If, as some science-fiction novels and movies would have us believe, the universe is teeming with intelligent aliens, that begs the question: where are they? If they're out there, why haven't they come to visit us yet (and please, for the love of God, don't refer me to stories of alien abductions and UFOs)?
That question has been pondered many times over the years (most famously by Enrico Fermi in 1950), by many scientists far smarter than a lowly computer geek like me. The heart of the matter comes down to one question: just how common is intelligent life?
In 1960, Dr. Frank Drake proposed a method for estimating the number of intelligent species present in the galaxy by use of a mathematical equation, as follows:
N = R* x fp x ne x fl x fi x fc x L
Okay, you're saying, but what the heck does that mean?
Well, N is the number of intelligent alien species currently in our galaxy with which we might hope to be able to communicate.
R* is the average rate of star formation in our galaxy.
fp is the fraction of those stars that have planets.
ne is the average number of earth-like planets that are capable of supporting life per star that has planets.
fl is the fraction of the above that actually go on to develop life at some point.
fi is the fraction of the above that actually go on to develop intelligent life.
fc is the fraction of civilizations that develop advanced enough technology to release detectable signs of their existence into space.
L is the length of time such civilizations release detectable signals into space.
By multiplying out the values you think are probably correct for those variables, you can get an estimate of the number of intelligent, advanced civilizations currently present in our galaxy. And the best part is, our knowledge of the universe is growing all the time, so we can refine the values used in this model as time goes on.
Drake and his colleagues used the following values in 1961:
R* = 10/year (10 stars formed per year, on the average over the life of the galaxy)
fp = 0.5 (half of all stars formed will have planets)
ne = 2 (2 planets per star will be able to develop life)
fl = 1 (100% of the planets will develop life)
fi = 0.01 (1% of which will be intelligent life)
fc = 0.01 (1% of which will be able to communicate)
L = 10,000 years (which will last 10,000 years)
If you work out the math, you'll find that Drake and his team estimated that there are 10 intelligent civilizations in the galaxy capable of producing advanced communication signals that we should be able to detect.
In the past 46 years, however, we've already learned enough to refine the model somewhat. For example, Drake estimated star formation (R*) at 10 stars per year, but studies by NASA and the ESA show that the number is closer to 6.
Drake himself wrote an article for Wired Magazine in 2004 about revising the information for his model as new data became available. Drake now believes that there are far more intelligent species in the galaxy than he did fifty years ago.
But if Drake is right, then Fermi's original question stands: Where is everybody?