Sometimes I jog my perspective on thorny physics issues by going back to earlier work. At our all too infrequent dinners together, Claudio Maccone used to tease me about this, saying that older scientific papers had inevitably been superseded by recent work which would, in any case, incorporate the early documents. But I find that looking at an idea afresh sometimes means re-living its inception, which puts things in context. It was in that spirit that I recently revisited a key paper by Ronald Bracewell.

The name Bracewell holds a certain magic, invoking as it does the era when SETI was just beginning and speculations about extraterrestrial civilizations were getting wider circulation outside the science fiction magazines. Bracewell (1921-2007) was Australian by birth, acquiring degrees in mathematics and engineering and joining in work on World War II era radar. Following completion of a PhD in physics at Cambridge, he continued his work in the 1950s with a position as senior research officer at the Radiophysics Laboratory of the Commonwealth Scientific and Industrial Research Organisation (CSIRO).

Image: Ronald N. Bracewell, Stanford, CA, March 1983. Credit: NRAO/AUI Archives, Sullivan Collection. Located through Wikimedia Commons.

Bracewell came to the U.S. in 1954 to lecture on radio astronomy at UC-Berkeley before joining the Electrical Engineering department at Stanford University. His contributions to interferometry and the calibration of radiotelescope instruments to achieve breakthrough results are substantial, as a quick look through NASA’s Astrophysics Data System under his name reveals. I’ve noticed in scanning through this body of work that his interest in interstellar probes was persistent as he continued to contribute to the science of exoplanet discovery.

Nestled within the ADS results from 1960 is the unusual paper titled “Communications from Superior Galactic Communities,” which ran in Nature in 1960. In this early era, we had just had the famous paper from Giuseppe Cocconi and Philip Morrison (citation below) that is widely regarded as the beginning of modern attempts to find extraterrestrial civilizations. Given that this paper ran in Nature, which Bracewell obviously knew well because he was writing for it, we can assume that Cocconi and Morrison triggered his decision to write about the SETI question.

I call SETI a ‘question’ in this case because what struck Bracewell about it was its impracticability. Remember, at this same time, Frank Drake had begun planning (in 1959) for the project that would become Ozma, listening to Tau Ceti and Epsilon Eridani in 1960, and it’s evident that Cocconi and Morrison spurred the conference at Green Bank in 1961 that led to the creation of the famous Drake Equation. So we are witnessing western (as opposed to Soviet, with its somewhat different perspectives) SETI beginning to emerge, and it seemed to Bracewell that its approach was off-center.

This comes across in the “Communications from Superior Galactic Communities” paper loud and clear. Going through the suggestion from Cocconi and Morrison that 1420 mHz was the ‘waterhole’ frequency around which radio-using civilizations in search of an audience would gather, Bracewell then mentions Drake’s plans, and points out how unlikely it is that ETI would find us. After all, we were at that time looking for a radio beacon singling us out:

Let us assume that there are one thousand likely stars within the same range as the nearest superior community. This makes it hard for us to select the right one. Furthermore, if this advanced society is looking for us, we can only expect to find them expending such effort as they could afford to expend on the thousand likely stars within the same range of them. It does not seem likely that they would maintain a thousand transmitters at powers well above the megawatt estimated by Drake as a minimum for spanning only 10 light years, and run them for many years, and we could scarcely count on them paying special attention to us. Remember that throughout most of tho thousands of millions of years of the Earth’s existence such attention would have been fruitless.

The alternative? Send probes to nearby stars designed to attract the attention of technological beings on any planets there. It is indicative of the optimism of the early space era that Bracewell should describe interstellar flight as “…what we ourselves are now discussing and are on the point of doing, probably during this century…” We now look to the possibility of an interstellar probe by the end of this century, but the physics says the idea is doable.

Unlike SETI, where we cope with the inverse-square problem of attenuation of the signal, we would be talking about a probe within no more than a few minutes or hours of communications time from its target. Travel times are obviously lengthy, but with an eye toward science delivery for coming generations Bracewell suggests ‘swarm’ strategies that would deliver probes to perhaps the thousand stars near enough to us to be of interest. Each probe could quickly learn key facts about life and technology on these worlds.

