled up" strings. The other possibility is that of a "brane world" scenario, or the notion that humans occupy a four-dimensional subspace of the universe and are unable to access the rest of the universe, which is ten-dimensional.