Review of Game 4: Lee Sedol's brilliant move reveals weaknesses AlphaGo
And how unimaginable resilient, persistent, and believing in himself Lee Sedol must be, after the new sledgehammer blow of the third game, to try to collect himself, to show that he is able to deliver, and to prove all humanity is right?
Dia. 1: Game 4, after black 25 (circle, Lee Sedol is white)
Dia. 3: Game 4, after black 47 (circle, Lee Sedol is white)
Where to play next, is a matter of 'looking' forward at least 20-25 moves in this ambiguous fight in the center and right edge. Lee Sedol's fabulous intuition and immense skills in assessing all options and starting points in such explosive positions, are herewith invaluable (Dia. 4). Both AlphaGo and Lee Sedol have to rely on a very crude estimate of how the position may develop gradually.
In short, it looks like this once more will be a major victory for AlphaGo: white's position seems completely hopeless. But then Lee Sedol puts forward a completely unexpected, wonderful, and brilliant move that suddenly sets the entire board on fire. All white center stones are activated at once and do cooperate effectively by exploiting the aji in black's center moyo in all conceivable means (Dia. 6).
With black 79, AlphaGo fails to play the best response and what happens is a sequence of moves that the program calculated as the strongest answers. In exchange for it's group lost on the right, AlphaGo just gets a poor sente move that the program also could have played immediately (Q13 in Dia. 7).
With move 110 (circle in Dia. 8), white secures his relatively weak stones in the center by attacking the black group on the left (center). In this manner, white creates enough options to connect with the group in the bottom left in case of emergency.
Direct comparison of Dia. 7 and --more than thirty moves later-- Dia. 8 immediately demonstrates that white has achieved the unimaginable: it is as if white's invading center stones were able to escape through an invisible tunnel (David Ormerod). Lee Sedol efficiently utilized the aji in black's center that he correctly estimated 25 moves earlier.
There is nothing left to AlphaGo other than to secure it's group on the middle left and to allow that Lee Sedol meanwhile connects his center group --in sente--. Then white further erodes black's center by connecting the three white stones on the upper side. Black's entire moyo has been neutralized (Dia. 9).
AlphaGo tries to catch up with active play but Lee Sedol plays magnificently sharp. An example of this is Lee Sedol's counter move in Dia. 9: white ignores black's threat at the bottom right corner and plays a larger counter threat (circle in Dia. 9; that also prevents black's sente moves in this area).
Dia. 9: Game 4, after white 136 (circle, Lee Sedol is white).
Another example is Lee Sedol's ignorance of a big endgame move by AlphaGo (triangle in Dia. 10) which he replies with an even more binding move (circle in Dia. 10).
Dia. 11: Game 4, after white 168 (circle, Lee Sedol is white).
Even though Lee Sedol played under ultra-high pressure for more than one hour (from move 90 on), using his last byo-yomi period up to the max each move, he was able to maintain and solidify his advantage built up in the middle game. This is the first time that AlphaGo lost a game against a top professional Go player (in the official match games without handicap). Lee Sedol's great victory is an extraordinary and historic achievement which has led to the deepest respect and admiration from all over the world.
Lee Sedol's win against AlphaGo shows --and does prove for the first time-- that deep learning AlphaGo does not play perfect all the time and sometimes has severe problems in judging correctly complex middle game positions. Despite the considerable pressure, Lee Sedol could play his own game and exposed a clear glimpse of AlphaGo's weaknesses.
During the press conference afterwards, the public and press were wild with enthusiasm and chanted: “Lee Sedol!” ”Lee Sedol!” “Lee Sedol!”. Several minutes flashing recorded a radiant, visibly overjoyed and genuinely pleased Lee Sedol: "I have never been congratulated so much just because I won one game. Winning this game is still valuable, the drive behind the win today is the trust and the compassion of you all. This one win is so valuable and I will not trade this for anything in the world".
And asked what Lee Sedol was thinking when he was played his brilliant move (move 78, Dia. 6): "I thought I would be able to gain some profit quite easily but that wasn't the case. It became more difficult than I had expected and at that point in time, at time of 78th move, that was the only move that I could see, there were no other moves, no other placement that I could think of. That was the only place I could place my stone, it was the only option for me, so I put it there. So I am quite humble for all the praise that I'm getting for that".
Demis Hassabis said: "AlphaGo made a mistake around move 79 but it realized it's mistake only after the 87th move, when it was too late to recover from. AlphaGo thought it was doing very well at the start of the complex center fight but was pressured by Lee Sedol in some mistakes. It is because of the incredible fighting spirit Lee Sedol showed that a creative genius like him is able to find out and exploit any weakness of AlphaGo. The loss by AlphaGo is very valuable: we will look at all the statistics and are looking forward to learn from these weaknesses and fix these problems".
David Silver, Google DeepMind's team leader, added: "We have designed AlphaGo to learn by itself by repeating self-plays. But such an algorithm must have holes in it that we cannot know about. Lee was able to push AlphaGo to expose such a weakness".
This fourth game of the match was decided by Lee Sedol's profound and fabulous move in a complex middle game fight that forced deep learning AlphaGo to a sequence of mistakes. After this, the game was completely turned around and AlphaGo was unable to defend against all weaknesses exposed in it's center position. After three successive losses against the program, this game Lee Sedol clearly outperformed AlphaGo for the first time: a great and historic achievement by Lee Sedol.
[Part 6: Review of Game 1: Lee Sedol underestimates AlphaGo's incredible fighting power]
[Part 8: Review of Game 3: Lee Sedol's opening mistakes due to enormous mental pressure]
[Part 10: Review of Game 5: AlphaGo unfamiliar with common tesuji in ultimate moyo game]