Mar 20 2011
The Drake Equation was humanity’s first serious attempt to think systematically about advanced extraterrestrial civilizations in our Galaxy. Devised by Cornell astronomer Frank Drake during the early 1960s Apollo Maslow Window, it was his ebullient goal to estimate their number and use radio telescopes to achieve contact.
Will interstellar probes, such as the one discovered on the Moon in the film “2001: A Space Odyssey,” ever really be found?
The number (N) of high-tech (e.g., communicative) civilizations in our Galaxy is traditionally estimated by considering 7 factors requiring stellar, planetary, biological, social, and technological information.
In 1961, Drake had good guesses about the astronomical factors, but little else. His surprisingly conservative estimate for N was 10 — hardly significant motivation for a radio search for ETs in a galaxy 100,000 light years across. But Carl Sagan made up for it; by 1974 his estimate for N was one million!
Today there are new data and ideas that illuminate the 3 biggest lingering mysteries involving N: 1) the abundance of Earth-like planets, 2) the origin of life and intelligence, and 3) the typical lifetime of high-tech civilizations.
This new information makes N seem more consistent with the Rare Earth Hypothesis of Peter Ward and Donald Brownlee (University of Washington).
Not only intelligent life, but even the simplest of animal life, is exceedingly rare in our galaxy and in the Universe … (However) life in the form of microbes or their equivalents is very common…
Two JPL scientists recently calculated that only about 2% of Sun-like stars have Earth-analog planets. The first four months of data on planet transits of 153,000 FGK stars, as observed by the NASA Kepler spacecraft, indicate that Earths are “relatively scarce.” (See: “Latest Data from NASA’s Kepler Mission Suggests Earths are ‘Relatively Scarce’.”)
Andrew Watson’s 2008 Astrobiology paper expands the anthopic model of Carter (1983) which assumed that an unknown number n of “critical steps” affect the timing and development of complex life and intelligence; the critical steps are
… defined as being intrinsically unlikely to occur in the time available.
Watson’s best guess is n=4 — i.e., appearance of prokaryotes, eukaryotes, cell differentiation, and homo sapiens — and that each event is separated by about 1 Gyr. If the probability for each step to occur either at or before the observed time (on Earth) is ~0.1, the cumulative probability of high intelligence developing on an Earth-like planet would be < 0.0001. This is consistent with Lineweaver and Davis (2002) who estimated that 13% of Earthilke planets older than 1 Gyr will experience biogenesis, based on the rapid appearance of life on Earth. The probability of 10(-4) seems optimistic considering biologist Ernst Mayr’s 1995 comment.
There have been perhaps as many as 50 billion species since the origin of life. Only one of these achieved the kind of intelligence needed to establish a civilization.
Longevity of High-Tech Civilizations
Princeton astrophysicist Richard Gott’s well-known and hotly-debated Copernican formula — aka the “Doomsday Argument” — was originally published in Nature in 1993. According to the New York Times (7/17/2007; J. Tierney) Gott has successfully used his technique to forecast the longevity of “Broadway plays, newspapers, dogs, … the tenure of hundreds of political leaders around the world.”.
In 2006 Gott’s approach received a vote of confidence from philosophers Bradley Monton and Brian Kierland in The Philosophical Quarterly who concluded that Gott’s technique is Bayesian and is a “useful tool for difficult situations” including those where little empirical data exists.
Gott can predict the future using only one piece of information: how long something has existed up to now. And he needs to be assured that there are no observational selection effects; i.e., there is nothing special about your location in time or space (the Copernican Principle). For example, using only the information that Homo sapiens has existed for 200,000 years, Gott predicted at the 95% confidence level that our species’ future duration is “between 1/39 and 39 times 200,000 years,” (5100 yrs and 7.8 Myrs).
A nuclear doomsday has only been possible since 1945 (66 yrs) so, at the 95% confidence level, it is unlikely to arrive in less than 1.7 yrs but most likely by 2574 yrs from now. An even shorter high-tech human civilization duration is suggested by the AI Singularity, described by Kurzweil and others, projected to arrive by 2045; this would give humans a total high-technology lifetime of only around 100 yrs. Note that the nuclear and singularity timeframes are less than the species lower limit, suggesting that our species will continue but possibly not with our nuclear or technological capability (at least under human control).
Estimating a 21st Century Value for N
We’ll use L — the longevity of a high-tech civilization in the Galaxy — as a parameter:
Using the values above, N = 1.4 x 10(-5) x L
(This assumes that the fraction of intelligent civilizations in the Galaxy that develop high technology is 100%.)
Therefore, N as a function of L (high-tech lifetime) is:
1) For the species UL (8 Myr), N = 112 (closest ETs are ~10,000 light years away)
2) For the species LL (205 Kyr), N = 2.8
3) For the Nuclear DD (2640 yr), N = 0.037
4) For the Singularity (100 yr), N = 0.0014
Initial Kepler results plus the Watson/Carter model of intelligence appear to preclude other intelligent ETs in our Galaxy unless their L’s are in the millions of years. This was attained only by our species upper limit, using Gott’s technique; the closest ETs would be ~10,000 light years away. Other high-tech civilization timescales — species LL, nuclear doomsday, and singularity — are consistent with the Rare Earth Hypothesis.