A Go board’s size means that the number of games that can be played on it is enormous: a rough-and-ready guess gives around 10170. Analogies fail when trying to describe such a number. It is nearly a hundred of orders of magnitude more than the number of atoms in the observable universe, which is somewhere in the region of 1080. Any one of Go’s hundreds of turns has about 250 possible legal moves, a number called the branching factor. Choosing any of those will throw up another 250 possible moves, and so on until the game ends. As Demis Hassabis, one of DeepMind’s founders, observes, all this means that Go is impervious to attack by mathematical brute force.

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