There has been light from the beginning. There will be light, feebly, at the end. In all its forms—visible and invisible—it saturates the universe. Light is more than a little bit inscrutable. Modern physics has sliced the stuff of nature into ever smaller and more exotic constituents, but light won't reduce. Light is light—pure, but not simple. No one is exactly sure how to describe it. A wave? A particle? Yes, the scientists say. Both. It is a measure of light's importance in our daily lives that we hardly pay any attention to it. Light is almost like air. It's a given. A human would no more linger over the concept of light than a fish would ponder the notion