The prospect of von neumann probes and the implications for the sagan-tipler debate

In the early 1980s, Carl Sagan and Frank Tipler published a series of articles in the pages of the Quarterly Journal of the Royal Astronomical Society on ETI that became a cause celebre at the time. Whilst reading for an MSc in Astronomy at the University of Sussex in the early 1990s, I expressed my interest in SETI as a staunch Saganite (influenced not only by the phenomenal Cosmos TV series, recently re-vamped by Neil de Grasse Tyson, but also by Sagan’s preceding Royal Institution Christmas lecture series) to Professor John Barrow who introduced me to the relevant chapter in the book The Anthropic Cosmological Principle that he co-authored with Frank Tipler (I still recommend the entire book for its visionary scope). I immediately went to source material and was struck by the von Neumann or self-replicating probe concept under discussion (I shall use the latter term to avoid conflation with von Neumann machine which has a well-established meaning in computer engineering). This was my road to Damascus in converting me from ETI believer to skeptic (but by the power of scientific argument rather than any epiphany). Thence, I decided to pursue a PhD in space engineering specialising in space robotics (though my research project was rather more mundane than self-replicating probes). Yet, despite its foundational importance, the Sagan-Tipler debate is almost forgotten today despite the fact that it exposes the root-and-branch of the SETI venture. However, every now and then, there is still the occasional paper on self-replicating probes but they appear to be sidestream to the SETI programme. This special issue seeks to re-focus the self-replicating probe back into the SETI mainstream where it belongs. There are four papers in this special issue. Dobler’s (2022) paper provides the ideal contextual discussion of the Fermi paradox that extant ETI are not here now. He suggests that any optimistic interpretation must accept that there must be a low probability of contact and that this state of affairs shall continue. He suggests that, although no one explanation can account for the Fermi paradox, perhaps a suite of explanations applicable to different categories of ETI are sufficient to cover the universality of the problem. And this will remain so. This is reminiscent of the “wedges” approach to combatting climate change (Pacala & Solow 2004). It does require that the explanatory wedges, even if they shift their weightings over time due to dynamic factors, must retain their universal coverage of non-contact. Nevertheless, from an instrumental viewpoint, this differs not from the assertion that ETI do not exist as Dobler (2022) points out. Only stable factors such as implausibility of interstellar travel are universal enough to account for the existence of ETI with the persistent lack of evidence. This dovetails neatly into Matloff’s (2022) paper who addresses issues associated with the selfreplicating probes including propulsion for interstellar flight. He analyses several plausible options of increasingly technological sophistication. The use of gravity assists, be they unpowered or powered, is well-established but such methods yield extremely long interstellar transmit times ∼10-10 y/ly. Such probes would almost certainly be overtaken by more advanced probes within the next century. Various forms of nuclear propulsion were considered to improve performance and reduce flight times to ∼1000 y/ly. Propellantless sails were also considered to offer similar performance.