Star Parasites: Carbon-Based Life and the Future of the Universe

  The universe is enormous, and yet it seems to follow certain patterns. What we are seeing today is the result of the dissipation of the enormous energy burst that came with the big bang, some 14 billion years ago. The dissipation occurs in steps, as it is typical of dissipative systems, forming a trophic chain of energy stocks that has some parallels with the kind we know in Earth's ecosphere. In this giant chain of beings, our role seems to be of "star parasites," growing on the light emitted by a star which, from the viewpoint of the star, is waste. Above: an image obtained by the Chandra X-ray telescope. This region shows hundreds of supermassive black holes, each one in a galaxy far beyond our own. (source: Ethan Siegel).

 

Nowadays, we are obsessed with the idea that we need to "produce energy." That is, of course, a wrong way to express the concept. Energy can't be produced: the first principle of thermodynamics tells us that. Energy can only be transformed from a kind of energy to another. And even that is not correct. You can only transform energy going in a specific direction, it is dictated by the second principle of thermodynamics. All you can do, and you can do no more, is to transform high energy potentials into low energy potentials. This is called "potential dissipation." 

An example: what we do when we claim that we "produce energy" is, mostly, to combine atmospheric oxygen with those long-chain carbon and hydrogen molecules that we call "fossil fuels," stored inside Earth's crust. The dissipation process starts with the chemical energy potential stored in crude oil (or gas, or coal), then it goes on in steps, always downhill, until we are left with low-temperature heat, plus water and carbon dioxide. What we covet from this transformation is heat that we then use to run engines and do more things. These intermediate steps we can call "dissipative structures," a term proposed for the first time by Ilya Prigogine.

Can we go back? It is possible, but we can't trick nature and go against the second principle. It is a steep uphill road that of recombining water and carbon dioxide to form again long-chain carbon molecules. We can only achieve that by dissipating an even higher potential, solar light. It is done all the time by plants, it is called "photosynthesis." It is the process that, long ago, created the carbon compounds we are so busy burning nowadays.

This is what makes us "star parasites." We, like the whole Earth's ecosystem, live by dissipating the potential of our star, the sun, and using the energy flow to build dissipative structures: plants, animals, and everything human-made. More correctly, we should say that we are "commensals," a technical term for those parasites that do not compete with their host for resources. From the viewpoint of the star, light is just waste discarded into space. We just intersect a minimal fraction of it and we re-emit it in a slightly lower potential form, infrared light. 

But how about the Sun? Is it a parasite of anything? Not in the same way, but the second principle of thermodynamics holds for the Sun, too. The potential that the Sun is dissipating originated long ago, with the big bang. At that time, an enormous energy potential was accumulated in a very small space. When the big bang came, this energy started being dissipated, a process that has been lasting for 14 billion years and is continuing now. As the universe expands, it cools down. Far from the enormous temperatures of its early life, the universe is now down to just 2.725 degrees Kelvin -- close to the absolute zero. In terms of radiative potential, it is by now dead. Too cold to create new dissipative structures.

But the universe can still create dissipative structures because matter can accumulate into gravity pits. These accumulations concentrate gravitational energy, creating new forms of potential dissipation. Stars are formed by accumulating a vast mass of interstellar dust in a relatively small space. Eventually, a star reaches conditions of temperature and pressure so high that it can start another process of dissipation, turning matter into energy. It is the fusion of hydrogen nuclei into helium ones, a process that, as far as we know, can happen only inside the depth of star cores. It is what makes us star parasites. There are surely other carbon-based lifeforms that do the same around other stars of our galaxy and other galaxies. (image from ESA)

Hydrogen fusion is not the only large-scale energy dissipation process in the universe. There is a sort of "trophic chain" out there, with entities that can be seen as existing at higher trophic levels. Black holes are predators that can devour anything that comes close to them, including stars, and turn it into more internal matter. Neutron stars can be eaten by black holes, but they can eat stars, too. 

Stars themselves have a life of their own. Most of them tend to become brighter and larger as they get older and consume their hydrogen stock. If they are very large, they end their life with the spectacular explosions called "supernovae." The remnant may be a neutron star or a black hole. The debris ejected into space will eventually coalesce to form a new star -- an "offspring" of the old one. So, stars reproduce but, as far as we know, all the information stored in the old star disappears in the cloud of gas that forms as a result. So, there is no transmission of information from a generation of stars to another and no evolution in the Darwinian sense (*). 

