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. It goes as, “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 spacefaring.
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 is that traveling from a 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 of 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 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 in order 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. 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 technological 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 would not have been 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, 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 efficiency of 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, in 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 huge 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 hypothesis 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 of the universe. A hydrogen-powered starship could refuel almost anywhere in the galaxy. Hopping from a 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 now 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 completely 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.
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