Science Puzzle
How Old Is the Light from Andromeda?
On a clear autumn night, far from street lights, you can spot a faint, fuzzy smudge in the constellation of Andromeda with your eyes alone. It's the Andromeda Galaxy, a spiral of hundreds of billions of stars, about 2.5 million light years away. It's one of the most distant things a person can see without a telescope.
When the light now reaching your eyes left Andromeda, what was living on Earth?
The Answer
No humans like us. The light set off about 2.5 million years ago, long before our species existed.
A light year is the distance light travels in a year: about 9.5 trillion kilometres. Light is the fastest thing there is, but it still takes time to cross space. If Andromeda is 2.5 million light years away, its light has been travelling for 2.5 million years to reach you.
At that time, Earth was entering a long run of ice ages. In Africa there were early human relatives, small-brained ancestors who walked upright and had begun to make simple stone tools. Our own species, Homo sapiens, only appeared around 300,000 years ago. The light hitting your eye tonight was already more than 2 million years into its journey by then.
So you never see Andromeda as it is now, only as it was. The same goes for everything in the sky: the Sun as it was 8 minutes ago, the nearest stars as they were years ago. The further away we look, the further back in time we see. Big telescopes such as the James Webb Space Telescope see galaxies whose light set off more than 13 billion years ago, not long after the universe began.
The principle: Looking back in time. Light travels at a finite speed, so the light from distant objects shows them as they were when it left, not as they are now.