In 1950, physicist Enrico Fermi raised a very important question about the Universe and the existence of extra-terrestrial life. Given the size and age of the Universe, he stated, and the statistical probability of life emerging in other solar systems, why is it that humanity has not seen any indications of intelligence life in the cosmos? This query, known as the Fermi Paradox, continues to haunt us to this day.
If, indeed, there are billions of star systems in our galaxy, and the conditions needed for life are not so rare, then where are all the aliens? According to a recent paper by researchers at Australian National University’s Research School of Earth Sciences., the answer may be simple: they’re all dead. In what the research teams calls the “Gaian Bottleneck”, the solution to this paradox may be that life is so fragile that most of it simply doesn’t make it.
It’s become a legend of the space age. The brilliant physicist Enrico Fermi, during a lunchtime conversation at Los Alamos National Laboratory in 1950, is supposed to have posed a conundrum for proponents of the existence of extraterrestrial civilizations.
If space traveling aliens exist, so the argument goes, they would spread through the galaxy, colonizing every habitable world. They should then have colonized Earth. They should be here, but because they aren’t, they must not exist.
This is the argument that has come to be known as “Fermi’s paradox”. The problem is, as we saw in the first installment, Fermi never made it. As his surviving lunch companions recall (Fermi himself died of cancer just four years later, and never published anything on the topic of extraterrestrial intelligence), he simply raised a question, “Where is everybody?” to which there are many possible answers.
Fermi didn’t doubt that extraterrestrial civilizations might exist, but supposed that interstellar travel wasn’t feasible or that alien travelers had simply never found Earth in the vastness of the galaxy.
The argument claiming that extraterrestrials don’t exist was actually proposed by the astronomer Michael Hart, in a paper he published in 1975. Hart supposed that if an extraterrestrial civilization arose in the galaxy it would develop interstellar travel and launch colonizing expeditions to nearby stars. These colonies would, in turn, launch their own starships spreading a wave of colonization across the galaxy.
How long would the wave take to cross the galaxy? Assuming that the starships traveled at one tenth the speed of light and that no time was lost in building new ships upon arriving at the destination, the wave, Hart surmised, could cross the galaxy in 650,000 years.
Even allowing for a modicum of time for each colony to establish itself before building more ships, the galaxy could be crossed in two million years, a miniscule interval on a cosmic or evolutionary timescale. Hart asserted that because extraterrestrials aren’t already here on Earth, none exist in our galaxy.
Hart’s argument was extended by cosmologist Frank Tipler in 1980. Tipler supposed that alien colonists would be assisted by self-reproducing robots. His conclusion was announced in the title of his paper ‘Extraterrestrial intelligent beings do not exist’.
Why is it important that Hart’s argument wasn’t really also formulated by the eminent Enrico Fermi? Because Fermi’s name lends a credibility to the argument that it might not deserve. Supporters of the search for extraterrestrial intelligence (SETI) want to search for evidence that alien civilizations exist by using radio telescopes to listen for radio messages that extraterrestrials may have transmitted into space. Interstellar signaling is vastly cheaper than a starship, and is feasible with technology we have today.
Hart drew public policy consequences from his argument that extraterrestrials don’t exist. His paper concluded that “an extensive search for radio messages from other civilizations is probably a waste of time and money”.
Our political leaders heeded Hart’s advice. When Senator William Proxmire led the successful drive to kill funding for NASA’s fledgling SETI program in 1981, he used the Hart-Tipler argument. A second NASA SETI effort was scuttled by congress in 1993, and no public money has been allocated to the search for extraterrestrial radio signals ever since.
Just how convincing is the Hart-Tipler conjecture? Like Hart, Carl Sagan was an optimist about the prospects for interstellar travel, and Sagan published his analysis of the consequences of interstellar travel for extraterrestrial intelligence a whole decade earlier than Hart, in 1963. Sagan and his co-author, the Russian astronomer Iosef Shklovskii devoted a chapter to the topic in their 1966 classic Intelligent Life in the Universe.
