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The spectral dimension is a real-valued quantity that characterizes a spacetime geometry and topology. It characterizes a spread into space over time, e.g. a ink drop diffusing in a water glass or the evolution of a pandemic in a population. Its definition is as follow: if a phenomenon spreads as t n {\displaystyle t^{n}} , with t {\displaystyle t} the time, then the spectral dimension is 2 n {\displaystyle 2n}. The spectral dimension depends on the topology of the space, e.g., the distribution of neighbors in a population, and the diffusion rate.
In physics, the concept of spectral dimension is used, among other things, in quantum gravity, percolation theory, superstring theory, or quantum field theory.