Bracewell, 1960 contemplates the methods to detect a signal from an advanced civilization. The paper can be considered a critique and response to Cocconi & Morrison’s seminal 1959 paper on interstellar communication. Bracewell argues that using narrow-band radio signals on Earth is a flawed approach to detect “superior communities” as these are not abundant. Bracewell posits that the inherent paucity of advanced lifeforms complicates their detection as such a society would need to search and select a correct target star potentially hundreds of light years away. Given the geological timescale for the formation of an Earth-like planet, monitoring various radio telescopes would require operating these facilities for millions of years. Furthermore, the distances would result in large time-delays for information exchange between civilizations.
Bracewell instead provides a novel solution for detection of intelligent life by considering the search from the perspective of an advanced society (arguably, much more advanced than humanity). A society with comparably primitive scientific capabilities, such as ours, would be most focused on exploring their respective planetary neighborhood for life. An advanced society could exploit this by sending durable solar-powered probes equipped with radio transmitters to candidate systems (i.e. solar-like stars) as a means to garner the attention of the primitive society. The use of probes would provide an expansive radio-relay network capable of informing the advanced society while removing the need to select the correct star to observe, as is required by Cocconi & Morrison’s proposal. Bracewell also suggests the probes would potentially house artificial intelligence and a cache of pictorial information capable of communicating with the primitive society. The probe would actively listen for signals around the system and repeat these signals to the society to make itself known.
One of the largest limitations to Bracewell’s probes are the technological requirements. To this blogger, the technological descriptions can only be relegated to the realm of science fiction. This was perhaps most apparent in Bracewell’s comment that the probe could televise an image of a constellation. The flaw in Bracewell’s arguments are the various assumptions made including that (i) life should be around sun-like stars, (ii) intelligent life can create devices capable of analyzing and using electromagnetic radiation, (iii) a primitive society will readily decipher a signal from a probe, and (iv) an advanced society has fundamentally reached a point of quasi-permanence, allowing it to persist through the millions of years of development for a primitive society. It is difficult to argue for or against the use of probes when they rely on factors outside the scope of humanity’s scientific understanding (i.e. see Figure 1). There is also the issue that Bracewell’s probe should reach out to the civilization once it detects some radio emission. As of this writing, there is no evidence of contact by such a probe.
It should be noted there have been searches for Bracewell probes (i.e. Spaewatch at KPNO; Lunan & Gatland 1973, Spaceflight, 15, 122; SETA as described in Freitas Jr. & Valdes 1985, Acta Astronautica, 12, 12, 1027). The popularity of Bracewell’s idea is also emphasized by Wikipedia, which provides examples of how these probes have permeated science fiction. The search for such probes are ongoing but, to this blogger, the search for extraterrestrial artifacts may be the wrong approach in the search for life. A probe does not necessarily need to make contact to detect life. With the growing field of astrobiology and the quest for biomarkers of life, the need for a probe to transmit and directly communicate with a primitive society appears unnecessarily complex. Logic dictates that the absence of evidence is not evidence of absence such that the search for Bracewell probes will persist until we find an advanced civilization.
One of the many images a quick Google search provides. The configuration of such a probe is limited by one’s imagination. Above is a self-replicating Bracewell probe that could theoretically propagate and populate an entire region of the galaxy. Source: David Daring, http://www.daviddarling.info/encyclopedia/B/Bracewellprobes.html