General Astronomy/The Drake Equation
The subject of extraterrestrial intelligence is one which has a fascinating mix of fact and speculation. In dealing with extraterrestrial intelligence there is one important fact which was first pointed out by Enrico Fermi, the scientist that built the first nuclear reactor, and it's an obvious fact that is so obvious that one doesn't realize how remarkable it is until one thinks about it. This is the fact that when one looks at the skies, there is no obvious sign of extraterrestrial life.
Where are they?
It's a remarkable fact, because even with our limited technology level, we've made the sun look very strange to an alien astronomer. Type G stars simply do not emit much radio radiation, and to any astronomer within a fifty light year radius of the earth, there is something blindingly weird about the sun, in that something is causing the sun to appear to emit vast amounts of radio waves. Once the sun has been identified as odd, it wouldn't take that much effort to start looking into the characteristics of the radio waves and through things like Doppler shift, quickly figure out that the radio waves didn't come from the sun but rather from a planet in orbit. Once that gets figured out, you can look at the details of the radio spectrum and piece together things like the rotation rate of the Earth (the amount of radio waves you will see will be sharply reduced when the Atlantic and Pacfic ocean are facing you in comparison to the continents), and someone who is sufficiently clever might be able to figure out how to decode the radio output of the earth.
The paradox becomes even more odd if you think ahead a thousand or two thousand years. Keep in mind that a thousand or two thousand years is nothing astrophysically speaking. If you can imagine what we ought to be able to do, assuming that rates of technological growth don't slow down, there ought to be many things that would be astrophysically obvious. For example, the ultimate answer to the energy resources is to build a series of solar satellites that trap solar energy and beam it to the earth or any other planet in the solar system. If you trap enough solar energy, this will start to become obvious in the solar spectrum, in that if you plot many stars in the HR diagram then you will soon find that there is something very odd about some of the stars that don't fit on the main sequence at all. Another technology that might be astrophysically significant is that of nanotechnology and self-replicating machines. It shouldn't be too hard in the next thousand years to produce autonomous robots that can feed off asteroids and reproduce. Once you have these robots roaming the stars, exponential growth makes it very likely that you will run into one of them.
So there is something odd to be explained. The problem here is that we have enough facts to establish that there is a puzzle, but not nearly enough facts to figure out what the solution is. There are many ways of resolving the Fermi paradox, and given the lack of facts to constrain the solution, we are more in the realm of science fiction than of science at this point.
Some of the solutions are extremely depressing. One solution is simply that technological civilizations destroy themselves. There is no end to the number of ways we could destroy ourselves now, and it's also possible that advancing technology could allow for ways of self-destruction that we haven't thought about. For example, what would happen if nanotechnology made it possible for any human being to create their own atomic bomb or a deadly virus?
There are also sociological explanations that are rather depressing. We happen to live in a society that for the most part thinks that technology and curiosity are good things. However, even looking at examples in our own history, it's not clear that the way that we look at the world is the most natural or the most stable. There are the classics of dystopian fiction 1984 and Brave New World, and one would imagine that these worlds aren't that conducive to stellar colonization.
It may be possible that when and if we start exploring alien worlds, we'll find that at least in our part of the galaxy, that some previous civilization managed to extinguish, for whatever reason, all traces of life and then destroyed itself. But then again maybe not.
Before one gets too depressing, let me just point out one other obvious fact that is even more amazing when you think about it. We do know that somehow, someway, we've made it this far. We don't know how probable or improbable life and alien civilization is, but we do know that somehow someway, it happened at least once on the Earth. We don't know what is the resolution of the Fermi paradox, but what we do find remarkable is that first that we might someday figure this out, and that somehow, at least to some people, it really matters.
Plus the sun is 93,000,000 miles away from earth.
- The Drake Equation
- Takes into account:
- Number of stars in a galaxy. (N*)
- Fraction of stars with planets. (fp)
- Number of planets in a system that reside within the life-zone for that system. (nLZ)
- Fraction of suitable planets on which life actully begins. (fL)
- Fractions of lifeforms which evolve to intelligence. (FI)
- Fraction of star's life during which lifeform is communicative. (FS)
The last factor is the hardest to estimate, along with the number of communicative civilizations per galaxy (NC). The probabilities cover several orders of magnitude.