Are we alone? N= R x Fp x Ne x Fl x Fi x Fc x L
- 11 Agu 2015
- 1 090
N= R x Fp x Ne x Fl x Fi x Fc x L
N = the number of civilizations in our galaxy with which radio-communication might be possible (i.e. which are on our current past light cone);
R = the average rate of star formation in our galaxy
Fp = the fraction of those stars that have planets
Ne = the average number of planets that can potentially support life per star that has planets
Fl = the fraction of planets that could support life that actually develop life at some point
Fi = the fraction of planets with life that actually go on to develop intelligent life (civilizations)
Fc = the fraction of civilizations that develop a technology that releases detectable signs of their existence into space
L = the length of time for which such civilizations release detectable signals into space
Rate of star creation in our galaxy, R
Latest calculations from NASA and the European Space Agency indicate that the current rate of star formation in our galaxy is about 7 per year
Fraction of those stars that have planets
Recent analysis of Microlensing surveys has found that fp may approach 1 -- that is, stars are orbited by planets as a rule, rather than the exception; and that there are one or more bound planets per Milky Way star
Average number of planets per star having planets that might support life
Here it is understood that satellites might also serve as good candidates
In November 2013, astronomers reported, based on Kepler space mission data, that there could be as many as 40 billion Earth-sized planets orbiting in the habitable zones of sun-like stars and red dwarf stars within the Milky Way Galaxy. 11 billion of these estimated planets may be orbiting sun-like stars Since there are about 100 billion stars in the galaxy, this implies Fp x Ne is roughly 0.4. The nearest planet in the habitable zone may be as little as 12 light-years away, according to the scientists.
Fraction of the above that actually go on to develop life.
Geological evidence from the Earth suggests that fl may be high; life on Earth appears to have begun around the same time as favorable conditions arose, suggesting that abiogenesis may be relatively common once conditions are right. However, this evidence only looks at the Earth, and contains anthropic bias, as the planet of study was not chosen randomly, but by the living organisms that already inhabit it (ourselves).
Fraction of the above that develops intelligent life.
This value remains particularly controversial. Those who favor a low value, such as the biologist Ernst Mayr, point out that of the billions of species that have existed on Earth, only one has become intelligent and from this, infer a tiny value for fi. Those who favor higher values note the generally increasing complexity of life and conclude that the eventual appearance of intelligence might be imperative, implying an fi approaching 1. Skeptics point out that the large spread of values in this factor and others make all estimates unreliable.
Fraction of the above revealing their existence via signal release into space.
For deliberate communication, the one example we have (the Earth) does not do much explicit communication, though there are some efforts covering only a tiny fraction of the stars that might look for our presence. (See Arecibo message, for example). There is considerable speculation why an extraterrestrial civilization might exist but choose not to communicate. However, deliberate communication is not required, and calculations indicate that current or near-future Earth-level technology might well be detectable to civilizations not too much more advanced than our own. By this standard, the Earth is a communicating civilization.
Another question is what percentage of civilizations in the galaxy are close enough for us to detect, assuming that they send out signals.
Lifetime of such a civilization wherein it communicates its signals into space.
Michael Shermer estimated L as 420 years, based on the duration of sixty historical Earthly civilizations. Using 28 civilizations more recent than the Roman Empire, he calculates a figure of 304 years for "modern" civilizations. It could also be argued from Michael Shermer's results that the fall of most of these civilizations was followed by later civilizations that carried on the technologies, so it is doubtful that they are separate civilizations in the context of the Drake equation.
Using lowest values estimates
N = 7 × 0.4 × 10^−11 × 10^−9 × 0.1 × 304 = 8 x 10^−20
Suggesting that we are probably alone in this galaxy, and likely the observable universe.
Using higher values estimates
N = 7 × 1 × 0.2 × 0.13 × 1 × 0.2 × 10^9 = 36.4 million
This result's 26 order of magnitude higher estimate that the foregoing provides motivation for funding and executing SETI research
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