by Adam Frank
fbt = flfifc
Finally, by asking about the total number of exo-civilizations that had ever existed, rather than limiting our interest to those existing now, we took the issue of the average lifespan of a civilization out of the problem. We didn’t care if the exo-civilization overlapped with our own. It didn’t matter. We just cared that they had existed at some point in cosmic history. Effectively, that allowed us to ignore the final factor—the pesky lifetime term, L—in Drake’s equation.
Our approach gave us a new form of Drake’s equation that looked a lot simpler:
A = fafbt
In this version of the equation, A was just the total number of civilizations that had ever existed. We thought of A as standing for “archaeology” because, in a weird way, that’s what we were interested in. Because we took the whole of cosmic history as our playing field, most of the civilizations we’d be describing in our approach would probably be long gone. But all that mattered to us was that they had existed at some point in cosmic space and time. That was the archeological bent our approach took. We saw that the Kepler data could tell us more about what had happened than what was happening right now.
Meanwhile, fa represented all the astronomical terms in the original equation. The important point was that, since all of those terms were now known, fa was also known. That left just the biotechnical probability (fbt). It represented all the unknown, life-oriented probabilities in Drake’s equation. This was what we were after.
By rewriting the equation without L and using the new exoplanet data, we then saw that we could recast the question of the probability of alien life in a way that turned our new form of the equation into a very specific and scientifically meaningful formulation. Our new question, therefore, was: What would the biotechnical probability per habitable zone planet have to be for humans to be the only civilization nature had ever produced over the entire history of the universe?
In other words, what were the chances that ours is the only civilization ever? Putting in the exoplanet data, we found the answer to be 10–22, or one in ten billion trillion.32 We called this number the “pessimism line,” for reasons we’ll unpack below. To me, the implications of this number are staggering.
To understand how to think about the pessimism line, imagine you were handed a very big bag of Goldilocks-zone planets. Our results say the only way human beings are unique as a civilization-building species would be if you pulled out ten billion trillion planets and not one of them had a civilization. That’s because Kepler has shown us that there must be ten billion trillion Goldilocks-zone planets in the universe. So the pessimism line is really telling us how bad the probability of a civilization forming would have to be in order for ours to be only one that has ever existed.
Ten billion trillion planets is a lot of worlds to go through without finding anything. The sheer size of that number is enough to make it seem like we are not the first time nature has ever created a civilization-building species. By comparison, think about getting killed by lightning, an event most of us think of as unlikely. The probability that you’ll be killed by a lightning strike in any given year is about one in ten million. But, based on the pessimism line, your lightning-induced death is a thousand trillion times more likely than humanity being the only civilization in cosmic history. Surely nature is not that biased against evolving civilizations? It can’t be that perverse. Or can it?
Drake’s question—How many civilizations exist now?—still can’t be answered. But our question—What limit can be placed on the odds that it’s ever happened?—could be. We could put a stake in the ground and say that if nature’s processes of evolution led to odds less than the pessimism line, then yes, ours is the only energy-intensive, technological civilization that’s ever existed. But if nature’s value for the biotechnical probability is higher than one in ten billion trillion, then we are not the first.
After our paper was published in the journal Astrobiology, I wrote an op-ed for the New York Times about our result. The Times ran the piece with the headline “Yes, There Have Been Aliens.” Within days, I was inundated with requests for interviews from outlets ranging from the large and established, like CBS, to small websites run by avid UFOlogists. Some of those folks might have been discouraged from contacting me if the headline had been closer to what we really meant, which was “Yes, Aliens Probably Existed.” But either way, our result was bound to generate controversy. The critiques are worth looking at closely, since interpreting the pessimism line correctly is critical.
Our goal, after all, is to see how astrobiology and the study of life on other planets can help us understand climate change and the project of civilization on our own world. In that pursuit, the pessimism line marks a critical boundary where we might see our project set against the stars. But to truly understand what the pessimism line can do for us in that endeavor, we must first understand what it cannot.
THE CRITIQUE
One of the principal objections raised to our paper (and the New York Times op-ed) was straightforward. Just because the probability that we’re the only civilization in cosmic history is low (10–22), that doesn’t constitute a proof that exo-civilizations have existed before us. This was the argument made by Ross Andersen, the science editor for the Atlantic, and Ethan Siegel, an astrophysicist who writes for Forbes.33 Andersen and Siegel are excellent thinkers, and their criticisms contained a lot of insight. Their essays cut to the heart of key issues in what Woody and I were trying to explore. Most of all, their skepticism made me think even harder about the ideas in our paper, and I was grateful for that.
There was one point in particular that Andersen took issue with, and it was embodied in this sentence from my Times op-ed: “The degree of pessimism required to doubt the existence, at some point in time, of an advanced extraterrestrial civilization borders on the irrational.”34 He was right to criticize that line. In spite of the bar set by the pessimism line, it’s not “irrational” to think we are unique in cosmic history. In fact, the only empirically valid claim Woody and I can make is this: we can say with certainty where the pessimism line lies. In the absence of more data, it is rationally possible to construct an argument that nature’s value for the biotechnical probability lies below 10–22.
