AlphaGo made history once again on Saturday, as the first computer program to defeat a top professional Go player in an even match.
In the third of five games with Lee Sedol 9p, AlphaGo won so convincingly as to remove all doubt about its strength from the minds of experienced players.
In fact, it played so well that it was almost scary.
The picture becomes clear
When AlphaGo defeated Lee Sedol in the first game, the result was shocking to many, but doubts still remained about its strengths and weaknesses.
It seemed like Lee had underestimated his opponent and chosen an experimental opening to test the machine’s strength. That did not go well.
Soon afterwards, he became embroiled in a complex tactical position, and eventually lost.
On the second day, Lee’s play was much better. His game plan was clearly to play solid and patient moves, and wait for an opportunity to strike.
It was a good test of the machine’s abilities.
Even though Lee never found that opportunity, it was a high quality game and it gave hope to everyone supporting ‘team human’.
For many, including An Younggil 8p and myself, game three crushed that hope.
We’re now convinced that AlphaGo is simply stronger than any known human Go player.
Searching for a weakness
After his second loss last Thursday, Lee and a group of strong professional Go players with whom he is friends stayed up until 6:00 AM reviewing the games.
They were looking for weaknesses, and helping Lee to devise a strategy for game three (Lee did rest on Friday, don’t worry).
Some players made the logical argument that, if AlphaGo were to be defeated, Lee would need to gain the upper hand in the opening and force it to overplay to catch up.
Since AlphaGo had yet to face a seriously challenging ko in its games against both Fan Hui and Lee Sedol, this theory was still largely untested.
A plan of attack
With his back against the wall, and armed with the results of his research, Lee took advantage of the first move by playing a fast paced and active High Chinese formation.
Things seemed to be going as planned when Black created a wide position up to Black 11 and AlphaGo entered Black’s large moyo with White 12.
Lee responded by denying White a base with Black 13, and the game became exciting.
It was the first time we’d seen AlphaGo forced to manage a weak group within its opponent’s sphere of influence. Perhaps this would prove to be a weakness?
This, however, was where things began to get scary.
No plan survives first contact with the enemy
Usually developing a large sphere of influence and enticing your opponent to invade it is a good strategy, because it creates a situation where you have a numerical advantage and can attack severely.
In military texts, this is sometimes referred to as ‘force ratio’.
The intention in Go though is not to kill, but to consolidate territory and gain advantages elsewhere while the opponent struggles to defend themselves.
Lee appeared to be off to a good start with this plan, pressuring White’s invading group from all directions and forcing it to squirm uncomfortably.
But as the battle progressed, White gradually turned the tables — compounding small efficiencies here and there.
Lee seemed to be playing well, but somehow the computer was playing even better.
In forcing AlphaGo to withstand a very severe, one-sided attack, Lee revealed its hitherto undetected power.
That sinking feeling
Move after move was exchanged and it became apparent that Lee wasn’t gaining enough profit from his attack.
By move 32, it was unclear who was attacking whom, and by 48 Lee was desperately fending off White’s powerful counter-attack.
I can only speak for myself here, but as I watched the game unfold and the realization of what was happening dawned on me, I felt physically unwell.
Generally I avoid this sort of personal commentary, but this game was just so disquieting. I say this as someone who is quite interested in AI and who has been looking forward to the match since it was announced.
One of the greatest virtuosos of the middle game had just been upstaged in black and white clarity.
AlphaGo’s strength was simply remarkable and it was hard not to feel Lee’s pain.
The gloves go back on
After being compelled to flex its muscles for a short time and gaining the upper hand, AlphaGo began to play leisurely moves.
By now, most observers know that this is a feature of the ruthlessly efficient algorithm which guides AlphaGo’s play.
Unlike humans, AlphaGo doesn’t try to maximize its advantage. Its only concern is its probability of winning.
The machine is content to win by half a point, as long as it is following the most certain path to success.
So when AlphaGo plays a slack looking move, we may regard it as a mistake, but perhaps it is more accurately viewed as a declaration of victory?
