An excerpt from “Distant Ruins,” Paul Gilster’s Aeon article about interstellar archaeology:

“Today, a small group of interstellar archaeologists is looking for evidence of those civilisations. They are tantalised by the possibility that the universe is not just a birthplace of alien cultures but also their necropolis.

We use the word ‘archaeology’ to describe this effort, because looking into deep space takes us deep into the past. The photons that strike our telescopes’ detectors take time to reach us: the light of Alpha Centauri, the nearest stellar system, is 4.3 years old when it arrives. It travels at 300,000 kilometres per second but has to cross 40 trillion kilometres to get here. Dig gradually into the soil and you push through layers accreted by wind, rain, construction, and flood. Dig deep into the sky, beyond local stars such as Alpha Centauri, and you push the clock back with the same inexorability. Epsilon Eridani, another nearby star, is seen as it was over 10 years ago. Light from the fascinating Gliese 667C, a red dwarf with three planets in its habitable zone, takes 22 years to make the journey.

In the cosmic scheme of things, these are trivial distances. Our green and blue world circles its star some 27,000 light years from the galactic centre. The glow we see at the Milky Way’s core began its voyage towards us at a time when prehistoric hunters were chasing mammoths across Europe’s ice sheets. The galaxy itself spans 100,000 light years, and its nearest equivalent, the great disc of Andromeda, is 2.5 million light years away. We see it as it looked when humanity’s ancestors walked the African savannah. When interstellar archaeologists tilt their telescopes to the sky, they are gazing into the deep history of the cosmos, but to find a civilisation more advanced than ours, they have to tilt their imaginations into the future. They have to plot out a plausible destiny for humanity, and then go looking for it in the cosmic past.”