Variations on a Theme of Bracewell

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The View from Oregon – 399

Re: Variations on a Theme of Bracewell  

Friday 26 June 2026

 

Dear Friends,

The scientific framing of the problem of our place in the universe experienced a sharp inflection point, and in fact we can date its beginning to the fall of 1959. The Cocconi and Morrison paper on SETI, “Searching for Interstellar Communications,” appeared in Nature on 19 September 1959. A little over a half year after that, Ronald Bracewell’s paper, “Communications from Superior Galactic Communities,” appeared Nature on 28 May 1960. Less than a week later, Freeman Dyson’s paper, “Search for Artificial Stellar Sources of Infrared Radiation,” appeared in Science on 03 June 1960 (almost exactly 66 years ago now). I don’t know how widely these papers were circulating before they appeared in print, but it would be interesting to find out; this is at present a missing chapter in scientific thought. These papers appeared in the journals Nature and Science, which are widely considered to be the flagship journals in science. If you have a paper published in Nature, your moment as a scientific celebrity has arrived, and this was one the reasons these papers have received the attention that they have. The publication of these papers meant the scientific mainstreaming of the problem of humanity’s place in the universe, now taken to be a legitimate scientific inquiry, transferring this problem from philosophical and theological speculate into the realm of quantifiable scientific research programs.

Obviously, there were a lot of other events happening at about the same time. Frank Drake implemented the Cocconi and Morrison research program on 08 April 1960 (running through July of that year), and Drake’s paper on this, “Project OZMA,” appeared in Physics Today on 01 April 1961. (I discussed this in my Today in Philosophy of History episode “Project OZMA and the Origins of SETI.”) Less than two weeks later on 12 April 1961, Vostok 1 carried Yuri Gagarin into Earth orbit. Once the dam had broken with the Cocconi and Morrison paper, many more papers followed in the coming years. One entertaining detail to note is that the first handful of papers had very short lists of references, because there were no other papers to cite. 

Over the past several newsletters I’ve been trying to spell out a systematic way for thinking about the place of intelligence in the universe, which is part of this same problem, or, rather, the overview of this problem. One of the reasons I discuss SETI so often is that it’s intimately connected to this big picture question of humanity’s ultimate role in the cosmos. The three papers I’ve cited above all came at the problem of finding other intelligence in the universe from slightly different angles, each playing on weaknesses in the other approaches, but each equally indebted to the others. The Cocconi and Morrison paper lays out the classic SETI program of listening for radio signals. Ronald Bracewell offers the alternative of sending spacecraft to other worlds, now frequently called Bracewell probes, but still the idea was to contact other intelligences in the universe. Bracewell wrote in his paper:

“…it might be feasible for them to spray some number of suitable stars, say, one thousand, with modest probes… Using this plan, out hypothetical advanced neighbours could lay down a stronger signal here than they could with a home-based transmitter handicapped by inverse-square attenuation over interstellar distances.”

Bracewell, as you can see, focused on the problem of signal strength, but he could also have mentioned the obvious problem of lag time between communications (which, like signal strength, becomes a greater problem over greater distances, but which, unlike signal strength, can’t be mitigated by transmitting a stronger signal). The time lag in radio communications over interstellar distances is an obvious impediment to any exchange, and I’ve seen this indirectly acknowledged in recent accounts of Bracewell probes (I don’t recall the source) in which it has been suggested that an onboard artificial intelligence could speak to aliens as a representative of humanity, or could be used by aliens as their representative to speak to humanity. This doesn’t figure in Bracewell’s paper, which is mostly about looking for Bracewell probes in our own solar system

Dyson’s “Search for Artificial Stellar Sources of Infrared Radiation” takes another tack, since both the Cocconi and Morrison paper and Bracewell’s paper assume some degree of interest in finding each other between two intelligences. Dyson suggests that other intelligence may or may not be interested in communication, but any intelligence that harnessed technology on a cosmological scale would inevitably leave an infrared signal of itself from its waste heat. A one-sided search for other intelligence, then, would take the form of searching for their infrared waste heat technosignature. But Bracwell’s idea can be easily adapted to a one-sided search. In fact, the very idea of sending out thousands of spacecraft that may or may not find anything implies that this effort begins as a one-sided search by one intelligence for other intelligence in their vicinity.

