This is Lorenz’s famous (and widely misunderstood) butterfly effect: a flap of a butterfly’s wing can cause a hurricane a month later, halfway round the world.If you think that sounds implausible, I don’t blame you. It’s true, but only in a very special sense. The main potential source of misunderstanding is the word ‘cause’. It’s hard to see how the tiny amount of energy in the flap of a wing can create the huge energy in a hurricane. The answer is, it doesn’t. The energy in the hurricane doesn’t come from the flap: it’s redistributed from elsewhere, when the flap interacts with the rest of the otherwise unchanged weather system.After the flap, we don’t get exactly the same weather as before except for an extra hurricane. Instead, the entire pattern of weather changes, worldwide. At first the change is small, but it grows – not in energy, but in difference from what it would otherwise have been. And that difference rapidly becomes large and unpredictable. If the butterfly had flapped its wings two seconds later, it might have ‘caused’ a tornado in the Philippines instead, compensated for by snowstorms over Siberia. Or a month of settled weather in the Sahara, for that matter. Calculating the Cosmos: How Mathematics Unveils the Universe