Since ancient times, our universe has been understood as a space with three dimensions, with a three-dimensional three-dimensional space composed of length, width and height. If we describe our space mathematically, we can describe the spatial position of an object by building three axes perpendicular to each other.
Until the beginning of the last century, einstein, a small clerk at the Swiss Patent Office and a recent phD graduate in physics, proposed an epoch-making theory of physics, special relativity, which made time and space a whole called space-time, and time became the fourth dimension in space-time.
However, although space-time is unified, the dimension has also increased to four, but time is added as a special dimension, time and space, although unified, still have essential differences, so in Einstein's university teacher Minkowski for the special theory of relativity to build a geometric model - Minkowski space, the time coordinates were transformed by imaginary numbers, with ict to measure the time dimension, where i represents the imaginary number, ct represents the spatial distance passed by the speed of light in unit time.
Therefore, in the space-time of special relativity unity, time and space are never the same thing, and space is still three-dimensional. When Einstein was studying the unified field theory of unified electromagnetic forces, a German mathematician, Theodore Karuza, tried to add a spatial dimension to the gravitational field equation, extending space-time to five dimensions. In five-dimensional space-time, there are several more sets of equations for gravitational field equations, in addition to the original gravitational field equations, there is also an extra set of equations, and Karuzza found that this extra set of equations is equivalent to Maxwell's electromagnetic equations, which means that when a spatial dimension is added to space-time, general relativity unifies the electromagnetic theory.
This is indeed a very good idea, but the question arises: where is the fourth spatial dimension? Since there is still a degree of spatial freedom, why can't we see or feel it? Another Swedish physicist, Oscar Klein, then proposed the hypothesis that the fourth dimension of space might be curled up, and that if it itself curled up at Planck's length, we would not be able to observe it in any way. At this point, a five-dimensional space-time theory that unified gravity and electromagnetic forces was born, and the scientific community named the Theory Karuza-Klein Theory after them.
But it is clear that this theory is incomplete for the simple reason that it ignores two other fundamental forces in nature: the strong nuclear force and the weak nuclear force. Therefore, with the development of the theory, this five-dimensional theory has gradually faded out of people's vision.
Later, the theory of symmetry between fermions and bosons: supersymmetry theory was proposed, and scientists began to upgrade the Kaluzza-Klein theory of five-dimensional unity on the basis of supersymmetry theory, until the dimensions of space were increased to ten dimensions, the dimensions of space-time were increased to eleven, and the theory of geometric gravitational finally contained all known forces. This eleven-dimensional theory of gravitational attraction is called the supergravity theory.
However, scientists soon discovered that the supergravitation theory had two fatal flaws: it would produce infinity and lose its signature (the term non-conservation under weak interactions is associated with this). So this complex and beautiful unified theory still ended in failure.
At the same time as supergravity theory, another set of high-dimensional theories was developed: superstring theory. Unlike supergravity theory, superstring theory unifies all known fundamental forces in nine dimensions of space, does not appear infinite, and does not lose its signs because the dimensions of space are odd. A perfect unified theory emerged? However, scientists have found at the same time that not one, but five... These five different sets of superstring theories can get the same result, and it is clear that these five sets of superstring theories are all correct, at least theoretically self-consistent, so who should we believe? At this time, a peacemaker appeared, he was the american genius physicist Edward Witten, he added a space dimension to the five sets of ten-dimensional superstring theory, and unified five different sets of superstring theories to become an eleven-dimensional superstring theory, of which ten space dimensions and one time dimension.
But you must have already discovered that the spatial dimension has become an even number again, and the signs of hand are gone... But Witten was more powerful, and he thought of a solution: space is still nine-dimensional, and the additional dimension added is a parallel spatial dimension connecting two nine-dimensional spaces, so that each individual spatial dimension maintains an odd number and raises the space-time dimension to eleven. And Witten found that this eleven-dimensional superstring theory automatically contained an eleven-dimensional supergravity theory, so that a unified theory of all known interactions (fundamental forces) in the universe was born! Witten calls this theory of inclusion of all things M-theory.
So how do we prove this great unified theory? How do we verify high-dimensional spaces? Unfortunately, there is no way to do it at this time... Since in theory only gravitational force can penetrate into high-dimensional space, other such as electromagnetic force, strong nuclear force, weak nuclear force can not be separated from three-dimensional space, therefore, our conventional measurement methods can not be used. And as for whether it can be measured by gravitational waves? It seems that the penetrating energy of gravitational waves is too strong to reflect signals, and current human technology does not have the ability to make gravitational wave signals that can be measured.
At present, the only thing that human technology can do may be to find the supersymmetric particles predicted by supersymmetric theory through the large hadron collider, but how high the energy state of supersymmetric particles needs to produce scientists know nothing, so that is to say, we do not know how big a hadron collider must be built to verify the supersymmetry theory... And it is only to verify supersymmetry theory, far from verifying the true string theory.
M Theory is successful because it very naturally unifies all known fundamental forces by adding spatial dimensions, but it fails because it fails because it cannot make any theoretical predictions that can be verified under existing technology, which is devastating for a scientific theory, because it means that the theory has lost its falsifiability, and can a theory that has lost its falsifiability still be regarded as a scientific theory?