To make the string theory mathematically consistent, the universe the strings exist in must have ten dimensions. This contradicts the experience that our real universe has four dimensions: three space dimensions (height, width, and length) and one time dimension. To “save” their theory, string theorists therefore added the explanation that the additional six dimensions exist but cannot be detected directly; this was explained by sophisticated mathematical objects called Calabi–Yau manifolds. The number of dimensions was later increased to 11 based on various interpretations of the 10-dimensional theory that led to five partial theories, as described below. Supergravity theory also played a significant part in establishing the necessity of the 11th dimension.