The same would seem to be valid for neutron stars, whose destiny is normally to become black holes. Then, black holes are supposed to evaporate over extremely long times by the slow emission of Hawking radiation, again losing all the information that may have been contained inside. (image below from NASA)

That's not the end of the great trophic chain. There is another energy dissipation mechanism: the decay of heavy radioactive nuclides, uranium and thorium. It generates energy that plays a fundamental role in keeping hot, at about 6000 K, the molten core of our planet. It probably does the same for billions of Earth-like planets of our galaxy. This heat is nearly impossible to detect at interstellar distances because it appears as very low temperatures at the surface, probably around a few tens of degrees K. Nevertheless, compared with the background temperature of the universe, Earth-like planets shine.

The decay of heavy nuclides is slow and not very spectacular, but it is fundamental for carbon-based life. A geologically active nucleus generates enough heat to keep the inner layers of Earth hot enough to create the structures we call "hydrothermal vents" at mid-ocean ridges. It is believed that life started at these undersea vents exploiting geothermal energy much before it ventured to the surface and learned how to exploit solar light. Then, life needs a constant supply of carbon dioxide, which is provided by outgassing from the hot mantle. A cold, solid nucleus would not be able to outgas anything and, in such case, the carbon dioxide in the atmosphere would be consumed by reacting with surface silicates and disappear in a few million years at most. A hot inner Earth doesn't just provide CO2 by outgassing. It actively controls its atmospheric concentration by removing carbon by silicate erosion and transporting it to the mantle by the subduction process that takes place at the edges of the crustal plates. And subduction can happen only because of the presence of convective currents in the semi-molten mantle.

So, no radioactive elements, no life. Or, at least, the lifetime of the ecosphere would be much shorter than it is, probably too short to generate complex, multicellular lifeforms.

There is more about radioactive elements: they have a trick that makes them go off in a burst of rapid energy dissipation. It is the neutron-catalyzed reaction that occurs for a sufficiently high concentration of a "fissile" nuclide. In the whole universe, only one fissile nuclide exists in significant amounts: the 235 isotope of uranium: most uranium and the other long-lived radioactive element, thorium, are not fissile, but "fissionable." They can undergo fission, but cannot sustain a chain reaction. 

Even though U(235) is rare, with its half-life of about 700 million years, in very ancient times it was less rare than it is nowadays. So, geological phenomena could accumulate a sufficiently large amount of it to generate a natural chain reaction. It happened at least once in Earth's history, some 2 billion years ago in Oklo, in the area we call today "Gabon," in Africa. Nothing spectacular happened at that time: the chain reaction went on and off several times, generating some heat over a few hundred thousand years. Probably, the place was rich of hot springs at the time, but there were no people who could have enjoyed them. Not even animals existed, they were to appear only one billion and a half years later. So this natural uranium reactor had little or no effect on the ecosystem. 

We don't know if other Oklo-like reactors existed on Earth but, if they existed, they must have been marginal phenomena (even though some people propose the rather improbable theory that the Moon was created in a nuclear explosion). What we know is that, in recent times, the creatures called "humans" managed to process minerals from Earth's crust in such a way to concentrate heavy nuclei at levels where chain fission could be maintained for a certain time. They even managed to "breed" fissile nuclei out of fissionable ones. The flow of energy created in this way is small, a few percent of the energy that humans are generating from fossil fuels, infinitesimal in comparison to the output of a star. But it is there. Could it become much larger in the future and became a significant mechanisms of energy dissipation in the universe? 

It is an especially interesting point because, as far as we know, the only way to run a nuclear chain reaction on a large scale is by having intelligent beings actively control it. We can say that since there is at least one such civilization in the universe (us) and there is no reason to think we are alone. The consequence is that carbon-based sentient beings might use the energy created by fission chain reaction to engage in major feats of planetary engineering and interstellar travel.

