Can the ‘Zoo Hypothesis’ Be Saved?

by Paul Gilster | Jan 5, 2024 | Astrobiology and SETI | 57 comments

Life Elsewhere? Relaxing the Copernican Principle

by Paul Gilster | Jan 3, 2024 | Astrobiology and SETI | 25 comments

Most people I know are enthusiastic about the idea that other intelligent races exist in the galaxy. Contact is assumed to be an inevitable and probably profoundly good thing, with the exchange of knowledge possibly leading to serious advances in our own culture. This can lead to a weighting of the discourse in favor of our not being alone. The ever popular Copernican principle swings in: We can’t be unique, can we? And thus every search that comes up empty is seen as an incentive to try still other searches.

I’m going to leave the METI controversy out of this, as it’s not my intent to question how we should handle actual contact with ETI. I want to step back further from the question. What should we do if we find no trace of extraterrestrials after not just decades but centuries? I have no particular favorite in this race. To me, a universe teeming with life is fascinating, but a universe in which we are alone is equally provocative. Louis Friedman’s new book Alone But Not Lonely (University of Arizona Press, 2023) gets into these questions, and I’ll have more to say about it soon.

I’ve thought for years that we’re likely to find the galaxy stuffed with living worlds, while the number of technological civilizations is tiny, somewhere between 1 and 10. The numbers are completely arbitrary and, frankly, a way I spur (outraged) discussion when I give talks on these matters. I’m struck by how many people simply demand a galaxy that is alive with intelligence. They want to hear ‘between 10,000 and a million civilizations,’ or something of that order. More power to them, but it’s striking that such a lively collection of technological races would not have become apparent by now. I realize that the search space is far vaster than our efforts so far, but still…

Image: The gorgeous M81, 12 million light years away in Ursa Major, and seen here in a composite Spitzer/Hubble/Galaxy Evolution Explorer view. Blue is ultraviolet light captured by the Galaxy Evolution Explorer; yellowish white is visible light seen by Hubble; and red is infrared light detected by Spitzer. The blue areas show the hottest, youngest stars, while the reddish-pink denotes lanes of dust that line the spiral arms. The orange center is made up of older stars. Should we assume there is life here? Intelligence? Credit: NASA/JPL.

So when Ian Crawford (Birkbeck, University of London) was kind enough to send me a copy of his most recent paper, written with Dirk Schulze-Makuch (Technische Universität Berlin), I was glad to see the focus on an answer to the Fermi question that resonates with me, the so-called ‘zoo hypothesis.’ A variety of proposed resolutions to the ‘where are they’ question exist, but this one is still my favorite, a way we can save all those teeming alien civilizations, and a sound reason for their non-appearance.

As far as I know, Olaf Stapledon first suggested that intelligent races might keep hands off civilizations while they observed them, in his ever compelling novel Star Maker (1937). But it appears that credit for the actual term ‘zoo hypothesis’ belongs to John Ball, in a 1973 paper in Icarus. From Ball’s abstract:

Extraterrestrial intelligent life may be almost ubiquitous. The apparent failure of such life to interact with us may be understood in terms of the hypothesis that they have set us aside as part of a wilderness area or zoo.

That’s comforting for those who want a galaxy stuffed with intelligence. I want to get into this paper in the next post, but for now, I want to note that Crawford and Schulze-Makuch remind us that what is usually styled the Fermi ‘paradox’ is in fact no paradox at all if intelligent races beyond our own do not exist. We have a paradox because we are uneasy with the idea that we are somehow special in being here. Yet a universe devoid of technologies other than ours will look pretty much like what we see.

The angst this provokes comes back to our comfort with the ‘Copernican principle,’ which is frequently cited, especially when we use it to validate what we want to find. Just as the Sun is not the center of the Solar System, so the Solar System is not the center of the galaxy, etc. We are, in other words, nothing special, which makes it more likely that there are other civilizations out there because we are here. If we can build radio telescopes and explore space, so can they, because by virtue of our very mediocrity, we represent what the universe doubtless continues to offer up.

But let’s consider some implications, because the Copernican principle doesn’t always work. It was Hermann Bondi, for example, who came up with the notion that we could apply the principle to the cosmos at large, noting that the universe was not only homogeneous but isotropic, and going on to add that it would show the exact same traits for any observer not just at any place but at any time. The collapse of the Steady State theory put an end to that speculation as we pondered an evolving universe where time’s vantage counted critically in terms of what we would see.

