This is known as the "Fermi Paradox" and is sometimes summarised in the three words,
"Where is everybody?"
The Fermi paradox (or Fermi's paradox) is the apparent contradiction between high estimates of the probability of the existence of life on other world, such as in the Drake equation, and the lack of evidence for such civilizations. The basic points of the argument, made by physicists Enrico Fermi and Michael H. Hart, are:
- Our Sun is a typical star, and there are billions of stars in our Milky way galaxy that are billions of years older.
- It appears very likely that, some of these stars will have earthlike planets, and if the earth is typical, some might develop intelligent life.
- Some intelligent life forms might develop a civilisation capable interstellar travel, a step the Earth is investigating now.
- Even at the slow pace of currently envisioned interstellar travel, this galaxycould be completely traversed in about a million years.
Attempts to explain this rapidly become rather depressing as scientists talk about the " Great Filter which might stop life forms from reaching the point where they could begin using interstellar communications or catching light from their stars on a large scale, or any other activity we might be able to detect.
The reason such speculation can be frightening or depressing is that if whatever filters act to reduce the vast number of potential sites where civilisations might emerge to the much smaller number so far seen are not in our history and already passed, they must be ahead of us and indicate a bleak future.
Supposing for example the reason we don't see evidence of more super-civilisations in the galaxy is that the vast majority manage to wreck their planet in a nuclear war within two centuries of discovering nuclear fission, or by triggering massive global warming within three centuries of an industrial revolution, or get destroyed by Alien Space Bats or Berserker robots from space within a century of developing powerful radios which alert the aliens/robots to their presence. In any of those scenarios our chances of surviving the next couple of centuries are not good.
One of the main elements of the Fermi Paradox and arguments for the Great Filter, up to now, has been that the absence of extra-terrestrial megascale engineering projects.
And now there is the faint possibility that astronomers may have found giant alien megastructures orbiting a star called KIC 8462852.
I'm told that all the scientists who have looked at this are quick to warn that although the readings from KIC 8462852 are extremely strange and very interesting, we are a long, long way from being able to confidently say what they are, and it is far too early to start jumping to the conclusion that we have found evidence of intelligent life. There is a tiny chance that these are alien megastructures but they are probably something else.
Still, even a tiny chance is encouraging. Perhaps someone 1,400 light years away, and therefore at least 1,400 years ago, may have passed the Great Filter. So we may yet pass it too ...