Finding Aliens

59 紙皮


Ancient people were fascinated by the dark sky, most of them wanted to understand the meaning or the mystery of the universe. Apart from that, they also wanted to find other lives which were similar to human. “What is it like out there?”, asked the ancient people when they looked at the starry sky. It was perhaps a simple question like such that ignited the very first desire for astronomical research in the human history. Among various subtopics like the form of existence of the universe, the possibly surviving outer space creatures have also been grabbing much attention. Specialised studies on such a topic using scientific approaches are known among astronomers as Search of Extra-Terrestrial Intelliengece (SETI).

Drake Equation

In 1960, a young radio astronomer Frank Drake performed the first SETI experiment. He conducted the world’s first microwave radio search, aiming for receiving signals from other planetary systems. Drake used an 85-feet West Virginia antenna to point to the direction of stars Tau Ceti and Epsilon Eridani, which are in the constellations Cetus (Whale) and Eridanus (River) respectively. These two stars were chosen because they were relatively close to the Sun. Although he was not able to detect any reply, this experiment still aroused others’ interest among the astronomical community. This experiment was later named “Drake’s Project Ozma”.

Figure 1:  The antenna used in the first SETI experiment, pointing to the direction of stars Tau Ceti and Epsilon Eridani

In 1961, Drake asked, “what do we need to know about to discover life in space?” This question helped him to set the “Drake equation”. According to the web page of SETI Institute, the annotations of the unknowns in the equation are as below.
N = The number of civilizations in the Milky Way Galaxy whose electromagnetic emissions are detectable
R∗ = The rate of formation of stars suitable for the development of intelligent life
fp = The fraction of those stars with planetary systems
ne = The number of planets, per solar system, with an environment suitable for life
fl = The fraction of suitable planets on which life actually appears
fi = The fraction of life bearing planets on which intelligent life emerges
fc = The fraction of civilizations that develop a technology that releases detectable signs of their existence into space
L = The length of time such civilizations release detectable signals into space

Figure 2: Drake Equation

Applying the equation given the estimated parameters, we could roughly get the estimation of the number of intelligent lives. Due to limitations in technology, the value N might not be as reliable as at that time, N was estimated to be 50000! However, the equation undoubtedly involves many factors which are difficult to be measured precisely. For instance, the first three variables might be affected by different research in astronomy, organic chemistry and evolutionary biology. Also, the latter variables related to the lifespan of civilization involve many uncertainties, such as the extent on economic, political and even social developments within the civilization. Although this equation consists of many uncertain factors and the estimation of number N is not rigorous, which might be very small, astronomers still believe that extra-terrestrial lives exist in outer space.

If N is as large as 50000 aforementioned, why can we not detect any signal from aliens? Some arguments describing this question were suggested If the Drake equation is true, the Earth should have already been visited by extra-terrestrial aliens. Two main hypotheses trying to explain this the question: Intelligent extra-terrestrial life is extremely rare and those civilizations have not contacted or visited the Earth.

Intelligent extra-terrestrial life is extremely rare

Biological evolution is rare or even unique to Earth. No other intelligence or life can be formed in other planets. The biological evolution requires different factors, such as whether a planetary system has those certain properties. For instance, if a right sized terrestrial planet exists in a circumstellar habitable zone,  simple life might evolve. Although it is possible that complex life is evolved from simple life, the probability of successful evolution to complexity is extremely low. Even if they are intelligent, they may not have advanced technologies to communicate with human beings.Along with non-intelligent life, those civilizations are therefore difficult for us to detect. Moreover, in extreme cases, if their technology is advanced, it is the nature of intelligence to destroy themselves or others, by using their technology to harm themselves or others. They might have declared wars, contaminated the environment or even created poor artificial intelligence to destroy the planets. Also, new life, no matter it is intelligent or not, will normally die out in the course of time. The species may die out due to the dynamic and varying planetary environments They may suffer from their planet’s catastrophes such as massive volcanic eruption or astronomical disaster like gamma-ray burst. The above arguments could possibly explain the reasons why no signal are received so far.

Intelligent civilizations have not contacted or visited the Earth

The other way to resolve the Fermi paradox is by stating that ET intelligence does exist but they have never contacted or visited us. Star may be too far away for meaningful two-way communication to be made. Right now, the furthest signal sent by the human has only travelled 50 light-years and no reply is received yet. This may imply that two intelligent civilizations are too far away that our signals have not reached them or they have not sent their reply. If the two civilizations are that far, say several thousand light years, the human might be possible to detect their existence but it might be less possible to communicate with them because of the far distance. A renowned speculation by astronomers Sagan and Newman suggests that if other civilizations exist, and keep transmitting signals to explore other civilizations, their signals and probes simply have not arrived yet. Another possible reason is that transmitting signals throughout the galaxy physically maybe very expensive. The recent scientific knowledge (special relativity) tells us that nothing can be accelerated beyond the speed of light in vacuum. The cost of materials and energy for interstellar ventures may be so high that it is unlikely to be affordable to any civilization. Even if their technology is feasible for communication, other civilizations may only receive but not understand the signal. Receiving the signal is easy but the solving the riddles it carries is difficult. Apart from that, “What should we say?”, “Who is going to speak for the Earth?” would be the most concerned. Because of these, consultations on different countries and transmission among different planets take time and a civilization may just become extinct in the process. Due to these factors, intelligent civilizations might not contact or visit the Earth.


Although astronomers currently have not detected any signals from aliens yet, their studies will continue. In fact, we have done some experiments on searching aliens. The electromagnetic waves to search for signals is mainly used in some famous experiments including sending Arecibo Message and directly observing the planet. A careful search for non-natural radio emissions from space may allow us to detect possible alien civilizations successfully. In the future, we might detect some “useful” signals and aliens might actually be in contact with us.

1.Drake, F. (1961). The Drake Equation. Retrieved from
2.Ward, P. D.; Brownlee, D. (2000). Rare Earth: Why Complex Life is Uncommon in the Universe (1st ed.). Springer
3.Webb, S. (2015). If the Universe Is Teeming with Aliens ... WHERE IS EVERYBODY? Springer
4.Zaitzev, A. (2006). The SETI Paradox. Retrieved from