 

Of course, we are not seeing anything like that in the universe. This is the essence of the so-called "Fermi Paradox," often understood as meaning that humans are the only technological civilization existing in the universe. In a previous post, I argued that the explanation is the result of two conditions. One is that energy production by controlled nuclear fusion is not possible outside stars, so it is not available to planetary civilizations. The other is that the amounts of radioactive elements available in the solar system are too small to provide sufficient energy to sustain a civilization for a long time. Not even fissionable elements are sufficiently abundant to create more that a transient "flare" of energy dissipation that then would rapidly decline before a civilization could engage in a sustained effort of interstellar exploration.

With the fissile/fissionable resources available, humans, or their extrasolar colleagues, could at most engage in a limited, local interstellar exploration program before running out of energy. More than that, we lack the capability to control what we are doing and it is likely that we'll soon be destroyed by the pollution we ourselves are generating, while at the same time doomed by the depletion of the mineral resources needed to keep civilization going. Or that we'll blow out ourselves using those chain fission reactions we are so proud of. Fortunately, it seems that some 50 years ago we missed the chance we had to embark in a dangerous and ultimately futile nuclear energy program. Now, it is probably too late to gather the resources that would be necessary. Given the current situation, dreams of galactic empires seem to be a little premature.

If we are lucky, we'll just fall back to our normal role of star parasites, after having run out of other forms of potentials to dissipate. That might not be such a bad destiny, considering that the flow of solar energy arriving on our planet is several thousand times larger than the flow of primary energy produced nowadays. But solar energy, although abundant, may not be easy to convert into forms that can power a starship.

So, are carbon-based civilizations destined to remain forever stuck to their home planets? Not necessarily. The universe is not static, it keeps changing albeit at a very slow pace and the availability of fissile materials may be different in the future. We saw that all that is happening in the universe nowadays is the result of the dissipation of the original energy of the big bang. Nuclear fission is no exception. The existence of radioactive nuclei is the result of processes believed to take place mainly in supernova explosions, although it seems that large amounts can also be created in the processes called "kilonovae," the fusion of two neutron stars that generates a black hole. Fissionable nuclei have a half-life of the order of billions of years, so that the universe (14 billion years old) is being progressively enriched with them. 

So, maybe there is a threshold at some moment in the future in which Earth-like planets will be normally endowed with sufficiently large resources of radioactive materials that carbon-based lifeforms might be able to engage in the exploration, and maybe even the colonization, of the galaxy. They might even be able to overcome the limitations of planetary resources by scooping heavy nuclides from the dust around neutron stars or black holes. Maybe we need to wait a few billion years but, from the viewpoint of the universe, a few billions of years are nothing.

This kind of reasoning is, of course, very tentative, but surely fascinating. The universe as a dissipation system appears as a thermodynamic machine. And when you deal with a machine, you can't avoid wondering what is its purpose. Why the universe is the way it is? Why do radioactive elements exist? Why is carbon-based life so dependent on their existence? Why do they exist in the small amounts that we see nowadays? Is the future accumulation of radioactive nuclei planned for some purpose? Are carbon-based civilizations destined to use this energy, someday? Are they supposed to be "filtered" by the danger of the power of nuclear fission? It is a theme that Isaac Asimov already explored in his 1957 short story "The Gentle Vultures." 

Surely, we need to avoid the mistake of the fleas that think that the dog was created for them. But, who knows? The flea God may be more powerful than what we can imagine.

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(*) It has been hypothesized that DNA-like structures could exist inside stars, formed by the combination of "cosmic strings" and "magnetic monopoles." But, for what we can say, these lifeforms cannot move out of their stars and maybe they can't even perceive the existence of the universe outside. Something similar holds for neutron stars, despite the attempt by Robert Forward in his novel "Dragon's Egg" (1980) to imagine living beings composed of neutronium. Black holes, then, tend to destroy information and have an extremely long life. Of course, there may exist things we can't even remotely imagine in the universe but for the time being that seems to be a safe assumption. Incidentally, I went back to read Forward's novel and I found it incredibly boring. Times are changing and the novel is by now an obsolete art form.

