Looking at What You Cannot See

Posted on December 25, 2021 by psu

The James Webb Space Telescope finally took off into orbit today. In the lead up to the launch I had read up some of the background for this device, and let me just say that what they are trying to do with this telescope is super ultra bonkers.

The super short summary is: fold a telescope mirror that is four times the area of the Hubble and a giant foil sun-shield that is even larger into a (relatively) tiny little tube and shoot it into an orbit a million miles from the Earth where it unfolds by remote control. The sun-shield keeps the sun off of it so it can stay cold enough to collect the infrared light that its mirror is optimized for.

At this point my brain twitched a bit. Infrared? Why optimize a telescope for infrared?

Well. You may recall the story I told once about telescopes and spectral lines. See, it turns out that when you shine light through a cloud of glowing gas (say) and look at it with an instrument called a spectrograph that breaks the light up into a spectrum, the spectrum will be dotted with either dark or bright lines at very specific frequencies. Like this:

The mechanism that causes these lines to appear where they do had no explanation until the early 20th century, and was one of the founding problems of quantum mechanics. But that’s a story for a future article.

The spectral lines of hydrogen and other atoms have taught us more than you might be able to imagine about how the universe works and what it is made out of. As I also mentioned in my older article, one of the most important things they tell us is that the universe is expanding. Let us review.

Since light, in some ways, acts like a wave, it is subject to an effect called the Doppler shift. This is (sort of) the same physics that makes the pitch of a sound seem to shift up if the sound (shorter frequencies) is moving towards you and down (longer frequencies) if the sound is moving away from you.

When you take the spectrum of an object that is emitting light, and that object is moving away from you very quickly, you will notice that all of the spectral lines shift towards the red part of the rainbow. In the 1920s Edwin Hubble used this fact to show that the universe is expanding by collecting light from far away galaxies and noting that the further away the object was, the stronger the red shift. This fact combined with other evidence like the existence of the cosmic microwave background radiation has led to all of our current cosmological models for the overall evolution of the universe.

With this context, the motivation for building a giant IR telescope is more clear. If you make the mirror big enough to see things even further away than the Hubble can see, then the red shifts will be even stronger and will eventually shift the light completely out of the visible and into the infrared. So, what the JWST will do is fly in an orbit a million miles from the Earth and sit behind a giant piece of tin foil blocking all the light and residual IR warmth coming from the Earth, the Moon and the Sun. Then it will point its giant mirror into the great void hoping to collect light that is more than 13 billion years old. This will give us a view of the oldest and most distant stuff that we can currently observe in the universe. These things are so far away and so old that you can’t really imagine it.

Here: the stuff is three or four times as old as the current age of the solar system, which is already older than you can possibly imagine. See?

Anyway, if all goes well over the next few months there will be a new robot telescope sitting in space continuing the grand tradition in modern science of being able to look at things that we can’t even begin to see or imagine in our every day existence. This is both the basic tool and the ultimate allure of science. I of course ended up working with computers, which are nothing if not strange and invisible puzzles. But they don’t tell you too much about the big questions, like how the universe began. At least not directly. I still like to try and keep up with astrophysics though. The ability of humans to look at and learn about things that are so far beyond what we can see around us in our immediate “real” world is the only reason to have any real hope for the future these days. So let’s hope it works.