How many technologically advanced civilizations exist in the Universe? The Drake Equation
"If the universe is teeming with aliens, where are they all?" It is said that the Italian physicist
Enrico Fermi asked himself this question in the context of a conversation on the evaluation of
the probability of coming into contact with intelligent extraterrestrial life forms.
Apparently you can only give an answer by working your imagination and setting out to
make excellent science fiction. But in the course of the video we will see that this question is
not so far-fetched: there was someone who tried to give a quantitative answer instead,
strenuously trying to stay within the context of pure scientific speculation.
He did it by elaborating an equation that, once solved, would allow us to have a reliable estimate
of how many other civilizations share with us this strange condominium that could be our galaxy. This
someone is astronomer Frank Drake, one of the pioneers in the radio search for extraterrestrial
intelligence. He devised a famous formula, known as the Drake equation, which should
help us estimate the number of technological civilizations potentially existing in our galaxy.
The equation was presented for the first time in a conference at Green Bank, in November 1961. The
initiative was intended to trigger a fruitful discussion on the real prospects of a search
for non-terrestrial intelligence by radio means. The formula that Drake presented
aimed to highlight with a mathematical relationship the specific factors that play
an important role in the birth and development of intelligent civilizations in the universe.
As it will be seen, each term of the equation represents a key element in the
evolution of a civilization and the resulting numbers have a purely theoretical value.
It should not be forgotten that any evaluation that can be made in such a context
has an exclusively probabilistic appearance, given that most of the values that can be assigned to
the various parameters are devoid of experimental relevance and therefore only theoretical.
Although a univocal solution to this equation is not currently conceivable,
it is of undoubted scientific interest, since it illustrates the potential of this type of
investigation very well. Drake's hypothesis starts from the consideration that the number
of advanced civilizations that may exist in our galaxy, possessing the interest and the ability
to communicate over interstellar distances, can be reasonably estimated using the following equation:
N = Nstar * fp * ne * fv * fi * fc * D
N is the number of intelligent civilizations presumably present in our galaxy,
capable of making contact with each other. We must pay attention to the fact that
here we are talking about civilization and not about life in the most general sense of the term.
The reason for such a distinction made by Drake probably lies in the current
technological impossibility to go and see for oneself if there are forms of life elsewhere.
So the hope of discovering if, for example, a civilization exists on Vega or not is entirely
placed in the possibility of receiving a "phone call" from there, a message organized in such
a way as to tell us in an unambiguous way that life hasn’t only taken its root there, but which
has also reached a degree of cultural evolution such as to allow it to communicate its presence.
Nstar represents the number of stars in our Galaxy, estimated at the order
of two hundred billion stars more or less similar to our Sun: that is a huge number,
and this enormity is the main reason why even the hope of not being alone is among the last to die.
The most optimists pose N = Nstar, that is, they hypothesize that forms of intelligent
life have developed around all the stars, but at the present state of our knowledge it is an
impossible position to defend on the basis of a good physical and historical-political sense.
In an alternative writing of Drake’s equation, instead of Nstar R appears, which is the average
annual rate of star formation during the entire life of the Milky Way; R is obtained by dividing
the number of galactic stars by the age of Galaxy. It represents the only parameter on
which it is possible to hazard estimates, because it is calculated from the number of existing stars
and the number of years of life of the Milky Way: currently its value is assumed to be 20.
fp represents the fraction of stars with a planetary system. This term is subject to
continuous revisions by astronomers who, thanks to the considerable boost given by new technologies,
in recent years are finding more and more unequivocal evidence of the presence of planets
around many of the stars examined, thanks to missions such as the satellites Kepler,
GAIA, TESS and other similar missions. However, double stars that are at least 50% have a hard
time supporting planetary systems due to celestial mechanics problems and reciprocal perturbations.
But we can also be optimistic because then other factors will rule out double or multiple
systems. fp could be assumed equal to 0.5, ie half of the stars have a planetary system.
ne represents the part of the fraction of these planets that have conditions similar to ours
(the subscript e stands for "Earth"), therefore potentially suitable for the development of life:
a distance from their Star that can provide a quantity of heat and light neither scarce nor
too intense (ie that places it in what is usually referred to as "habitable zone"),
in an area of the planetary system where the tidal effects of the other planets
are negligible; a planet in which there is the presence of the essential elements for
the birth of life and with such gravity as to be able to retain an atmosphere;
an atmosphere that is able to induce a greenhouse effect so as to protect the soil from harmful
ultraviolet radiation. The estimation of ne is more complicated and it will be Kepler, TESS, GAIA
and the other similar space missions to help us. So far we have discovered a prevalence of giant,
often gaseous planets very close to the central stars. Several rocky planets have also been
discovered, but with greater difficulty than the first due to their smaller size. ne then depends a
lot on the type of star. The small and cold ones have a very narrow and close habitable zone and
therefore the "useful" planets should huddle close to them. But in this case the tidal force would
"block" them like the Moon with the Earth and they would be destined to always show the same
face to the central star with obvious dramatic consequences. Giant stars give more space but
live much less, and so on: each star has its own problems of "urbanization". A decent estimate for
this parameter could be 1 (ie one habitable planet for each star with planetary system).