 

The Long Term Perspectives of Nuclear Energy: Revisiting the Fermi Paradox

This is a revisitation of a post that I published in 2011, with the title "The Hubbert hurdle: revisiting the Fermi Paradox" Here, I am expanding the calculations of the previous post and emphasizing the relevance of the paradox on the availability of energy for planetary civilizations, and in particular on the possibility of developing controlled nuclear fusion. Of course, we can't prove that nuclear fusion is impossible simply because we have not been invaded by aliens, so far. But these considerations give us a certain feeling on the orders of magnitude involved in the complex relationship between energy use and civilization. Despite the hype, nuclear energy of any kind may remain forever a marginal source of energy. (Above, an "Orion" spaceship, being pushed onward by the detonation of nuclear bombs at the back).

 

 

The discovery of thousands of extrasolar planets is revolutionizing our views of the universe. It seems clear that planets are common around stars and, with about 100 billion stars in our galaxy, organic life cannot be that rare. Of course, "organic life" doesn't mean "intelligent life," and the latter doesn't mean "technologically advanced civilization." But, with so many planets, the galaxy may well be teeming with alien civilizations, some of them technologically as advanced as us, possibly much more.

The next step in this line of reasoning is called the "Fermi Paradox," said to have been proposed for the first time by the physicist Enrico Fermi in the 1950s: "if aliens exist, why aren't they here?" Even at speeds slower than light, nothing physical prevents a spaceship from crossing the galaxy from end to end in a million years or even less. Since our galaxy is more than 10 billion years old, intelligent aliens would have had plenty of time to explore and colonize every star in the galaxy. But we don't see aliens around, and that's the paradox. 

One possible interpretation of the paradox is that we are alone as sentient beings in the galaxy, perhaps in the whole universe. There may be a bottleneck, also known as the "Great Filter," that stops organic life from developing into the kind of civilization that engages in space-faring. 

Paradoxes are often extremely useful scientific tools. They state that two contrasting beliefs cannot be both true, and that's usually powerful evidence that some of our assumptions are not correct. The Fermi paradox is not so much about whether alien civilizations are common or not, but about the idea that interstellar travel is possible. It may simply be telling us that traveling from one star to another is very difficult, perhaps impossible. It is not enough to say that a future civilization will know things we can't even imagine. Any technology must obey the laws of physics. And that puts limits to what it can achieve. 

The problem of interstellar travel is not so much about how to build an interstellar spaceship. Already in the 1950s, some designs had been proposed that could do the job. An "Orion" starship would move by riding nuclear explosions at its back, and it was calculated that it could reach the nearest stars in a century or so. Of course, it would be a daunting task to build one, but there is no reason to think that it would be impossible. More advanced versions might use more exotic energy sources: antimatter or even black holes.

The real problem is not technology, it is cost. Building a fleet of interstellar spaceships requires a huge expenditure of resources that should be maintained for a time sufficiently long to carry out an interstellar exploration program - thousands of years at least. An estimate of the minimum power that a civilization needs to engage in sustained interstellar travel is of the order of 1000 terawatts (TW). It is just a guess, but it has some logic. The power installed today on our planet is approximately 18 TW and the most we could do with that was to explore the planets of our system, and even that rather sporadically. Clearly, to explore the stars, we need much more.

Of course, we are not getting close, and we may well soon start moving in the opposite direction. John Greer and Tim O'Reilly may have been the first to note that the "great filter" that generates the Fermi paradox could be explained in terms of the limitations of fossil fuels on Earth-like planets. Because of the "bell-shaped" production curve of a limited resource, a civilization flares up and then collapses. I dubbed this phenomenon the "Hubbert Hurdle" in 2011. The hurdle may be especially difficult to overcome if the Seneca effect kicks in, making the decline even faster, a true collapse.

But let's imagine that an alien civilization, or our own in the future, avoids an irreversible collapse and that it moves to nuclear energy. Let's assume it can avoid the risk of nuclear annihilation. Can nuclear energy provide enough energy for interstellar travel? There are many technical problems with nuclear energy, but a fundamental one is the availability of nuclear fuel. Without fuel, not even the most advanced spaceship can go anywhere.