Our position in time matters. So, for that matter, does our position in the galaxy.

But physics seems to work no matter where we look, and the assumption of widespread physical principles is essential for us to do astronomy. So as generalizations go, this Copernican notion isn’t bad, and we’d better hang on to it. Kepler figured out that planetary orbits weren’t circular, and as Caleb Scharf points out in his book The Copernicus Complex: Our Cosmic Significance in a Universe of Planets and Probabilities (Farrar, Straus and Giroux, 2014), this was a real break from the immutable universe of Aristotle. So too was Newton’s realization that the Sun itself orbits around a variable point close to its surface and well offset from its core.

So even the Sun isn’t the center of the Solar System in any absolute sense. As we move from Ptolemy to Copernicus, from Tycho Brahe to Kepler, we see a continuing exploration that pushes humanity out of any special position and any fixed notions that are the result of our preconceptions. I think the problem comes when we make this movement a hard principle, when we say that no ‘special places’ can exist. We can’t assume from a facile Copernican model that each time we apply the principle of mediocrity, we’ve solved a mystery about things we haven’t yet proven.

Consider: We’ve learned how unusual our own Solar System appears to be; indeed, how unusual so many stellar systems are as they deviate hugely from any ‘model’ of system development that existed before we started actually finding exoplanets. This is why the first ‘hot Jupiters’ were such a surprise, completely unexpected to most astronomers.

Is the Sun really just another average star lost in the teeming billions that accompany it in its 236 million year orbit of the galaxy? There are many G-class stars, to be sure, but if we were orbiting a more average star, we would have a red dwarf in the sky. These account for 75 percent, and probably more, of the stars in the Milky Way. We’re not average on that score, not when G-class stars amount to a paltry 7 percent of the total. Better to say that we’re only average, or mediocre, up to a point. If we want to take this to its logical limit, we can back our view out to the scale of the cosmos. Says Scharf::

The fact that we are so manifestly located in a specific place in the universe — around a star, in an outer region of a galaxy, not isolated in the intergalactic void, and at just this time in cosmic history — is simply inconsistent with ‘perfect’ mediocrity.

And what about life itself? Let me quote Scharf again (italics mine). Here he works in the anthropic idea that our observations of the universe are not truly random but are demanded by the fact that the universe can produce life in the first place:

…a Copernican worldview at best suggests that the universe should be teeming with life like that on Earth, and at worst doesn’t really tell us one way or the other. The alternative — anthropic arguments — require only a single instance of life in the universe, which would be us. At best, some fine-tuning studies suggest that the universe could be marginally suitable for heavy-element-based-life-forms, rather than being especially fertile. Neither view reveals much about the actual abundance of life to be expected in our universe, or much about our own more parochial significance or insignificance.

So when we speculate about the Fermi question, we need to be frank about our assumptions and, indeed, our personal inclinations. If we relax our Copernican orthodoxy, we have to admit that because we are here does not demand that they are there. Let’s just keep accumulating data to begin answering these questions.

And as we’ll discuss in the next post, Crawford and Schulze-Makuch point out that we’re already entering the era when meaningful data about these questions can be gathered. One key issue is abiogenesis. How likely is life to emerge even under the best of conditions? We may have some hard answers within decades, and they may come from discoveries in our own system or in biosignatures from a distant exoplanet.

If abiogenesis turns out to be common (and I would bet good money that it is), we still have no knowledge of how often it evolves into technological societies. An Encyclopedia Galactica could still exist. Could John Ball be right that other civilizations may be ubiquitous, but hidden from us because we have been sequestered into ‘nature preserves’ or the like? Are we an example of Star Trek’s ‘Prime Directive’ at work? There are reasons to think that the zoo hypothesis, out of all the Fermi ‘solutions’ that have been suggested, may be the most likely answer to the ‘where are they’ question other than the stark view that the galaxy is devoid of other technological societies. We’ll examine Crawford and Schulze-Makuch’s view on this next time.

