Nib asked
How do telescopes look back in time?
Telescopes have no time machine: light simply takes time to travel, so the farther away something is, the older the light that reaches you. A big telescope's job is to collect that faint, ancient light.
5 steps, drawn by Nib. Read it here, or watch him draw it. Updated .
1light takes time
Light is fast but not instant: it travels about 300,000 kilometers every second. The Sun is 150 million kilometers away, so its light takes about eight minutes and twenty seconds to reach you. If the Sun vanished this moment, you would go on seeing it for eight more minutes. Every glance at the Sun looks into the past.
Light takes time to arrive, so you see the Sun as it was eight minutes ago.
a common mix-upWe do not see the sky as it is now; we see old light.
2far means old
Every distance in space is also a time. Moonlight is about 1.3 seconds old. The nearest star beyond the Sun, Proxima Centauri, is 4.2 light-years away, so we see it as it was 4.2 years ago. The Andromeda galaxy is about 2.5 million light-years away, and its light left before humans existed. A light-year is the distance light covers in a year, about 9.5 trillion kilometers.
A distance in light-years is also how old the light is in years.
The Andromeda galaxy is 2.5 million light-years away. How old is the light we see?
Show the answer
2.5 million years old.
3a light bucket
Looking far back is not only about distance: the light has spread out so much that almost none of it reaches you. A telescope's mirror works like a bucket in the rain: the wider it is, the more light it catches. The James Webb Space Telescope has a mirror 6.5 meters across, made of 18 gold-coated segments, and it can add up a faint trickle of light over hours.
A bigger mirror catches more of the faint, old light.
Why does a telescope need a big mirror?
Show the answer
To collect enough of the faint light from far-away, old objects.
4the farthest
The most distant galaxies Webb has measured are seen as they were less than 300 million years after the Big Bang. The universe is about 13.8 billion years old, so that light has been travelling for roughly 13.5 billion years. Webb is looking at galaxies as babies, long before our own galaxy took shape.
Webb sees galaxies as they were under 300 million years after the Big Bang.
If the universe is 13.8 billion years old and a galaxy is seen at 0.3 billion years, how long has its light travelled?
Show the answer
About 13.5 billion years.
5the whole idea
Telescopes look back in time because light travels at a finite speed. The Moon is seen one second ago, the Sun eight minutes ago, the nearest star four years ago, and the farthest galaxies Webb can find as they were over thirteen billion years ago. The night sky is a snapshot made of many different dates at once, and a big mirror is what lets us catch the faintest, oldest light.
Telescopes see the past because light is slow, and big mirrors catch the oldest light.
Every point of light you see tonight is a letter from the past, and the farther the sender, the older the letter.