In quantum gravity, a lower-dimensional space is a space that has fewer dimensions than the usual four (three spatial and one temporal) that we observe in our universe. A lower-dimensional space can be thought of as a projection or a slice of a higher-dimensional space, where some of the information or degrees of freedom are lost or hidden. For example, a two-dimensional space is a lower-dimensional space of a three-dimensional space, and a one-dimensional space is a lower-dimensional space of a two-dimensional space. Lower-dimensional spaces are useful for studying some aspects of quantum gravity, such as holography, entanglement and black holes.

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