Classical SETI 2. SETI Science (The Drake Equation) 

Browse any major SETI website and you will find a highlevel topic called "SETI Science". Click it to learn something of the scientific foundation for the SETI program. I have found that invariably this brings the reader to the Drake Equation page at The SETI Institute. SETI science is The Drake Equation. The Drake Equation is actually a mathematical statement of the SETI fundamental assumptions and is part of the SETI paradigm to be discussed in a later section. The equation is supposed to provide an estimate of the number of communicative technological civilizations in our (Milky Way) Galaxy  obviously something that would be highly desirable to know! It is ubiquitous in the SETI literature so I will not take the trouble to repeat the usual description and explanation of its factors. But just to remind the reader of the equation's form  the product of guessed numbers and fractions  which I do wish to discuss, here it is as found the SETI Institute's page:
The SETI Institute does give a complete explanation and even provides a handy calculator by which anyone can make their own estimates. But the equation has some important limitations. The first has to do with the estimated error of the result. Clearly, the error in the result is going to be at least as large as the largest error in any of the factors, and many of the factors at this time represent nothing more than the wildest guesses. Estimated values of some of the factors can vary over a range of two orders of magnitude. The range of possible results is then many orders of magnitude. But actually the Drake equation is not necessarily used to estimate numbers of civilizations. It has been used in a sort of reverse reasoning process to infer relationships between some of its factors, given that the number of technological civilizations (the equation's result) seems to be very small. That number is constrained to be small by inference from the lack of evidence of technological civilizations. (See Fundamental Assumptions for a description of the circular reasoning involved here.) David Brin's seminal review paper (1982) provides an excellent example of working with the Drake equation in this manner. I would have no objection to using the equation this way, except for the fact that the process is driven by highly erroneous data! There is another very large problem with the Drake equation. It is that the factors in the equation are presumed to be independent of one another. That is, the value of any one factor is assumed to be unaffected by the value of any other. And this would be true if space travel does not occur. Consider, for example, some of the fractions in the equation: the fraction of planets where life develops, the fraction of life sites where intelligence develops, the fraction where technology develops, and also the number that gives the lifetimes of communicating civilizations, and so forth. These factors are defined within the present paradigm of independent development of civilizations. They simply don't work in the presence of colonization. Brin understands the problem clearly and has formulated a modified Drake equation that includes the effect of contact between civilizations and defines humankind's contact crosssection with each expanding Galactic civilization. His model introduces a few new parameters, providing the opportunity to reason mathematically with what had been philosophical concepts. It is a commendable effort but it is out of touch with realities discussed elsewhere in this paper. Furthermore, one wonders what has come of it. Today's SETI proponents  many of them the same individuals who were guiding the program when Brin wrote his paper  appear oblivious to the contact considerations. The Drake equation is still promoted in its original form. SETI philosophy is as sterile as it ever was. 

