Light

A tale from light of faraway objects

Humans have five external senses: hearing, smell, taste, touch, and sight. If something is so far away that we cannot hear, smell, taste, or touch it, sight is often the only way of gaining knowledge about it. Our vision works by light entering our eyes, where it strikes cells called photoreceptors. These sensory cells send a signal to the brain about the light that has hit them, and from this the brain constructs an image of our surroundings.

When we "see" a lamp, what we are seeing is light from the lamp that has travelled through the air and into our eyes, and from this we can gain information about the lamp. The same happens when we look at stars, as they are far too distant for us to use any other senses on them. They are also far too distant for us to send space missions out to explore them. But even though the stars are far away, we can see the light from them, and that can provide a lot of information on for example how far away, large, hot, on what kind of stars they are, and on whether they have planets around them. (Read more about Stars and Exoplanets.)

Light is immeasurably important in astronomy, as it is often the only thing that can travel the entire long distance through space and reach us here on Earth. It is therefore often the only way we can gain knowledge about very distant objects. Astronomy is therefore mainly a study of light! But what is it exactly?

Light is energy

Light is a form of energy that can travel through space. It is also what we call electromagnetic radiation, so if you have ever heard that term before, know that it is simply the same thing as light! Light is very difficult to describe because it is incredibly small, and when things become that small they start to behave strangely. Light has neither a size nor a mass, and it can behave both as a particle (like a tiny ball flying through space) and as a wave (like waves in the ocean, or sound waves moving through air). When it is described as particles, we talk about photons, while when it is described as a wave, we describe its energy in terms of how long the waves are, what we call the wavelength.

Light is the fastest thing we know of in the universe, and it travels at 300,000,000 metres per second (299,792,458 m/s to be more precise!). This is its speed in a vacuum, meaning in completely empty space, which is what most of the universe is made of. Light can actually slow down when passing through a medium like water or glass, which is what causes lenses and prisms to bend light and why rainbows form! However, in the vast emptiness of space nothing slows it down. We know this because scientists have measured the speed of light very accurately and believe it is the ultimate speed limit in the universe. The speed of light is the same regardless of where it is observed or measured from, and this gives rise to a whole range of remarkable phenomena.

The speed of light was first described by the Danish astronomer Ole Rømer in 1676, who was studying the movement of Jupiter's moon Io. He noticed that it took longer for light from Jupiter to reach Earth when Earth was further from Jupiter in its orbit than when it was closer, and from this he was able to calculate that light must have a fixed speed. He calculated that light travels at 230,000,000 m/s, which is actually remarkably close to the value we know today!

It takes around 8 minutes for light to travel from the Sun to Earth, so we say that distance is 8 light-minutes. It takes 4 hours for light to travel from the Sun to Neptune, so we say that distance is 4 light-hours. It takes 4 years for light to travel from the nearest star, Proxima Centauri, to the Solar System, so we call that distance 4 light-years. So if you have ever heard the term light-year, now you know it is simply the distance light can travel in one year!

Visible and invisible light

Different light has different energy, and that is what creates colours! When we see different colours, we are seeing light with different amounts of energy. This becomes very clear when we look at a rainbow. In a rainbow, light has been sorted by its energy, or its wavelength. At one end of the rainbow we have light with a lot of energy and a short wavelength, which gives blue and violet light. At the other end we have light with less energy and a longer wavelength, which gives red light.

The rainbow shows all the light we can see with our eyes. The sensory cells (photoreceptors) in our eyes have evolved over many millions of years to detect light (photons) with exactly these energies, because this is the light we receive most of from the Sun, making our eyes excellently suited to seeing in sunlight. 

The visible light we see in a rainbow is not the only light we receive, however, either from the Sun or from the rest of the universe. There is much more light that is invisible to us. If light has more energy than visible violet light, it becomes ultraviolet. Ultraviolet, or UV light, is what causes us to need sunscreen in summer. Even though we cannot see UV light with our eyes, its energy can damage our skin, which is what causes sunburn and in the worst cases skin cancer.

Light with even more energy we call X-ray light, and it can travel through the body. This is why we use X-rays in hospital scanners, where they make it possible to photograph things like broken bones.

The most energetic light with the shortest wavelength is called gamma radiation, which can travel through both people and some metals. It is very dangerous for the body.! Fortunately, very little gamma radiation reaches us from the Sun, so we are not harmed.

