His Dark Materials is based on the notion of parallel worlds. We know that there must be other universes parallel to our ours, because (as I understand it) an experiment which has been a cornerstone of twentieth century physics leaves us with no alternative explanation…The only problem is how to get from one universe to another. In a sense, this is a basic problem of science fiction: how to get from here, where we live, to there, where strange things happen. And there are as many ways of solving it as novelists. So when I needed a way of getting from one universe to another, I didn’t waste time inventing anything fancy: I just cut a hole. The young boy Will has acquired a knife, known as the subtle knife, which is sharp enough to cut through the molecules of the ait, and he’s now learning how to use it. Once you have the subtle knife, you can open the way to any other world, the and the problem of travel is dealt with at once.