We turn now to the third of the three branches of science that underpin our changing climate: radiant energy – the energy carried by light and other electromagnetic waves – and the ways it interacts with the gases and particles in our atmosphere. In this chapter we will outline the necessary science, and especially the laws of thermal emission. It turns out that we are bathed in electromagnetic radiation that we can’t see. We will then show how these laws lead to the counter-intuitive outcome that the ‘temperature’ of our planet is actually well below freezing! Fortunately, however, our Earth has a ‘good’ atmosphere, containing some very important gases – greenhouse gases – which act as a blanket, keeping the surface at a suitable temperature for liquid water, and hence for life. A simple model will help you gain a basic understanding of the greenhouse effect.

The introductory chapter places radio astronomy in the context of the broader astronomical environment. The transformational discoveries made by radio astronomy and the circumstances surrounding these discoveries are summarized with an emphasis on the role of serendipity and its impact on science.

Electromagnetic radiation is the primary source of astronomical information.

In particular, until the early 1930s astronomy was all based on the use of telescopes

that extended the power of the human eye, but were restricted to the

collection of visible light. In general, the sources of astronomical electromagnetic radiation and other sources of astronomical information are what we call visible matter. This chapter introduces some key concepts and notation that characterize light and the collection of light for astronomical purposes. It addresses the main types of information that we may extract from the observations, by means of imaging and spectroscopy, recalling the difference between apparent and intrinsic properties of the astronomical sources and the fact that the light from distant sources is often a mixture of photons from different stars or different

components. This serves as an excuse for a quick introduction to important

concepts, such as stellar populations, mass-to-light ratios, mean motions, and

velocity dispersions. In closing the chapter, a method is described to measure the distance to a stellar system based on the application of a very simple dynamical model to a suitable set of observations.

Chapter 7 examines the mindset of Newton Minow, the Federal Communications Commission (FCC) chairman who summed up the regulator’s view of television by calling it a “vast wasteland.” Minow championed public interest regulation of the broadcast medium based on the theory that the electromagnetic spectrum is scarce and that the government must allocate broadcast licenses and regulate the content of programming.But the spectrum is no more scarce than any other economic good, and the events that led to federal control over broadcasting were contrived to extend government control over the medium. Minow and other like-minded regulators deny that this type of control is censorship, but their efforts caused diminished diversity in programming and dampened innovation. Further, the tenets of broadcast regulation were undermined as new technologies emerged, although that fact did not deter Minow and other like-minded regulators from advocating more government control. Since then, the law and the culture have moved on, rendering the positions that Minow espoused obsolete.

We now consider why stars shine with such extreme brightness. Over the long-term (i.e., millions of years), the enormous energy emitted comes from the energy generated (by nuclear fusion) in the stellar core, as discussed further in Chapter 18. But the more immediate reason stars shine is more direct, namely because their surfaces are so very hot. We explore the key physical laws governing such thermal radiation and how it depends on temperature.

Here, I outline the nature of the electromagnetic spectrum. I note that various living organisms can detect ultraviolet, visible, and infrared wavelengths, but that none can detect the longer wavelengths of radio. This may be partly due to the fact that there is little to be gained from evolving such an ability and partly because unrealistically large eyes would be required to ‘see’ these long wavelengths. I then turn to human use of radio, which began in the early twentieth century. I consider the question of when the ‘radio age’ started, from the perspective of our transmissions into space – both accidental and deliberate. Leakage of broadcasts could have occurred since the 1920s; messages specifically aimed into space began in the 1960s. When the radio age will end is hard to predict; guessing how long other broadcasting civilizations will last is even harder. However, using both optimistic and pessimistic figures, I use the Drake equation to guestimate how many broadcasting civilizations there are in the Milky Way right now. The result: anything from just one (us) to about a quarter of a million. I end by stressing the difference between radio waves and radio signals.