In what even I, who predicted an AlphaGo series win, regard as a bit of a surprise, AlphaGo defeated Lee Sedol in the first game of the five-game match. AlphaGo won by Lee’s resignation, which came 186 moves into the game, up by (the consensus seems to be) between three and seven points at the time.

Our liveblog has our detailed commentary from the first 120 or 130 moves of the game, after which I went to bed. Here, though, are some highlights and post-game thoughts.

• AlphaGo is much stronger now than it was before. Michael Redmond, a 9-dan pro who was engaged to commentate, remarked as much: AlphaGo didn’t make any serious mistakes.
• One of AlphaGo’s suspected weaknesses before the game was coming out ahead after complicated fights; we thought this because it avoided getting involved in very many serious fights against Fan Hui. It turns out that AlphaGo can indeed fight, and fight well, as we saw with the fight at the top center of the board that developed early on in the running. AlphaGo came out ahead there, or at the very least, ended up with a strong enough position that she¹ could hold it and use it as a base to fight over the center of the board, while strengthening her position in the top left.
• Lee Sedol seemed a little flustered by the level of AlphaGo’s play. He, like a lot of go experts, seemed to make the twin assumptions that: 1) AlphaGo would not improve dramatically between October and today, and 2) AlphaGo was not vastly stronger than Fan Hui in October. AI experts suspected that the first item was false, and they turned out to be correct. That said, it’s hard to fault go experts for lacking experience with AI systems, and not realizing that six months is an eternity for a learning system to improve. It is much easier to fault go experts for the second point, which is one of simple math. A 9-dan pro should beat a 2-dan pro (like Fan Hui) 95%-99% of the time. Although the known AlphaGo-Fan Hui games are too small a sample to draw firm conclusions from, it is telling that AlphaGo defeated Fan Hui by large margins or resignation in all of the long-time games, losing only during quick play; either way, its record suggests that AlphaGo is properly probably at least 5-dan, and may be up at the 9-dan level.
• It’s possible that a lot of AlphaGo’s added strength came from the extra time to think, but it’s difficult to do much more than speculate on that front. Suffice it to say that this could be a contributing factor.
• One thing we haven’t seen very much of is AlphaGo’s ability in close endgames. Given that endgames are basically tests of reading, and AlphaGo has proven to be superb at that so far, I wouldn’t expect its endgames to be any worse than its middle games, but Lee Sedol may nevertheless aim to explore AlphaGo’s endgame capabilities with a more relaxed early game, next time out.
• One of my prematch predictions seems to have turned out true: computers play differently than do humans, and it may be that humans play more optimally than humans. In the late midgame, around move 120 to 150, in the bottom right and center right of the board, AlphaGo played very cautiously, keeping stones alive (particularly in the bottom right) and denying Lee Sedol territory, without aiming to decisively win the corner itself. Go experts would denounce this sort of play as ‘slow’, given that it seems to yield tempo, but if AlphaGo is confident that she’s already ahead, she need not play to do any more than deny Lee the win.

That’s all for now. We’ll be back with more at the end of the weekend, or after any particularly interesting games, and we might liveblog the final game in the series on Monday night. If so, you’ll hear about it here.