In Part I I established the claim that technosignatures must be less prevalent than biosignatures, and showed that while that certainly could be true, the opposite is actually quite plausible, and by a huge factor.

In Part II we looked at the longevity term and, again, found that even though technology has been on Earth for much less time than life has, it’s still possible, and even plausible, that it typical lifetime in the Galaxy is actually much longer than that of life.

In this part, we look at two more criteria: detectability, and ambiguity.

Detectability

How detectable are technosignatures?  Except for a few things like radio and laser transmissions, it’s not actually very clear. Most technosignature strengths have not been worked out in detail!  An ongoing project led by Sofia Sheikh is to determine what Earth’s detectability is because of our technosignatures.

Héctor Soccas-Navarro proposed a nifty metric called the inchoscale that compares a technosignature strength to that produced by Earth today. So, Earth today has, by definition, i=1 for all of its technosignatures.  How does their strength compare to our biosignatures?

If you ignore one-offs like the Arecibo Message, it’s actually not clear what our “loudest” technosignature is.  To stars that see Earth transit, they could try to measure our atmospheric composition, and Jacob Haqq-Misra has worked out roughly how hard it would be to detect our CFCs, and Ravi Kopparapu has done something similar for NO_xs.  Both would be very challenging to detect…but then, so would our ozone and methane.  Which is stronger? I’m not sure.

I do know that the full SKA is supposed to be strong enough to have a shot at detecting our regular aircraft radar emissions at interstellar distances in coming decades. This means that being able to detect ichnoscale=1 technosignatures is a few decades out, and that feels similar to the time before we could detect biosignatures around an Earth analog.

The bottom line is that we don’t know whether Earth’s technosignatures are more or less detectable than its biosignatures with Earth technology from nearby stars, but it’s probably a close call, and it could easily be that technosignatures win.

Ambiguity

The ambiguity of technosignatures depends on the signature. Waste heat from Dyson Spheres any circumstellar material should generate waste heat. A narrowband radio signal, however, can only be technological (although its origin could be ambiguous).

Dynamic spectrum of the Voyager I carrier wave—a clear example of an unambiguous technosignature

So technosignatures run the gamut. Clearly, searching for an unambiguous one is better on that score, but ambiguous ones may require less contrivance—waste heat is an inevitable consequence of energy use, but there’s no reason aliens would have to use narrowband radio transmitters. Balancing this requires thinking about the axes of merit of technosignatures.

But the same is true for biosignatures! There are examples of what an unambiguous detection would look like (microbes swimming in Europa’s subsurface ocean), but there are plenty on the other end, too, especially for remote detection: detecting oxygen or methane in an alien atmosphere is a potential biosignature, but both species can also be generated abiotically.

Even iidentifying something that would serve an “agnostic” (not specific to Earth life) and unambiguous biosignature is a major challenge in astrobiology. The most probable path to success, IMO, is identifying a “constellation” of ambiguous biosignatures that together suggest strong disequilibrium chemistry maintained by metabolism (oxygen and methane together for instance).

So as far as ambiguity goes, biosignatures and technosignatures share the same problems, and neither has a clear advantage. Both have many examples of ambiguous signatures, and both can offer examples of clean detections.

Conclusions

This last point illustrates something important: biosignature searches and technosignature searches have a lot in common. Both search for the unknown, trying to balance being open-minded about what there is to find while letting what we know about Earth life to inform us. Both struggle with identifying good signatures to hunt for, how to handle ambiguity, and how to interpret null results.

But the communities don’t talk about these much between one another. Indeed, astrobiologists have called for and launched an ambitious project to nail down standards of life detection, without acknowledging or even mentioning the significant work on the topic over in SETI. Similarly, technosignature search would benefit from a this sort of rigorous exercise.

I hope our new paper will inspire better cross-pollination between the two communities, and a better balance of effort between the two methods of finding life. Since we don’t know which has a better chance of success, we should follow a mixed strategy to maximize our chances.

Our paper, written with Adam Frank, Sofia Sheikh, Manasvi Lingam, Ravi Kopparapu, and Jacob Haqq-Misra, is now published in Astrophysical Journal Letters.