Putting the machine through its paces
As AlphaGo gradually allowed the pressure to slacken, giving false hope to some observers, Lee took the opportunity to give the machine a workout.
He was already clearly behind, and he knew it, but it was such an important game for him and it was too early to resign.
Lee soldiered on with commendable fighting spirit, probing the computer’s weaknesses all over the board.
He tried a clever indirect attack against White’s center dragon with Black 77, but AlphaGo’s responses made it feel like it knew exactly what Lee’s plan was.
Next he tried a cunning probe inside White’s territory, with move 115, attempting to break a ladder in sente or live inside White’s territory, but White responded firmly.
He attempted to make good of his probe by living inside White’s territory with sharp tactics, but White was unperturbed.
Finally, he even tried forcing a complicated ko. At this point, AlphaGo once again showed just how strong and detached its play is by ignoring the ko fight to play honte at White 148.
This move also removed any possibility of a double ko after Black at 148 (which may have been what Lee was planning).
Having answered many questions about AlphaGo’s strengths and weakness, and exhausting every reasonable possibility of reversing the game, Lee was tired and defeated.
He resigned after 176 moves.
Brief analysis of game three
Here is An Younggil 8p’s preliminary analysis of the game. Further game commentary will be posted over the coming week.
Could 31 be the losing move?
Black 15 was an active way of playing, but White resisted with White 16, which was strong.
White 26 was a good move, preserving White’s shape, and Black 29 would have been better at K13.
Black 31 was too much, and (based on how the game progressed) it might have been the losing move.
White 32 was an unexpected counter attack. It was razor sharp.
White’s sequence from 34 to 46 was natural and flawless, and the game was already good for White.
Black resisted with 47, but White 48 was calm, and it came to pass that Black’s left side was destroyed.
Black raises the stakes
Black 49 looked like an overplay, but White 58 was gentle, and Black was able to connect under with 61. However, White was still in the lead.
Black 77 and 79 were typical of Lee’s dynamic style. His intention here was to make the game complicated, but White’s responses up to 86 were excellent.
Black 89 was a mistake. Black should have turned at 90.
White 90 was the vital point for eye shape, and suddenly White had managed to simplify the game.
Black 91 and White 98 were miai, and White built a huge territory at the bottom.
AlphaGo consolidates its advantage
Black 99 was also questionable. Lee should have played at E18, to chase White’s center group first.
White’s play from 102 to 112 was sophisticated, and White consolidated its lead.
Black 113 seemed like a misread. It might have been better to harass White’s right side group with R12.
Black 115 was a tricky probe, but White 116 was the right response, and the game was practically decided.
A desperate invasion
The invasion starting with Black 125 was a desperate strategy, and White’s responses were all accurate.
Black 131 was brilliant, and Black could have created a ko with K4, W J6, B L4, W M2, B N2, W N3, B N4, but he was short of ko threats.
White 148 was an eye popping tenuki, and it was soon proven to be correct because there weren’t any serious problems for White at the bottom.
Black fought the ko with 151, but White’s responses were flawless and Lee resigned afterwards.
After the game, Lee made an unnecessary apology to everyone for losing the match. He also said that he would continue trying to win in games four and five.
Korean commentator Lee Hyunwook 8p said that Lee Sedol had the “strongest heart” of any Go player he knows and that Sedol was the best person to challenge AlphaGo in this match.
Michael Redmond 9p suggested that we might be on the verge of a “third revolution of the opening,” driven by the arrival of AlphaGo. The first two were instigated by the famous players Dosaku and Go Seigen.
Though AlphaGo has technically won the match already, the full five games will still be played.
Lee hopes that Go fans will continue to watch the match and support him during the remaining two games.
Beyond that, we as a community of Go players will have to adapt and make the most of this new force in the world of Go, and perhaps it’s time for broader discussion about how human society will adapt to this emerging technology.
The match continues
Game four of the match will be played on Sunday March 13.
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Lee Sedol vs AlphaGo – Game 3