Like Hart, Sagan concluded that “if colonization is the rule, then even one spacefaring civilization would rapidly spread, in a time much shorter than the age of the galaxy, throughout the Milky Way. There would be colonies of colonies of colonies…”. So why didn’t Sagan, like Hart, assert that extraterrestrials don’t exist because they aren’t already here?
The answer is that Sagan, unlike Hart, considered unlimited colonization as only one of many possible ways that extraterrestrial spacefarers might act. He wrote that “habitable planets lacking technical civilizations will frequently be encountered by spacefaring civilizations. It is not clear what their response will be…Perhaps strict injunctions against colonization of populated but pre-technical planets are in effect in some Codex Galactica. But we are in no position to judge extraterrestrial ethics. Perhaps attempts are made to colonize every habitable planet…A whole spectrum of intermediate cases can also be imagined”.
Besides assuming that interstellar travel is feasible, Hart’s argument is based on very specific and highly speculative ideas about how extraterrestrials must behave. He assumed that they would pursue a policy of unlimited expansion, that they would expand quickly, and that once their colonies were established, they would last for millions or even billions of years. If any of his speculations about how extraterrestrials will act aren’t right, then his argument that they don’t exist fails.
The evolutionary biologist Stephen Jay Gould was scathing in his criticism of Hart’s speculation. He wrote that ”I must confess that I simply don’t know how to react to such arguments. I have enough trouble predicting the plans and reactions of the people closest to me. I am usually baffled by the thoughts and accomplishments of humans in different cultures. I’ll be damned if I can state with certainty what some extraterrestrial source of intelligence might do”.
In 1981, Sagan and planetary scientist William Newman published a response to Hart and Tipler. While Hart used a very simple mathematical argument, assuming that an alien civilization would spread almost as fast as its ships could travel, Newman and Sagan used a mathematical model like the ones that population biologists use to analyze the spread of animal populations to model interstellar colonization.
They concluded that the rates of expansion assumed by Hart are highly unrealistic. Expansion will be drastically slower, for example, if civilizations control their population growth rates on any given planet to avoid ecological collapse, if colonies have a finite life span, and if alien societies eventually outgrow expansionist tendencies. Hart’s assumption that an alien civilization would spread almost as fast as its ships can travel isn’t plausible. It’s possible to walk across Rome in a day, Newman and Sagan noted, but Rome wasn’t built in a day. It grew much more slowly.
If the evolution of intelligent life is at all likely, other civilizations could emerge before any hypothetical first wave of expansion swept slowly over the galaxy. If several worlds produced waves of colonization, they might encounter one another. What would happen then? Nobody knows. The history of the galaxy can’t be predicted from a few equations.
For Newman and Sagan, the absence of extraterrestrials on Earth doesn’t mean that they don’t exist elsewhere in the galaxy, or that they never launch starships. It just means that they don’t behave in the way Hart expected. They conclude that “except possibly in the very early history of the Galaxy, there are no very old galactic civilizations with a consistent policy of conquest of inhabited worlds; there is no Galactic Empire”.
So, Enrico Fermi never did produce a powerful argument that extraterrestrial intelligence probably doesn’t exist. Neither did Michael Hart. The simple truth is that nobody knows whether or not extraterrestrials exist in the galaxy. If they do exist though, it’s possible that discovering their radio messages would give us the evidence we need. Then we could stop speculating and start learning something.
It’s become a kind of legend, like Newton and the apple or George Washington and the cherry tree. One day in 1950, the great physicist Enrico Fermi sat down to lunch with colleagues at the Fuller Lodge at Los Alamos National Laboratory in New Mexico and came up with a powerful argument about the existence of extraterrestrial intelligence, the so-called “Fermi paradox”. But like many legends, it’s only partly true. Robert Gray explained the real history in a recent paper in the journal Astrobiology.
