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“Where Are They?” A New Estimate of Alien Civilizations Is Both Exciting and Scary

Elon Musk tweeted a link to a new article on the odds of alien civilizations existing (or once having existed), and asked: “Where are the aliens?”.

In only a few hours, he’s already received thousands of retweets and likes. So, people are interested in the question!

And that question has a name.

“Where Are They?”

Enrico Fermi, the famous Italian theoretical and experimental physicist, was having lunch with his colleagues at Los Alamos labs in 1950, when he suddenly blurted out, “Where are they?”.

Quickly jotting down some basic principles, he noted that our galaxy alone contains billions of stars that are similar to our Sun, that these are often billions of years older than Earth, and if alien life arose on planets in a habitable zone around these suns, some percentage of it must develop intelligence.

He further calculated that if even only a few of these intelligent civilizations began to explore other stars, within a short galactic time-frame (a million years say) these alien spacecraft or artifacts would have arrived at Earth. Since we have no convincing evidence of that, his arithmetic then poses a logical conundrum.

This is the Fermi Paradox.

Some Possible Solutions to the Fermi Paradox

There are many possible ways to explain this paradox.

Carl Sagan once remarked, “absence of evidence is not evidence of absence”. Maybe we aren’t looking in the right places, or aren’t sophisticated enough to recognize intelligent alien life when we see it. Maybe it is happening at quantum levels rather than at giant scales that we would expect to see and recognize.

In the early 1900s, the great Russian thinker Konstantin Tsiolkovsky came up with an early version of the question that became known as the Fermi Paradox. His solution was that there was a kind of cosmic zoo, and Earth was being deliberately protected from alien contact until our civilization was mature enough to deal with it. If you’ve ever watched Star Trek, you’ve heard of this idea, which is called the Prime Directive.

“Ancient Aliens” on History Channel

Another theory: Maybe we have been/are being visited after all! Popular culture is full of conspiracy theories about alien visitation, including Roswell, Chariots of the Gods, the Prometheus movie, and so many others. If aliens are visiting Earth, then obviously there is no longer any Fermi Paradox.

I personally don’t believe in alien visitation, since to quote the very wise Carl Sagan again: “extraordinary claims require extraordinary evidence”. If there was extraordinary evidence of alien visitation, we would see it and know it. Sorry, History Channel.

There are also other (usually grim) theories about what might happen to alien civilizations when they reach a certain level of technology. One concept originated by Robin Hanson is called the Great Filter, and his article “The Great Filter – Are We Almost Past It?” was then elaborated upon by Nick Bostrom in “Where Are They? Why I Hope the Search for Extraterrestrial Life Finds Nothing“, an existentially terrifying essay.

If the Great Filter exists, then we haven’t been visited by all these aliens because something/someone is systematically destroying them (and we may be next!). Basically think super predatory aliens that would snuff out our existence without us even knowing about them, and with no chance of defending ourselves by uploading a software virus to their spaceship. Sorry, Independence Day.

It’s also true that, as a species that is relatively young and certainly of limited lifespan (for now), we have trouble grasping the immensity and age of the galaxy, and how that might prevent the kind of alien exploration that would eventually reach Earth.

Still, Fermi’s original question has galvanized thought about how prevalent intelligence alien life is in our galaxy, and how likely it is to visit Earth.

The Drake Equation

The equally-famous Drake Equation was a more rigorous calculation that followed along the same lines as Fermi, but injected the concept of interstellar communications, rather than actual visits by alien tourists with selfie sticks/probes.

At the time, Frank Drake was coming up with an agenda for a meeting on detecting and communicating with extraterrestrial civilizations using radio signals.

As I planned the meeting, I realized a few day[s] ahead of time we needed an agenda. And so I wrote down all the things you needed to know to predict how hard it’s going to be to detect extraterrestrial life. And looking at them it became pretty evident that if you multiplied all these together, you got a number, N, which is the number of detectable civilizations in our galaxy. This was aimed at the radio search, and not to search for primordial or primitive life forms.  (Source: Wikipedia

Screenshot from Wikipedia entry on the “Drake Equation”

SETI@Home program analyzing radio signals (

Although the Drake Equation focuses on radio signals, it is very similar to Fermi’s in that it crunches numbers based on the prevalence of Earth-like planets leading to the rise of intelligent extraterrestrial civilizations, and then produces a calculation indicating how “detectable” they are.

Again, this equation is subject to criticism based on its assumptions (use of radio technology), as well as the variance and accuracy of the numbers used to determine the resulting calculation.

However, regardless of criticism, it is certainly true that the Drake Equation and the Fermi Paradox have become influential and important thought frameworks in which to explore the existence of intelligent extraterrestrial life.

What About Unintelligent Alien Life, or Aliens without Tools?

Both of these equations assign a likelihood to the existence of “unintelligent” alien life, which is a necessary prerequisite leading to the rise of extraterrestrial civilizations.

