We live in four dimensions of space-time, three-dimensional space plus one-dimensional time, so how many dimensions does the universe have?
This is an unanswerable question, because we can only perceive four-dimensional space-time, and no matter how many dimensions exist in the universe, we cannot realize what the higher dimensional space looks like. But although we can't perceive on our own, there is one theory that gives us an answer, and that is string theory and the superstring theory that developed from it. String theory suggests that the universe exists in 26 dimensions, while superstring theory, while optimized, also indicates that the universe has 10 dimensions. Does the universe really have 10 dimensions? What is string theory and why is string theory proposed? Because science needs a big unified theory. The universe as a whole must have a unified law, but the reality is that the known laws have local limitations and are incompatible.
At present, human beings have found four basic forces in the universe, namely strong force, electromagnetic force, weak force and gravitational force, after the efforts of physicists, in the 80s of the last century, strong force, weak force and electromagnetic force were finally unified, these three forces can finally be explained by the Standard Model, but only gravity is still incompatible with them.
Not only that, but we haven't even found particles that are the medium of gravitational propagation. The propagation of strong, weak and electromagnetic forces require corresponding media particles, and we have also discovered their media particles, but only gravitational media particles have not progressed, but scientists still believe in the existence of such media particles, so they named this particle that has not yet been discovered "graviton".
This is where the conflict between general relativity and quantum mechanics lies.
General relativity holds that gravity is a geometric effect that is achieved through the curvature of space, so it does not require any media particles. Quantum mechanics, however, insists that even though gravity is achieved through the curvature of space, the curvature of space must rely on a medium particle to transmit gravity. So in the absence of the discovery of gravitons, scientists substituted this hypothetical "graviton" into the Standard Model, trying to prove the correctness of the theory through calculations, but the result was an infinitely large solution. This means that attempts to use the Standard Model to explain gravity have failed utterly. This means that general relativity cannot be unified with quantum mechanics, neither of which is the ultimate law of the universe, and we need to find a great unified theory that is perfectly compatible with quantum mechanics and general relativity, and can unify the four fundamental forces.
The universe is made up of matter, large matter is made up of small matter, small matter is made up of smaller matter, and the smallest unit of matter is what we call elementary particles.
In the Standard Model, particles can be divided into two categories, those with integer spins are called bosons, and those with spins that spin half integers are called fermions. The media particles of the three fundamental forces and the hypothetical graviton are inseparable elementary particles. Now it seems that gravitons are incompatible with other particles, but what if particles are not the smallest structure of matter? What if the particles still have internal structures and can still be split? On smaller scales, then, they are likely to be compatible. Under this vision, string theory was born.
String theory holds that all particles are made of a small string, the string can vibrate, and different vibrational methods produce different waves, which also show different particles.
Is string theory true? No one knows, because it doesn't have any evidence of reality, and it's a complete conjecture. String theory is a conjecture with the greatest potential to become a grand unified theory, but it is not widely accepted, because it can be calculated that all vibrational forms exhibit the characteristics of bosons, but cannot form fermions like electrons. Even more outrageous is that the world described by string theory is not the same as the four-dimensional space-time we know it to be, and can only be applied in the 26-dimensional universe. To solve this problem, scientists who study string theory have added a fine-tuning mechanism to the theory.
With the addition of the fine-tuning mechanism, the problem of string theory not being able to produce fermions was solved, and the number of fermions and bosons became equally numerous, which is supersymmetry.
With the addition of supersymmetry, string theory developed into superstring theory. Compared to string theory, which only applies to the 26-dimensional universe, superstring theory only requires the universe to have 10 dimensions, that is, 9-dimensional space plus one-dimensional time, which is much more acceptable, after all, it is only 6 dimensions more than the space-time we feel. Does the universe have ten dimensions? This cannot be answered, at least not now, but if the universe really has ten dimensions, then superstring theory can be perfectly applied, and it is likely to become the grand unified theory we are looking for. But looking back, it seems more difficult to perceive the extra 6 space-time dimensions than to find the Grand Unification Theory.