When Gary Kasparov lost to chess computer Deep Blue in 1997, IBM marked a milestone in the history of artificial intelligence. On Wednesday, in a research paper released in Nature, Google earned its own position in the history books, with the announcement that its subsidiary DeepMind has built a system capable of beating the best human players in the world at the east Asian board game Go.

Go, a game that involves placing black or white tiles on a 19x19 board and trying to remove your opponents’, is far more difficult for a computer to master than a game such as chess.

DeepMind’s software, AlphaGo, successfully beat the three-time European Go champion Fan Hui 5–0 in a series of games at the company’s headquarters in King’s Cross last October. Dr Tanguy Chouard, a senior editor at Nature who attended the matches as part of the review process, described the victory as “really chilling to watch”.

“It was one of the most exciting moments of my career,” he added. “But with the usual mixed feelings … in the quiet room downstairs, one couldn’t help but root for the poor human being beaten.”

It’s the first such victory for a computer program, and it came a decade before anyone expected it. As recently as 2014, Rémi Coulom, developer of the previous leading Go game AI, Crazy Stone, had predicted that it would take 10 more years for a machine to win against a top-rated human player without a handicap.

AlphaGo beat all expectations by approaching the challenge in a completely different way from previous software. Building on techniques DeepMind had employed in other feats of artificial intelligence, such as its system that could learn to play retro video games, AlphaGo used what the company calls “Deep Learning” to build up its own understanding of the game. It could then pick the moves it thought most likely to win.

When teaching a computer to play a game, the simplest method is to tell it to rank every possible move over the course of the game, from best to worst, and then instruct it to always pick the best move. That sort of strategy works for trivial games such as draughts and noughts and crosses, which have both been “solved” by computers that have fully examined every board state and worked out a way to play to at least a draw, no matter what the other player does.

However, for complex games such as Chess, the simple approach fails. Chess is just too big: in each turn there are approximately 35 legal moves, and a game lasts for around 80 turns. Enumerating every board position becomes computationally impossible very quickly, which is why it took so many years for IBM’s team to work out a way to beat Kasparov.

Go is bigger still. The definition of easy to learn, hard to master, it essentially has just two rules governing the core play, which involves two players alternately placing black and white tiles on a 19x19 board. The stones must be placed with at least one empty space next to it, or part of a group of stones of the same colour with at least one empty space, and if they lose their “liberty”, they are removed from the board.

While a game of chess might have 35 legal moves each turn, a game of Go has around 250 (including 361 legal starting positions alone); where Chess games last around 80 turns, Go games last 150. If Google had tried to solve the game in the same way noughts and crosses was solved, it would have had to examine and rank an obscene amount of possible positions: in the ballpark of 1,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000 of them.

That renders an exhaustive search impossible, and even a selective search, of the style used by Deep Blue to defeat Kasparov, tricky to run efficiently.

Adding to the woes of those trying to master Go is the fact that, unlike chess, it’s very difficult to look at the board and mathematically determine who is winning. In chess, a player with their queen will probably beat a player whose queen has been taken, and so on: it’s possible to assign values to those pieces, and come up with a running score that roughly ranks each player’s prospects. In Go, by contrast, counters are rarely removed from the board, and there’s no simple mathematical way to determine who is in the stronger position until the game is very far progressed.

So AlphaGo focused on a very different strategy. As David Silver, DeepMind’s co-lead researcher on the project, puts it: “AlphaGo looks ahead by playing out the rest of the game in its imagination, many times over.” The program involves two neural networks, software that mimics the structure of the human brain to aggregate very simple decisions into complex choices, running in parallel.

One, the policy network, was trained by observing millions of boards of Go uploaded to an online archive. Using those observations, it built up a predictive model of where it expected the next piece to be played, given knowledge of the board and all previous positions, that could accurately guess the next move of an expert player 57% of the time (compared to a previous record of 44.4% from other groups).

This “supervised learning” was then backed up by a bout of “reinforcement learning”: the network was set to play against itself, learning from its victories and losses as it carried out more than 1m individual games over the course of a day.

The policy network was capable of predicting the probability that any given move would be played as next, but the system also needed a second filter to help it select which of those moves was the best. That network, the “value network”, predicts the winner of the game given each particular board state.

Building AlphaGo isn’t just important as a feather in DeepMind’s cap. The company argues that perfecting deep learning techniques such as this are crucial for its future work. Demis Hassabis, DeepMind’s founder, says that “ultimately we want to apply these techniques in important real-world problems, from medical diagnostics to climate modelling”.

For now, the DeepMind team is focused on one final goal on the Go board: a match against Lee Se-dol, the world champion. Lee says that “regardless of the result, it will be a meaningful event in the baduk (the Korean name for Go) history. I heard Google DeepMind’s AI is surprisingly strong and getting stronger, but I am confident that I can win at least this time.”