Questions were also raised about the values for the individual pieces that make up our biotechnical probability. Some argued that the probability of making just simple forms of life would be too low to allow civilizations to ever form. Or perhaps it was the probability of life evolving its way up to intelligence that was really low. But these considerations don’t change our result. Our biotechnical probability, fbt, does not hide the fact that each of the life-centric terms in the Drake equation might be small on its own. We didn’t establish our pessimism line by ignoring possibly small values for the individual life-centric terms. Instead, our reworking of the Drake equation bundled them all together. Our approach let us go for the whole enchilada at once: the entire evolutionary process, from abiogenesis up to the creation of a technological civilization. No matter how improbable you think each individual step is, it’s the total probability of other civilizations existing that matters. That’s what you have to pay attention to, and that’s what the pessimism line represents.
We called our result the pessimism line for good reason. The whole history of the debate about life beyond Earth is an argument between optimists and pessimists. It’s a debate that began with the opposition between Aristotle and Epicurus, extended through the 1800s to Flammarion versus Whewell, and took its modern turn with the Drake equation, through which the battle between pessimism and optimism became quantitative.
Since the 1961 Green Bank meeting, many scientists have argued that exo-civilizations are rare. What is rarely specified, however, is exactly what “rare” really means. Scratch below the surface, and you’ll see that many self-described pessimists’ version of rare is way above our pessimism line. That’s why the history of the debate can’t be ignored.
; Looking across that debate since the Drake equation appeared, we see that optimism always has a clear upper limit. You can’t get more optimistic about the possibility of life evolving on an exoplanet than if you say it always occurs (that would mean setting the value of fl at 1). The same holds true for the other life-centric terms in the Drake equation. You can’t assign a value greater than 1 to the probability of intelligence—or high technology—evolving. Making all these choices implies that every exoplanet in the habitable zone will create life that goes on to form an intelligent technological civilization.
But pessimism is another story. How low is low? How pessimistic do you have to be—expressed in terms of the Drake equation—to be truly pessimistic about exo-civilizations? That was what Woody and I were asking. Our answer provided a line marking the limit of true pessimism. If nature had a biotechnical probability that was lower than our limit—one in ten billion trillion—then human beings had to be the only example of a high-tech civilization in the history of the observable universe. In that case, we’d be truly and deeply alone in the most absolutely cosmic sense of the word. But if the forces of evolution led to a number higher than the pessimism line, then what’s happened with us on Earth has happened before.
Of course, we still don’t know what nature has chosen. But to see what our next steps might be in thinking about exo-civilizations and our own fate, we can look at how our pessimism line compares with what actual pessimists have proposed for the biotechnical probability.
Pessimist #1: Ernst Mayr. Chief among the exo-civilization pessimists was the renowned German evolutionary biologist Ernst Mayr. Mayr was a brilliant scholar who was instrumental in linking classical ideas from Darwin to the revolution in genetics that occurred after the discovery of DNA. But Mayr never bought Carl Sagan’s optimism about SETI or the existence of other intelligent forms of life. In 1995, the Planetary Society gave both men the chance to voice their opinions on the subject and respond to each other’s criticism. While Mayr never provided an explicit value for the biotechnical probability, from his essay35 we can extract an estimate of his pessimism.
Mayr had no doubts about life forming on other planets. Of the probability that life exists elsewhere in the universe, he wrote, “Even most skeptics of the SETI project will answer this question optimistically.” Because molecules necessary for the formation of life had been found in cosmic dust, he conceded that it was very possible there was life elsewhere.
The development of intelligence, however, is where Mayr’s pessimism kicks in. Looking at the history of Earth, Mayr wrote, “Only one of these [approximately fifty billion species that have existed on Earth] achieved the kind of intelligence needed to establish a civilization.” And on the subject of intelligence leading to a civilization, Mayr wrote, “Only one of [the twenty or more civilizations that have risen in the past ten thousand years] . . . reached a level of technology that has enabled them to send signals into space and to receive them.”
From Mayr’s statements, we can estimate what he thinks the biotechnical probability might be. Given that he argues that the formation of life is not a hard step, let’s assume he would be happy with a value of one in a hundred for that factor (10–2). After all, something that happens once every hundred times is not really very rare.
Given his statement about the total number of species evolved on Earth versus the single one that became intelligent, we can infer that Mayr might say that the odds of evolving intelligence on any given exoplanet with simple life would be one in fifty billion (or about 10–11). That certainly seems pretty pessimistic. Finally, from his statements about civilizations becoming high tech, we might infer he’d consider the probability for that term to be one in twenty. Let’s err on the side of pessimism and call this one in a hundred (10–2).
If we put all of these together, we would find that Mayr seems to be arguing that the value of the biotechnical probability is around one in a thousand trillion (10–15). That is certainly pretty small. Recall that if Mayr is right, you would have to sort through a bag of one thousand trillion planets to find a single technological civilization. Given that there are “only” one hundred billion stars in our galaxy, Mayr’s brand of pessimism would mean we were alone in our galaxy.
