A Case for Unified Theory?
For some time now, superstring theory has been cited as a candidate for a unified theory in physics. Superstring theory is a theory of forces, including gravity, and matter, in which everything can be described by "strings." The reason for using such a theory is "the desire to think about the entire universe simply," and I would like to explain this ambitious desire below.
Most microscopic phenomena in the natural world can be explained by the Standard Model of particle physics. This model includes dozens of types of elementary particles, such as the electrons that make up matter and the photons that mediate electromagnetic force, as well as dozens of parameters representing their masses and the strengths of forces. Many people, frustrated by the complexity of the model that arises from its high explanatory power, think, "I want a simpler model."
Furthermore, there is dissatisfaction that this model can only explain about 5% of the entire universe. The other 95% of the universe is of unknown nature, and you may have heard the terms dark matter and dark energy. Dark matter is thought to be the source of gravity that forms galaxies, which contain stars, and galaxy clusters, which are collections of galaxies. It is also known that the volume of our current universe is undergoing accelerated expansion over time. Compared to gravity, which is a universal attractive force, dark energy could be described as a "universal repulsive force" that pushes the entire universe to accelerate its expansion. The reason these components, which are crucial to the universe (and to us), remain unidentified is likely because we observe them through the weak force of gravity. (For example, comparing the gravitational and electromagnetic forces between two electrons, their magnitudes differ by a factor of more than 10 40 .) While general relativity is thought to be correct for gravity on a macroscopic scale, tracing the expanding universe back in time suggests that the early universe was extremely small. If we want to understand the entire universe, we need to explore the microscopic properties of gravity, but these are not well understood due to its weakness. This means we do not even know if only the known elementary particles existed in the early universe, or if space was three-dimensional as it is today. However, if this "darkness" exists, the universe likely contains many unknown degrees of freedom. Since a universe containing numerous known elementary particles, gravity, and the unidentified (and numerous) "dark" components is complex, many people ambitiously desire "a simple, understandable principle of the universe."
Independently of the observations mentioned above, there is superstring theory—a theory that might allow us to understand everything the universe could contain as strings—and it may be human nature to jump at it. In superstring theory, elementary particles are tiny strings, and the distinctions between gravity and different particles arise from differences in how the strings are wound or vibrate. This means that, just like known elementary particles, candidates for gravity and the "dark" components can also be handled, which seems to solve everything... or so one might think. However, due to the high degrees of freedom, an infinite number of ground states for the theory emerge. At the risk of being misunderstood, this means countless parallel universes appear, and we do not know which one is our universe, or even if there is only one. This problem is thought to stem from our inability to solve the complex (seemingly unsolvable) equations of motion.
The current situation is that this ambitious and fascinating unified theory, precisely because of its simplicity, has become complex by encompassing too broad a range of phenomena. How to reconsider research methods and directions, and whether a unified theory can be verified, are likely challenges for the future.