Introduction

Perhaps more than any other science, the fundamental distinction between the things that we observe and the things that exist is made clear in astronomy. We explored the distinction between observables and things that exist in Module 2 through Plato’s cave allegory. We noted that in science we develop theories based on hypotheses about things that exist, and we use those theories to derive descriptions of things we might observe if our hypotheses are correct.

Scientists don’t have direct access to the actual things that exist. Even when you touch something, say the top of a desk, on a subatomic level what’s happening is that charged particles in your hand are being repelled by charged particles in the desk. In fact, everything we perceive as being “solid” is almost completely vacuous space. The only thing keeping you from falling through the Earth right now is that the charged particles in the atoms that make up your body are repelled by the charged particles in the Earth.

Did you know that every second the Sun emits so many neutral particles that 65 billion of them pass through one square centimetre of earth? These particles, called neutrinos, move nearly at the speed of light, and because the Earth is mostly just empty space they pass right through it as if it weren’t even there. They pass through it because they have no charge and because the Earth is mostly empty space, while you don’t budge past the surface of the Earth because you and the Earth are made up of electrically charged particles. In the subatomic world, the poking and prodding that scientists do when they “touch” things takes on a whole new meaning.

Such is the sense of touch, which we use to explore the world around us. But astronomers don’t normally have the opportunity to touch the objects they investigate. The only way astronomers can know anything about the objects that we study is by seeing them. And much like the sense of “touch,” in which nothing actually touches in the way that we commonly think of it, when we see things we also do not have immediate experience with them.

Right now, you are looking at a computer monitor, which you know is there right in front of you. However, technically what you are seeing is not the computer monitor itself, but light emitted by the computer monitor that enters your eye, which causes a signal to be sent to your brain with the image of the words you are reading. Because the computer monitor is so close to you, it is easy to think that you are having a more direct experience with the thing itself. Any change that occurs in it, say if you move your cursor across the screen, shows up immediately. This is because the light coming from the monitor is moving incredibly fast—at 300,000 km/s.

In astronomy, where the distances to the objects that we study is so much greater than the distance to the monitor in front of your face, the distinction between the images that we see and the things that exist is important. As mentioned in Module 1, the Sun is eight light minutes away. This means that when you see the Sun in the sky you are not looking at the Sun in the same way that you’d look at a thing like the monitor that’s right there in front of you. When you look at the Sun, you’re seeing an eight minute-old image of the Sun. If you look at the Sun right now, you’re not seeing a thing that exists right now, but an image of a thing as it existed roughly when you started to read this module’s Learning Material.

The further away something is, the older the image is that we actually see. When astronomers observe a galaxy a billion light-years away, they’re seeing what that galaxy looked like a billion years ago.

Through this brief introduction, you should be starting to see how important it is for astronomers to understand the nature of the thing that we actually observe—i.e. the nature of light, which is our only source of information about astronomical objects. For if we do not understand that, we could hardly use light as a tool to learn about the distant objects that we see.

In this module, we are going to focus on describing the nature of light and on the way that telescopes are used—to collect greater amounts of it than our eyes can, to resolve finer details in the things we observe than our eyes can resolve, and to zoom in on parts of those images that we may be particularly interested in. Once we’ve learned a little bit about light, we’ll see that there are different forms of it to be observed, and that we can design different telescopes to observe light that can’t be seen with our eyes. Finally, we’ll investigate the means used by modern astronomers to record the images our telescopes collect, so they can be used to learn about the natures of the objects that emitted the light we observe.

By analysing those images, which are our only source of information about the cosmos, we learn about the nature of astronomical objects. But in order to do that, we need understand more than just the nature of light and how we can collect it—we need to know something about how light actually interacts with matter, since those interactions determine the properties of the light we observe. Those interactions will be the subject of Module 5, but already some indication of the way that works can be given as a result of our discussion so far in this introduction.