More games than atoms

There are more possible chess games than atoms in the observable universe. It's not a metaphor: it's a mathematical estimate with a name of its own.

In 1950, Claude Shannon, the father of information theory, published a pioneering paper on how to program a machine to play chess. In it he estimated that the number of possible games is around a 1 followed by 120 zeros. The atoms in the observable universe are reckoned at roughly a 1 followed by 80 zeros. The difference isn't double or triple: it's forty orders of magnitude.

That figure, the Shannon number, explains something important: no computer will ever win at chess by calculating everything, because everything doesn't fit inside the universe. The machines that crush us today don't explore it whole: they learn to look only where it matters.

Source: Philosophical Magazine (paper by Claude Shannon, 1950)