Five-dimensional space is an abstract concept that often appears in mathematics. In physics and mathematics, the sequence of n numbers can be understood as representing the position in N-dimensional Euclidean space. Whether the universe has five dimensions is also a controversial topic.
Most of the early work on five-dimensional space tried to develop a theory that unified four basic interactions in nature: strong nuclear force and weak nuclear force, gravity and electromagnetic force.
German mathematician Theodore Kaluza and Swedish physicist Oscar Klein independently developed Kaluza-Klein theory in 192 1, and unified gravity and electromagnetic force by using the fifth dimension.
In order to explain why this dimension cannot be directly observed, Klein suggested that the fifth dimension be rolled into a tiny dense ring of about 10-33 cm.
Under his reasoning, he imagines light as a disturbance caused by higher-dimensional ripples that humans can't perceive, just as fish in a pond can only see the shadow of ripples left by raindrops on the water surface.
Although it cannot be detected, it will indirectly imply that there is a connection between seemingly unrelated forces. Due to the appearance of superstring theory and supergravity, Kaluza-Klein theory experienced a revival in 1970' s: reality is a concept composed of vibrating energy chains, which can only be mathematically feasible in ten or more dimensions. The superstring theory then evolved into a more general method called M theory.
M theory can explain the weakness of gravity relative to other basic natural forces. For example, when a needle is lifted from a table by a magnet, the magnet can overcome gravity and relax the whole earth.
Mathematical methods were developed in the early 20th century, and the fifth dimension is considered as a theoretical structure. These theories refer to Hilbert space, which is a concept that assumes an infinite number of mathematical dimensions to allow an infinite number of quantum states.
Einstein, Bergman and Bargmann later tried to extend the four-dimensional space-time of general relativity to an extra physical dimension to include electromagnetism, although they failed.
In the paper 1938, Einstein and Bergman first put forward modern viewpoints, namely, four-dimensional theory and long-distance Einstein-Maxwell theory, which originated from a five-dimensional theory that is completely symmetrical in all five dimensions. They believe that electromagnetism is produced by the "polarized" gravitational field in the fifth dimension.
The main novelty of Einstein and Bergman is that they regard the fifth dimension as a physical entity, rather than an excuse to combine the metric tensor with the electromagnetic potential. But they later went back on their word, revised the theory and broke its five-dimensional symmetry.
As edward witten said, their reasoning is that a more symmetrical version of the theory predicts the existence of a new long-range field, which is massless and scalar, which requires a fundamental revision of Einstein's general theory of relativity. Minkowski space and Maxwell equation in vacuum can be embedded into Riemann curvature tensor in five-dimensional space.
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Scientists still have not reached an agreement on the dimensions of real existence. String theory holds that everything in this universe is absolutely the expression of a single object-a tiny string. The way it vibrates determines whether it is a photon or an electron, and everything is part of the unified concept.
Because there is not enough deviation to explain all the particles and forces in the universe, string theory needs at least six additional dimensions besides the known four dimensions.