Caleb Scharf’s The Copernicus Complex: Our Cosmic Significance in a Universe of Planets and Probabilities is a superb read, highly recommended. The Ball paper is “The Zoo Hypothesis,” Icarus Volume 19, Issue 3 (July 1973), pp. 347-349 (abstract). The Crawford & Schulze-Makuch paper we’ll look at next time is “Is the apparent absence of extraterrestrial technological civilizations down to the zoo hypothesis or nothing?” Nature Astronomy 28 December, 2023 (abstract).

Talking to Starglider

by Paul Gilster | Dec 6, 2023 | Astrobiology and SETI | 30 comments

When we’ve discussed interstellar ‘interlopers’ like ‘Oumuamua and 2I/Borisov, the science fiction-minded among us have now and then noted Arthur Clarke’s Rendezvous with Rama (Gollancz, 1973). Although we’ve yet to figure out definitively what ‘Oumuamua is (2/I Borisov is definitely a comet), the Clarke reference is an imaginative nod to the possibility that one day an alien craft might enter our Solar System during a gravitational assist maneuver and be flung outward on whatever its mission was (in Rama’s case, out in the direction of the Large Magellanic Cloud).

Since we’ll never see ‘Oumuamua again, we wait with great anticipation the work of the Legacy Survey of Space and Time (LSST), which will be run via the Vera Rubin Telescope (first light in 2025). Estimates vary widely but the consensus seems to be that with a telescope capable of imaging the entire visible sky in the southern hemisphere every few nights, the LSST should produce more than a few interstellar objects, perhaps ten or more, every year. We probably won’t find a Rama, but who knows?

Meanwhile, I’m reminded of another Clarke novel that rarely gets the attention in this regard that Rendezvous with Rama does. This is 1979’s The Fountains of Paradise (BCA/Gollancz). Although known primarily for its exploration of space elevators (and its reality-distorting geography), the novel includes as a separate theme another entry into the Solar System, this time by a craft that, unlike Rama, is willing to take notice of us. Starglider is its name, and it represents a civilization that is cataloging planetary systems through probes scattered across a host of nearby stars.

Starglider has a 500 kilometer antenna to communicate with its home star (humans name this Starholme), and in the words of a report on its activities within the novel, it more or less ‘charges its batteries’ each time it makes a close stellar pass. Having explored the Alpha Centauri trio, its next destination after the Sun is Tau Ceti. The game plan is that each stellar encounter will gather data and open communications with any civilization found there as a precursor to long-term radio contact and, presumably, entry into some kind of interstellar information network.

This is rather fascinating. For Starglider is smart enough to have studied human languages and is able to converse, after a fashion. From the novel:

It was obvious from its first messages that Starglider understood the meaning of several thousand basic English and Chinese words, which it had deduced from an analysis of television, radio, and especially broadcast video-text services. But what it had picked up during its approach was a very unrepresentative sample from the whole spectrum of human culture; it contained little advanced science, still less advanced mathematics, and only a random selection of literature, music, and the visual arts.

Like any self-taught genius,therefore, Starglider had huge gaps in its education. On the principle that it was better to give too much than too little, as soon as contact was established, Starglider was presented with the Oxford English Dictionary, the Great Chinese Dictionary (Mandarin edition), and the Encyclopedia Terrae. Their digital transmission required little more than fifty minutes, and it was notable that immediately thereafter Starglider was silent for almost four hours — its longest period off the air. When it resumed contact, its vocabulary was immensely enlarged, and more than ninety-nine percent of the time it could pass the Turing test with ease — that is, there was no way of telling from the messages received that Starglider was a machine, and not a highly intelligent human.

Clarke slyly notes the cultural differences between species as opposed to the commonality of, say, mathematics, saying that Starglider had little comprehension of lines like this from Keats:

Charm’d casements, opening on the foam
Of perilous seas, in faery lands forlorn…

And it drew a blank on Shakespeare as well:

Shall I compare thee to a summer’s day?
Thou art more lovely and more temperate…

Well, these are aliens, after all. We have enough trouble with cross-cultural references here on Earth. Humans broadcast thousands of hours of music and video drama to Starglider to help it out, but here, of course, we run into the messaging problem. Just how much do we want to reveal of ourselves to a culture about which we have all too little information other than that it is markedly more advanced than our own? You’ll find that aspect of the METI debate explored as a core part of the Starglider subplot.

