An endless blanket of darkness illuminated by a plethora of stars — one glance at this glorious night sky is enough to remind us of the sheer magnitude of this universe. And alongside it, the gloomy realization that in the millions of miles that this universe stretches through, we are truly alone. Over the years, many scientists have questioned the fact that if there are billions of planets capable of supporting life, and if there are technological advanced species in this universe, then how come no one has visited Earth yet?
This question was pondered over by the Italian physicist Enrico Fermi as well. On a fine Sunday afternoon lunch with his fellow professors, Fermi was discussing the possibility of extraterrestrial life. Amid this discussion, he wondered out loud: Where is everybody? Through this seemingly simple question, Fermi was questioning the great truths. Where is extraterrestrial life in our universe? Why have they not contacted humans yet? For many, this may seem like an absurd question. The simple reality that we accept is that aliens do not exist. Fermi, however, had other ideas. Following this conversation, he began working out the possibility of the existence of extraterrestrial life. Through a series of calculations concerning the probability of numerous factors like Earth-like planets, intelligent life, and development of technology by other life, he came to the conclusion that there is a high probability of the existence of intelligent life. Not only that, but Earth should have been visited long ago, and frequently, by extraterrestrial species.
This perplexing contradiction sums up the essence of the Fermi Paradox. Even though the likelihood of extraterrestrial life is high, there is no concrete evidence of its existence. But in order to truly understand the basis of this contradiction, we must delve into the intricacies of our universe. In the Milky Way, there are approximately 200-400 billion stars. Among these, around a billion stars are similar to the Sun and there is a high probability that some have Earth-like planets. If life was to exist on a small percentage of these planets, there would be a huge number of civilizations in our universe. Secondly, Earth and the Sun are part of a relatively young planetary system. It implies that on many Earth-like planets that may exist in the universe, intelligent life must have developed long ago. Humans, in their relatively short period of existence, have already started to investigate interstellar travel. Hence, based on the human development and technological advancements in our young planetary system, Fermi concluded that interstellar travel would have been achievable for a civilization developed around a billion years before us.
These high estimates are what lead to the contradiction that Fermi outlined. Although he died in 1954, his idea was subsequently explored and furthered by many. An astrophysicist Michael Hart wrote a paper in 1975 examining a number of reasons for this absence of extraterrestrials. He stated that aliens may not have arrived because of the physical difficulties of interstellar travel — maybe, we are in some desolate corner of the galaxy, too far to be reached by other civilizations. He also argued that alien civilizations might have arisen only recently and do not yet have the technology to travel. Lastly, and most confusingly, he speculated that aliens might have visited Earth in the past and we did not observe them.
Over the years, the explanations for this paradox have become more sophisticated. One such line of reasoning involves the concept of the Great Filter. Scientists speculate that there is an inevitable phenomenon that occurs naturally during the evolution of civilizations; this filter wipes out life forms before they can develop to a point where they can contact us or travel through space. The Great Filter defines that all intelligent life forms are evolved through various critical steps before they are capable of interstellar travel, and there is at least one hurdle that virtually no species can clear it. Developed by Robin Hanson, the Great Filter theory is biased on relative probability of occurrence of critical events in species development.
There are two main speculations about the nature of the Great Filter. One of them is that the Filter is a simple evolutionary change that compares the inherited traits of successive generations of the potential extraterrestrial life with the characteristics of our own natural development. In this case, it is considered to be the advancement of prokaryotic cells to eukaryotic cells. If this is true, then the existence of extraterrestrial life that this paradox draws upon could be true. However, this life is neither intelligent nor advanced, which would explain the lack of contact with it. The other possibility is that the Great Filter could be an incredibly destructive force, like gamma ray bursts or electromagnetic explosions, which can cause a cataclysmic natural event, wiping out all life forms. If this is indeed the case, it would explain the complete absence of aliens. Thousands of civilizations may have dwelled before us, but they were wiped out long before ours came into existence.
While these ideas and speculations are indeed fascinating, we must not forget that most of them are mere assumptions. We are yet to discover evidence of intelligent life besides us. This is precisely why many renowned researchers object to the idea of the Fermi Paradox. Whatever the truth may be, we cannot deny that the question presented by Enrico Fermi is truly an enigma and has altered the course of astronomical investigations. One can only hope that as our technologies develop further, we may be able to find a qualitative answer. Will this answer be a fruition to our explorations, or will we only hear the echoes of our own voice? Only time will tell.