Lee Sedol Beats AlphaGo in Game 4

I suppose beating humans isn't AlphaGo's primary motive though - learning to play a perfect game of Go in general is probably more difficult than playing the perfect game against a particular person.

Generally I'd guess you're right though.

> This was when things got weird. From 87 to 101 AlphaGo made a series of very bad moves.

It seems to me, that these bad moves were a direct result of AlphaGo's min-maxing tree search.

According to @demishassabis' tweet, it had had the "realisation" that it had misestimated the board situation at move 87. After that, it did a series of bad moves, but it seems to me that those moves were done precisely because it couldn't come up with any other better strategy – the min-max algorithm used traversing the play tree expects that your opponent responds the best he possibly can, so the moves were optimal in that sense.

But if you are an underdog, it doesn't suffice to play the "best" moves, because the best moves might be conservative. With that playing style, the only way you can do a comeback is to wait for your opponent to "make a mistake", that is, to stray from a series of the best moves you are able to find, and then capitalize that.

I don't think AlphaGo has the concept of betting on the opportunity of the opponent making mistakes. It always just tries to find the "best play in game" with its neural networks and tree search – in terms of maximising the probability of winning. If it doesn't find any moves that would raise the probability, it picks one that will lower it as little as possible. That's why it picks uninteresting sente moves without any strategy. It just postpones the inevitable.

If you're expecting the opponent to play the best move you can think of, expecting mistakes is simply not part of the scheme. In this situation, it would be actually profitable to exchange some "best-of-class" moves to moves that aren't that excellent, but that are confusing, hard to read and make the game longer and more convoluted. Note that this totally DOESN'T work if the opponent is better at reading than you, on average. It will make the situation worse. But I think that AlphaGo is better in reading than Lee Sedol, so it would work here. The point is to "stir" the game up, so you can unlock yourself from your suboptimal position, and enable your better-on-average reading skills to work for you.

It seems to me that the way skilful humans are playing has another evaluation function in addition to the "value" of a move – how confusing, "disturbing" or "stirring up" a move is, considering the opponent's skill. Basically, that's a thing you'd need to skilfully assess your chances to perform an OVERPLAY. And overplay may be the only way to recover if you are in a losing situation.