Searching for fish in the cosmic ocean.
Encyclopaedia Galactica.
In COSMOS chapter 12, Carl
Sagan introduced us in an exquisite way to the question: are we alone in the
Universe? We know that the laws of
physics are aplicable in all the Universe. Therefore, it is possible that, in
another place, another civilization has emerged with technological
capabilities. What we don’t know, is how probable this is.
It is remarkable that
after decades, the message in COSMOS continues to be valid. There are just a
couple of updates I would like to point out.
For the case of the
Drake equation, we have now a bit more knowledge. Back then, we didn’t know any
planet around other stars. Their existence was speculated from indirect
information. Nowadays, the study of exoplanets has revolutionized our
understanding of this topic. It has been less than a decade since we know
statistically that every star has at least one planet, and about one in five
stars has a planet in the habitability zone (1). This same zone where our “pale
blue dot” is sheltered by our Sun. If tonight we see the stars, we can then
imagine the great number of planets that can potentially have the capacity to
harbor life.
Just as in COSMOS times
we wondered about the prevalence of planets in other stars based on indirect
studies. Now, we ask ourselves in the same way about the origin and evolution
of life that resulted in our species. These fields of science are beginning to
have their own revolutions, and perhaps in the decades to come they will help
us solve some of the other variables in the Drake equation.
Nevertheless, there
exist more unknowns in the evolutionary process that give rise to a
civilization on our planet capable of communicating with the Voyager probes
(See more of Voyager in chapter 6.3).
But then, are there
other civilizations in our Galaxy?
Whatever the
probability of creating intelligent life is. What evolutionary processes
(biological or not) would be necessary for the creation of a civilization with
scientific and technological advances like our own? On Earth, the invention of
agriculture, mathematics and astronomy took place on various parts of the world
independently. Could this be the consequence of some universality in
evolutionary processes of technological civilizations, having reached a
sufficient cognitive capacity? Or could it be a coincidence specific to humans?
What could we say about other later processes such as urbanization? If any of
these universalities existed, then they would serve us as the Rosetta stone
served to understand Egyptian hieroglyphs, as Sagan entertainingly narrated in
COSMOS.
Since the times of cave
paintings in remote places, we have wondered about our origins, and about our
current comic solitude. In COSMOS, we saw how the Drake equation was used to
estimate the reality of our solitude. Unfortunately, we cannot go very far
without first assuming and speculating too much. Given the unknowns of various
factors, it is equally likely that there are millions of civilizations in the
Milky Way or that we are the only ones. In order to actually get closer to the
answer, a rigorous, systematic and scientific study is necessary.
In 1959, Guiseppe
Cocconi and Philip Morrison, prestigious physicist from Cornell University,
published the article: “Searching for Interstellar Communications”. There, for
the first time in history it was boldly pointed out that radio waves were a
reasonable means of interstellar communication, and with this the SETI1
field of research was created. The reasons for using radio waves are clear: 1)
they travel at the speed of light2; 2) they are minimally affected
by the interstellar medium; 3) they are cheap to manufacture; and 4) their
manipulation to send information is straightforward. As an illustration of the
potential of radio waves we show signals we collected from the Voyager 1
spacecraft at the outskirts of the Solar System. The spacecraft has a power no
greater than that of a home refrigerator (Figure). What, then, would an
advanced civilization be then capable of?
Voyager 1 spacecraft signal routinely detected
in September 2016 by the Green Bank radio telescope in West Virginia, USA. The
strong central signal is a tone without information, its purpose is to be
easily located. The information arrives in the sidebands at a known “distance”
from the central signal. The change in
frequency with respect to time described in chapter 6.3 is evident here. Image
courtesy of the author.
It is not by chance
that today our telecommunications technology permeates our everyday lives in
countless ways. These properties radio has, offer versatility in its use and
even more importantly, they provide the ability to create a message that would undoubtedly
be recognized for its artificial characteristics. Such a message would give us
the certain but indirect proof of intelligent life somewhere else in the
Universe, and as Carl Sagan said in COSMOS, “the history of our species […]
would change forever”.