Does the Martian meteorite ALH84001 show signs of primitive life? c. JPL/NASA

In this context “unintelligent” could mean simple life such as bacteria, or it could refer to complex species that have little or no intelligence.

“Intelligent” life is assumed to be making and using tools and technology (this assumption is implicit in the Fermi argument, and made explicit in the Drake Equation).

We have plentiful examples on our own planet of intelligent creatures that do not create or use tools and are therefore incapable of interstellar travel or signalling, such as dolphins (unless they are hiding something?).

It is theoretically possible that an intelligent alien species that does not use tools or sends signals, is still somehow detectable by us and/or capable of arriving at Earth. However, both the Drake Equation and the Fermi Paradox rely on statistical assumptions, and including such an unlikely possibility would bring the calculations into the realm of fantasy.

The Goldilocks Zone

The Fermi Paradox was based on calculations of “Earth-like planets” that could support intelligent life. Drake assigned a variable likelihood to the habitability of planets which doesn’t specifically require them to be Earth-like.

The Search-for-Extraterrestrial-Life (SETI) research field began in the 1950’s and 60’s when we had much less understanding of the universe’s rate of planet-formation, and had not even discovered an extra-terrestrial planet. We could only base the statistical possibilities on what we could observe and understand in our own system, and there was a natural bias to conditions for life to thrive on Earth (especially as we then understood them).

Our early understanding therefore led SETI researchers to formulate the theory of the “Goldilocks Zone”, which is an area in a planetary system where conditions are hospitable enough, and for long enough, for life to flourish. This zone was originally thought to require moderate temperatures, liquid water, and a plentiful solar energy source. The name is taken from the child’s story of Goldilocks and the Three Bears, because the zone had to be “not too hot, not too cold, but just right”. NASA has more on this here.

Recent discoveries have dramatically changed our understanding of how tough life on Earth is, how many ecosystems it supports, and led us to consider the possibility of alien life that relies on chemical and not solar energy (such as that around deep sea vents, or deep underground). We now have (indirect) observations of subsurface oceans on many of the moons in our solar system, and even potentially on Pluto! The ingredients of life have been found to be common in the universe, floating around in giant dust clouds or carried around on comets.

These discoveries show that the Goldilocks Zone is much wider than it was once thought. Therefore, the likelihood of alien life existing might be very high, and that in turn pushes up the likelihood of intelligent alien life, and then in turn the likelihood of it signaling, or exploring Earth.

NASA just announced the discovery of many potentially habitable planets around a nearby star, including 3 within the habitable zone (

Optimists vs Pessimists

Taking all that history into account (phew!), we return now to Elon Musk’s tweet.

The internet article he quoted, “Drake Equation Revision Hugely Ups Odds Intelligent Extraterrestrial Life Exists“, contains a great overview of an Astrobiology journal article called “A New Empirical Constraint on the Prevalence of Technological Species in the Universe” by Adam Frank and W.T. Sullivan III.

In their abstract for the journal article, the authors state:

…We address the cosmic frequency of technological species. Recent advances in exoplanet studies provide strong constraints on all astrophysical terms in the Drake equation. Using these and modifying the form and intent of the Drake equation, we set a firm lower bound on the probability that one or more technological species have evolved anywhere and at any time in the history of the observable Universe. We find that as long as the probability that a habitable zone planet develops a technological species is larger than ∼10−24, humanity is not the only time technological intelligence has evolved. This constraint has important scientific and philosophical consequences.

That’s 1 in 10 billion trillion, and as Neel Patel in writes: “That’s incredibly small — which means the odds that another intelligent species has evolved are very, very good.” He quotes Astrobiology journal co-author Adam Frank:

“Think of it this way,” said Frank. “Before our result you’d be considered a pessimist if you imagined the probability of evolving a civilization on a habitable planet were, say, one in a trillion. But even that guess, one chance in a trillion, implies that what has happened here on Earth with humanity has in fact happened about a 10 billion other times over cosmic history!”

Neel Patel reminds us that over the enormous volume and age of the universe, even 10 billion intelligent alien civilizations could be greatly separated by space and time.

But! here we travel back to the Fermi and the Drake Equation:

Drake’s final variable, L, is “the length of time over which such civilizations release detectable signals.”. The longer this length of time is, the more possibility any of the (hypothetical, statistical) 10 billion alien civilizations would have to send signals across the “vasty deeps” of space. And, the more time at least one motivated alien species would have to cover the universe with its probes and ships.

This new calculation implies an enormously high number of alien civilizations have existed since the beginning of the universe, yet we have provably encountered precisely zero. Does this calculation therefore also increase the likelihood of a cosmic Great Filter that prevents them from existing long enough to contact us, or be contacted? If so, what is the Great Filter, and can Humanity survive it?

This is why I’m afraid of math.

Neel Patel concludes his article by saying: “There’s certainly never been a better time to be an E.T. optimist.”

That’s one thing we can all agree on.

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