It’s been a tough run for Go champion Lee Se-dol who lost game after game in what could very well be the match of the century, against Google’s AlphaGo algorithm. He went down 3-0 in the best of five before winning a game, showing that the AI is not out of reach just yet.

Go players in Shanghai demonstrate the traditional technique of holding a stone.

AlphaGo is now 3-1 up in the series after a rather strange game. Commentators noted several unusual moves which they would have classified as mistakes, but given that AlphaGo played extremely unusual moves previously and won, they kept a healthy dose of moderation.

According to tweets from DeepMind founder Demis Hassabis, however, this time AlphaGo really did make mistakes. The AI “thought it was doing well, but got confused on move 87,” Hassabis said, later clarifying that it made a mistake on move 79 but only realized its error by 87. AlphaGo adjusts its playing style based on its evaluation of how the game is progressing, according to the Verge.

Lee entered the post-game press conference to rapturous applause, remarking “I’ve never been congratulated so much just because I won one game!” But he didn’t just win a game – he showed why he’s the 18-time world champion and a living legend. He showed that at least in Go, humanity still has the resources to strike back.

“Lee Se-dol is an incredible player and he was too strong for AlphaGo today,” said Hassabis, adding that the defeat would help DeepMind test the limits of its AI. “For us this loss is very valuable. We’re not sure what happened yet.”

While in its essence, it’s simpler than chess (with all stones being equal), in terms of possible moves and overall strategies, Go is way, way more complex than chess. The total number of moves in Go is estimated at 10761 compared for example to the estimated 10120possible in chess. It would take me about 20 rows just to write all the zeroes 10761 . Just so you get an idea, the total number of atoms in the universe is estimated at around 1080. For this reason, it was thought that AIs would be very slow in reaching human level, but AlphaGo surpassed estimates by some ten years.

Lee was competing for a $1 million prize put up by Google, but DeepMind’s victory means the sum will be donated to charity.

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