Image: Ronald Bracewell (left), with Stanford’s Von Eshleman, a major figure in early research into gravitational lensing. Here the two are examining the horn antennae that Bracewell used in 1969 to determine that the Sun is moving relative to the cosmic background radiation. Credit: Linda Cicero/Stanford University.

In 1974, Bracewell would investigate the prospect of a galactic ‘network’ of civilizations, one that we could perhaps join, but even here in 1960 he homes in on the idea. He imagines our world joining a perhaps galaxy-spanning ‘chain of communication,’ and thus dealing with civilizations that have been through the contact scenario many times on many worlds. These would, obviously, be superior technologies from which we could learn new science.

Bracewell’s probes, then, are designed for contact, and meant to be identified by ETI. He would expand these ideas in his 1974 book The Galactic Club: Intelligent Life in Outer Space. The version of this title most likely to be available in used book stores is the 1976 printing from the San Francisco Book Company, and it’s a good thing for any interstellar enthusiast to track down.

Here the method reminds us that not long after the time Bracewell was writing, Carl Sagan was negotiating with Russian astronomer Iosif S. Shklovskii to reprint the book that would become in its western edition Intelligent Life in the Universe (Holden-Day, 1966). The story of that collaboration is itself interesting, as Shklovskii didn’t realize Sagan would not just publish his book Universe, Life, Mind in the west, but would also heavily annotate it with his own brand of science popularization. That disharmony apart, Sagan’s awareness of Bracewell becomes apparent given the method of communications that ETI uses with Earth to announce their presence in the novel Contact, the re-broadcast of radio messages from our past.

Bracewell had suggested something similar, though using radio:

Such a probe may be here now, in our solar system, trying to make its presence known to us. For this purpose a radio transmitter would seem essential. On what wave-length would it transmit, and how should we decode its signal ? To ensure use of a wave-length that could both penetrate our ionosphere and be in a band certain to be in use, the probe could first listen for our signals and then repeat them back. To us, its signals would have the appearance of echoes having delays of seconds or minutes such as were reported thirty years ago by Størmer and van der Pol and never explained.

I don’t want to get caught up in the famous delayed-echo story of the 1920s, but the short version is that amateur radio operator Jørgen Hals observed echoes of a Dutch shortwave station in 1927 and took the matter to Norwegian physicist Carl Størmer and Dutch physicist Balthasar van der Pol. The echoes became the subject of work by Scottish writer Duncan Lunan, who explored them as possible signs of a Bracewell probe operating in the Solar System. The claim became controversial, to say the least, and has since been refuted, although Lunan continued to investigate it. And it is also true that long-delayed echoes have been attributed to various natural sources but remain enigmatic.

In any case, Bracewell advocated remaining alert to a possible interstellar origin for signals that are unusual, for the benefits of joining in an interstellar conversation would be immense. He calculated that even if there were few civilizations that outlived their adolescence (remember, this was in the Cold War era, with nuclear destruction always on our minds), there might still be a few that survived and went on to long lifetimes. The paper continues:

Presumably such an ancient association would be very able indeed technically, and might seek us out by special means that we cannot guess. Whether they would be interested in rudimentary societies which, in their experience, would usually have burnt themselves out before they could be located and reached, is hard to say. Such communities would be collapsing at the rate of two a year (10³ in 500 years), and they might already have satisfied the!r curiosity by archreological 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.

Bracewell’s place in the early SETI literature, including Michael Hart and Frank Tipler, can’t be examined without bringing in John von Neumann, whose self-reproducing machines would likewise have spurred Bracewell’s imagination, though his own concept did not include this capability. I want to try to fit some of these pieces together and likewise bring back Sagan and Shklovskii in the next essay. What we’re juggling here is the very concept of what Sagan called ‘mediocrity,’ which he described as ‘the idea that we are not unique.’ Do we sometimes stretch our Copernican understanding of the cosmos too far?

The paper is Bracewell, “Communications from Superior Galactic Communities,” Nature Volume 186, Issue 4726 (1960), pp. 670-671. Abstract. The Cocconi & Morrison paper is “Searching for Interstellar Communications,” Nature 184 (4690) (1959), pp. 844–846. Full text.