With 200 billion trillion (former) stars in the universe and 13.7 billion years since everything began, you might be wondering where all the alien civilizations are. This is the basic question behind the Fermi Paradox, the tension between our doubts about the potential for life in the universe (given planets in habitable zones, etc) and the fact that we have only found one planet with an intelligent (former) species. . inhabiting it.
There are many proposed explanations for the paradox, from a galactic zoo and everyone going silent so as not to be destroyed, to large filters stopping the progress of life at various stages. A new paper has looked at the paradox from a new angle and concluded that the simplest explanation may be the best; it is possible that we are entirely (or almost entirely) the only intelligent civilization in our galaxy.
The paper begins with a thought experiment proposed by the physicist Edwin Jaynes in 1968. Imagine walking into a laboratory and finding a line of large beakers filled with water into which you will place “substance X” to see if it dissolves.
In such a scenario, you would expect the substance to either dissipate almost 100 percent of the time, or nearly zero percent of the time. Either this substance dissolves in water at room temperature or it doesn’t. If it dissolved about half the time, this would mean that small changes in temperature and pressure in the laboratory were enough to change the result and that the conditions were “fine tuned” for the substance to dissolve.
We can apply the same kind of reasoning to the hunt for alien life and civilizations.
“Consider a group of Earth-like planets across the cosmos—worlds with similar gravities, compositions, chemical inventories, and climatic conditions,” the team wrote in their paper, which has not yet been peer-reviewed. “Although small differences will certainly exist in space (like beakers in a laboratory), life must either be expected to appear almost all the time under such conditions, or almost never. As before, it would seem contrived for life to appear in roughly half of the cases—again motivated by a fine-tuning perspective.”
We do not have enough information to apply this reasoning to lower levels of life, such as microbial life. It may be that microbial life appears almost every time it can arise, or almost never. We just don’t have enough data about planets and exoplanets to know both ways, although looking at our planet, we know that multicellular life has only existed for about 600 million years, which might suggest that the jump from cells to alone in multicellular life is rare.
First author on the paper David Kipping explains more.
We also cannot use our existence as proof that we are living in the scenario where intelligent species are abundant. We may just be on one of the very rare worlds where life arose.
But we have little information that we can use to narrow down the search for advanced alien civilizations. Although we have looked for possible Dyson spheres and other signs of advanced alien civilizations, all of them (where explanations have been found) have turned out to be natural phenomena, e.g. hot dogs.
If we were in a galaxy where intelligent life almost always appears (over large time scales and making reasonable assumptions about lifetimes), then you’d expect to see signs of alien civilizations all over it, as the team pointed out using a modified Drake equation. We just don’t see it, leading us to conclude that we are in the scenario where we are in a galaxy in which intelligent life almost never appears, rather than one where it is abundant.
That’s a pretty bleak conclusion, but the team says there’s still potential reason to be optimistic about the scenario where intelligent life rarely appears but spreads quickly when it does, the so-called “aliens abducted” scenario.
“Here, one can imagine that ETIs appear rarely, but when they do they often continue to colonize their region in a short time. In such a Universe, most regions are full and thus F ≈ 1. “The fact that we don’t see F ≈ 1 in place is because humanity must necessarily have exited a pocket of space where this wave has not yet reached, via the weak anthropic principle,” the team explains.
“Such a scenario lends itself to inverting the normal SETI picture—instead of looking locally, we have to look at regions far apart from us. Such a hypothesis has the advantage of being, in principle, verifiable via extragalactic SETI.”
The paper has been posted to the arXiv preprint server.