String theory is a theoretical framework in which the point-like particles of particle physics are replaced by one-dimensional objects called strings. It describes how these strings propagate through space and interact with each other. On distance scales larger than the string scale, a string looks just like an ordinary particle, with its mass, charge, and other properties determined by the vibrational state of the string.
String theory was first proposed in the late 1960s as a theory of the strong nuclear force. However, it was soon abandoned in favor of quantum chromodynamics. In the 1980s, the theory was revived as a possible theory of everything, a single theory that could describe all of the fundamental forces of nature. The theory has undergone a number of revolutions, and it is now a vast and complex subject with many different branches.
String theory is a controversial subject in the physics community. Some physicists believe that it is the most promising candidate for a theory of everything, while others are skeptical of its claims. The theory has not yet been experimentally verified, and it is not clear whether it will ever be possible to do so. However, the theory has had a major impact on mathematics, and it has led to a number of important discoveries in that field.