This is one of the biggest thought experiments in science today – as we look for life elsewhere in the universe, what should we be looking for, exactly? Other stellar systems are too far away to examine directly, and even our most powerful telescopes can only resolve points of light. So how do we tell if there is life on a distant exoplanet? Also, how could we detect a distant technological civilization?

Here is where the thought experiment comes in. We know what life on Earth is like, and we know what human technology is like, so obviously we can search for other examples of what we already know. But the question is – how might life different from life on Earth be detected? What are the possible signatures of a planet covered in living things that perhaps look nothing like life on Earth. Similarly, what alien technologies might theoretically exist, and how could we detect them?

A recent paper explores this question from one particular angle – are there conditions on a planet that are necessary for the development of technology? They hypothesize that there is an “oxygen bottleneck”, a minimum concentration of oxygen in the atmosphere of a planet, that is necessary for the development of advanced technology. Specifically they argue that open air combustion, which requires a partial pressure (PO2) of oxygen of ≥ 18% (it’s about 21% on Earth), is necessary for fire and metallurgy, and that these are necessary stepping stones on the path to advanced technology.

There are a lot of assumptions in this argument, but they do a descent job of defending it. I’ve long thought an aquatic species, even if very intelligent, would not be able to develop advanced technology under water. This is a similar argument. They also point out that oxygen-based life itself can exist with much lower PO2, so there may be many planets out there with a biosignature of oxygen compatible with life, but too low to be a technosignature. They suggest we should focus our efforts to find other technosignatures on planets with high PO2.

They consider other types of atmospheres that might be compatible with life, but none are compatible with open air combustion – hence the notion of the oxygen bottleneck for the development of advanced technology.

While this is all a very interesting and potentially useful thought experiment, it is frustrating that we completely lack data. This is the same problem we have with the search for biosignatures – it challenges our imagination as to what is possible. The universe is an awfully big place, which provides for trillions of opportunities to experiment. This means that even extremely unlikely scenarios are still likely to exist, given enough opportunities. Therefore, saying that something is unlikely is not enough. We will likely find examples of everything out there in the universe unless it is so unlikely that it is essentially impossible.

It is useful to examine the physics of a hypothesis to determine if a certain type of life or technology is possible within the laws of physics. If it is truly impossible, then we can rule it out. But if it is possible, even if extremely unlikely, then we can’t.

But the big problem I have with this approach is that it is ultimately limited by our imagination. Even though the human imagination is quite expansive, it is still nothing compared to the scope of the universe. It is hard to capture in a thought experiment the real world experiment of trillions of worlds. Perhaps, for example, there might be other pathways to advanced technology, just not a type of technology that we are familiar with.

Here evolution is a good example. Biology has come up with countless clever solutions and workarounds to the challenges of life, ones that would be difficult to imagine and design from the top down. But given the countless opportunities for evolutionary experimentation, strange and unlikely solutions emerge. Similarly, perhaps most aquatic species never develop spacefaring technology, but if there are enough of them out there it may happen occasionally, through some weird path we have not imagined.

But even if the “oxygen bottleneck” is not absolute, it still may be a useful statistical realization. Open air combustion may not be necessary for the development of technology, but it does provide a relatively easy and high-probability pathway. Therefore we may be more likely to detect technosignatures on exoplanets with high oxygen levels. If we have a lot of choices we might as well start with the low-hanging fruit.

There is also the notion that we are more likely to recognize technosignatures for familiar technology. We can better imagine technology that results from open air combustion and what that might look like, and therefore know what the technosignatures might be. If an alien species did develop an alien technology through an alternate pathway, we might not even recognize it when we see it.

As long as we are stuck with an N of 1 when it comes to life and technology, all we can do is thought experiments, and keep looking for some actual data.