Physicists are not necessarily "sure" about the existence of a single unified theory, often referred to as the theory of everything. Instead, they have a strong motivation and desire to discover such a theory. Here are some reasons why physicists are driven to seek a theory of everything:
The pursuit of simplicity and elegance: Physicists have observed that the laws of nature seem to possess underlying simplicity and elegance. They believe that a fundamental theory should be able to explain a wide range of phenomena with a small set of fundamental principles and equations. The idea of a unified theory is appealing because it would provide a simple and elegant framework to describe the entire universe.
Unification of fundamental forces: Currently, our understanding of the universe is described by two major theoretical frameworks: the theory of general relativity, which explains gravity on large scales, and the Standard Model of particle physics, which describes the electromagnetic, weak, and strong nuclear forces. Physicists aim to unify these two frameworks into a single, consistent theory that can explain all the fundamental forces of nature.
Explanation of fundamental particles and interactions: The Standard Model successfully describes the behavior of a wide variety of particles and their interactions. However, it leaves unanswered questions such as why there are so many different particles and why their masses and properties take specific values. A theory of everything could potentially provide answers to these questions by revealing a deeper underlying structure of the universe.
Resolution of quantum gravity: General relativity, which describes gravity, is a classical theory, while the Standard Model is a quantum theory. Physicists believe that a theory of everything should unify these two frameworks into a consistent theory of quantum gravity. This would enable us to understand the behavior of the universe at the most fundamental level, including the physics of black holes and the very early moments of the Big Bang.
Despite these motivations, it is important to note that a theory of everything remains elusive, and physicists are actively engaged in research to uncover its nature. The pursuit of such a theory is a challenging task that requires theoretical insights, experimental data, and potentially revolutionary breakthroughs. The final verdict on the existence and nature of a theory of everything will ultimately depend on the progress of scientific discovery.