The evidence for strings in physics or cosmology is indirect and primarily based on theoretical arguments rather than direct observational or experimental evidence. String theory is still a highly speculative and active area of research, and its definitive experimental verification remains an open question. However, here are some lines of evidence and motivations that support the study of string theory:
Consistency with quantum mechanics: String theory naturally incorporates quantum mechanics and provides a framework that reconciles quantum mechanics with general relativity. This is important because both quantum mechanics (which governs the behavior of particles at the microscopic scale) and general relativity (which describes gravity and the behavior of spacetime) have been independently confirmed by a wide range of experiments.
Resolution of the ultraviolet divergences: String theory has the potential to resolve some of the mathematical difficulties encountered in other quantum field theories, such as the ultraviolet divergences (infinities) that arise when trying to calculate certain physical quantities. The finiteness of string theory calculations suggests that it may provide a consistent and self-contained mathematical framework.
Explanation of particle properties: String theory offers the possibility of explaining the properties of elementary particles, such as their masses and charges, by the vibrational patterns and modes of the strings. The specific vibrational states of strings could correspond to different types of particles, giving rise to the diverse spectrum of particles observed in nature.
Unification of fundamental forces: String theory aims to unify all the fundamental forces of nature, including gravity, electromagnetism, the strong nuclear force, and the weak nuclear force. By treating these forces as different aspects of string vibrations, it offers the potential for a unified description of all fundamental interactions.
Connection to black hole physics: String theory provides insights into the behavior of black holes and has been successful in reproducing some of the known properties of black holes, such as their entropy and Hawking radiation. This connection between string theory and black hole physics suggests that string theory may shed light on the fundamental nature of black holes and gravity.
It is important to note that while these arguments provide motivations for studying string theory, they do not constitute direct empirical evidence. Experimental verification of string theory remains a significant challenge due to the extremely high energy scales involved, which are currently beyond our technological capabilities. As a result, string theory remains a subject of active research and debate within the physics community.