How Does The James Webb Telescope See Into The Past
Humanity now has an infrared sensor that can see into the outer reaches of the universe. Here’s how it works.
The JAMES WEBB Space Telescope also known as JWST was launched December 25, for its 930,000 mile journey from Earth. This will be the next generation of the Hubble Space Telescope. Hubble has been taking amazing photos for more than 30 years. But it’s time to do something better.
The JWST will use its infrared cameras to search the darkest and most difficult-to-see areas of the sky. It will also assist with the search for exoplanets, and the exploration of the early days of the universe. This seems like a good opportunity to review the most important scientific concepts related to space telescopes.
Why put a telescope in space?
With just a pair of binoculars and a consumer telescope, you can see all kinds of cool stuff from Earth, including nebulae, comets, and more. However, if you are looking for high-quality images of distant galaxies you will need to use air. Although you might believe that air is transparent, this is only part of the truth.
Light can be described as an electromagnetic wave. It can also have different wavelengths. The wavelength range that people can see is very narrow. It can be seen from 380 nanometers (10 -9 meter) up to 700. The longer wavelengths are interpreted by our brains as being red, while the shorter ones are interpreted as being violet. These wavelengths can pass through the atmosphere without causing a decrease of brightness, so we can say that the air is transparent to visible lights.
The air is opaque for wavelengths of light we cannot see with our eyes. Consider the infrared area of the electromagnetic spectrum, or wavelengths that are longer than red. This means that much of the light can be absorbed in the atmosphere by water vapor and carbon dioxide. This is exactly what happens with global warming. When visible light hits Earth’s surface, it increases temperature and radiates infrared. The atmosphere absorbs some infrared from carbon dioxide, which further increases the temperature. This can cause bad effects for humans.
A ground-based infrared telescope is also susceptible to light absorption. It’d be like looking at the sky through clouds.
This problem can be solved by simply placing the telescope in space, where there is no air. (Of course every solution has its challenges. This is an extreme case where you have to mount a highly sensitive scientific instrument on a rocket, and then launch it. It’s a brave move.
Why does the JWST look at infrared light?
The JWST examines two infrared ranges: the near and mid-infrared. Near infrared light has wavelengths that are very similar to visible red light. It is the wavelength your TV remote uses. If you can’t find it, it’s likely under the couch cushions.
It is common to associate the mid-range infrared with heat. Everything produces light, it turns out. You are actually making light. Temperature determines the wavelength of light an object emits. The wavelength of light that an object emits depends on its temperature. While you cannot see mid-infrared light, you can feel it.
This is how it works: Turn on your stovetop in the kitchen and place your hand over the burner. Infrared light is produced when the element heats up. Although you cannot see the infrared light, it can be felt as heat when it touches your skin.
An infrared camera can see this type of light even though you cannot see it. This infrared photo shows me preparing a hot cup.
This is a false color image. The camera used infrared light to map colors, from yellow to purple. The hotter parts, such as the coffee pot, are represented by the brighter yellow areas. The darker purple parts represent colder things. Reality is more complex than that (you can also have infrared light), but the idea is there.
Great. But how does JWST view infrared light. Doppler effect is the reason.
The Doppler effect is something you already know. It is evident when a train or car passes you at high speed. The frequency of the sound changes because the source is moving first towards you and then away from you. When the vehicle is coming towards you, its sound will have a shorter wavelength and a higher pitch. It will then move away from you and produce a longer wavelength and lower pitch. (Here is an older post that provides more information.)
You can also create a Doppler effect by using light, but because the speed of light is so fast (3 x 10 8 m/s), it isn’t obvious in most situations. Due to the expansion of our universe, almost all galaxies visible from Earth are now moving away. Their light appears to have an extended wavelength to us. This is called a redshift. It means that the wavelengths appear to be longer and are therefore more red. This redshift is large enough to make it difficult for distant objects to see the interesting stuff in the infrared spectrum.
