Go

AlphaGo

Go is a board game for two players that originated in China more than 2,500 years ago. The game is noted for being rich in strategy despite its relatively simple rules. According to chess master Edward Lasker: ‘The rules of Go are so elegant, organic, and rigorously logical that if intelligent life forms exist elsewhere in the universe, they almost certainly play Go.’

The two players alternately place black and white playing pieces, called ‘stones,’ on the vacant intersections (called ‘points’) of a grid of 19×19 lines. The object of the game is to use one’s stones to surround a larger total area of the board than the opponent.

Once placed on the board, stones may not be moved, but stones are removed from the board if captured. As the game progresses, the board gets divided up into areas of territory, as outlined by groups of stones. These areas are then contested in local battles, which are often complicated, and may result in the expansion, reduction, or wholesale capture and loss of the contested area. It is often the case that a certain kind of ‘trade’ goes on, where a player’s loss in one part of the board can be compensated for or mitigated by a gain in another part of the board.

A basic principle of Go is that stones must have at least one ‘liberty’ to remain on the board. A liberty is an open ‘point’ (intersection) next to a stone. An enclosed liberty (or liberties) is called an ‘eye,’ and a group of stones with at least two separate eyes is said to be unconditionally ‘alive.’ Such groups cannot be captured, even if surrounded. ‘Dead’ stones are stones that are surrounded and in groups with poor shape (one or no eyes), and thus cannot resist eventual capture.

The general strategy of Go is to expand one’s territory where possible, attack the opponent’s weak groups (groups that can possibly be killed), and always stay mindful of the ‘life status’ of one’s own groups. The liberties of groups are countable. Situations where two opposing groups must capture the other to live are called capturing races. In a capturing race, the group with more liberties (and/or better ‘shape’) will ultimately be able to capture the opponent’s stones. Capturing races and questions of life and death are examples of what makes go challenging.

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