Fundamental Astronomy

esearch, or OSETI, has already been started, but it is
still behind the radioastronomical SETI.
In 1974 Frank Drake used the Arecibo radio telescope
to send a message towards the globular cluster
M13 (which is not a good place for life). The message
contained 1679 pulses. This number has exactly two
factors, 23 and 73. Thus the receiver, who obviously
must understand some mathematics, could guess that
the message contains a two-dimensional picture. If we
ever detect such a message, we can be pretty certain of
its artificial origin, even if we were not able to interpret
the message.
The first serious SETI project (Search for ExtraTerrestrial
Civilizations) was carried out in 1960, also by
Frank Drake. This project Ozma observed two nearby
stars, τ Cet and ε Eri, at the 21 cm wavelength. Since
then radio technology has improved enormously, and
currently we are able to listen to a huge number of
channels at the same time.
There are two basic search strategies. In a targeted
search we listen to a few objects that are potential candidates
for harbouring life. In a wide-sky survey large
areas of the sky are scanned. Most current projects
belong to the latter category.
Observing time with large telescopes is expensive,
and priority is given to projects that can be expected
to produce positive results. In some projects, like
SERENDIP, this problem is avoided by having the
receiver sit piggyback on some other instrument and
listen to whatever the telescope happens to be observing.
Thus the SETI project does not need any dedicated
time of its own. The drawback is, of course, that many
of the target areas may not be interesting in the SETI
sense.
Detecting a potentially artificial signal among the
huge amount of data requires a lot of computing power.
The seti@home project has connected millions of computers
to an enormous virtual machine to analyze
the data. Anyone with a computer connected to the
Internet can load a screensaver program that will automatically
fetch packets of data and send back the
results.
Thus far not a single message sent by another civilization
has been confirmed. There have been some
interesting cases, but they have been single bursts of
unknown origin. They have not been detected later, not
even with more sensitive instruments.
20.11 Number of civilizations
20.11 Number of civilizations
Although no extraterrestrial civilization has been found,
we can try to estimate their number. The SETI pioneer
Frank Drake suggested a formula for calculating the
number of civilizations in the Milky Way capable of
communication at a given instant:
N = R × fp × fh × fl × fi × fc × L, (20.1)
where N is the total number of communicating civilizations
in the Milky Way, R is the annual birth rate of
stars, fp is the fraction of stars possessing planets, fh is
the fraction of planets being habitable, fl is the fraction
of habitable planets having some kind of life, fi is the
fraction of these planets having intelligent life, fc is the
fraction of intelligent civilizations that have developed
means for interstellar communication, and finally L is
the time in years that such a civilization has been communicating.
All the f -factors are probabilities that are
in the range [0, 1].
The astronomical factors (R, fp and fh) are the
only ones that are known with any accuracy. The biological
factors, fl and fi, involve a lot of guesswork.
The last two factors, fc and L, are even harder, since
they are related to the sociological behaviour of the
exocivilization.
Actually, the formula was intended as the basis of the
agenda of an influential SETI meeting held in Green
Bank in 1961. It splits the problem nicely into smaller
subproblems that can be discussed separately. But using
the formula to find the actual number of civilizations is
not very meaningful, since so many of the factors are totally
unknown. In the most “optimistic” case we could
find out that the distance between neighbouring civilizations
is just a few parsecs, but giving the probabilities
small (possibly more realistic) values, their product might
be so minute that we ought to be alone in the Milky Way.
At least the formula shows how little we know.
Even if favourable conditions and evolution of communicating
civilizations were relatively common, the
last factor may turn out to be the limiting one. If the
lifetime of a civilization is short compared to the age
of the universe, the chances of hearing a message from
another star are poor.
Earlier many astronomers seemed to think that exocivilizations
would not be that rare, while biologists
showed that the evolution of life had so many obsta-
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