Another reason to use infrared for the JWST is that it’s difficult to see distant celestial objects unobstructed due to the gaseous and dust from old stars. They can scatter visible light much more quickly than they can infrared wavelengths. Infrared sensors can see through these clouds more effectively than visible light telescopes.
Scientists will require everything to be darkest around the JWST, as it is infrared-spectrum observing. To avoid emitting infrared radiation, the telescope must be extremely cold. It has a sunshield to protect it from the sun. It will block sunlight from the main instruments, allowing them to stay cool. It will also block out excessive light so that the telescope can pick-up the dimly lit exoplanets while they orbit their brighter host stars. It would be like trying not to see in darkness while someone shines a flashlight into your face.
What Does the JWST See Back in Time?
Light is a wave which travels very fast. Light could travel around the Earth seven times faster in a matter of seconds.
We must consider the time taken for light from a celestial object to reach our telescope or eyes. It takes 4.37 years for light from Alpha Centauri’s star system to reach Earth. It is 4.37 years back, so if you can see it in the sky you are actually looking into the past.
In reality, all you see is from the past. The past is 1.3 seconds long. Mars is approximately three minutes away from Earth when it is closest to Earth.
JWST is expected to be able see further than 13 billion years back in time, at the moment when the universe was evolving. It’s amazing, if you stop to think about it.
What is a Lagrange Point?
The Hubble Space Telescope can be found in low-Earth orbit. This is a nice feature because astronauts have been able to service it whenever needed. The JWST will be located at the L2 Lagrange Point. What exactly is a Lagrange Point?
Let’s take Hubble as an example of Hubble orbiting Earth. Any object that moves in a circle must have a centripetal force. This is a force that pulls it towards its center. The tension in the string is what pulls a ball around your head. This centripetal force, according to Hubble is the gravitational force that results from the Earth’s interaction with it.
The strength of the gravitational force decreases as an object moves further away from Earth. The centripetal force would decrease if the telescope was placed in a higher orbit (a greater circular radius). Hubble would need to spend longer to orbit in order to remain in a circular orbit. We would suggest that it has a lower angle velocity.
Although the JWST orbits around the sun, it is not the Earth, the same principle applies. The orbital distance between the sun and the Earth determines how long it takes to complete an orbit. What if the JWST is to orbit the sun further away than the Earth, and complete the solar orbit in the same timeframe as the Earth? To make it easier to control, you would need to keep the telescope in the same location relative to Earth. You need a trick to make this happen.
This trick is called a Lagrange Point, which is a place in space where the Earth and sun both exert gravitational forces in the same direction. Two gravitational forces pull on an object at this point to cause it to move in a circular motion. It can orbit the sun at a greater angular speed because of this. It also maintains it at a fixed position relative to the planet.
Five Lagrange points are available for the Earth-sunsystem. If there is an L2, then there should at most be an L1 right? The L2 Lagrange Point is approximately 1.5 million kilometers away from Earth. This is quite a distance compared to the 400 kilometers of low Earth orbit.
These are the other four Lagrange points of the Earth-sun solar system (not scaled):
The JWST will not be right at the L2 point. It will instead be in a slow orbit. It seems strange that an object could orbit somewhere there is nothing. But, keep in mind that the telescope will not orbit the L2 point. Instead, it will orbit the sun. It will appear to be orbiting L2 only from the Earth’s rotating reference point.
Why should humans spend billions on the JWST
The telescope cost $8.8 billion, with another billion being planned for operating costs. It might seem expensive to some. You could actually convince me that there are many. But the JWST remains a good idea. It is an investment in basic science. Science is like literature, art, and sports. It’s one of the things that makes us human. Our curiosity about the universe is part of our human nature. Using the telescope, we might be able to see what the cosmos looked like after the Big Bang.
The telescope will allow us to locate more planets around other stars, and even search for signs of life. We will learn about the formation of the first galaxies and their history. The James Webb Space Telescope will provide answers to many of the unanswered questions.
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