The first search for
radio signals was conducted in 1961 by Frank Drake (whom I have the honor to
meet) and known as Project OZMA (2). This search as well as others conducted in
the following two decades were focused around specific frequencies also known
as “magic frequencies” because of their relation to known astrophysical
processes, and thus a reference in common with other civilizations. The most
common example is the emission line of the hydrogen atom at 1,420 MegaHertz.
The technological limitations
of the time allowed radioastronomers to search only on a reduced number of
frequencies or “channels” simultaneously. By the 1980s, this number increased
to a few thousands frequencies, and further technological advances increased
this number to millions of frequencies by the end of the 20th century. More
over, the frequency bands for the search were also widen. These days it is
possible to study billions of channels simultaneously, equivalent to giving
each person in the world their own frequency to tune into.
The search has also
increased in other aspects as well over the decades. Project OZMA was a modest
search of two neighboring Sun-like stars. Later projects made observations of
dozens, then hundreds, and these days thousands of stars have been searched for
signals. Moreover, the search has also diversified to nearby galaxies and other
regions of the sky.
COSMOS has been a
continuous inspiration since my childhood, and directed me to the field of
astronomy. It was a privilege for me to publish in 2017 the first results of
the Breakthrough Listen project, at that time were the most comprehensive
results in the history of SETI (3). It was an exciting experience, where we
found some peculiar signals which origin, we realized, was our own human technology,
telecommunication satellites orbiting the Earth.
Not all searches have
been on radio waves, other projects are looking for optical and infrared lasers signals with conventional telescopes. Other searches
have been of putative technological processes of great scales, as is the
well-known example of Dyson’s spheres3. Today we call the set of all
these types of posible signatures (including radio) as technosignatures4.
All these different types of searches have the same premise: to find signals or
signatures that nature cannot create by itself.
Multiple projects over
the years have been carried out by looking for these different types of
technosignatures. But after all these searches there hasn't been found any
definitive sign still. An unavoidable question that many have asked is: after
searching all these decades, can we say that we are alone in the Universe?
It is actually too
early to say this. The largest searches are still small. In the Milky Way there
are between 100 and 400 billion stars, if there were a million civilizations in
the Galaxy, as mentioned in COSMOS, then we could say that on average one out
of every 100,000 or 400,000 stars has a civilization, about 100 times more than
the number of stars that have been searched to date. Assuming that the nearest
civilization wanted to communicate with us, and assuming that it wanted to do
it with radio waves, what frequency would they use? Here we would have to
extend our search to other frequencies, to cover the entire possible range, we
would have to increase this by another factor of 100. And what if they are not
transmitting continuously, but sporadically? This would mean that we had to
always observe them. Today the stars are generally observed for a few minutes,
hours at most. Searches would have to increase by another factor of 1,000.
Multiplying these factors and ignoring other possible ones, we see that we need
to increase by a factor of 10 million, and this is an optimistic scenario
assuming a million civilizations in our Galaxy. As Jill Tarter, a radio
astronomer by profession and one of the most iconic characters in SETI history,
put it, “All
of the concerted SETI efforts, […] are equivalent to scooping a single glass of water
from the oceans. And no one would decide that the ocean was without fish on the
basis of one glass of water” (4).
The great search is yet
to begin, but now is not the time to get discouraged. The technological
advances foreseen in the coming decades will give us the appropriate tools for
the search. What we have to do is to keep “listening” and increase the search. In the past, there have been ups and
downs in the level of public or private support for this cause. In general it
has only been a handful of scientists involved, and the current projects still
lack the manpower. So there are many aspects to improve in the future.
I
conclude with what Guiseppe Cocconi and Philip Morrison in 1959 aptly said in
their inaugural article: "The probability of success is difficult to
estimate; but if we never search, the chance of success is zero”.
Notes :
1 SETI or the Search for Extraterrestrial Intelligence;
originally called CETI, or Communication with Extraterrestrial
Intelligence.
2 The speed of light is the speed limit in the Universe
as Albert Einstein discovered in 1905 with his theory of special relativity (See chapter 8.5).
3 First proposed in 1960 for Freeman Dyson, a talented
physicist from Princeton.
4 A subset of biosignatures, term used in Astrobiology.
Thus updating the somewhat obsolete SETI term.
Bibliography:
J.
Emilio Enríquez Rascón.
PhD
in Astrophysics.
Previously scientific
researches at the University of California Berkeley.
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