Light of the Stars Read online

  While the Drake equation was all about making contact with other civilizations, our perspective was straightforward: the exoplanet data now lets us make a reasonable argument that there have been many other civilizations before ours. If you agree that the pessimism line is low enough to make those other civilizations far more probable than we could have known before, then you can also take the step of considering them worthy of serious consideration. With that step, something remarkable can follow in facing the challenge of the Anthropocene.

  Before we go any further, let me be clear that you don’t have to make that step. Remaining deeply agnostic about the existence of other civilizations in cosmic history is certainly a stance that science cannot argue against. So, if you don’t think it’s worth considering those other civilizations seriously, that’s fine. Everything we have already explored about astrobiology and the Anthropocene will still hold true. Our understanding of the climate change we are driving today must still be seen as grounded in what we learned by studying other planets in the solar system. And our questions about what to do next must still be informed by the understanding we gained by studying our own planet’s long history of coevolution between the biosphere and Earth’s other coupled systems. We know what we know because we have already learned a lot about what it means to think like a planet. That means there are rules to the game when it comes to the evolution of the Earth, including the Earth with us on it. That perspective alone undercuts the arguments of climate denialists and represents a fundamental shift in how we understand ourselves and the challenges before us.

  But if you are willing to see the pessimism line as the universe’s invitation to consider other civilizations seriously, then we can begin to ask what other civilizations mean for us. The purpose here is not to consider them as the source of science fiction stories, but to recognize that we are probably not the first experiment in civilization building the universe has run.

  Throughout all of history, our mythologies have told us who we are, what we are, and where we stand in relation to the cosmos. But those stories ignored the possibility that we are one of many. Our stories did not—because they could not—include the possibility that our civilization was a planetary phenomenon that was not unique. That is why the exoplanet revolution and all the astrobiology we have explored so far can be a kind of wake-up call for us. It can be part of our coming of age as a civilization.

  The discovery that the universe is teeming with habitable-zone planets connects the challenges we face in the Anthropocene directly with the questions Fermi, Drake, and Sagan asked fifty years ago. The pessimism line tells us that the universe has had lots of opportunities to do what it did on Earth. With that information, we can begin to seriously consider that there have been many other stories, meaning many other histories, beyond our own. It’s an invitation to begin putting ourselves and our choices into a more accurate and fully cosmic context. If we take that step, then everything we’ve learned about planets and climate and biospheres becomes relevant to those other civilizations, too. We can treat those other civilizations as objects of study. That is why a science of those civilizations—a theoretical archaeology of exo-civilizations—is the territory we must explore next.

  CHAPTER 5

  THE FINAL FACTOR

  MANY WORLDS, MANY FATES

  Two decades into the twenty-first century, we find ourselves facing the existential challenge of creating a sustainable version of human civilization. The scale of human activities is pushing hard on the tightly linked planetary systems that make up Earth’s climate. As the planet begins to move off into a different climate state, our project of civilization will, at the very least, find itself under stress. At worst, Earth’s changes may make our project impossible to maintain.

  We urgently need to adapt civilization so that it can continue for the long term, so that it can become fully and globally sustainable. But before we can start working toward that goal, there’s an equally urgent question that often goes unstated: How do we know that’s even possible? How do we know there is such a thing as a long-term version of our kind of civilization? Most discussions of the sustainability crisis focus on strategies for developing new forms of energy or the projected benefits of different socioeconomic policies. But because we’re stuck looking at what’s happening to us as a singular phenomenon—a one-time story—we don’t think to step back and ask this kind of broader question. To even in pose it seems defeatist. But it must be addressed if we are to make the most informed, intelligent bets on the future.

  Let’s be clear about what our question implies. Maybe the universe just doesn’t do long-term, sustainable versions of civilizations like ours. Maybe it’s not something that’s ever worked out, even across all the planets orbiting all the stars throughout all of space and time. Maybe every technological civilization like ours has been just a flash in the pan, lighting up the cosmos with its brilliance for a few centuries, or even a few millennia, before fading back to darkness.

  This question speaks directly to Fermi’s Paradox. Perhaps the bottleneck we face today explains the Great Silence of the stars. Our question points to the final factor in Drake’s equation—the average lifetime of civilizations. Even if every planet orbiting every star in the universe evolved a civilization, it would still be possible that none lasted very long. That kind of fate might be universal for exactly the same reasons we find our own future challenged.

  So, does anyone make it past the challenge we now face?

  Staring down that question is where the rubber really meets the road in the astrobiology of the Anthropocene. The pessimism line tells us that, unless the universe is highly biased against the appearance and evolution of civilizations, others came before us. Each of those civilizations will have had a trajectory of development in terms of their growth and their impacts on their planets. Those trajectories are what we want to understand. Given what we have learned about planets and climate, there are good reasons to argue that many planets evolving a young, energy-intensive civilization will be driven into an Anthropocene-like transition. If there have been exo-civilizations before us, we’ve already learned enough about “thinking like a planet” to see if the conditions leading to Anthropocenes are common or rare. So, how can we use the science we know, gained from the planets we have seen, to begin a science of the civilizations we haven’t?

