The Diameter of the Rubik's Cube Group Is Twenty

We give an expository account of our computational proof that every position of the Rubik's Cube can be solved in 20 moves or fewer, where a move is defined as any twist of any face. The roughly $4.3 \times 10^{19}$ positions are partitioned into about two billion cosets of a specially chosen subgroup, and the count of cosets required to be treated is reduced by considering symmetry. The reduced space is searched with a program capable of solving one billion positions per second, using about one billion seconds of CPU time donated by Google. As a byproduct of determining that the diameter is 20, we also find the exact count of cube positions at distance 15.