This in an excerpt from this book
String theory corresponds to a theoretical (As the name implies) framework introduced in physics. This framework share similarities with particle physics, as it presents point like particles (from particle physics) as one-dimensional objects, these objects are said to be string. They are stored in the framework described above. String theory describes the behavior of these strings through space. It also determines how these strings interact with each other. It a string is observed on a larger scale than standard string scale, it may look very similar to any ordinary particle. The particle that is being observed will hold mass, charge and other string properties that are determined according t their vibrational state. Graviton is corresponded by one of many vibrational states of string theory. Graviton is simply said to be a quantum mechanical particle, which is responsible for carrying gravitational force. According to this statement, it can be said that string theory focuses mainly on quantum gravity thus it is a theory of quantum gravity.
The main goal of string theory is an attempt in order to address in-depth questions about fundamental physics. It is said to be a broad subject that holds a wide range of concepts. Concept of black hole, cosmology, nuclear physics, matter physics etc are some of those concepts in which string theory is applied in order to solve a variety of questions. In the field of pure mathematics, many developments are carried out successfully by the application of string theory. String theory is said to be a theory for everything, since it contains descriptive concepts of gravity and particle physics. It literally covers all fundamental forces and concept of matter physics, which makes it a self-contained mathematical structure.
Despite the theory being too much descriptive you may face certain challenges while trying to practically apply the theory. One of the major reasons behind these problems is that this theory does not contain a perfect definition that can be applied in all the situations and circumstances. It becomes complicated to develop particle physics theory based on string theory, since string theory describes a number of landscapes of possible universes. This fact can be determined as another limitation in application of string theory. As a response to these limitations, many communities criticize this theory for its various approaches in the field of physics. Many theorists and experts also question about the legality of the research that is being continued while using string theory as a base.

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