Fermi's paradox (from the physicist Enrico Fermi in the years 1940-1950) reads as follows :
- the Earth is clearly younger than the Universe (by several billion years)
- if technologically advanced extraterrestrial civilizations exist (or have existed) in the Galaxy, then at least one of them has started an interstellar trip/colonization
- we can demonstrate that the colonization of the Galaxy requires only a few million years
- so we should see some signs of this colonization around us
- however, we have not noticed any signs! (most of the UFO stories can be explained by "terrestrial" or "human" causes, and the signs we talk about should be obvious, not just fleeting appearances)
- ... therefore ...
- the starting hypothesis is wrong, and we are the only technologically advanced civilization (and likely the only intelligent one) in the Galaxy!
Some objections and counter-objections :
- Not all civilizations are expansionist (agressive, colonizing), so they could have chosen not to colonize other systems.
Human beings are expansionist, and only one civilization would be sufficient. And if the principle of biological exclusion ("two species can not share the same ecological niche") can be applied to intelligent species, then there would be only one in the end!
- The galaxy is huge, and colonization would take an extremely long time.
Even with the current rocket technology, only a few centuries would be a sufficient amount of time to reach planets orbiting the closest stars (under hibernation or with colony-spaceships). If we add a few more centuries to build a new civilization, similar to the previous one, and continue the expansion, each step requires between 1000 and 10000 years. If we double the number of colonized planets every 10000 years, then less than one million years is necessary to colonize 10 billion planets, i.e. all the Galaxy!
Note that this process (expansion, colonization, "strengthening" phases, and so on) has indeed been applied by the inhabitants of the archipelagos in the pacific islands!
- Human beings wouldn't risk their lives in huge numbers to wander through space for centuries searching for planets.
We can send robots to learn about the terrain!
- The cost of such an enterprise would be prohibitive.
Only the first step would be costly, since the next step would be carried out by the next civilization. This first step could be made (with robots) at a slower speed, thus reducing significantly the propulsion costs (which are dominant).
Furthermore, the mathematician J. Von Neumann has demonstrated that it is theoretically possible to design an "intelligent" machine (in the sense of artificial intelligence, in an advanced version), and self-reproductive (on the DNA model). It could exploit the arrival planet to build copies of itself, including rockets, re-program and send them carrying on the colonization.
Finally, the cost of the radio communication attempts (like SETI, see below) for several thousand years would also be prohibitive and less "profitable".
- The machines would deteriorate and couldn't reproduce ad infinitum without any programming errors or anything else.
We already have efficient automatic correction methods for the errors.
- Intelligent machines could threaten their creators ("Frankenstein" complex).
We could design security operations such as Asimov's "three laws of robotics" ... or also consider that this new "intelligent race" would have the right to expansion independent of its creators!
- The technology required is far from being accessible.
This is wrong with regard to propulsion (and probably hibernation for a longer term). For artificial intelligence, the extrapolation is hazardous.
- Furthermore, maybe there is an intrisic limit (such as chaotic behavior) for the performance of the systems required for such an enterprise. Recent and numerous spatial failures show the weak current reliability.
Other speculations include the possibility that, actually, extraterrestrials just arrived (a few years ago) in the Solar System, or that they have been here for a long time studying humanity without being revealed (hypothesis of the "cosmic zoo"). Some people suggest that the asteroid belt, between Mars and Jupiter, rich in metals, is a preferred target to extract the ore necessary for the duplication of the machines proposed by Von Neumann. This activity would result in an infrared emission (note that there is an infrared emission from the asteroid belt, but it is most likely natural!)
Pvie ? Pint ? The debate remains open!