 
Let's start with the technology we know: nuclear fission. Fissile elements (more exactly, "nuclides") are those that can create the kind of chain reaction that can be harnessed as an energy source. Only one of these nuclides occurs naturally in substantial amounts in the universe: the 235 isotope of uranium. It is a curious quirk of the laws of physics that this nuclide exists, alone. It is created in the explosions of supernova stars and also in the merging of neutron stars. It has accumulated on Earth's surface in amounts sufficient for humans to exploit to build tens of thousands of nuclear warheads and to currently produce about 0.3 TW of power. Fission could power a simple version of the Orion spaceship, but could it power a civilization able to explore the galaxy? Probably not. 

 

The uranium reserves on Earth are estimated at about 6 million tons. Currently, we burn some 60.000 tons of uranium per year to produce 0.3 TW of energy.  It means we would need 200 million tons per year (600,000 tons per day) to stay at the 1000 TW level estimated as needed for interstellar travel. At this rate, and with the current technology, the reserves would last for about 10 days (!). 

 

This is no surprise: it was already known in the 1950s that the uranium reserves are not sufficient even to keep our current civilization going using the fission of U(235) nuclides. Imagine engaging in the colonization of the galaxy! But, of course, we know that we are not limited to U(235) for fission energy. There also exist "fissionable" nuclides that cannot sustain a chain reaction, but that can be turned ("bred") into fissile nuclides when bombarded with neutrons (usually generated by fissile isotopes). We never deployed this technology on a large scale, but we know that it can work at the level of prototypes. So, in principle, it could be expanded and become the main source of energy for a civilization.

 

The naturally occurring fissionable nuclides are isotopes of uranium and thorium: U(338) and Th(232), both much more abundant than U(235). Let's say that, using these nuclides, the resources needed for energy production could be increased by a factor of 100 or 1,000 in comparison to what we can do now. But, even in the most optimistic estimate, at an output of 1000 TW, we would simply pass from 10 days of supply to a few decades. No way!

We can think of ways to find more uranium and thorium, but it is hard to think that bodies in the solar system could be a source. You need an active plate tectonic condition in order for geological forces to accumulate ores and, on bodies such as the Moon and the asteroids, there are no uranium ores. Only extremely tiny amounts, of the order of parts per billion. And that makes extracting it an impossible task. We also know that there are some 4 billion tons of uranium dissolved in seawater, an amount that would change the game, at least in principle. But the hurdles are enormous: uranium is so diluted that you are thinking of filtering quintillions (10^18) of tons of water to get at those dispersed amounts. Would a planetary civilization destroy its oceans in order to build interstellar spaceships? 

Maybe we can stretch things in more optimistic ways, but within reasonable hypotheses, we remain at least a couple of orders of magnitude short of what is needed. Fission is not something that can sustain an interstellar civilization. At most, it can sustain a few interstellar probes, just like fossil fuels have been able to create a limited number of interplanetary probes. (BTW, the Oamuamua object might be one of these probes sent by an alien civilization). But, sorry, no fission-based galactic empire. 

There is one more possibility: nuclear fusion, the poster child of the Atomic Age.  The idea that was common in the 1950s is that nuclear fusion was the obvious next step after fission. We would have had energy "too cheap to meter." And not only that: fusion can use hydrogen isotopes, and hydrogen is the most abundant element in the universe. A hydrogen-powered starship could refuel almost anywhere in the galaxy. Hopping from one star to another, a fusion-based galactic empire would be perfectly possible. 

 

But controlled nuclear fusion turned out to be much more difficult than expected. In more than half a century of attempts, we have never been able to get more energy from a fusion process than we pumped into it. And, as time goes by, the task starts looking steeper and steeper.

 

Maybe there is some trick that we can't see to get nuclear fusion working; maybe we are just dumber than the average galactic civilization. But we may have arrived at a fundamental point: the Fermi paradox may be telling us that controlled nuclear fusion is NOT possible.

All this is very speculative, but we arrived at a concept entirely different from the one that is at the basis of the Fermi paradox: the idea, typical of the 1950s, that a civilization keeps always expanding and that it rapidly arrives to master energy flows several orders of magnitude larger than what we can do now (sometimes called the "Kardashev Scale."). 

Maybe we'll arrive to exploit solar energy so well that we'll be able to use it to build interstellar spaceships, but we are talking of a future so remote that we can't say much about it. For the time being, we don't have to think that the Fermi Paradox is telling us that we are alone in the universe. It just tells us that we shouldn't expect miracles from nuclear technology.