ABSTRACT
Imagine being trapped in a room. The room has no windows, and no doors. The only way you can learn about the outside world is to listen. If you listen with your ears, you can perhaps learn about what is happening immediately outside the room. You might hear a violent thunderstorm taking place a few miles away, if it were very loud and you listened very carefully. Now suppose that you could build a receiver which would help your hearing by amplifying distant sounds. You could maybe hear people talking several buildings away, or cars on a distant highway. The better your receiver was, the more you could hear and the more you could learn. Now suppose you are an astronomer, trying to understand the forces that created the stars, the galaxies that contain them and the planets that orbit them. You too are trapped, not in a room but on the Earth. You cannot ask the stars what shaped them, nor can you experiment on a galaxy. All you can do is sit and watch.
