Light of the Stars

by Adam Frank

  In the language of math, the predator (wolf) and the prey (bunny) populations are coupled. They depend on each other. The two equations must be also solved together, which makes the problem tricky from a technical point of view. Volterra worked out this solution, and it showed him that the wolves and bunnies can end up cycling back and forth from high to low populations and back again. What was truly surprising, though, was the timing.

  In an environment where both the bunny and wolf populations start low, the model predicted that only the prey begin increasing rapidly. The bunnies start reproducing first, and their numbers climb. The wolf population only begins increasing after enough bunnies are around to make them easy to find and catch.

  Eventually, the bunny population peaks as the rapidly growing number of wolves starts having its impact. After that, the bunny numbers drop and they start to grow scarce. The wolf population, however, takes some time to feel the change. Only later do their numbers peak and then start dropping. Eventually, the wolf population gets low enough for the bunnies to recover, and the cycle begins anew.

  What D’Ancona saw during the war was that the sharks (the predators) were still on the upswing, while the mackerel (the prey) were already past their peak and in decline. Volterra’s model predicted the lag between population peaks, so it explained why the shark numbers would be seen increasing while the mackerel were falling. In this way, Volterra’s theory—meaning his mathematical model—let D’Ancona get to the root of his apparent paradox.9 The theory revealed the essential biology of predator-prey interactions.

  What emerged from the work of Volterra and other pioneers was a true form of theoretical biology. In this setting, theory doesn’t mean a hypothesis, like a detective’s notion of who committed a murder. Rather, in science, theory means a large body of knowledge resting on mathematical principles that have been thoroughly verified through experience. The theory of population biology (also called population ecology) that Volterra and others founded was powerful enough that it could be applied to an ever-growing range of problems. Today, population biologists, ecologists, and their compatriots use mathematical models to study everything from the spread of disease to the propagation of invasive species.10 Their approach would, eventually, even find its way to the study of human civilizations.

  EASTER ISLAND, EASTER EARTH

  Easter Island is a long way from anywhere. Located more than two thousand miles west of Chile and four thousand miles southeast of Hawaii, it’s an isolated outpost of land surrounded by seemingly boundless expanses of ocean. The skilled Polynesian sailors who colonized the Pacific thousands of years ago didn’t reach Easter Island in their long canoes until sometime around 400 CE. When they did, they found an island rich in fertile soil, as well as plant and animal life. It was a promising beginning to a story that would end in ruin.

  When Dutch explorers discovered Easter Island on Easter Sunday in 1722, they found a “barren place with a few thousand people living in abject poverty and fighting over meager resources.”11 The island was devoid of trees, and the ground was covered only with unproductive scrub. But the huge stone monuments dotting the island and fashioned in the shape of human sentinels spoke of a very different past. Many of the “stone heads” were thirty feet high and weighed more than fifty tons. The silent faces of the monuments reflected a time when Easter Island had hosted a vibrant civilization with a population that may have peaked at more than ten thousand people.12 Whatever culture existed before the Dutch arrived, it was technologically advanced enough to carve the monuments from rock located at the volcanic core of the island and transport them across miles of rugged terrain.

  The mystery of what happened to Easter Island’s civilization has haunted generations of writers and scientists. Erich von Däniken, in his 1973 bestseller Chariots of the Gods? went as far as to suggest an alien civilization was the only explanation.13 How, he asked, could the islanders have moved the massive stone monuments when there were no trees around to use as rollers? But ancient aliens were not required. The answer to Easter Island’s mystery turned out to be far simpler, and far more depressing.

  There are no trees on Easter Island because the Easter Islanders cut them all down. They deforested their island in the building and transportation of those giant stone heads. In the process of deforesting the island, they also started a downward spiral that drove their civilization to collapse.

  The iconic statues on Easter Island are evidence of a thriving civilization that collapsed before the Dutch landed there in 1722.

  While there remains debate about the exact trigger for Easter Island’s fall, environmental degradation driven by the inhabitants’ own activity played an essential role. Easter Island serves as an object lesson for the interaction between an isolated, habitable environment and a civilization using that environment’s resources: they did it to themselves. The parallel to our current situation on Earth seems clear.

  In his 2007 bestseller, Collapse, anthropologist Jared Diamond unpacked that parallel.14 His work explored the trajectories of a number of human civilizations that disappeared at the height of their vibrancy and power. Diamond’s examples included the Anasazi of the American southwest, the Maya, and the Norse colony on Greenland. In each case, the civilization overshot the carrying capacity of its environment. Their populations grew as the society became ever more ingenious at extracting resources from its surroundings. Eventually, the limits to growth were hit. A short time after running into those limits, each civilization fell apart. Easter Island was the poster child for Diamond’s story.

