Physicist Enrico Fermi is credited with saying after a pointless search, “Where is everybody?” which remains the bane of any contemporary search of extraterrestrial intelligence.
Recent formulation of the problem by astrophysicist Robin Corbet retains the drama but replaces the traditional suspects. The disappearance of the alien radio calls chorus does not need conspiracy, disguised art or a cosmic agenda of denying Earth. It is even able to rest snugly within the most intractable restriction in the universe, the laws of physics. When intelligent life is usual, the simplest explanation as to why we still feel it is still absent is that the signals which people would anticipate to hear are frequently too weak, too narrow, too brief, or too poor aimed to be heard amidst the noise.
This is where the Fermi Paradox gains its survival. According to one survey, “where is everybody?” captures an incompatibility between plausible-sounding assumptions about a galaxy full of life, on the one hand, and the fact that no unambiguous artifact, probe, or transmission has been established, on the other. That mismatch, however, is based upon an observation that is exceptionally local. The absence of detection by Earth-based listening is not necessarily empty, but merely could be, as are equally the search being shallow, the targets being inaccurate, and the signatures being unlike those induced by human technology.
One practical method of de-mythologizing alien silence is simply to begin with a more clumsy question: what does present-day Earth detect, assuming that Earth is the target? In cross-wavelength analysis of “Earth Detecting Earth,” scientists had experimental results of signaling distance of various human-made signals using current tools. They discovered that Earth has its strongest detectable techno signatures not in common-or-garden broadcasts but in high-intensity, narrow-beam emissions like planetary radars; some of which might be seen at a distance of even 12,000 light-years. That is big until it is compared to the fine print: the common communication leakage is much less obvious, and even that which is conspicuous, does not mean that it has been identified without the appropriate survey plan, at the appropriate time and with the appropriate faith that what is being caught is not ground-based interference.
Atmospheric technosignatures are important in the same study since they do not travel in the same manner as radio. Nitrogen dioxide is a less feasible target since industrial effluents are much harder to detect, and the paper reports that an observatory of the future may be able to sense such emissions to 5.7 light-years away. That is a sober figure: it puts serious work in terms of “planet-as-a-whole” technosignature into a neighborhood of the galaxy, the amount of volume of the galaxy. Stated otherwise, even optimistic instrumentation cannot simply make the Milky Way transparent in a glass, where all the exhaust plumes of all the civilizations can be clearly seen.
Radio searches have also their engineering reality, which is that the sky is loud in the same ways that human technology is loud. Contemporary SETI pipelines will be constructed to isolate narrowband signals, which are usually regarded as more likely to be designed, and broad natural emission. But “narrowband” also characterizes a massive portion of the interference of the earth itself. A single Breakthrough Listen machine-learning research paper puts the magnitude of the task in a set of numbers that are difficult to remember: researchers processed an amount of about ~1011 spectrograms, which were subsequently filter-checked on the most interesting ones approximately 20,000, none passing a cursory examination. The non-detection is not a joke; it is a quantification of the difficulty in locating a rare and unspecified needle in an industrial haystack.
It is that challenge that makes “silence” the erroneous mental model. A civilization does not have to be silent in order to be hiding. Instead, it might just be not interested in erecting wasteful beacons, might have a mode of communication that is not leaky, may run at power levels requiring hours-to-days integration times to detect, longer than many surveys can afford on a target. In planning techno signature strategies even in the literature planning techno signatures on its own planet, detectability is limited by the power of the transmitter, bandwidth, and stare time of a telescope; smaller powers can require integrations of orders of magnitude larger than standard observing periods.
The second method of puncturing expectations is seeing what happens when astronomers do receive a rare “visitor” to subject to scrutiny. As the interstellar object 3I/ATLAS travelled in the inner solar system, Breakthrough Listen used the Green Bank Telescope of 100 meters over 1-12 GHz and did not find any artificial radio emission associated with the object, even at very high sensitivity. The valuable lesson about the editorial is not the object itself; it is the workflow. Current day searches are becoming more and more able to promptly transform odd targets into systematic, multi-instrument test cases which either generate a signature or aid in refining the boundaries as to what might have been present.
Here all this instrumentalization and methodology has Corbet squarely in its middle: he is trying to apply the alien silence like an engineering effect instead of a narrative turn. The universe may be alive and yet not light the receivers on Earth, since distance is a penalty on power, since time is a penalty on coincidence, since those messages one can most easily conceive are the least effective messages to convey. That rationale can be banal, yet the suggestion is strenuous: improved searches require less insistence on noisier hope and more on improved assumptions, extended perceptions, more clean data, and treating the technosphere on the planet itself as the reference point.
