From this new point of view, the universe I had inhabited became an object I could perceive in its entirety. It was a hypersphere embedded in a cloud of alternative states-the sum of all possible quantum trajectories from the big bang to the decay of matter. "Reality”-history as we had known or inferred it-was only the most likely of these possible trajectories. There were countless others, real in a different sense: a vast but finite set of paths not taken, a ghostly forest of quantum alternatives, the shores of an unknown sea.