We live in a galaxy populated by hundreds of billions of stars. Earth-type planets are supposed to be in the billions. So, why in this vast space, have we not (yet) had contact with extraterrestrial civilizations?
Is there life elsewhere than on Earth or are we absolutely alone in the universe? This question has taunted humanity for centuries. The idea of other forms of life in the Universe is not far-fetched; there are hundreds of billions of stars in our galaxy — and hundreds of billions of galaxies in the universe — and most of them are surrounded by planets.
In addition, 13 billion years have passed since the formation of the Milky Way, which leaves a lot of time to life to emerge on a multitude of planets. So, as the famous Fermi paradox states, if there is life and extraterrestrial civilizations elsewhere in the Universe, why have not these beings come to find us? Where are they?
We are alone
The very first possibility is that we are absolutely (and desperately) alone in the entire universe. There would be no life anywhere but on Earth. All other worlds would be sterile. Or else, life could have started and then be annihilated by a cosmic event. This is possible but it is hard to believe when we know, according to statistics, that there would be at least 40 billion habitable exoplanets just in our galaxy. We would rather want to think, on the contrary, that life abounds…
There is no intelligent extraterrestrial life
First, how do we define intelligent extraterrestrial life? Are we part of it? After all, there is perhaps life elsewhere, but of a primitive nature… microbial for example. Or plants and animals that would not (yet) have the ability to communicate beyond their planet (for the Earth, this was the case until recently).
Extraterrestrials do not use technology
To detect our presence on Earth, an Extraterrestrial civilization would need some advanced technologies. Similarly, to detect them, they must live traces behind for us to find such as radio waves. So a extraterrestrial species, which does not have the technology to communicate with other worlds is almost impossible to detect.
Furthermore, their evolution could be very different from ours, so they could very well not be interested in technology. Or, do not want to use it…
The extraterrestrials have a much more advanced technology than ours
Another case: their technology could be much more advanced than ours. It is possible that they use modes of communication that we do not know (yet) and that are impossible to decipher using our current tech.
Extraterrestrial civilizations self-destruct
When we see the challenges that humanity faces, especially since the middle of the twentieth century, it is possible to think that elsewhere, on other planets, so-called “intelligent” civilizations could have caused their own disappearance. As far as we are concerned, there are many threats of collapse and / or extinction: nuclear fire at the end of a world conflict, severe global warming (famines, droughts, diseases, wars, collapse of democracies, etc.) combined with the sixth massive extinction that is currently happening, epidemics, etc. Nevertheless, we can hope that man will be able to avoid such issues.
The universe is a very dangerous environment
As we saw above, a sixth mass extinction is currently happening on Earth. While the latter is caused by our species, the previous ones all had a natural origin: natural climatic changes and also… asteroids, as was the case for the previous biological crisis, 65 million years ago.
Today, we know that asteroids may one day endanger life on Earth and we also know that there are still other cosmic events that could annihilate us, such as surpernovae, quasars and also, near us, violent solar eruptions. It is therefore possible that elsewhere, and frequently, life has not had time to develop. Consider, for example, that the red dwarfs around which rocky planets are often found (see Trappist-1 and its 7 planets) are obviously ravaging stars. Their repetitive anger greatly reduces the chances of life to develop.
The Milky Way is very big
To explain why a contact with an advanced extraterrestrial civilization has not yet taken place, another possibility is that our galaxy – and still more, the universe – is so big that it has not yet been able to happen. With a diameter of 100,000 light-years, we can imagine that signals emitted at the other end of the Milky Way will take several millennia to arrive to earth. Everything depends on the distance and also when it was done. And then, let’s not forget that there are hundreds of billions of stars. We simply have not been spotted… yet. We may not even be on their lists. The choice can be immeasurable. For example, if they emitted a signal 100 years ago and are 10,000 light years away, it will still take 9,900 years.