Where "N" is the number of stars in the Milky Way galaxy; "fp" is the fraction with planets; "ne" is the number of planets per star capable of supporting life; "fl" is the fraction of planets where life evolves; "fi" is the fraction where intelligent life evolves; "fc" is the fraction that communicates; and "fL" is the fraction of the planet's life during which the communicating civilizations live.
This serious-looking equation gave SETI serious footing as a legitimate intellectual inquiry. The problem, of course, is that none of the terms can be known, and most cannot even be estimated. The only way to work the equation is to fill in with guesses. And guesses--just so we're clear--are merely expressions of prejudice. Nor can there be an "informed estimate." If you need to state how many planets with life choose to communicate, there is no way to make an informed estimate. Your estimate will be simply prejudice.
As a result, the Drake equation can have any value from "billions and billions" to zero. An expression that can mean anything means nothing. Speaking precisely, the Drake equation is literally meaningless, and has nothing to do with science. I take the hard view that science involves the creation of testable hypotheses. The Drake equation cannot be tested and therefore SETI is not science. SETI is unquestionably a religion. Faith is defined as the firm belief in something for which there is no proof. The belief that the Koran is the word of God is a matter of faith. The belief that God created the universe in seven days is a matter of faith. The belief that there are other life forms in the universe is a matter of faith. There is not a single shred of evidence for any other life forms, and in forty years of searching, none has been discovered. There is absolutely no evidentiary reason to maintain this belief. SETI is a religion.
One way to chart the cooling of enthusiasm is to review popular works on the subject. In 1964, at the height of SETI excitement, Walter Sullivan of theNew York Times wrote a breathless book about life in the universe entitledWe are Not Alone . By 1995, when Paul Davis wrote a book on the same subject, he titled itAre We Alone? (Since 1981, there have in fact been four books titledAre We Alone? ) More recently, we have seen the rise of the so-called "Rare Earth" theory which suggests that we may, in fact, be all alone. Again, there is no evidence either way.
Back in the sixties, SETI had its critics, although not among astrophysicists and astronomers. The biologists and paleontologists were harshest. George Gaylord Simpson of Harvard sneered that SETI was a "study without a subject," and it remains so to the present day.
But scientists in general have been indulgent toward SETI, viewing it either with bemused tolerance, or with indifference. After all, what's the big deal? It's kind of fun. If people want to look for aliens, let them. Only a curmudgeon would speak harshly of SETI. It wasn't worth the bother.
And of course it is true that untestable theories may have heuristic value. Of course it is true that extraterrestrials are a good way to teach science to kids. But that does not relieve us of the obligation to see the Drake equation clearly for what it is--pure speculation in quasi-scientific trappings.
The fact that the Drake equation was not greeted with screams of outrage--similar to the screams of outrage that greet each new Creationist claim, for example--meant that now there was a crack in the door, a loosening of the definition of what constituted legitimate scientific procedure. And soon enough, pernicious garbage began to ooze through the cracks.
Now let's jump ahead a decade to the 1970s, and the theory of Nuclear Winter.
In 1975, the National Academy of Sciences reported on "Long-Term Worldwide Effects of Multiple Nuclear Weapons Detonations," but the report estimated the effect of dust from nuclear blasts to be relatively minor. In 1979, the Office of Technology Assessment (OTA) issued a report on "The Effects of Nuclear War" and stated that nuclear war could perhaps produce irreversible, adverse consequences on the environment. However, because the scientific processes involved were poorly understood, the report stated it was not possible to estimate the probable magnitude of such damage.
Three years later, in 1982, the Swedish Academy of Sciences commissioned a report entitled "The Atmosphere after a Nuclear War: Twilight at Noon," which attempted to quantify the effect of smoke from burning forests and cities. The authors speculated that there would be so much smoke that a large cloud over the northern hemisphere would reduce incoming sunlight below the level required for photosynthesis, and that this would last for weeks or even longer.
The following year, five scientists, including Richard Turco and Carl Sagan, published a paper inScience called "Nuclear Winter: Global Consequences of Multiple Nuclear Explosions." This was the so-called TTAPS (Turco, Toon, Ackerman, Pollack, and Sagan) report, which attempted to quantify more rigorously the atmospheric effects, with the added credibility to be gained from an actual computer model of climate.