|
Essentially, Drake came up with an equation to calculate the number of alien civilisations in our galaxy. The results depend on a series of variables which are currently mostly unknown; however, estimates of these values usually return a large number of alien civilisation.
Fermi said that if that were the case, why haven't we detected or met any of these civilisations yet? The galaxy has been around a long time, and we haven't found one piece of evidence that extra-terrestrial life ever existed.
The arguments are more complex than that, but that's the gist.
There are quite a few theories put up to explain Fermi's Paradox. It could be, for instance, that any sufficiently advanced civilisation eventually discovers technology that will likely lead to its destruction. Perhaps nuclear weapons, perhaps something we haven't yet uncovered. Or perhaps intelligent life is such a fluke, such an unlikely event, that Earth is the only planet in the galaxy it has occurred upon.
In the novel Accelerando, my current favourite work of sci-fi, the author puts forward the theory that sufficiently advanced intelligences become so large that they lack the resources to spread out to other solar systems. Other novels have put forward the suggestion of "berserker" Von Neumann machines that exterminate any intelligent life they come across.
It could be that alien civilisations progress to an unknown and undetectable form of communications before we can detect them. If they're only using radio waves for a few hundred years, then switch to, say, quantum entanglement, then the chance of two civilisations of that tech level intercepting each others radio transmissions would be fairly low.
It could also be that more advanced civilisations have no interest in us, and have long since outgrown the need for planets with air and water.
|