[This post is the second in a series exploring major works in the field of space archaeology. Read the first part here.]
In 1959, Giuseppe Cocconi and Philip Morrison proposed what would become the mainstream strategy of the modern search for extraterrestrial intelligence: scanning for interstellar radio signals intentionally beamed towards the Solar system by advanced civilisations.
Ronald Bracewell of Stanford University, however, questioned some of the assumptions on which the nascent SETI program was founded:
- that interstellar communication was only practical using electromagnetic waves;
- that civilisations would transmit ‘on spec’ over geological timespans; and
- that a civilisation would be close enough that we would be among its targets.
In late May, 1960, Nature published his response, ‘Communications from Superior Galactic Communities‘ (paywalled). In it, he suggested that a civilisation might ‘spray some number of suitable stars, say, one thousand, with modest probes … armoured against meteorites and radiation damage, and stellar powered.’
These would take up an orbit within a stellar habitable zone and either attempt to gain the attention of indigenous sophonts or listen for their transmissions and report back (a response might already be on the way, he suggests!). The latter is the premise for Arthur C. Clarke’s short story ‘The Sentinel’, which inspired 2001: A Space Odyssey.
The advantage of this method of contact are that a local signal could be stronger than one attenuated by interstellar distance and that the plan does not rely on the emerging species to correctly guess the location and wavelength to search (a probe could listen for our signals and transmit back on the same).
Bracewell proposes that astronomers look for signs of probes within the Solar System – the first call to search for extraterrestrial artefacts since the collapse of the martian canal hypothesis – or be alert to the possibility that unexpected signals may be interstellar communications.
Maintaining a belief in the ‘Galactic Club’ vision, he expected that we would only find a single probe from our nearest neighbour, as the superior communities of the galaxy would act in concert to avoid duplication of effort. This Galactic Club (the term would be coined later by Bracewell, but fits here) would have long experience in contacting emerging communities.
This probe might even contain an ‘elaborate store of information and a complex computer’ so that it could converse with us – saving communication time.
Bracewell concludes with some discussion of the frequency and lifespan of superior communities. If we search and do not find a probe, perhaps the nearest superior community is beyond the range where successful communication is likely. Perhaps the mortality rate for civilizations is high.
Even so, some may achieve durability or quasi-permanence, he argues, and be searching for ‘rudimentary societies’ before they ‘burn out’:
They might already have satisfied their curiosity by archaeological inspection made at leisure on sites nearer home. On the other hand, the prospect of catching a technology near its peak might be a strong incentive for them to reach us.