In string theory, strings are considered fundamental objects that cannot be broken or destroyed in the usual sense. They are one-dimensional entities with no substructure, meaning they do not have smaller constituents. The concept of breaking a string into smaller pieces is not applicable within the framework of string theory.
However, strings in string theory can undergo various processes and interactions that might be described as "splitting" or "joining" of strings. These processes involve the interaction and exchange of energy and momentum between strings, leading to their transformation or combination.
For example, in certain scattering processes, two strings can approach each other, interact, and then separate again. The interaction can change the properties and behavior of the strings involved, but it does not result in the destruction or annihilation of the strings themselves.
It's important to note that the behavior of strings in string theory is governed by the laws and equations of the theory, which incorporate quantum mechanics and the principles of quantum field theory. These laws describe the dynamics of string vibrations and interactions and determine the possible outcomes of string processes.
While strings in string theory are generally considered to be indestructible in the conventional sense, it is crucial to remember that our understanding of string theory is based on theoretical models and mathematical formulations. Direct experimental evidence for strings or their properties is currently lacking, and further research and advancements are needed to fully understand the behavior of strings within the context of string theory.