Getting Redder All the Time

How a New Space Telescope Will Let Us Look Back in Time

In honor of the first images from the new James Webb Space Telescope (JWST), we’re going to take a closer look at some of the science behind its creation.

What is JWST?

JWST launched in December of last year, and is just getting ready to release its very first images.

James Webb has four main instruments, and all of them are designed to detect infrared light. This is light that has a wavelength a little bit longer than the red light we can see with our eyes. But why are we interested in detecting infrared light?

Understanding Redshift

Detecting infrared light is important because it’s the only way we can get pictures of super distant galaxies. The farther out we can see, the more we can learn about the history of the universe. Detecting infrared light from faraway galaxies could help us understand how the universe came to be.

To understand why, let’s explore how different kinds of waves, including light waves, change as they travel through space.

To start, imagine you are observing an object that makes sound waves, for example an ambulance siren.

The frequency of the sound waves (how many waves pass by every second) is what determines the pitch of the sound, or how high or low it sounds. If the waves are close together, the pitch of the sound is high while if the waves are far apart, the pitch is low.

The siren is going to put out a consistent number of waves per second.
Let’s see what happens when the siren is moving away from you.

If we compare the waves of the stationary siren to the waves of the moving siren, we can see that the waves seem to be more spread out when the siren is moving away from you. This means that the wavelength of the sound wave is longer, which corresponds with hearing a lower pitch.

If the source of a sound is moving away from you, you hear a lower pitch than you would hear if the source was standing still. This is why it sounds like an ambulance siren is getting lower as it moves away from you.

This is known as the Doppler Effect and it applies to any kind of moving wave, not just sound. For instance, light is also a wave. So we can also see the Doppler Effect when looking at a moving light source. The Doppler Effect tells us that when the source of a wave is moving away from us, the wavelength of that wave gets longer. The faster the source is moving, the more the wavelength changes. When talking about sound, this corresponds with hearing a lower pitch, but with light, this corresponds with seeing a different color.

The wavelength of a light wave determines what color you see, with shorter wavelengths appearing as purple or ultraviolet light and longer wavelengths appearing as red or infrared light.

So, when we see light from an object that is moving away from us, the wavelength of that light becomes longer. A longer wavelength makes the light shift toward the red end of the light spectrum. This phenomenon is known as redshift.

Redshift is really important in modern astronomy since the universe is expanding. From our point of view here on Earth, it looks like everything is moving away from us as the universe expands. The further away a star or galaxy is, the faster it seems to be moving away from the Earth. As light from the most distant stars and galaxies goes through the Doppler Effect, its wavelength gets longer and redder than the visible light that we see coming from stars that are relatively close to us.

Is Seeing Redder Really Better?

This is where JWST comes in: since most of the light from the most distant stars and galaxies will be infrared light once it reaches us, we need a different kind of telescope to help us observe them. While the Hubble Space Telescope, in use since 1990, was designed for capturing visible and ultraviolet light, JWST is specifically designed to look at the light from distant stars and galaxies that appear in the infrared part of the light spectrum.

JWST will therefore help us to look even further out into the universe. The further out we look, the more we can learn about the very beginning of the universe. The light we see coming from the most distant stars takes billions of years to travel all the way to Earth. So when we observe those stars and galaxies, we’re seeing what they looked like billions of years ago. Since JWST will let us see more distant galaxies than we’ve ever seen before, it’ll help us look further back in time to see how galaxies looked and how they were formed in the early universe.

The first images of these distant and ancient stars and galaxies from JWST will be released on July 12, 2022.