Where Is Everyone? Exploring the Fermi Paradox

The designer of the world's first nuclear reactor and a key member of the Manhattan Project, Enrico Fermi was one of the most brilliant theoretical physicists of his generation, but he is also well-known for defining the eponymous “Fermi Paradox.” According to the story, sometime in the summer of 1950, Fermi and his colleagues at the Los Alamos National Laboratory were discussing the probability of the existence of alien life on their lunch break — much as you or I might discuss the weather or the previous night’s sports results. There are so many stars, they reasoned, surely some of them are inhabited. With at least 100 billion stars in our galaxy and roughly 100 billion galaxies in the universe, a quick back-of-the-envelope calculation revealed that there might be up to 10 sextillion stars in the universe. That’s 100 billion trillion stars. 1022. That’s a one followed by 22 zeros. 10,000,000,000,000,000,000,000 stars. That’s a lot of stars.

Fermi and his colleagues assumed that many of these stars probably have rocky planets in the so-called habitable zone (the recent discovery that exoplanets are very common proves that this assumption was correct, and this study claims that there could be 300 million of these potentially habitable worlds in the Milky Way alone). The discussion then followed this chain of reasoning:

1. There is nothing special about our planet, so we can assume the appearance of life is inevitable in the right conditions;

2. therefore, given the unfathomably large number of stars in the universe, life could plausibly exist on millions of planets;

3. given the age of the universe, life could have developed on planets around the older stars billions of years ago;

4. given the pace of evolution and technological advancement seen on Earth, such alien civilisations would have had time to explore the galaxy by now.

Having reached this logical conclusion, Fermi supposedly blurted out “so, where is everyone?” Thus was born the Fermi Paradox.

The Fermi Paradox seeks to make sense of this apparent contradiction between the seeming inevitability of extraterrestrial life1 and the absence of credible proof of such life. Since Fermi first articulated this problem, many people have sought to find solutions to it. This blog is intended as a non-exhaustive review of these explanations, some of which are quite troubling.

The Rarity of Life

The most obvious answer to Fermi’s question is that his first assumption is simply wrong: while habitable planets do appear to be common, we have no idea how likely the emergence of life (abiogenesis) is since the process is not well understood. This is a form of selection bias called the “anthropic principle,” where we assume that the conditions on Earth, which are conducive to life, are universal because we live on Earth, but as Steven Weinberg aptly noted: “Where else could we be, except on a planet that can sustain life?” The emergence of life might require highly specific and perhaps rare conditions that not easily replicated.

The Rarity of Advanced Civilisations

Just as abiogenesis may be rare, the emergence of intelligent life might also be extremely rare. It took around 3.5 billion years ago for homo sapiens to evolve from the earliest single-celled organisms. Complex life forms only appeared around 600 million years ago, and technological advanced civilisations only appeared in the last few hundred years — so advanced civilisations have only been around for about 0.000007% of the history of life on Earth.

Even among mammals, which have existed for around 200 million years, only a few species have developed higher intelligence and complex social behaviour. Given what we now know about animal intelligence, it is likely that early mammals and birds had some level of intelligence, but evolution is random and does not have a predetermined direction, so there is no reason that it necessarily results in large brains, opposable thumbs, and complex language — which, in our case, enabled an intelligent species to become a space-faring species. To paraphrase Charles Lineweaver, dolphins have had 20 million years to develop a space programme and have failed to do so. There may be millions of intelligent species confined to their home planets by physical or social constraints2. 

Mass Extinction Events

There have been five major so-called “mass extinction events” in the last 500 million years, each one eliminating between 50 and 80% of all species on earth. These events range from runaway global warming and ocean acidification caused by high volcanic activity, to oxygen depletion, sea level change, and asteroid impacts.

It is plausible that mass extinction events are even more common on certain planets due to a higher frequency of asteroid impacts, greater volcanic or tectonic activity, or more powerful solar flares or gamma-ray bursts from their local stars.

Each such event creates a fork in the evolutionary tree and potentially sets back the emergence of civilisation by hundreds of millions of years, so it’s easy to imagine this has been a temporary or permanent setback to the emergence of technologically advanced civilisations in many cases.

The Fragility Of Advanced Technological Civilisations

Maybe the darkest possible hypothesis is that every civilisation that becomes technologically advanced enough to destroy itself, does so. This hypothesis would have been all too easy for someone like Fermi — the "architect of the atomic bomb" — to envisage.

However, the argument that all technologically advanced civilizations destroy themselves may be based on a parochial view of evolution. Why would an evolutionary process taking place on another planet in different conditions necessarily bring about bellicosity and a fear of the outgroup?

A more optimistic take would be that some species would tend more towards cooperation than fear and violence. Sadly, it does not appear that our species is one of them.

The Dark Forest Hypothesis

This term was coined by Chinese writer Liu Cixin in the second book of his Three Body Problem trilogy. Various other versions of this argument have been put forward, but the basic premise is that technologically advanced civilisations lie in wait, unseen to other civilisations, like a hunter in a dark forest, and when a potential rival civilisation blinks into existence they are wiped out in a pre-emptive strike before they become a threat. As a result, it would also be rational for any civilisation that develops this theory to not want to make itself known. Hence the absence of evidence for extraterrestrial life.

The Zoo Hypothesis

The final hypothesis I want to address today — and the most fun — is that they are here. The so-called Zoo Hypothesis can be summarised like this: aliens have visited Earth, but for some reason they don’t want us to see them. Advanced civilizations might have a moral code akin to Starfleet’s “Prime Directive” in Star Trek, which states that advanced societies should avoid interfering with the natural evolution of less advanced ones.

They might want to avoid contact for sociological or scientific reasons, to study the flora and fauna on our planet without having undue influence, or they might want to monitor us to ensure that we don’t represent a threat to them.

If an alien species has visited Earth, it means they are capable of interstellar travel, so it is not a great stretch to imagine that they would also be capable of easily remaining hidden in ways we cannot image. As futurist Arthur C. Clarke said, “any sufficiently advanced technology is indistinguishable from magic.”

Of course, proponents of the Zoo Hypothesis might point to UFOs (now known as UAP, or unidentified anomalous phenomena) as evidence of this theory, but that does seem somewhat contradictory to a major part of the theory, i.e. that they want to remain hidden. Why would they appear to some people some of the time if they don’t want us to know they’re here?

Maybe it is a combination of some or all of these theories that explains the absence of evidence. Which theory do you think is most plausible?

1 The Drake Equation is an attempt to work out how many such alien civilisations there are. It helpfully puts that number somewhere between 1,000 and 100,000,000 for our galaxy alone.

2 For example, in his 1993 novel, A Fire Upon the Deep, Vernor Vinge imagines a species of dog-like creatures which, although highly intelligent, cannot remain in close proximity to one another. This species would have difficulty becoming a space-faring civilisation because large-scale collaboration is difficult for them.