On the television series Cosmos (1980), American astronomer Carl Sagan first introduced the concept of the “Cosmic Calendar”. The Cosmic Calendar is a method to visualize the chronology of the universe by scaling it down to the size of a single calendar year. On this calendar, Jan.1, 00:00 represents the Big Bang, on Jan. 22, galaxies are formed and the Milky Way is formed on May 12th. While the Earth is formed on September 16th, modern humans don’t appear for quite some time–not until December 31, at 2 minutes from midnight.
Consider that within that brief window of time, our species has advanced from rudimentary stone tools to jet-fuelled rockets– and it’s not unreasonable to imagine that within 30 seconds of Cosmic Calendar time (roughly 13 500 years), we might acquire the capability to send space probes out into the depths of our galaxy.
Moreover, the universe contains at least one hundred billion galaxies, each containing at least one hundred billion stars. It is estimated that roughly 7.6% of these stars are similar to our own. A quarter of those are likely to have at least one Earth-like planet in their habitable zone. This means that there are likely more than 19 sextillion (or 19 billion trillion) planets in the universe that are theoretically capable of supporting life. How large is a sextillion? If a sextillion people were stacked on top of one another, their height would reach upwards of 180,000 light years–that’s larger than the diameter of the Milky Way!
With this in mind, there seems to be a strong case to be made that some form of extraterrestrial life must have developed at some point, somewhere in the universe. Further, if alien life did come about, it is extremely probable that it would have done so much earlier than humans, given our place in the Cosmic Calendar. If intelligent aliens began to develop merely a day before humans on the Cosmic Calendar, this would equate to roughly a 37.8 million year head start!
Assuming that such aliens would have access to similar resources as us, how they could advance technology in 30 million years is unimaginable to us. What is imaginable, however, is that an alien civilization could easily develop space technology that would allow them to colonize galaxies and send probes out into the far reaches of the universe.
This naturally begs the question: Where are they?
It was the Italian physicist Enrico Fermi (better known as the creator of the first nuclear reactor) who first proposed this question as an offhand remark during a lunch discussion in 1950. As the story goes, he and his colleagues were in the midst of discussing the possibility of alien societies when Fermi made this casual observation. He noted that if such aliens had an appetite for imperialism, given enough time (he gave 10 million years as an example), they would only need relatively moderate rocket technology in order to rapidly colonize entire galaxies.
Though he never followed up on it himself, Fermi’s innocent remark spread throughout the scientific community leaving a mass of puzzled scientists in its wake. (As a side note, how it took the name “Fermi’s Paradox” is unknown and is often a source of confusion, as it is not really a paradox, it’s more of a problem.) Several theories have been proposed, but none satisfactory enough to mollify those mystified by the problem.
One theory suggests that alien civilizations may exist somewhere in the universe, but they are prevented from reaching us due to technological limitations– that they’re simply too far away. However, critics note that if humans can advance from stone tools to space probes in a few thousand years, then given millions of years, an advanced alien civilization would surely be able to conquer the vast distances through space. Perhaps, they might even discover the wormholes that could act as shortcuts through space and time as Einstein once postulated.
Another theory maintains to those assuming an alien civilization would be as interested in exploration as humans are committing the anthropomorphic fallacy; that is, the mistaken tendency to attribute human characteristics to non-humans things. They claim that there is no reason to suspect that an alien civilization would possess the human proclivity for expansion and colonization. Rather, it is perfectly plausible to imagine that such aliens would be content within the confines of their own solar system.
However, others note that even if an alien civilization had no intention to colonize, they might still traverse galaxies out of necessity, as an advanced civilization would surely exhaust massive amounts of resources over millions of years. Also, even if they were confined to one planet, it is likely that they would use some sort of radio signal to communicate across the planet’s surface (it is hard to imagine a sophisticated alien society using landlines and rotary dials). Given millions of years for those signals to travel across space, surely we would have been able to pick-up at least a faint whisper of those signals by now, considering that our signals have already reached 200 light-years in diameter.
Some are less convinced by the force of Fermi’s Paradox. Instead, they are content to suppose that we are the only intelligent life that exists or that has existed in the entire universe. But taking the premises of Fermi’s Paradox into consideration–particularly the large number of habitable planets–it would be rather strange, even eerie, if we were the only form of life ever to have existed in a universe that is 13.8 billion years old and that spans 93 billion light-years. So strange, in fact, that it makes room for some rather bizarre theories to seem more plausible, such as the simulation hypothesis recently popularized by Stephen Hawking and Elon Musk.
The most popular explanation for Fermi’s Paradox, however, was presented in a 1996 article by economist Robin Hanson titled, “The Great Filter–Are We Almost Past It?” Instead of looking to answer the question of whether or not alien societies exist, The Great Filter asks: If they don’t exist, why is that? Assuming that no sophisticated alien civilization has ever arisen, there must be a factor that acts like a filter and halts the development of alien life at a certain point. The problem is we don’t know at what stage of the development this filter may be.
It could be that the particular conditions are so specific that even the rudiments of life are unable to develop on any other planet in the universe other than our own. Thus, the Great Filter may precede even the early stages of the development of life. Alternatively, the Great Filter may be somewhere near the early stages of life, preventing alien life from progressing past, say, the single cell stage, for example. Lastly, it is possible that the Great Filter may not lay behind us in our current stage of evolution, but rather, ahead of us. It’s possible that alien civilizations may have surpassed our current stage of development but were wiped out by some unknown cause before they were able to reach us.
Consider the implications of that possibility. Suppose that countless alien societies have developed past our current stage but were all eliminated by some factor: Are we next? Moreover, perhaps we have already discovered the instrument of The Great Filter, whether it be climate change, nuclear weapons or artificial intelligence. Perhaps, unbeknownst to us, our time on the Cosmic Calendar is coming to a close we will join our predecessors in the cosmic civilization graveyard.