The Drake Equation
About this video
What are the chances of finding intelligent life in our galaxy?
Narrated by Carl Sagan, this video details the individual elements that are used to calculate N in the Drake Equation (where N equals the number of civilisations in our galaxy with whom communication might be possible):
N = N* fp ne fl fi fc fL
Devised by astronomer Frank Drake in the early 1960s, the equation was formulated as part of SETI to pull together all the elements required to predict how hard it might be to detect extraterrestrial intelligence.
The problem with the equation is that many of the dependent terms from which N is calculated cannot be known accurately.
Many of these are related to the 'Fermi paradox', the apparent contradiction between the high probability of extraterrestrial civilisations and the lack of evidence for such civilisations existing.
Even for the parameters that can be calculated, these values are constantly changing, as we advance our knowledge of science and understanding of the universe.