Assuming that the Earth was observed by an extraterrestrial intelligence in a galaxy 70 million light years away, our planet would look as it was in the time of the dinosaurs. Others more distant would perhaps witness the first aquatic life or a young planet with great volcanic activity. What is this about?
Past events are the only ones we can perceive. The reason is due to the time it takes for information from the objects around us to reach us, be it the light they reflect or the sound they emit. The difference between the two is abysmal: light travels at 300,000 kilometers per second (km / s), while sound travels at 343.2 meters per second (m / s).
That is why we see the explosion of a pyrotechnic before we hear its roar.
As outer space is an almost empty environment, sound has no way to be transmitted, so our knowledge about the universe is based mainly on the light that reflects matter (planets, stars, galaxies, etc.)
The Sun, our closest star, is located at a distance of 150 million km, therefore it takes 8.3 minutes for the reflected light to reach Earth. That is, we see the Sun as it was 8.3 minutes in the past.
On the other hand, to calculate the distance of more distant objects, scientists use as a reference the ‘light-year’ which is the path of light during a year in space and represents 9 trillion kilometers. For that reason, as we look further we also look further back in time.
For example, when we look at the Andromeda galaxy, located 2.5 million light years away, we are seeing it as it was 2.5 million years ago. Also, if it suddenly disappears, it would take that same amount of time for us to notice it.
The situation would be similar if we are the observed objects.
In this way, an advanced civilization in the center of the Milky Way (25,000 light years away) equipped with a telescope capable of looking through the Earth’s atmosphere, would capture nomadic human groups sleeping in caves and hunting animals. Pleistocene.
If they watched us from the galaxy NGC 2525 located 70 million light years away, and its telescopes also managed to penetrate the atmosphere, they would see the dinosaurs dominating the face of the Earth.
And if someone in the galaxy cluster Abell 370 Located 4.6 billion light-years away, you point your telescope where we are, you probably won’t find anything, since Earth was formed 4.543 million years ago.
Now we can understand why the exploration of very distant objects – that is, very old – with telescopes, allows scientists to study what the universe was like in its early days.