Has it been experimentally disproven or are other reasons for its apparent obscurity?Although this allows the construction of a string theory in 4 spacetime dimensions, such a theory usually does not describe a Lorentz invariant background.
String theory in 4 dimensions is a theoretical framework in which the point-like particles of particle physics are replaced by one-dimensional objects known as strings. These strings can vibrate at different frequencies, and their vibrational modes correspond to different particles. The "4 dimensions" refer to the three spatial dimensions and one time dimension that we experience in our everyday lives.
Higher-dimensional string theories, such as those with 10 or 11 dimensions, include additional spatial dimensions beyond the familiar three. These extra dimensions are often compactified or curled up in such a way that they are not observable at low energies. String theory in 4 dimensions focuses on scenarios where the extra dimensions are not explicitly considered, simplifying the mathematical framework but often at the cost of losing some of the richer structure provided by higher dimensions.
One of the main challenges is that string theory naturally predicts a higher number of dimensions (usually 10 or 11). Reducing these to 4 dimensions while maintaining consistency and the correct physical properties is non-trivial. This often involves complex mechanisms like compactification on Calabi-Yau manifolds or flux compactifications to hide the extra dimensions. Additionally, ensuring that the theory matches observed physical phenomena, such as the Standard Model of particle physics, adds another layer of complexity.
In principle, string theory aims to be a "theory of everything" that unifies all fundamental forces, including gravity, electromagnetism, and the strong and weak nuclear forces. However, achieving this in a 4-dimensional framework is particularly challenging. While string theory provides a promising approach to unification, a fully consistent and experimentally verified 4-dimensional model that includes all known forces and particles has not yet been formulated.
As of now, there is no direct experimental evidence that supports string theory in any number of dimensions, including 4. String theory remains a highly theoretical construct with many compelling mathematical features, but its predictions are often at energy scales that are currently inaccessible to experiments. Researchers continue to look for indirect evidence and develop new ways to test the theory, but a definitive experimental validation is still lacking.