Enrico Fermi was the winner of the 1938 Nobel Prize for physics, led the team that developed the world’s first nuclear reactor at the University of Chicago, and was a key contributor to the Manhattan Project that developed the atomic bomb during World War II. The Los Alamos Lab where he worked was founded as the headquarters of that project.
The line of reasoning often attributed to Fermi, in his lunchtime conversation, runs like this: There may be many habitable Earth-like planets in our Milky Way galaxy. If intelligent life and technological civilization arise on any one of them, that civilization will eventually invent a means of interstellar travel. It will colonize nearby stellar systems. These colonies will send out their own colonizing expeditions, and the process will continue inevitably until every habitable planet in the galaxy has been reached.
The fact that there aren’t already aliens here on Earth was therefore supposed to be strong evidence that they don’t exist anywhere in the galaxy. This argument actually isn’t Fermi’s and was published more than 25 years later by astronomer Michael Hart. It was elaborated in a paper published by the cosmologist Frank Tipler in 1980.
Fermi’s lunch conversation really did happen. Although he died just four years later of cancer, physicist Eric Jones published the recollections of the physicist’s luncheon companions more than thirty five years later. Among these companions were Edward Teller, Emil Konopinski, and Herbert York, all eminent physicists and veterans of the Manhattan Project. Teller played a central role in the development of the hydrogen bomb. Konopinski studied the structure of the atomic nucleus, and York became director of Lawrence Livermore National Laboratory.
During the walk to the Fuller Lodge, the physicists discussed a recent spate of UFO sightings, and a cartoon in the New Yorker Magazine depicting aliens and a flying saucer. Although the topic of conversation moved on as the group sat down for lunch, Edward Teller recalls “in the middle of the conversation, Fermi came out with the quite unexpected question ‘Where is everybody?’…The result of his question was general laughter because of the strange fact that in spite of Fermi’s question coming out of the clear blue, everybody around the table seemed to understand at once that he was talking about extraterrestrial life”.
In his account of the famed luncheon, Teller wrote “I do not believe much came from this conversation, except perhaps a statement that the distances to the next location of living beings may be very great and that, indeed, as far as our galaxy is concerned, we are living somewhere in the sticks, far removed from the metropolitan area of the galactic center”.
York recalled a somewhat more expansive discussion in which Fermi “followed up with a series of calculations on the probability of earthlike planets, the probability of life given an earth, the probability of humans given life, the likely rise and duration of high technology, and so on. He concluded on the basis of these calculations that we ought to have been visited long ago and many times over”.
According to York, Fermi supposed the reason we hadn’t been visited “might be the interstellar flight is impossible, or if it is possible, always judged not worth the effort, or technological civilization doesn’t last long enough for it to happen”.
So Fermi, unlike Hart, wasn’t skeptical about the existence of extraterrestrials, and didn’t view their absence from Earth as paradoxical. There is no Fermi paradox, there is simply Fermi’s question “Where is everybody?”, to which there are many possible answers. The answer that Fermi preferred seems to be that, either interstellar travel isn’t feasible because of the enormous distances involved, or Earth simply had never been reached by alien travelers.
Interstellar distances are truly vast. If the entire solar system out to the orbit of Neptune were reduced to the size of an American quarter, the nearest star, Proxima Centauri, would still be about the length of a football field away. A practical starship would either need to travel very fast, at an appreciable fraction of the speed of light, or be capable of supporting its crew for a very long time. While either is theoretically possible, interstellar travel seems to present day humanity to be such a grandiose undertaking that it’s not clear whether any civilization would be able or willing to muster the enormous resources needed.
Where did the confusing of Fermi’s question with Hart’s argument come from? Carl Sagan mentioned Fermi’s question in a footnote to a 1963 paper. After the publication of Hart’s paper in 1975, Fermi’s question and Hart’s speculative answer became associated in many writer’s minds. Fermi’s question seemed to beg Hart’s answer, and “Fermi’s paradox” was born. According to Robert Gray, the term was coined by D. G. Stephenson, in a paper published two years after Hart’s.
