7. DRAKE EQUATION
It isn’t only the beauty of the night sky that thrills me. It’s the sense I have that some of those points of light are the home stars of beings not so different from us, daily cares and all, who look across space with wonder, just as we do.
— Frank Drake, the American astronomer and astrophysicist and originator of the Drake equation
In 1961, Frank Drake conceived of an equation to estimate the number of civilizations, N, in our Galaxy with which communication might be possible:
|R* =||average rate of star formation per year in our Galaxy.|
|fp =||fraction of those stars that have planets.|
|ne =||average number of planets that can potentially support life per star that has planets.|
|fl =||fraction of the above that actually go on to develop life at some point.|
|fi =||fraction of the above that actually go on to develop intelligent life.|
|fc =||fraction of civilizations that develop a technology that releases detectable signs of their existence into space.|
|L =||length of time such civilizations release detectable signals into space.|
Clearly, to obtain a reasonable estimate for N, one has to make some drastic but educated assumptions and Drake originally estimated there should be between 1,000 and 100,000,000 detectable civilizations in our Galaxy. Scientists now have a better understanding of some of the variables in his equation and they have been continually revising his numbers. A conservative estimate today is that there are between two and 50,000. More than N, Drake equation is also extremely useful for identifying the areas one must focus on in the search for alien life.
8. FERMI PARADOX
Where is everybody?
— Enrico Fermi, Italian-born American scientist Nobel Laureate in physics who was one of the chief architects of the nuclear age
The above quote is attributed to the famous Italian born American scientist, Enrico Fermi, one of the most influential physicists of the twentieth century who, among other landmark achievements, directed the first controlled chain reaction involving nuclear fission.
If any among these 50,000 civilizations produce cultures that are capable of colonizing over interstellar distances, even at a small fraction of the speed of light, the Milky Way Galaxy should have been completely colonized in no more than a few million years. Since our Galaxy is billions of years old, Earth should have been visited and colonized long ago. In fact, they should have already completed the expansion to fill the Galaxy before the emergence of life from the ocean. The absence of any evidence for such visits is the Fermi paradox.
Interstellar distances are massive, perhaps too vast to be conquered by living beings whose lifetimes are only finite. A technologically advanced civilization would therefore be faced with the challenge of finding the most efficient way to travel interstellar or interplanetary distances. However, they should be able to construct self-reproducing, autonomous robots to colonize the Galaxy. Mathematically, the most efficient way of exploring the hundreds of billions of stars in the Galaxy is via the Von Neumann probe. The Hungarian-born American mathematician John von Neumann was the first to develop a mathematical theory of machines that could make exact copies of themselves. Famed futurist and physicist Michio Kaku describes a Von Neumann probe in his blog in the following way:
“A Von Neumann probe is a robot designed to reach distant star systems and create factories which will reproduce copies themselves by the thousands. A dead moon rather than a planet makes the ideal destination for Von Neumann probes, since they can easily land and take off from these moons, and also because these moons have no erosion. These probes would live off the land, using naturally occurring deposits of iron, nickel, etc. to create the raw ingredients to build a robot factory. They would create thousands of copies of themselves, which would then scatter and search for other star systems.
Similar to a virus colonizing a body many times its size, eventually there would be a sphere of trillions of Von Neumann probes expanding in all directions, increasing at a fraction of the speed of light. In this fashion, even a Galaxy 100,000 light years across may be completely analyzed within, say, a half million years.”
Back in the 1940s, Fermi realized that the aliens should have had more than enough time to colonize the Galaxy. The apparent absence of evidence of extraterrestrials in the vicinity of the Earth is thus clearly a paradox. SETI has been going for nearly fifty years, employing increasingly powerful telescopes, faster computers and data mining techniques. But the reality is, it has so far consistently come up with null results. Where is everybody?
There are several possible answers or solutions to Fermi’s paradox. Physicist Brian Cox has explained some of them in his article “Were we contacted by aliens in 1977?” on BBC online:
“Space is too vast. We can’t survey every part of the night sky, and it’s possible that radio signals will become too weak as they cross our Galaxy to be detected.
Civilizations don’t last. Advanced civilizations may tend to destroy themselves, through war or environmental catastrophe. If not, a disaster like an asteroid strike might wipe them out.
Our planet is unique. According to the Rare Earth theory, even if simple life has arisen elsewhere, the chance events that led to intelligent life on our planet is highly improbable.”
Clearly, the reason for the lack of evidence for extraterrestrial civilization or the Great Silence, as it is sometimes called, falls under the realm of abstract speculation. Fermi paradox will be revisited later in this article (Section 10), but first one needs to be able to rank the detectable civilizations as guessed by Drake’s equation, which is an important working tool in the field of SETI, to better understand what distinguishing features they might possess. That will then tell us how to properly set the search criteria.