  By the time Diamond brought historical examples of environmental collapse to the public’s attention, scientists had already begun the mathematical modeling of Easter Island’s fall. Using the same kinds of biological population models as those pioneered by Volterra and others, these researchers developed equations to explore the islanders’ trajectory from vibrancy to collapse.

  It began in 1995 with a paper by environmental economists James A. Brander and M. Scott Taylor.15 Brander and Taylor set out two equations. The first described the change in the human population over time. The second described the change in the availability of the island’s resources over time. Just as in Volterra’s predator-prey models, the two equations were coupled. As the islanders used the island’s resources for food and technology, their numbers grew. The resources, like trees, were renewable, and the equations could describe them growing back at natural rates, even as they were harvested by the islanders. But when Brander and Taylor solved their equations for the coupled trajectories of both the human population and the island’s resources, their model tracked the islanders’ fate with a grim certainty.

  As the population grew, the resources could not keep up. Overharvesting pulled resources down, and eventually, the island’s inhabitants went with them. Peaking sometime around 1200 CE, the human population of Easter Island then experienced a gradual die-off, ending with just a few thousand inhabitants left by the time the Dutch arrived. The mathematical model got the general trend in the history right.

  Other researchers soon followed up on Brander and Taylor’s work. They changed the assumptions in the model by adding new terms to the equations or changing the form of the terms to reflect different kinds of interactions. A 2005 study by Bill Basener and David S. Ross16 looked at the problem slightly differently. They assumed that the island had a carrying capacity for humans, as well as for the island’s resources (like trees or animals). In their models, they then made the human carrying capacity explicitly dependent on the resources. As the resource levels declined, the ability of the island to host a human population would drop as well. When Basener and Ross solved these new equations for Easter Island’s history, they saw something different from the gradual die-off Brander and Taylor found. The population climbed to its peak and then dropped like a stone—a true collapse.

  Theory building regarding the history of Easter Island continues, with new studies appearing each year. There are many open issues
that researchers must struggle with, since some of the data about the island before the Dutch arrival remains open to interpretation. But the basic path of the islanders’ fate seems well captured in the models.

  That success shows us the way forward for thinking about our own planetary fate in its proper cosmic context. What is true for an isolated island, its ecosystems, and its inhabitants should also be true for planets in the isolation of space.

  A THEORETICAL ARCHAEOLOGY OF EXO-CIVILIZATIONS

  In 1959, Carl Sagan took the greenhouse effect, a theory developed sixty years earlier for the Earth, and applied it to the distant planet Venus. In 1983, James Pollack and his collaborators took detailed models of dust storms on the distant planet Mars and applied them to Earth’s own climate after a nuclear war. In the midst of the current exoplanet revolution, astronomers are taking knowledge gained from studying Venus, Mars, and Earth and applying it to the habitability of distant worlds orbiting distant suns.

  For the last five decades, our knowledge of planets as generic cosmic phenomena has exploded. Data from these different worlds has been cross-pollinated with our understanding of Earth, helping us to understand other worlds, both in their own right and in relation to our own. This cross-pollination is so robust that scientists are now creating detailed models of possible biospheres on exoplanets. They want to be ready with predictions when soon-to-be-completed telescopes give them next-generation views of exoplanet atmospheres.

  But if we are already creating theoretical models of biosphere-harboring exoplanets, what keeps us from carrying out the same process for worlds harboring civilizations? If we ask the right kind of questions, nothing stands in our way; we can get started now. By uniting our understanding of planets with population ecology—in the spirit of Volterra and those who followed—we can take a first stab at exploring the coupled trajectories of civilizations and their planets as generic cosmic phenomena.

  It’s a project that might be called a theoretical archaeology of exo-civilizations.17 Anything we do concerning exo-civilizations will have to be theoretical. This is true not only because we don’t have data, but also because our method will start from basic ideas about life and environments, as Volterra did in developing his predator-prey model. We want to let physics, chemistry, and population ecology guide us in unpacking the possible histories of exo-civilizations. Our goal with this theoretical archaeology of exo-civilizations is to see what could have happened to them, so that we can get a better handle on what might happen to us.

  Given both the audacity and possible absurdity of anything calling itself a theoretical archaeology of exo-civilizations, let’s boil the idea down to its core elements.

  Step 1: Other Civilizations, Other Histories. As the pessimism line indicates, unless the universe has a really strong evolutionary bias against creating civilizations, we are not the first. If we are willing to take the existence of those other civilizations seriously, then we will recognize that each will have its own history in terms of interactions with its host planet.

  Step 2: It’s All about the Averages. We’re really interested in things like Drake’s final factor: How long, on average, does a technological civilization last? That means the results of a single theoretical model don’t really tell us much. What we need are statistics compiled by modeling a large number of exo-civilizations. Thanks to the pessimism line, we know what that means.

