All these tattered old bits and pieces have a history and a meaning. A group of them together can seem like the traces left by an ionizing particle i… - Philip Pullman

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All these tattered old bits and pieces have a history and a meaning. A group of them together can seem like the traces left by an ionizing particle in a bubble chamber: they draw the line of a path taken by something too mysterious to see. That path is a story, of course. What scientists do when they look at the line of bubbles on the screen is work out the story of the particle that made them: what sort of particle it must have been, and what caused it to move in that way, and how long it was likely to continue. Dr. Mary Malone would have been familiar with that sort of story in the course of her search for dark matter. But it might not have occurred to her, for example, when she sent a postcard to an old friend shortly after arriving in Oxford for the first time, that that card itself would trace part of a story that hadn't yet happened when she wrote it. Perhaps some particles move backward in time; perhaps the future affects the past in some way we don't understand; or perhaps the universe is simply more aware than we are. There are many things we haven't yet learned how to read. The story in this book is partly about that very process.