  HOW NOT TO DO A SCIENCE OF EXO-CIVILIZATIONS

  Prosthetic foreheads. That’s what you want to avoid—the Klingons, the Vulcans, the UFO aliens with the big heads. Science fiction has given us enduring images of alien races. Not surprisingly, most of them look a lot like us, but with different kinds of foreheads or ears or a different number of fingers on their hands.

  In developing our science of exo-civilizations, we’re not interested in what aliens might look like or how they might behave. We’re going to avoid the specifics of their biology and their sociology because science provides us little to work with on those issues.

  So, what issues can science help us with? There are three terms from Drake’s equation that make up the biotechnical probability. They involve basic biology (the origin of life), evolutionary biology (the rise of intelligence), and sociology (the development of socie­ties). When it comes to what might happen on other planets, we are on murky ground for each of these terms. But if we ask the right questions, there are principles that constrain our theoretical explorations. These constraints are like guide rails keeping our theoretical bowling balls from plunging into the gutter.

  For the basics of life, for example, we are going to have to rely on our knowledge of chemistry. But we already know that chemistry works the same way in distant regions of the cosmos as it does on Earth. From observations of interstellar clouds, planet-forming disks, and even exoplanet atmospheres, we can see physics and chemistry playing out exactly as they do down here on Earth. So, no matter what surprises life on other worlds may have for us, it must still utilize the same basic laws of physics and chemistry that app
ly on Earth. Based on this cosmic uniformity, scientists are already exploring what alternative biochemistries might look like.1 There are even studies of how photosynthesis might work on planets with very different kinds of suns.2

  On the question of intelligence, things get shakier. That’s because there are so many steps needed for its development. Worse, we don’t know which steps are essential and which happened to be specific to how intelligence worked out on Earth. In dealing with the evolution of intelligence, however, we do at least have a principle we believe should be general across all planets. The genius of Darwinian evolution is its ubiquity. Darwin proposed that all life on Earth was shaped by the same set of simple processes: mutation, adaptation, and survival of the fittest. Simply put, whatever organism is best adapted to the environment will outlast its competition. It’s a principle that applies to everything from the first self-replicating molecules to modern, fully-formed biological organisms. It should even apply to future self-replicating robots if we ever make them.

  So, when it comes to evolution on other worlds, this kind of uniformity should prove useful, particularly when we think globally in terms of biospheres. Darwinian evolution, in terms of population growth and competition in ecosystems, gives us a constraint for our ideas as we follow them to their consequences.

  The science of sociology and the question of the formation of civilizations seem to be another story entirely. We cannot assume that sociological truths we’ve observed on our world will hold true across time and space. Do other civilizations have political parties? Do they worship a god or gods? We can tell stories about how an exo-civilization might organize itself, but our descriptions would always be just that—a story. Here, I am specifically referring to questions of their morality or economics or religion. Have they, for example, created institutions that value altruism over conflict, or conflict over altruism? Does the idea of institutions even make sense in their civilizations?

  Unlike the foundational laws of physics and chemistry or the potential for Darwinian evolution to be cosmically general, it’s hard to see what kind of universal principles exist that would allow us to constrain something like alien economics. When it comes to sociology, I don’t believe such constraints exist.

  So, while we are now in a position to begin building a science of exo-civilizations, the questions we can meaningfully take on must be limited. We need to avoid science fiction stories. That means speculation about whether civilizations are warlike or peaceful, or whether cultures focus on empire building or are content to stay at home, is out of bounds. Trying to answer questions about any of these dichotomies is close to a hopeless task. Extending our knowledge from the seen to the unseen requires something that keeps our theory building within nature’s possible bounds. No matter how far we want to reach, there has to be some ground for our feet to stand upon. For the time being, that means sticking with the physics and chemistry of planets (things like climate) and the parts of biology one can reasonably argue should be common. In developing our science of exo-civilizations, we should try to avoid questions about culture. That will be the challenge in building our astrobiology of the Anthropocene.

  Of course, this strategy cuts out a whole lot of questions that many people want to know about exo-civilizations. For example: What are aliens like? Do they have two sexes, or twenty-three? Have they built a society on logic or on love? Are they traders or warriors? And of course: Do they look like us? If those are the questions you want to ask, I’m afraid you’re out of luck as far as our scientifically bounded theorizing is concerned.

  But there is one specific kind of question about those civilizations that our science of exo-civilizations can address directly. By sticking to the laws of planets we learned through Carl Sagan, Jack James, Lynn Margulis, James Lovelock, and thousands of others, we can now ask the question that matters most to our project of civilization: How common is the Anthropocene? How often do civilizations trigger climate change on their planets? And, most important, how easy is it for a civilization to make it through its Anthropocene bottleneck?