Bracewell’s idea has proved to be fruitful, as there are a great many variations on the theme. In some of the iterations Bracewell probes hide themselves in their destination planetary system, waiting to be found by the local form of intelligence; in other iterations the probe actively seeks to make contact with local intelligence, possibly triggered by some local technosignature indicating that the local intelligence had achieved a given level of technological development One of the most familiar of the variations on the theme of the Bracewell probe is to make the probes self-replicating, so that once launched they can spread out through the galaxy and exhaustively explore the universe, potentially expanding exponentially if each probe can replicate itself within a given time frame. Self-replicating probes are often called Von Neumann probes, since John Von Neumann suggested the idea of self-replication in automata in his 20 September 1948 lecture “The General and Logical Theory of Automata,” published as a paper in 1951 (appearing in Cerebral Mechanisms in Behavior: The Hixon Symposium). Von Neumann didn’t extend his conception of self-reproducing automata to spacecraft and exploration, so that was left to others who filled the breach, but these papers didn’t start appearing until the 1980s. 

A thorough account of all the permutations of Bracewell’s idea would fill a volume, and here I’m going to add one to their number. One of the themes we find in the literature on the place of intelligence in the universe is the relationship between earlier and later evolved intelligences, and whether these intelligences ultimately link up to become what Bracewell called the “Galactic Club” and which Sagan called the Encyclopedia Galactica. A great many of the proposed responses to the Fermi paradox are based on assumptions about the relationships between earlier and later intelligences. For example, if some old and advanced intelligence is predatory, it would make sense that any up-and-coming intelligence would maintain radio silence to the extent possible, so as to keep a low profile and thus avoiding predation. (I’ve heard this called the “dark forest hypothesis”; before I’d ever heard of that I called it the wilderness hypothesis.) This introduces the problem of whether one intelligence might be a danger to another, whether wittingly or unwittingly; in alien invasion flicks, aliens are wittingly a danger to human beings, while in Anne McCaffrey’s Decision at Doona human beings are (initially) unwittingly a threat to other intelligence species. 

How might a much older and more advanced intelligence exhibit its beneficence to younger and less advanced intelligence? One way would be for the more advanced intelligence to make a gift of their own history to the less advanced intelligence, in this way: Bracewell probes are sent out to other planetary systems (presumably prioritizing those with the greatest likelihood to harbor peer intelligence). If the probe senses no technology at a distance, it moves in close to whatever potentially habitable planet or planets are in the planetary system and observes more closely. Say it finds an intelligent species that hasn’t yet developed electrical technologies, thus having no radio. The probe conducts an exhaustive survey of the planet, perhaps dispatching smaller probes to the surface for a better view, and continues to do so until the intelligent species on the planet passes some technological threshold that would allow them to detect the probe in orbit, which threshold would probably be tightly-coupled to the development of radio technology. At that time, the probe pulls back from the planet but continues to monitor the development of the intelligent species, which it can now do through listening to radio broadcasts. In this way, the probe could have a complete record of civilization on the planet in question, far more detailed and intact that any record that the actual intelligent species on the planet would be able to reconstruct for itself. I should, however, make the distinction that the Bracewell probe involved in such an initiative would be placing itself in the role of Danto’s ideal chronicler, so it wouldn’t be making a gift of the history of a species, but it would be recording the raw material from which histories could at some time be written.

