Well, it turns out if you look closely that usually the arms aren't exactly the same. Which is rather a clue: the coordinating mechanism isn't some kind of quan

Well, it turns out if you look closely that usually the arms aren't exactly the same. Which is rather a clue: the coordinating mechanism isn't some kind of quantum magic that enables the structure in the centre to determine exactly what the arms are like. Rather, (1) the arms all start out roughly the same because the crystal structure of ice has 6-fold symmetry, and then (2) the way growth happens at any place and any given moment is largely determined by (a) the structure there and (b) the external conditions, which vary as the snowflake is forming because it's being blown around. But because the snowflake is quite small, those external conditions are generally much the same for all six arms; so similar arms + similar conditions for each arm --> similar incremental growth for all arms --> still-similar arms, so the symmetry is preserved as the crystal grows. See, e.g., http://www.ams.org/notices/200504/fea-adam-3.pdf http://www.ams.org/notices/200504/fea-adam-3.pdf .