How the number of civilizations in our galaxy can be calculated
It’s been some time since we started spending time thinking about life outside the Earth, always with many questions. On one hand, the chances of the existence of life outside the Earth are very high, but in another hand, this is often confronted with the fact that we never had contact with those lives. This contradiction between the high probabilities of the existence of extraterrestrial civilizations and the lack of evidence for them is what characterizes the phenomenon called “ The Fermi paradox.” But there are some mechanisms that we can use to set a possible rate for how many smart civilizations there are in our galaxy.
“The universe is a pretty big place. If it’s just us, seems like an awful waste of space.”- Carl Sagan
Imagine the number of intelligent civilizations in our galaxy as the letter N. If we want a number not necessarily equal but close to N, we will necessarily have to go through many factors that influence that number. Based on the parameters we have from Earth for a planet to find itself in suitable conditions for life, the existence of a star in its system would be essential. Therefore, the first of these influential factors can be considered as the rate or number of stars present in the galaxy, which can be called R. There’s a lot of other factors that should be considered. Some of them are the fraction of stars that have planets in their orbits functioning similarly to our sun (Fp), the rate of planets that orbit these systems that have managed to develop an environment conducive to life (Ne), the fraction of planets with an adequate environment in which life really came to exist (Fl), the fraction of the inhabited planets that have managed to develop intelligent life (Fi), the fraction of planets in which intelligent beings develop a technical communicative civilization with long-term technologies (Fc), and the life span of civilizations with the communicative ability and call it L.
All these factors can be brought together in an equation, and the relation of all these numbers would result in an approximate rate of that initial N number. This equation is known as the Drake equation.
Carl Sagan was one of the people who used this equation to make his own estimates and we can use his analysis to replace those terms with some estimated values. On his estimations, the number of stars in the Milky Way is around 400 billion; The Fp number of stars that have a planetary system is about ¼.; Ne is taken in a more conservative as 2; the Fl or the rate of planets on which life actually appeared is considered to be half or ½.
The partial result that we have to multiply the terms so far is 100 billion, and from here the values start to be more difficult to be stipulated since the technological path that extraterrestrial societies can take involves many factors that we don’t know and this path wouldn't necessarily be the same to that taken by humans here on Earth. But Sagan considers Fi as 1/10 and Fc as 1/10. Multiplying all the values so far we have the number of 1 billion environments in which intelligent life has emerged at least once. Lastly, this number is multiplied by the value L, which’s considered by Sagan by the extremely small number of 1/100 millionths.
As a result, we have the number 10. About 10 smart civilizations existing in the presence of our galaxy. But Sagan also used to consider that we can think of an alternative in which societies have a relatively low rate of extinction on the part of their development in which, unlike the previous survey, the life span of civilizations with the communicative ability (L) would instead of 1/100 millionths, be 1/100 Centesimus. In this more optimistic alternative view, the number N would be in the millions. There are other equations more recent than Drake’s one and predictions in which in general find different results because they consider different elements in their methods.
Now if we think about the fact that we have never had contact with these civilizations, we can consider some main causes. One of them could be a tendency in self-extinction civilizations after reaching a high technological level, that considers that all those projections of the future of humanity that involve our self-extinction could be a tendency not only for us but for most highly developed civilizations. In this case, we can think that there were very intelligent civilizations in the past but they were extinct at some point, or that they are even alive in the present but will be extinct in the future before they even reach the necessary level of development to build contact with our planet. And vice-versa.
Another explanation is that these civilizations don’t yet exist because maybe we are the pioneer, which can be possible since we do not have this external reference to know whether our existence was early or late in relation to others. There’s also the very popular and accepted answer maybe this contact hasn’t happened yet for just a distance factor, which considers that the distances that separate us are very large, and perhaps that is the main cause.
Whether it’s 10 or 10 million these numbers show that our forecasts in these fields still variate a lot according to the estimates we make based on information we don’t have, and even when we stipulate some valor that seems to be reasonable, the margin of error is still very high and that is why we are still far from exact statements on the subject. If science makes us know much more about the universe, this same knowledge brings us much more questions that we don’t have the answers to yet.