Is there a theory that could unite all fundamental forces of the Universe, a so-called Theory of Everything? The need for this answer arose in 1970s after the strong force was successfully described in the same theoretical framework with the electroweak interaction, which is the unified electromagnetic force and the weak interaction. There are still unsolved problems with unifying the strong force to the electroweak interaction, but what really poses a problem is the lack of theoretical framework that would unite all four forces.
Two theories that describe these forces, the Standard Model, that describes the strong force and the unified weak and electromagnetic force, and the theory of general relativity, that describes gravitation, are not compatible. The Theory of Everything is the theory in the new field of quantum gravity that would unite all forces into a single theoretical framework. At the moment, no theory has claimed the title “Theory of Everything”, but one candidate sticks out.
In the past few decades, the string theory has emerged as the best way to unite all four forces and become the Theory of Everything. In the string theory, particles are replaced with one-dimensional string as the ultimate elementary building block of the Universe. These strings vibrate or oscillate and it is this oscillation that gives particles their properties. If viewed as one-dimensional strings, gravitons are fundamentally same as quarks, they just oscillate in a different manner, giving them different properties like mass, spin and charge. In order for the theory to be right, there would have to be 10 or 11 dimensions in the Universe, where those dimensions we cannot perceive are simply too small.
Problem with the string theory is that it failed to predict the results of any significant experiment until now, something that Einstein’s theory of general relativity and the Standard Model have done over and over again. Another problem is that mathematics behind the string theory are very complex. For this reason, it has taken decades, until mid-1990s, for the theoreticians to realise previously five known string theories were part of a single theory.
Finding the Theory of Everything will be a monumental achievement, whenever it happens. However, it is good to keep in mind that there are possibly other theories waiting to be found in the future, theories that will upgrade the Theory of Everything or even make it obsolete, as it was case with theories in the past replacing theories that were held as correct for a long time.