The classical approach to estimating the present number of technological civilizations in a volume around Earth (usually, the Galaxy) is based on the famous Drake equation which, in its most basic form, can be written as:
N = ΓL
where Γ is the average rate of appearance of communicating civilizations and L is their average longevity. In steady-state, N represents the average number of communicating civilizations at any epoch in the volume under examination. Usually, the rate of appearance Γ is written as the product of the probabilities attached to the various processes that are deemed necessary for the arising of communicating civilizations.
Now, Balbi and Cirkovic in their recent paper, showed that the chances of success of SETI depend on the longevity of technological civilizations or, more broadly, on the duration of the signs of their existence, or technosignatures. They examined the longevity of technosignatures and showed how this factor affects the probability of success in the search for extraterrestrial intelligence.
In early SETI, longevity had a very restricted meaning and essentially was synonymous with the overall lifetime of technological civilizations. Therefore, although the importance of the factor L in the Drake equation was recognized from the very beginning, the subtleties involved in its definition and in the underlying statistical features were not explored in detail. This resulted in a situation in which an empirical success of SETI observations seemed predicated on a large number of coexistent Galactic civilizations at a vaguely similar stage of development (the famous “Galactic Club”). The improbability of such state of affairs has been masterfully exploited by opponents of SETI, resulting not only in well-known funding issues, but also in widespread confusion surrounding the whole enterprise.
In their paper, Balbi and Cirkovic followed a more general approach, and looked at longevity under a wider angle, decoupling the lifetime of technosignatures from that of the species that produced it. A strong general conclusion of their simulations is that the minimum value of τ compatible with detection is of order ∼ 106 years. This is what most likely type of technosignatures that we can detect, but this does not tell us anything about what is the most likely type of technosignatures that exists. It may be possible that, in reality, the set of technological civilizations or artifacts with τ > 106 years is empty. Alternatively, while very old technosignatures could indeed exist, their unequivocal detection might be impossible due to the signal-to-noise ratio declining sharply with time (for example, due to the interstellar scintillation for radio or galactic magnetic field distortions for cosmic-ray particle signals). Eventually, the success of any search for evidences of technological activity beyond Earth hinges on the possibility that long-lived technosignatures are possible, or even probable.
They also proposed a broad classification scheme for technosignatures, based on their duration, that can be useful to guide discussions and strategies for future searches.
• Type A: technosignatures that last for a duration comparable to the typical timescale of technological and cultural evolution on Earth, τ ∼ 103 years
• Type B: technosignatures that last for a duration comparable to the typical timescale of biological evolution of species on Earth, τ ∼ 106 years
• Type C: technosignatures that last for a duration comparable to the typical timescale of stellar and planetary evolution, τ ∼ 109 years.
Unlike the famous Kardashev’s energy-based classification, this is not meant to classify civilizations, but only their technosignatures. Thus, in this context, longevity should be intended in a general sense, and it may or may not coincide with the survival time of a technological species: rather, it indicates the persistence of its artifacts or of detectable evidences of its existence. This is a major departure from the classical approach to SETI, where the factor L (i.e. longevity) was explicitly linked to the duration of communicating civilizations. Also, it extends the scope of the search to locations that are not necessarily identified with planetary systems: for example, an interstellar probe emitting a detectable technosignature would be classifiable according to the same scheme, and could be treated along the lines as I discussed above.
Another appealing feature of the proposed classification is that it allows one to treat technosignatures as generic astronomical sources, partially decoupling their phenomenology from the complex bio-sociological trajectories of the civilizations that created them. For example, while some long duration technosignatures can only be produced by civilizations that are high in the Kardashev scale (notably, those requiring energy-intensive planetary engineering), others can easily be left over by societies in the early stages of their technological development (for example, persistent atmospheric pollutants, space debris, or chemically propelled interstellar probes). Even a relatively short-lived civilization low on Kardashev scale can, in principle, produce a large number of technosignatures; launching many cheap interstellar probes is one manner of doing so. It is likely that development of even modest Solar System industrial infrastructure in the course of the next century or so will demonstrate many additional ways of doing so which are unconceived at present. Also, the same civilization can produce technosignatures of various types, over the course of its history. For instance, our species has not yet produced Type A technosignatures, if we only consider the leakage of radio transmissions or the alteration of atmospheric composition by industrial activity; but its artifacts, such as the Voyager 1 and 2, Pioneer 10 and 11, and New Horizons probes, could in principle become type B or even C in the far future, even if our civilization should not survive that long. Similarly, a Type C technosignature can equally be produced by a very long-lived civilization, or by one that has gone extinct on a shorter time scale but has left behind persistent remnants, such as a beacon in a stable orbit or a Dyson-like megastructure.
A major conclusion of their work is that any Galactic technosignature that we can detect at present is most likely very long-lived, with a duration ⪰ 106 years. This is irrespective of the underlying distribution and abundance of technosignatures in the Galaxy. Short-lived (∼ 103 years) technosignatures can be detectable only if they are extremely common (at least one per each stellar system in our neighborhood) or if some global astrophysical mechanism synchronized their appearance at late times in the history of the Galaxy, so that they are essentially coeval with us. This suggests that an anthropocentric approach to SETI is flawed: it is rational to expect that the kind of technosignatures we are most likely to get in contact with is wildly different, in terms of duration, from what has been produced over the course of human history. This conclusion strengthens the case for the hitherto downplayed hypothesis (which is not easily labeled as “optimistic” or “pessimistic”) that a significant fraction of detectable technosignatures in the Galaxy are products of extraterrestrial civilizations which are now extinct.
They also showed that short-lived technosignatures are extremely unlikely to be detectable. The situation improves for Type B, where only ∼ 103 technosignatures are needed, and it gets even better for Type C: only a handful of them, distributed over galactic history, would be enough to have one in contact with us.
“We gave a first sketch of a possible model of long duration technosignatures based on the transition from an initial high-mortality rate to a more stable long-term behaviour. This should be seen as a prompt for further explorations of more complex and detailed models, whose outcome should not only orient the direction of technosignature studies, but would also be relevant for the future prospects of humanity.”— Concluded authors of the study.
Reference: Amedeo Balbi, Milan M. Ćirković, “Longevity is the key factor in the search for technosignatures”, Astronomical Journal, pp. 1-21, 2021. https://arxiv.org/abs/2103.02923
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