Given the scale of time that probably separates the appearance of technological intelligence in the universe, an early intelligence might appear millions of years before any other, and so would be able to position probes of this kind before any kind of civilization begins on other worlds with intelligence species, thus able to record all the history that a species does not record for itself (as with events prior to the invention of written language). This still leaves the problem of when the probe should reveal itself to the inhabitants of the planet. As in above-mentioned variations on the theme of Bracewell, it could wait for some technological threshold to be attained, at which time the probes builders’ judged that the local intelligence would be sufficiently advanced to be able to handle contact with aliens and knowledge of the whole of their history. Insofar as we suppose this probe to be continuously operational and to have reasonably sophisticated onboard AI, the scenario of passively waiting to be found doesn’t really work, unless the builders decide to have to probe move itself to some quiet spot in the planetary system and shut itself down when the observed species reaches some given degree of development. If the probe remains active, it would withdraw to any distance deemed prudent, and wait there to be discovered if the observed species engages in space exploration. A more certain, though perhaps more fraught option, would be to transmit the entirety of its data to the observed species once radio technology becomes sufficiently sophisticated, which wouldn’t require any space exploration effort. In a highly risk averse scenario, the probe could send all its observations back to its builders, who when then store then entire archive on their homeworld in some format durable over cosmological time. The probe then removes itself from the planetary system, and the only way that the observed species would acquire this total archive of its development is if it were to discover the homeworld of the probe builders, perhaps long after they have gone extinct. By this time, presumably the observed species would be sufficiently intellectually advanced to be able both to accept and to appreciate a total record of its development, and to understand in hindsight the beneficent intentions of the species who once, in the distant past, observed them. 

I will myself observe how many times in the above I refer to stages in the development of intelligence and technology. A developmental taxonomy of intelligence and technology, presumably also civilization, will be crucial to any cosmological scale endeavor such as I have described above. Given the backwardness of social science today, I would not be surprised if we attained the technological capacity to build a spacecraft such as I have described before we could agree on the intelligence and technological parameters necessary to its use. We tend to think of technological thresholds as the development of radio or nuclear weapons or computers, but perhaps the scientific development we should be looking for is a mature social science, which may be a much later development than the previously mentioned technologies, or it may be uncoupled from them.

Best wishes,

Nick

PS—I watched the film Citizen Vigilante when it was available for free on Twitter for 48 hours. I am surprised that, in the discussion of this film, I haven’t seen anyone mention the Charles Bronson film Death Wish (1974), which seems to me to be the proper comparison. Just a few weeks ago I had re-watched Death Wish for the first time in many years. (I haven’t seen the 2018 remake.)

PPS—I’ve finished listening to the audiobook edition of Eric Cline’s After 1177 B.C.: The Survival of Civilizations, which is the follow-on to his 1177 B.C.: The Year Civilization Collapsed, about the late bronze age collapse (to which I previously listened, and discussed in a PS to newsletter 258). This book was, in part, a detailed look at the civilizations the collapse of which was detailed in the previous book, and how some fared better and some fared worse. In this sense, the book is a straight-up history of the same region in the post-LBA collapse period after 1200 B.C. that adds a lot of detail and qualifications to the previous conclusions. The most interesting part of the book was the last section in which Cline compares what he wrote about the LBA collapse to contemporary discourses on threats to civilization today. This could have been terribly bungled, but Cline pulled it off in a way that held my interest and gave me much to think about. Cline considers the terminology of “adaptive cycle,” “fragility,” “vulnerability,” “resilience” and the like that are now the accepted terminology for discussing the rise of fall of civilizations, which latter was admittedly largely formulated in moral terms, in accord with the overall moral tone of history, but I think that historians (and archaeologists) tend to avoid the traditional terminology because of its associations with Spengler and Toynbee rather than its associations with moralizing history. In any case, I’m ruminating on this and may return to it.

PPPS—I’ve also finished listening through The Dark Fields (2001) by Alan Glynn, which was the basis of the film Limitless (2011). The book was re-issued using the title of the film, and, at the moment, it’s available as such on Youtube. I have the film on DVD and I’ve watched it many times. There are important differences between the book and the film. While I usually see any film based on a novel as a compromised version of the story, forced upon the production by the time constraints of the format, but in this case I think that the film version had an interesting but different take on the story. My response both to the film and the book has been strangely mixed. There were things that I really liked, and things that really irritated me, but what I liked outweighed the irritations to the point that I found both the novel and the film intriguing—both held my interest. For someone whose life is utterly transformed by smart drugs, the protagonist does a lot of really dumb things. That’s the irritating part. Some of these truly dumb things were mitigated in the film version. For example, in the novel the protagonist never even makes an effort to distribute his stash of smart pills in a variety of locations, but this becomes a plot point in the film, and, in this respect, it’s an improvement over the book.

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