Discovering an Invisible Universe

Before 1931, to study astronomy meant to study the objects visible in the night sky. Indeed, most people probably still think that’s what astronomers do wait until dark and look at the sky using their naked eyes, binoculars, and optical telescopes, small and large. Before 1931, we had no idea that there was any other way to observe the universe beyond our atmosphere.

In 1931, we did know about the electromagnetic spectrum. We knew that visible light included only a small range of wavelengths and frequencies of energy. We knew about wavelengths shorter than visible light Wilhelm Rontgen had built a machine that produced x-rays in 1895. We knew of a range of wavelengths longer than visible light (infrared), which in some circumstances is felt as heat. We even knew about radio frequency (RF) radiation, and had been developing radio, television, and telephone technology since Heinrich Hertz first produced radio waves of a few centimeters long in 1888. But, in 1931, no one knew that RF radiation is also emitted by billions of extraterrestrial sources, nor that some of these frequencies pass through Earth’s atmosphere right into our domain on the ground.