Circa 2019
According to string theory, all particles and fundamental forces arise from the vibrational states of tiny strings. For mathematical consistency, these strings vibrate in 10-dimensional spacetime. And for consistency with our familiar everyday experience of the universe, with three spatial dimensions and the dimension of time, the additional six dimensions are “compactified” so as to be undetectable.
Different compactifications lead to different solutions. In string theory, a “solution” implies a vacuum of spacetime that is governed by Einstein’s theory of gravity coupled to a quantum field theory. Each solution describes a unique universe, with its own set of particles, fundamental forces and other such defining properties.
Some string theorists have focused their efforts on trying to find ways to connect string theory to properties of our known, observable universe—particularly the standard model of particle physics, which describes all known particles and all their mutual forces except gravity.
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