Why is it important that Hart’s argument was never really made by the eminent physicist Enrico Fermi? Did Michael Hart and Frank Tipler really make a compelling case that extraterrestrial civilizations don’t exist in our galaxy? We’ll answer those questions in the second installment.
References and Further Reading:
F. Cain (2013) How Could We Find Aliens? The Search for Extraterrestrial Intelligence (SETI). Universe Today.
R. H. Gray (2012) The Elusive WOW, Searching for Extraterrestrial Intelligence, Palmer Square Press, Chicago, Illinois.
One answer to the Fermi Paradox is the idea of the Great Filter; the possibility that something wipes out 100% of intelligent civilizations. That why we’ve never discovered any aliens… they’re all dead. Is that our future too?
In a previous episode, I presented the idea of the Fermi Paradox. If space is huge, like space huge, not aircraft carrier huge, and there are billions upon billions of stars, AND there seem to be lots of habitable planets around those stars, where are all the damn aliens?
There are plenty of theories about what might be the solution for the Fermi Paradox, like there aren’t a lot of aliens out there and we’re too far apart to bother with, or maybe they just don’t want to talk to us because of our meat cooties, or maybe we’re really in a cosmic zoo and if you break it, you bought it.
It’s possible that we’re the first intelligent civilization to exist in the entire Universe, but I’ve never been a fan of that idea. If we’re the best the universe can do in billions of years, I seriously need to make some heavy expectation adjustments to my view of everything.
There is still another theory, one that you might find troubling. It’s called the Great Filter, and it says that something prevents intelligent civilizations from ever forming, in a darkly mysterious Philip K Dick kind of way.
Consider the long series of steps that happened to get from the early Earth to where we are now: A planet with the right combination of atoms needed to have liquid water long enough for organic molecules to form, those molecules needed to somehow be able to reproduce, eventually becoming the first organisms, which became multicellular organisms, then learning to reproduce sexually, evolving tool use, and eventually becoming intelligent life, and all the while managing to survive a planetary extinction or two. And then, at some point in the future, this intelligent life goes on to colonize an entire nearby galaxy.
Since humanity has passed all those previous steps, we know they’re not impossible. Maybe really really improbable, but not impossible. As we imagine the future, there’s nothing in the laws of physics that’ll stop us from building machines that can help us colonize the entire galaxy. Pretty machines with blinking lights, possibly incorporating meat parts from future generations of humans. If we can do it, any race could do it.
If the universe has been around for 14 billion years, and we’ve done it in a fraction of that, there’s been plenty of time for this to have been done. And yet, still no aliens. So maybe the Great Filter is still waiting for us. No matter how hard we try to extend beyond our Solar System, something will stop us.
So what could the Great Filter be? There are lots of ideas. Maybe all civilizations discover the most horrible of weapons and eventually destroy themselves. They could develop virtual reality technology and choose to spend their future in a simulated reality. They could create an exotic type of matter that destroys their home planet. Perhaps they create robotic servants who inevitably overthrow their masters in a planetary robotalypse. Perhaps someone creates a super plague that wipes out all life, the civilization ruins their environment and its ability to support life by filling their oceans with plastic and their atmosphere with CO2 until their planet becomes a pressure cooker. Or maybe they just watch too much reality tv and just get too dumb to put food in their own mouths and die of stupid.
Whatever the cause is, here’s the haunting idea behind it… Whatever this Great Filter is, it must hit 100% of intelligent civilizations. Because if even 1% of aliens are able to avoid it, they’d go on to colonize the galaxy. And still even to this day, yes, we have no aliens.
There could be some high probability absolute devastation event in our future, which will happen just before we become a space-faring civilization…. And there’s no way we can predict or prevent it. The idea that all advanced civilizations are doomed is unsettling.
Here’s hoping that the Great Filter is wrong. Either we’re the first advanced civilization in the Milky Way, or perhaps we’ll figure it out, and avoid the catastrophe that killed off all the other aliens in the galaxy.
