To plagiarise Wikipedia shamelessly, the Fermi Paradox is the apparent contradiction between high estimates of the probability of the existence of extraterrestrial civilizations and the lack of evidence for such civilizations. Fermi is supposed to have asked “Where are they?”.
The extreme age of the universe (nearly 14,000,000,000 years) and its vast number of stars (around 70,000,000,000,000,000,000,000) suggest that if the Earth is even reasonably typical, extraterrestrial life should be common. Nobody knows whether intelligence is likely or unlikely to evolve once some kind of life arises, but even very low estimates suggest that there could be at least some intelligent aliens in our galaxy.
Current Earth technology can’t quite aspire to interstellar travel yet, but it’s clearly not an impossibility. Modest increments of our capabilities would make “slow” interstellar travel feasible, say several decades to reach the nearest star, but even at these snail-like speeds it would only take from 5 million to 50 million years to colonize the entire galaxy. The age of the universe has been long enough for that to happen 300 times.
So where is everybody?
You might have heard of SETI, the Search for Extra-Terrestrial Intelligence. You might even be running SETI@home on your computer instead of a screensaver. It’s software distributed to over 350,000 computers around the world, and which is supposed to be analyzing data from the Arecibo radio telescope. Although the source code has not been released, so it could be cracking credit card codes for all we know. Anyway, it hasn’t found anything. Nor has any other SETI project.
For myself, I think this is because they’ve been barking up the wrong tree. In the wrong forest. In the wrong country. On the wrong continent. All the SETI schemes to date have tried to detect alien radio signals. Messy, wasteful, analogue signals like twentieth-century television from Earth. Are all alien civilizations going to be stuck in the sixties, and permanently broadcasting their version of Flipper or Ready, Steady, Go! to all and sundry? I don’t think so.
Humans are moving to directional transmission channels such as cable, optical fibre, narrow-beam microwave and lasers, while radio with non-directional antennas is increasingly reserved for low-power, short-range applications such as mobile phones and Wi-Fi networks. Those signals are far less detectable from space.
Analogue television signals contains strong “carriers” that are very easily detected yet do not convey any information beyond their highly artificial nature. Every SETI project includes looking for carriers for just this reason, but advances in technology on Earth are replacing analogue TV with digital television which uses spectrum more efficiently precisely by eliminating or reducing components such as carriers that make them so conspicuous.
Using our own experience as an example, we could set the date of radio-visibility for Earth as 1901, when Marconi sent radio signals from Cornwall to Newfoundland. Visibility is now ending, or at least becoming orders of magnitude more difficult, as analog TV is being phased out. And so, if our experience is typical, a civilization remains radio-visible for approximately a hundred years.
Perhaps advanced alien civilizations even evolve beyond broadcasting at all in the electromagnetic spectrum and communicate by principles of physics we can’t manipulate or don’t yet understand.
SETI projects are probably listening for the jungle drums while the aliens talk to each other by mobile phone. That still doesn’t explain why they haven’t come visiting us though.