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Horizon effect

The horizon effect, also known as the horizon problem, is a problem in artificial intelligence whereby, in many games, the number of possible states or positions is immense and computers can only feasibly search a small portion of them, typically a few plies down the game tree. Thus, for a computer searching only a fixed number of plies, there is a possibility that it will make a poor long-term move. The drawbacks of the move are not "visible" because the computer does not search to the depth at which its evaluation function reveals the true evaluation of the line. The analogy is to peering at a distance on a sphere like the earth, but a threat being beneath the horizon and hence unseen.

Examples
In chess, assume a situation where the computer only searches the game tree to six plies and from the current position determines that the queen is lost in the sixth ply; and suppose there is a move in the search depth where it may sacrifice a rook, and the loss of the queen is pushed to the eighth ply. This is, of course, a worse move than sacrificing the queen because it leads to losing both a queen and a rook. However, because the loss of the queen was pushed over the horizon of search, it is not discovered and evaluated by the search. Losing the rook seems to be better than losing the queen, so the sacrifice is returned as the best option whereas delaying the sacrifice of the queen has in fact additionally weakened the computer's position. As another example, while some perpetual checks quickly trigger a threefold-repetition draw, others can involve a queen chasing a king around across the board, varying the position each time, meaning the actual forced draw could be many moves into the future. If a quiescence search playing out the checks isn't done, then the AI might not detect the possibility, and can let the engine blunder a winning position into a drawn one. In Go, the horizon effect is a major concern for writing an AI capable of even beginner-level play, and part of why alpha-beta search was a weak approach to Computer Go compared to later machine learning and pattern recognition approaches. It is a very common situation for certain stones to be "dead" yet require many moves to actually capture them if fought over. The horizon effect may cause a naive algorithm to incorrectly assess the situation and believe that the stones are savable by calculating a play that seems to keep the doomed stones alive as of the move the search tree stops at. While the death of the group can indeed be delayed, it cannot be stopped, and contesting this will only allow more stones to be captured. A classic example that beginners learn are Go ladders, but the same general idea occurs even in situations that aren't strictly ladders. ==See also==
Source: Wikipedia ↗
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