So what do you think? What’s it gonna be? What will wipe us out? Tell us in the comments below.
“One of the main things we’re focused on is the notion of existential risk, getting a sense of what the probability of human extinction is,” said Andrew Snyder-Beattie, who recently wrote a piece on the “Great Filter” for Ars Technica.
As Snyder-Beattie explained in the article, the “Great Filter” is a response to the question of why we can’t see any alien civilizations. The “Great Filter” deals with similar issues as the Drake Equation, which talks about the probability of communicating civilizations outside of Earth, and the Fermi Paradox, which asks where the civilizations are.
Simply speaking, the idea is that if a civilization continues to expand (especially at the technological pace we humans have experienced), it wouldn’t take all that long in the lifespan of the universe for artificial processes to be visible with our own telescopes. Yes, this is even taking into account a presumed speed limit of no more than the speed of light. So something could be preventing these civilizations from showing up. That’s an important part of the Great Filter, but more details about it are below.
Here are a few possibilities for why the filter exists, both from Snyder-Beattie and from the person who first named the Great Filter, Robin Hanson, in 1996.
‘Rare Earth’ hypothesis
Maybe Earth is alone in the universe. While some might assume life must be relatively common since it arose here, Snyder-Beattie points to observation selection effects as complicating this analysis. With a sample size of one (only ourselves as the observers), it is hard to determine the probability of life arising – we could very well be alone. By one token, that’s a “comforting” thought, he added, because it could mean there is no single catastrophic event that befalls all civilizations.
Advanced civilizations are hard to get
Hanson doesn’t believe that one. One step would be going from modestly intelligent mammals to human-like abilities, and another would be the step from human-like abilities to advanced civilizations. It only took a few million years to go from modestly intelligent animals to humans. “If you killed all humans on Earth, but you left life on Earth — and the animals have big brains — it wouldn’t necessarily be that long before it came back again.” Some of the filter steps leading up to that would have taken longer, though, including the emergence of multicellular animals and the emergence of brains, roughly on the timespan of a billion years each per stage.
‘The Berserker Scenario’
In this scenario, powerful aliens sit hiding in wait to destroy any visible intelligence that appears. Hanson doesn’t believe that would work because if there were multiple berserker species, there would be opposing parties. “As an equilibrium, you’d have these competing teams of these berserkers all trying to smash each other.”
Maybe natural activities are masking the extraterrestrials
Maybe the big natural activities of those beyond Earth just happens to look exactly as if they are not there. Hanson said it seems rather unlikely, as it would be a “remarkable coincidence” if advanced artificial processes were actually responsible for all the astronomical phenomena we do explain from natural occurrences,- from pulsars to dark matter
A natural disaster
There certainly is an inherent risk to just being an Earthling. One asteroid strike, a stream of radiation from a nearby supernova, or a large enough volcano could end civilization as we know it — and possibly much of life itself. “But the consensus is we have a track record of surviving these things. But it’s unlikely that all life would be destroyed forever. “If those humans who were left, it took them 10,000 years to come back to civilization, that’s hardly a blink of an eye, that doesn’t do it,” Hanson said. The next is that statistically speaking, although these events happen, they don’t happen often. “It is unlikely one of these very rare events would happen in the next century or 300 years,” Snyder-Beattie said.
A ‘fundamental technology’ that ends civilization
This is open to complete speculation. For example, climate change could be the catalyst, although it would seem extraordinary for all civilizations to encounter such similar political failures, Snyder-Beattie said. More generalized possibilities could be the rise of machine intelligence or distributed biotechnology, a force that is self-replicated. Hanson, however, points out that even that has its limitations — presumably then it would be the robots that head out through the cosmos and leave traces of civilization themselves.
For the fate of our own civilization, the key is to focus on what we can control, Hanson says. This means drawing up a list of the things that could kill us — however theoretical — and then work on ways of addressing those.
The question of why other civilizations are not visible still persists, however. What are your thoughts about the Great Filter? Let us know in the comments.