Each exists but for a short time, and in that time explored a small part of the entire universe. But humans are an alien species. We wonder, we seek answers. Living in this vast, lively and cruel world, staring at the immense sky above, people have always asked many questions: How can we understand the world in which we find ourselves? How does the universe behave? What is the nature of reality? Where does all this come from? Does the universe need a creator? Most of us don't spend most of our time worrying about these questions, but almost all of us do worry about them sometimes.