  Scientists usually like to have more than a thousand data points for whatever they’re studying (this is true even in political polling). With that much data, quantities like averages make sense. So long as nature’s choice for the biotechnical probability is one thousand times greater than the pessimism line, a thousand exo-civilizations will have already lived out their histories across cosmic space and time. Given the already tiny value of the pessimism line, it’s not much of a leap to imagine that a thousand civilizations have already run their course. This would require a biotechnical probability of just one in ten thousand trillion (10–19), which is still much smaller than most historical pessimists have feared.

  Step 3: There Is No Free Lunch. Now we enter the territory where our astrobiological view of planetary science and climate studies comes into play. In the public debate about sustainability, the focus is often on switching our civilization’s energy source from fossil fuels to something with less of a planetary impact. There is nothing wrong with such a goal, but the message often gets mangled in public debate from “less impact” into “no impact.”

  If we take the astrobiological view and start thinking like a planet, we see there’s no such thing as “no impact.” Civilizations are built by harvesting energy and using that energy to do work. The work can be anything from building buildings to transporting materials to harvesting more energy.

  Without technology, each human being gets one human being’s worth of energy each day. But with technology, we vastly expand the energy at our disposal. The average American uses the equivalent of about fifty servants just to power their home.18 If we add in the energy needed for driving, flying, and other activities, the number of virtual servants gets much, much higher. Since this is just a matter of physics, what’s true for us in terms of energy, power, and work must be true for any civilization-building species. The whole process of building a technological civilization is really an exercise in harvesting energy from the surroundings—in other words, from the planet.

  So you can’t build the kind globe-spanning, energy-intensive civilization we’re interested in without having some impact on your planet. In fact, the laws of physics demand that you have an impact. Specifically, the Second Law of Thermodynamics is the culprit.

  The Second Law tells us that energy can’t be perfectly converted into useful work. There is always some waste. So any civilization-building species on any planet, using any form of energy, must produce waste. As that waste builds up, it turns into feedbacks on the planetary systems. From this perspective, the CO2 produced by our burning fossil fuels can be seen as a kind of waste product of our civilization building. So, while the waste can take many forms, all of it will affect the planet. The states of the atmosphere, oceans, ice, and land will all change as the waste accumulates. That’s the real scientific story of climate change and the Anthropocene.

  Now, you might counter with the argument that civilizations more advanced than ours will find ways around the Second Law. Most physicists will tell you, “Good luck with that.” The Second Law is baked into the structure of the universe, and being able to skirt it entirely is very unlikely.

  But what capacities a highly advanced civilization might possess is an extremely important question for our theoretical archaeology project. It’s so important, in fact, that our archaeology of exo-civilizations is designed explicitly to avoid speculating about it. And that leads us to the next step.

  Step 4: Planets Come with a Limited Number of Energy Sources. In building our archaeology of exo-civilizations, we are going to focus explicitly on young technological civilizations. That means civilizations at our stage of development. This focus makes sense for two reasons. First, the whole point of this enterprise is to see what we can learn by treating our predicament as a general and generic phenomenon. The challenge humanity faces in the Anthropocene would not be so compelling and existential if we already had warp drives and other super-technology. Understanding our immediate fate is one good reason to keep our thinking focused on young civilizations. But the emphasis on youth is also essential for creating a project with strong scientific constraints.

  One of the greatest impediments to thinking about exo-civilizations (or our own deeper future, for that matter) is technological progress. How can we anticipate what kind of technology a civilization that’s a million years older might have at its disposal? Societies that mature might have found entirely new forms of energy that come from thin air. How can our theoretical modeling of exo-civilizations account for unknown sources of energy we haven’t discovered?

  The answer is, it can’t
. But luckily, it doesn’t have to.

  The development of technology is like climbing a ladder. You can’t make a steel blade until you know how to make an iron blade. The Babylonians simply didn’t have the capacity to build the metal-alloy components of a modern wind turbine. Each civilization must climb up the ladder of technological sophistication as it discovers the physical and chemical principles of the world around it.

  For our project, that means a young civilization will have a limited number of energy sources available. Crucially, we know what those forms are. The laws of physics, chemistry, and planetary evolution tell us what resources might be at the ready for an intelligent species building its way up the technological ladder. Here is a pretty complete list of the energy resources a planet might offer:

  •Combustion. This means burning stuff. It could be fossil fuels that are burned if the planet went through the right kind of geologic epoch, or it could just be biomaterials, like wood on our world.

  •Hydro/Wind/Tides. If the planet has fluids or gases flowing on its surface, then those movements can be tapped to generate energy.

  •Geothermal. Heat from the planet’s interior can also be captured and used to do the work of civilization building.

  •Solar. Sunlight can be trapped in both low-tech (heat) and high-tech (electric current) ways.

  •Nuclear. The energy locked up in atomic nuclei can be used as long as there are reserves of radioactive elements like uranium around. Nuclear energy is obviously higher on the technological ladder than other modes of energy harvesting, but given that our society has made use of it, it’s fair to think that others might as well.