But being alone in the galaxy and being the only civilization the universe has ever produced are two different things. Comparing Mayr’s pessimism with the limit expressed by the pessimism line Woody and I derived shows something remarkable.
Even if civilizations were as rare as Mayr proposes, there is still a vast gulf between Mayr’s “one in a thousand trillion” and the pessimism line’s “one in ten billion trillion.” To be exact, even if Mayr is correct, there will still have been ten million high-tech civilizations appearing across space and time. That means ten million individual stories of a species waking up to itself. Ten million different versions of science being harnessed to harvest a planet’s resources and build a civilization. Ten million different histories of civilizations either going on to become long-lasting or collapsing under the weight of their own choices.
If you tried to imagine the history of each of these civilizations, giving each one an hour of your time, it would take 1,140 years to get through them all. That’s how many exo-civilizations would have existed in what Mayr thought to be a pessimistic universe.
Pessimist #2: Brandon Carter. In 1983, the physicist Brandon Carter developed an absolutely ingenious argument against exo-civilizations. Carter was famous for using simple observations to infer immensely vast conclusions about the universe and our place in it.
His thinking about exo-civilizations began with the simple observation that the time required for intelligence to arise on Earth was close to the total age of the Sun. In particular, while the Earth has been habitable for four billion years, it will only remain so for another billion or so years because the Sun is continually heating up. It will eventually grow so hot that the Earth’s orbit will no longer be in the habitable zone. Thus, a technological civilization (ours) has only appeared on Earth close to the end of its period of habitability. Using this one fact, Carter made the case that intelligence must have required evolution to pass through a series of “hard steps.” Fulfilling each of these hard steps would itself be highly improbable.36
Looking at Earth’s evolutionary history, Carter argued that there were ten evolutionary hard steps. These included the evolution of oxygenic photosynthesis or of multicelled animals. Based on these ten hard steps, he devised a calculation to predict the probability of exo-civilizations, which was just our biotechnical probability by a different name. Carter’s value came out to be 10–20. He claimed this was “more than sufficient to ensure that our stage of development is unique in the visible universe.”
What is wonderful about Carter’s calculation is that it leads to an explicit number for the biotechnical probability. The number he calculated was so small, it implied to him that no technological civilization other than our own could ever have existed across all cosmic space and time.
But that’s not what Carter’s number implies! A comparison of the pessimism line Woody and I found with Carter’s result shows that his 1983 calculation still allows for one hundred exo-civilizations. Carter intended his calculation to be hyper-pessimistic, but it turns out to be optimistic instead. Carter’s original argument still leaves us with the remarkable conclusion that we are not the first. If Carter is correct, a hundred other civilizations had passed through the processes of civilization building that we find ourselves in now.
It should also be noted that researchers who have followed Carter’s line of reasoning now believe only five hard steps exist, if any exist at all.37 This consideration, combined with the other values in Carter’s original paper, implies a biotechnical probability of 10–10. Compare that with our pessimism line, and you end up with a trillion exo-civilizations across cosmic history. Allowing for the existence of a trillion other civilizations is anything but pessimistic.
Pessimist #
3: Hubert Yockey. Of course, one can find ways to argue for a hyper-hyper-pessimistic viewpoint. This is exactly what Hubert Yockey did in a 1977 paper. Yockey was a physicist and information theorist. His argument focused on the first life-oriented term in Drake’s equation—the probability of life forming on an exoplanet. What are the odds, he asked, that random chemical combinations would produce the right kind of self-reproducing molecule for getting life started? His answer was less than an astonishing one in a trillion trillion trillion trillion trillion (his actual value was 10–65).38 This number is certainly below our pessimism line, and if Yockey is right, then we represent the only time in cosmic history that life of any kind has emerged.
But this kind of argument is balanced by the fact that there are strong counterarguments that life’s emergence may not be so hard to achieve. Many of these responses come from advances in biology. For example, biologist Wentao Ma and collaborators used computer simulations to show that the first replicating molecules could have been short strands of RNA (a molecule closely related to DNA and an integral part of cellular machinery). These are much easier to form than what Yockey was thinking about.39 Many researchers also take the fact that life appeared so quickly after the Earth’s formation as an indication that abiogenesis may not be extremely hard to achieve. Either way, Yockey’s hyper-hyper-pessimism seems to be an outlier in the debate about alien life.
THE BIG STEP
The pessimism line doesn’t prove that other civilizations on other worlds ever existed. It doesn’t help us in our search for signals from other civilizations that may overlap with our own. So what, exactly, does it allow us to say or to do?
More than anything, what Woody Sullivan and I did was use exoplanet science to raise a key philosophical point that drew its potency from real observations. It was an opening into a way of thinking about our place in the universe and the challenges of our Anthropocene moment in a radically different way from what we’re doing now.