Some have panned Starglider’s appearance in the novel because it seems intrusive to the plot (although I suppose I could argue that autonomous probes cataloging stellar systems almost have to be intrusive to get their job done). But in the midst of the Starglider passages, we learn that the chatty aliens, now freely talking to humans via radio, catalog the civilizations they find on a scale based on their technological accomplishments. Is this Clarke channeling Nikolai Kardashev?

Whatever the case, Clarke as always takes the long view, and the long view by its very nature always pushes out into mystery. Consider the scale used by Starglider:

I. Stone Tools

II. Metals, fire

III. Writing, handicrafts, ships

IV. Steam power, basic science

V. Atomic energy, space travel

VI. “…the ability to convert matter completely into energy, and to transmute all elements on an industrial scale.”

On this scale of one through six we can place our species at level 5, as Starglider sees us. But are there further levels? Clarke is wise to imply their existence without exploring it any further, as this lets the reader’s imagination do the job. He’s expert at this:

“And is there a Category Seven?” Starglider was immediately asked. The reply was a brief “Affirmative.” When pressed for details, the probe explained: “I am not allowed to describe the technology of a higher-grade culture to a lower one.” There the matter remained, right up to the moment of the final message, despite all the leading questions designed by the most ingenious legal brains of Earth.

When the University of Chicago’s Department of Philosophy transmits the whole of Thomas Aquinas’ Summa Theologica to Starglider, all hell breaks loose. I turn you to the novel for more.

Image; Hubble took this image on Oct. 12, 2019, when comet 2I/Borisov was about 418 million kilometers from Earth. The image shows dust concentrated around the nucleus, but the nucleus itself was too small to be seen by Hubble. We are on the cusp of a windfall of ‘interstellar interloper’ data as the LSST comes online within a few years. Will we ever find a Rama, or a Starglider, amidst our observations? Credit: NASA, ESA and D. Jewitt (UCLA).

As I mentioned, some critics fault The Fountains of Paradise for Starglider’s very presence, noting that there are essentially two plots at work here. In fact there are in fact three plots taking place on different timescales here, one of them dating back several thousand years, and recall that the voyage of Starglider itself spans millennia, the mission having began some 60,000 years before the events of the main part of the novel – construction of the space elevator – take place. This kind of chronological juggling, allows Clarke to inspire deeper reflection on humanity’s place in the universe and I find it enormously effective.

Wonders fairly pop out of Clarke’s early novels and much of his later work. On that score, I likewise refuse to fault him severely because he cannot achieve complex characterization. A case can be made (James Gunn makes it strongly) that science fiction of Clarke’s ilk needs to put the wonder first. Rich, strange and complicated characters confronting rich, strange and wondrous events may lead to one richness too many. For we, the readers, to absorb the mystery, we need to see how a relatively straightforward character reacts. It’s that contrast that Clarke aims to mine.

That’s only one way of doing science fiction, but much science fiction of the 1950s, which I consider the genre’s true golden age (with a nod to the late 1930s, as one must) often operated with precisely this conceit. And that’s okay, because when writers of greater literary style began to emerge – writers like Alfred Bester, say, with his staggering The Stars My Destination (1956) we were able to see complex characters confronting the deeply strange in ways that simply added depth to the experience. Look at Robert Silverberg in the 1960s as an exemplar of an almost magical insight into what makes the individual human tick. Once you’ve begun on that journey, the field is altered forever, but that doesn’t negate its rich past.

In fact, none of this subsequent growth nullifies Clarke’s accomplishment in the realm of big ideas. Consider him a writer of a kind of SF that flourished and fed a mighty stream into what has now become a river of wildly untamed ideas and insights. And sometimes only Clarke will do. Thus when i read, for the umpteenth time, The City and the Stars, I’m again dazzled by the very title, and the first few pages take me back into a realm where there are suns not quite our own casting a numinous glow over landscapes we learn to navigate through characters who learn with us. Like Stapledon’s, like Asimov’s, Clarke’s is a voice we’ll celebrate deep into the future.