At the other end of the spectrum we have low energies. Light with less energy than visible red light is called infrared light. Infrared light is not only emitted by the Sun, but actually by everything that has warmth! We humans therefore constantly emit infrared light, or heat radiation, because of the warmth of our bodies.

If light has even less energy and longer wavelengths, it becomes microwaves. Microwave light is what we use in microwave ovens to heat our food! Microwaves have long wavelengths compared to visible light, and we can actually see evidence of this on our microwave ovens. If you look at your microwave, you will see that its door has a kind of mesh grid in front of the window. This is so that visible light can pass through the small holes in the mesh, allowing you to see your food being heated. Microwave light, on the other hand, has such long wavelengths that it cannot pass through the mesh, which is fortunate, because otherwise we would be cooked along with our food!

The light with the least energy and the longest wavelengths is radio waves. Radio waves can travel incredibly long distances without being disrupted, which is why we use them in our wireless electronics, such as phones, Wi-Fi, GPS, and of course radio. Every time you make a phone call, your signal travels through the air using radio waves. Radio waves are not harmful to humans at all, and they are everywhere all the time!

Light in the universe

When we observe the universe, we want to look at all the light, both visible and invisible. We therefore build telescopes that can observe light across many different energies and wavelengths, and our telescopes can see things the human eye would never be able to see.(learn more about Telescopes and Observatories).

By observing light at different wavelengths we are able to observe different phenomena in the universe. In the image below you can see how the Andromeda galaxy looks when observed with different instruments designed to detect light at different wavelengths.

Top left, we see how the Andromeda galaxy looks in visible light, which is how the galaxy would appear if we looked at it with our own eyes. Here we mostly see the light from the many stars in the galaxy, which makes sense since it is precisely a star's (the Sun's) light that our eyes are designed to see. If you look carefully, you can see some dark rings in the galaxy. These are regions with a great deal of dust and gas, and they appear dark because the dust and gas block the visible light, like a dark cloud in the sky.

Top right (or the yellow light in the middle image) we see Andromeda in infrared light, the light that comes from heat and has lower energies and longer wavelengths than visible light. This light comes mostly from the large clouds of dust and gas in the galaxy, where stars are being born (read more about Galaxies and the birth of Stars).  If you compare this image with the visible light image, you may notice that the areas that are dark in visible light glow brightly in the infrared. This is because the clouds do not block infrared light in the same way as they do visible light, and the infrared light can therefore pass through the clouds and reach us. Bottom right (or the blue light in the middle image) we see Andromeda in X-ray radiation, light with very high energies and very short wavelengths. X-ray radiation is emitted from the most energetic regions of the galaxy, such as areas around supernovae, very energetic stars, or the galaxy's centre (learn about Supernovae).

If we combine the different forms of light into one image, we get what we see bottom left. By observing the universe across all these different wavelengths, we can gain knowledge not only about what is visible to our human eyes, but also about what is invisible to us, and we need all of it if we are to solve the mysteries of the universe!

The faster you go, the stranger it gets

As mentioned earlier, light is the fastest thing we know of in the universe, and it travels at a constant speed of around 300 million m/s regardless of where we observe it from. It takes four years for light to travel just to our nearest neighbouring star, which is quite a long time if we ever want to travel between stars in a spaceship in the future. In science-fiction films we therefore often see spaceships travelling faster than light, but researchers actually do not believe this is physically possible! If you travel faster and faster until you approach the speed of light, strange things begin to happen.

So let us say you are sitting in a spaceship travelling so fast that it begins to approach the speed of light, and see what happens. First, time begins to slow down. It will feel as though time is passing normally, but if you compared your clock on the spaceship with a clock back on Earth, you would see that your clock is running more slowly. What feels like a few months to you could be several years back on Earth. If you get even closer to the speed of light, your time will simply stand still as seen from the outside.

Second, it becomes harder to increase your speed the closer you get to the speed of light. It is as though you become heavier and need more and more energy to go faster. Eventually you would need an infinite amount of energy to accelerate further, and so you can never actually reach the speed of light. Light however does not have this problem, since it has no mass at all and is made entirely of energy.

The famous scientist Albert Einstein described these phenomena in his theory of relativity. He describes how time and space are linked together in what we call spacetime, and how it can all be bent by gravity. The one thing that cannot be bent or stretched is the speed of light, it will always be the same, and it will always be the fastest thing in the universe.

One of the most remarkable phenomena described by Einstein's theory of relativity is Black Holes.