Why is a carbon-carbon double bond shorter than a carbon-carbon single bond?

A carbon-carbon double bond is shorter than a carbon-carbon single bond because it consists of two pairs of shared electrons rather than one. Specifically, a double bond is composed of both a sigma bond and a pi bond.12

The presence of these two components makes the double bond both stronger and shorter than a single bond, which contains only a sigma bond. In terms of valence bond theory, the formation of a double bond involves hybridization where carbon atoms use $s{p}^2$ hybrid orbitals to form the sigma bond, while unhybridized $p$ orbitals overlap side-by-side to create the pi bond. This additional electron density and the nature of the pi interaction pull the carbon nuclei closer together compared to the single-bond configuration found in alkanes, where carbon is $s{p}^3$ hybridized.1234