  OF PREDATORS AND PREY

  The Adriatic Sea has fed Italy’s eastern shores for eight thousand years. From Venice in the north to Brindisi in the south, its warm waters have provided a livelihood for more than a hundred generations of fishermen. There are 450 different species of fish swimming in the Adriatic, many of which end up as food on Italian tables.3 But those tables have always been demanding. Human fishermen are the Adriatic’s top predator, and many of the sea’s species are currently in danger of collapse from overharvesting.

  But the beat of fishermen’s oars or the buzz of their motors in the Adriatic has not been uniform across history. Conflict can slow the pace of fishing, as fleets of warships patrolling coasts make the work even more dangerous than normal. In World War I, the Adriatic became a battle zone. The new efficiency of mechanized navies gave Italy’s enemies a long enough reach that commercial fishing in the Adriatic almost ground to a halt.

  For all its hardship, that lull in fishing proved to be an unlikely gift to science. By slowing the human draw on the Adriatic’s fishing stocks, a paradox surfaced that reshaped how biologists thought about animal populations, ecology, and the nature of their own work.

  In the years immediately after the war, a young marine biologist named Umberto D’Ancona was working himself to exhaustion studying fish populations and their evolution. Through long, diligent work, D’Ancona amassed statistics on sales at fish markets in cities like Trieste, Fiume, and Venice, across the length of Italy’s Adriatic coast. His data bracketed the war years, beginning with 1910 and ending in 1923. Poring over the numbers, D’Ancona saw something that defied explanation.

  During the war years, when fishing had been reduced, the number of predators such as sharks seemed to soar. This might have made sense if the numbers of prey fish, like mackerel, had also climbed, as D’Ancona had expected. More prey should mean more predators. But the numbers of prey fish didn’t rise during the war. Instead, they dropped. The statistics in front of D’Ancona told him that less fishing led to fewer prey fish and more predators. The young scientist puzzled over his paradox until, in desperation, he brought his biology problem to an unlikely consultant: the great mathematician and physicist Vito Volterra.4

  Volterra was a world leader in solving hard physics problems. His work had touched on everything from the structure of crystals to the behavior of fluids.5 But Volterra’s reputation was not the main reason fate and D’Ancona brought him into the domain of biology. D’Ancona was also marrying the professor’s daughter. Luisa Volterra was herself a scientist, with a specialization in ecology—the biological study of populations and their environment.

  Physicist Vito Volterra (third from left) developed the predator-prey model of population ecology for his son-in-law, marine biologist Umberto D’Ancona (far right). D’Ancona's wife, ecologist Luisa Volterra (daughter of Vito), stands next to him (circa 1930).

  At the time Volterra took up the problem, mathematical “modeling” of the kind found in physics was not yet in the biologists’ toolkit. Biologists certainly dealt with statistics, but modeling is something different. Modeling is essentially a theoretical enterprise. It’s a process that begins by choosing a set of assumptions about how the world works. Those assumptions then get turned into mathematical equations, and those equations are what scientists mean what they talk about a model.

  As we have seen in building climate models for Earth or Mars, the essential step in mathematical modeling is solving equations. Those solutions are descriptions of the world’s behavior over time. They are, therefore, predictions. So, whatever equations Volterra came up with for D’Ancona’s fish problem, their solutions needed to predict how the predator and prey populations changed with time.

  Physicists have been making mathematical models ever since Newton devised his laws of mechanics back in the 1600s, giving physics its deeply theoretical emphasis. But biologists in the early twentieth century saw
their work in a different way. The kind of modeling physicists routinely carried out didn’t seem up to the task of explaining the complexity of living systems and their interactions. As complicated as the orbits of planets might be, the complexity of a single cell, or even a simple food chain, puts astronomers to shame. For biologists, fieldwork always led the way.6

  By the time Volterra began thinking about his son-in-law’s problem, however, things were changing. A movement had already begun to bring theory, in the form of mathematical models, into biology. That work had begun in the 1800s, when Pierre Verhulst of Brussels discovered what he claimed was a law of populations.7 Consider, for example, a few bacteria introduced to a pond. Their numbers will climb rapidly as each cell divides into two new “daughter” cells. The two daughter cells then divide, leading to four granddaughter cells. The process continues, yielding eight cells, and then sixteen, and so on. Soon, the bacteria population is skyrocketing. But it’s a process that can’t continue forever. Limitations on food and space mean that at some point the bacteria population reaches an environmental limit. That limit is called the environments’ carrying capacity. The population starts low, rises quickly, and then flattens out at the environment’s carrying capacity.

  A century later, Volterra (and others) took theoretical biology further by creating what is now known as the classic predator-prey model.8 It begins with two equations. One tracks the prey population, which could be something like the number of bunnies in a forest. The second follows the predator population, which we could imagine as the number of wolves in the same forest. The important point for modelers to capture is that the two populations are tied together. The wolves eat the bunnies, and that changes the bunny population. But eating bunnies lets the wolves reproduce, adding to their population. So, the bunny population affects the wolf population, too. In these linked equations, there’s a part (a “term”) that describes how the bunnies get eaten by wolves, and another that describes how the wolves have more babies by eating bunnies.