From Robert Worden
You tell the life story of a water molecule from a comet to the present day far too simply (14 November, p 31). There is in fact no such thing as the history of a single water molecule. We know from quantum mechanics that every time two water molecules collide in the ocean – one going left and the other going right – there is no true account of which one went left and which one went right. In colliding, they become a mixture and lose their identity – inasmuch as they ever had one.
So a water molecule in the full stop of your journal does not have one history. Through its previous collisions, it has 2, 4, 8, 16… and many more histories – the same huge set of histories as almost every other water molecule on the planet. A water molecule in my body did not pass through just one dinosaur. It passed through every dinosaur that ever had a bodily function.
Sutton, Cambridgeshire, UK
