This is a Great Filter only for old and slow alien civilizations. By old, we mean civilizations that are on a planet that was formed in the first generation of rocky planet formation, still during the time when the galaxy was forming and taking on its shape. By slow, we mean civilizations that needed longer time than we did to reach intelligence. Our sun is about 4.6 billion years old, planetary formation took 0.3 billion years, so life took 4.3 billion years on Earth to form. The first few hundred million years of that may have been too violent a time for life to get started, with the solar system having many more asteroids flying around and occasionally smacking into our planet. Each smack made a lot of dust and dirt, and in general wrecked the place, and it took some time for things to calm down and become stable enough for life to form again. We don’t know how life originated, nor do we know if it had to originate over and over again because of various planetary upsets that happened. Asteroids might have been the least of Earth’s worries, as the materials making up the core of the planet had to settle out, liquefy or solidify, rise to the surface, separate into mineral concentrations, vent heat, and so on. Planets likely don’t form easily, and gigantic upheavals may be the norm. Geological time scales are of the order of a hundred million years, which is not in conflict with the delay in forming life here on Earth.
We live in a good neighborhood in the galaxy and in a good time. When the galaxy was forming, 13.2 billion years ago, there was more gas, and it was busy concentrating itself into clouds that could form stars. Because there was more gas, there would have been more large stars, meaning more stars that would live short lives and then explode. This was good for us, as it made all the nice higher elements we like to use. This was not good for planets forming earlier in the galaxy, as a supernova bombardment would have interfered with life like a good asteroid impact would. Furthermore, the motion of stars in the galaxy wasn’t as regular during formation as it is today. Now, stars are arranged in a thin disk, which rotates, differentially, around a core. Before that formed, there was more turbulent motion of the gas clouds and the stars, which means more stellar encounters, or even encounters between a star and its solar system with a primordial star, in the process of condensing. This might disturb planetary orbits, and take one out of the habitable zone. Infalling gas to the star would have affected its output, again adjusting the habitable zone. Thus, it may have been more difficult for life to get started in the early days of the galaxy than it was 4.6 billion years ago. That might translate into a longer time for life to form.
Fast forward to the time when the planet has had life around for long enough for it to become intelligent, and in a galactic blink of an eye, Sir Francis Bacon has worked his magic and it’s doing science. So, the civilization invents wood-burning and uses it for energy, powering home fires but also a start to industry. Then somebody decides on coal, and industry and science blooms, and soon oil and gas are used. Time goes on, and they are looking for the next big step. How about uranium? Some measurements are done and they decide that uranium isn’t going to work, as there has to be enough U235 to make a reactor and there isn’t. There is only 0.09% of it in natural uranium of their world. Just not enough to make a reactor out of and too hard to enrich. Civilization hits a Great Filter.
Without uranium, nuclear fission is not used and nuclear science doesn’t get promoted. Fusion is never conceived of. Without fusion, society eventually gets short on fossil fuels and powers down. Science never starts the run-up to advanced technology. The excitement in science that nuclear fission made here never happened on this alien planet.
The problem is that uranium is made in stellar burning, and especially in supernovas. The ratio of U235 to U238 is the same, as it depends on the nuclear cross-section of predecessor elements, not on the conditions in the supernova. Once uranium is made, U235 starts decaying, much faster than U238, meaning the ratio of U235 to U238, called the enrichment, drops. At our current era, we have 0.7% U235, barely enough to get a reactor made with natural uranium going. If we had started a billion years earlier, it would have been much easier. If an alien civilization took longer to develop, by a couple of billion years, it might find that the uranium enrichment is too low and their civilization hits a roadblock.
Are there any civilizations that meet these circumstances? If so, the number of old civilizations that achieve star-faring capability would be less, and the age of those that do might be less. If we are worrying about finding all the habitable worlds already occupied, we might do well to accurately measure whatever we can about the enrichment of uranium on other worlds. This is probably so far beyond any capabilities we could ever have, even with asymptotic technology, that direct measurements would take a visit to find out. But maybe there are secondary clues that give good indications.
As we gradually develop more observational technology, and start to speculate about the history of life in the galaxy, prior to finding it, it would do us well to consider the pathways to asymptotic technology closely. Is there a way that low enrichment planets would ignore uranium and go directly to fusion research? They would certainly figure out, early on, as we did, that it is not gravitation that powers the sun’s output, but something else. This led us to the realization that fusion occurred and later that we might use it. But without fission as a bridge, could they get to a fusion power plant or a fusion powered propulsion system?
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