"What is that other universe?" she said. "One of uncountable billions of parallel worlds. The witches have known about them for centuries, but the first theologians to prove their existence mathematically were excommunicated fifty or more years ago. However, it's true; there's no possible way of denying it. "But no one thought it would ever be possible to cross from one universe to another. That would violate fundamental laws, we thought. Well, we were wrong; we learned to see the world up there. If light can cross, so can we. And we had to learn to see it, Lyra, just as you learned to use the alethiometer. "Now that world, and every other universe, came about as a result of possibility. Take the example of tossing a coin: it can come down heads or tails, and we don't know before it lands which way it's going to fall. If it comes down heads, that means that the possibility of its coming down tails has collapsed. Until that moment the two possibilities were equal. "But on another world, it does come down tails. And when that happens, the two worlds split apart. I'm using the example of tossing a coin to make it clearer. In fact, these possibility collapses happen at the level of elementary particles, but they happen in just the same way: one moment several things are possible, the next moment only one happens, and the rest don't exist. Except that other worlds have sprung into being, on which they did happen. "And I'm going to that world beyond the Aurora," he said, "because I think that's where all the Dust in this universe comes from. You saw those slides I showed the Scholars in the retiring room. You saw Dust pouring into this world from the Aurora. You've seen that city yourself. If light can cross the barrier between the universes, if Dust can, if we can see that city, then we can build a bridge and cross. It needs a phenomenal burst of energy. But I can do it. Somewhere out there is the origin of all the Dust, all the death, the sin, the misery, the destructiveness in the world. Human beings can't see anything without wanting to destroy it, Lyra. That's original sin. And I'm going to destroy it. Death is going to die."