fv is the fraction of suitable planets, in which life actually develops and evolves towards very
complex forms. This variable takes into account how many of these life-forming planets were
actually able to do so. Life as and perhaps more than any other event within our Universe
is a complex phenomenon, subject to many, too many variables and the physical, chemical and
biological path that leads to its development can be diverted or interrupted by a whole series of
factors, some currently known and many others unknown. Among the former we cannot but think,
for example, of the fall to the ground of one or more asteroids the size of the one that here on
Earth is supposed to have abruptly interrupted the existence of dinosaurs. A fairly sensible
estimate could lead to a value of 0.2 (i.e. one planet out of 5 would allow life to evolve).
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fi represents the fraction of these planets on which intelligent life forms develop.
This term must take into account how many, among all those lucky planets that have developed life,
have been able to guard it and allow it to evolve as intelligent life. What actually
makes life evolve up to intelligence? It certainly takes time, fairly constant climatic conditions,
few external disasters (falling asteroids, exploding supernovae, etc.).
fc is the fraction of planets where intelligent life forms develop
interest in interstellar communications. His estimate is also quite complicated:
a very dense belt of asteroids that would prohibit space travel or orbital telescopes.
Either a much denser cloud cover, or even cosmic modesty, intolerance towards others,
agoraphobia or simply a civil evolutionary stage but not technologically adequate for interstellar
communication could all be elements that make the value of this fraction increase or decrease.
D is the average duration in years of a technologically advanced civilization.
It tells us that civilizations are born, grow, reach a cultural apogee and economic prosperity;
but it also says that sooner or later they suffer the catastrophe of often self-destructive
decline. If an alien civilization were willing to communicate with other civilizations scattered
throughout the cosmos, but in its evolutionary hysteria the nightmare of a total war was opposed
to destroy the entire race or to leave a few survivors unable for various reasons to resume an
discourse oriented towards cosmic communication, we would never know of its existence.
It should be noted that the most delicate aspect of the calculation consists precisely in the
evident difficulty of assigning reasonably approximate values to the parameters,
as there is no experimental evidence that allows a sufficiently precise assessment
of their value to be made. Everyone can advance personal hypotheses and practice
calculating the value of N by inserting more or less reliable data on the basis of the results
deriving from the most recent discoveries in astronomy and exobiology. One can be
fundamentally pessimistic or optimistic: the remarkable fact is that, in many cases,
the estimate leads to a probability of the existence of intelligent life within
our galaxy very close to one. In recent years, following important astronomical discoveries,
it has been gradually possible to assign more and more precise values to some variables. However,
at least three of the parameters of the Drake equation remain entirely theoretical.
According to some scholars, however, it is possible to give a fairly precise estimate
of the value of the individual terms of the Drake equation, an assessment that seems to lead some
to the conclusion that life exists elsewhere, convincing many others instead of its absence.
The beauty of this equation is ultimately its most obvious flaw: that its apparently
rigorous scientificity attracts many serious scholars who are able to make extremely
varied and often strongly conflicting numerical evaluations on the possible value of its result!
The orbital data and the mass of exoplanets discovered so far are constantly increasing and
this data is closely linked to the availability of new and more sophisticated instrumental
technologies which, in the near future, would allow us to directly reveal the existence of
terrestrial-type planets. However, the scientific community is quite in agreement in considering
the common existence of terrestrial-type planets orbiting around solar-class stars (spectral class
G2 V of the Hertzsprung-Russel diagram). With similar arguments it is reasonable to hypothesize
that the stars of the main sequence, of a spectral class approximately similar to the Sun,
have a similar distribution of planets, with orbits stable enough to allow the
existence of liquid water indispensable for the origin and development. of life.
Drake’s equation tells us that we will find civilization only where conditions are the same or
slightly different from those we find here on Earth. The claim to scientificity of this
equation and of the theory that supports it could then seem to waver if one looked at it
as a multiplication of unknowns, but one would make the mistake of not considering the great
correctness of the method adopted to conduct the search for the value of its single terms:
it is precisely to protect this correctness of the scientific method that it has been chosen
to restrict the field of action of this research to what we know for certain about life
and its requests from our direct experience as living beings.
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