And then the light of an older heaven was in my eyes
and when my vision cleared, I saw Titans.
— Alan Moore
Today’s Occasional Paper comes to us from the James Webb Space Telescope.
So let’s start with some basics: nothing can travel faster than the speed of light. So when a telescope looks out into space, it’s also looking back into time. Look at the moon? You’re seeing it as it was when the light left it’s surface about 1.5 seconds ago. Look at the Sun? You’re seeing it as it was 8 minutes ago. The Sun could have exploded 5 minutes ago, and there’s no way you could possibly know about it until 3 minutes from now.
Okay, so keep going. Look at the nearest star? You’re seeing it as it was about four years ago. Look at the center of our galaxy? 30,000 years. The light from there left around the high point of the last Ice Age. Look out of our galaxy, at our neighbor galaxy Andromeda? About 3 million years.
Now it starts to get weird and interesting. Because as we start to look at things that are billions-with-a-b light years away — very distant galaxies — things start to change. That’s because we’re looking back into the distant past of the Universe. And the Universe is only 13.5 billion years old, so… yeah. In theory, if you had a strong enough telescope, you could see back to the Big Bang and the beginning of everything.
Of course it’s not that simple. The Universe is expanding. Distant galaxies are receding from us. More distant galaxies are receding faster, often at significant fractions of the speed of light (from our perspective). This means that the distance to them is greater than you might expect. It also means that their light is “red shifted” by the Doppler effect. Also, while the Big Bang was very bright, once it cooled down the Universe was just a hot dark cloud of gas, mostly hydrogen with a bit of helium mixed in. In that earliest pre-dawn epoch, there was not much to see, and no light to see with… until the first stars switched on.
And now for a brief historical digression.