Cometary Impacts: Looking for Life in the Right Places

by Paul Gilster | Nov 29, 2023 | Astrobiology and SETI | 11 comments

If you had to choose, which planetary system would you gauge most likely to house a life-bearing planet: Proxima Centauri or TRAPPIST-1? The question is a bit loaded given that there are seven TRAPPIST-1 planets, hence a much higher chance for success there than in a system that (so far) has produced evidence for only two worlds. But there are other factors having to do with the delivery of prebiotic materials by comet, which is the subject of a new paper from Richard Anslow (Cambridge Institute of Astronomy). “It’s possible that the molecules that led to life on Earth came from comets,’’ Anslow reminds us, “so the same could be true for planets elsewhere in the galaxy.”

So let’s untangle this a bit. We don’t know whether comets are vital to the origin of life on Earth or any other world, and Anslow (working with Cambridge colleagues Amy Bonsor and Paul B. Rimmer) does not argue that they are. What their paper does is to examine the environments most likely to be affected by cometary delivery of organics, which in turn could be useful as we begin to study exo-atmospheres for biosignatures. If we can narrow the kind of systems where cometary delivery is likely, that could explain future findings of life signs as opposed to systems without such mechanisms, ultimately supporting the comet delivery model.

Image: Comets contain elements such as water, ammonia, methanol and carbon dioxide that could have supplied the raw materials that upon impact on early Earth would have yielded an abundant supply of energy to produce amino acids and jump start life. Credit: Lawrence Livermore National Laboratories.

The idea of delivering life-promoting materials by impacts is hardly new. We’ve learned from asteroid return samples like those from Ryugu and now Bennu that the inventory of prebiotic molecules is rich. We’ve also found intact amino acids in meteorite samples, showing the survival of these materials from their entry into the atmosphere. The authors point out that comets contain what they call ‘prebiotic feedstock molecules’ like hydrogen cyanide (HCN) along with basic amino acids, and some studies suggest that by way of comparison with asteroids, comets have delivered two orders of magnitude more organic materials than meteorites because of their high carbon content. But survival is the key, and that involves impact velocity upon arrival.

HCN is particularly useful to consider because its strong carbon-nitrogen bonds may make it more likely to survive the high temperatures of atmospheric entry. What Anslow and team call the ‘warm comet pond’ scenario demands a relatively soft landing. This is interesting, so let me quote the paper on it. A ‘soft landing’:

…excavates the impact point and forms a dirty pond from the cometary components. Climatic variations are thought to cause the episodic drying of these ponds, promoting the rapid polymerisation of constituent prebiotic molecules. It is thought this wet-dry cycling will effectively drive the required biogeochemical reactions crucial for RNA production on the early-Earth, and therefore play an important role in the initial emergence of life… Relatively high concentrations of prebiotic molecules are required for there to be sufficient polymerisation, and so this scenario still requires low-velocity impacts. Specific prebiotic molecules are more (or less) susceptible to thermal decomposition by virtue of their molecular structure, and so the inventory of molecules that can be effectively delivered to a planet is very sensitive to impact velocity.

The question that emerges, once we’ve examined the delivery of molecules like HCN, is what kind of stellar system is most likely to benefit from a cometary delivery mechanism? To address this, the authors construct an idealized planetary system with planets of equal mass that are equally spaced to study the minimum impact velocity that can emerge on the innermost habitable planet. They then use N-body simulations to model the necessary interactions between comets and planets in terms of their position and velocity through time. The snowline marks the boundary between rocky and volatile-rich materials in the disk, as below:

Image: This is Figure 1 from the paper. Caption: Schematic diagram of the idealised planetary system considered in this work with equally spaced planets (brown circles, semi-major axis a_i ) scattering comets (small dark blue circles) from the snow-line. The blue region represents the volatile-rich region of the disc where comets occur, and the green region represents the habitable zone. Low velocity cometary impacts onto habitable planets will follow the lower arrows, which sketch the dynamically cold scattering between adjacent planets. The dynamically hot scattering as shown by the upper arrows, will result in high velocity impacts.

That equal spacing of planets is interesting. The authors call it a ‘peas in a pod’ system and note what other astronomers have observed, that “…individual exoplanet systems have much smaller dispersion in mass, radius, and orbital period in comparison to the system-to-system variation of the exoplanet population as a whole.” And indeed, tightly packed systems with equal and low-mass planets have been shown to be highly efficient at scattering comets into the inner system and hence into the habitable zone. Giant outer planets, it’s worth noting, may form in these tight systems, their effects helping to scatter comets inward, but they are not assumed in the author’s model.

The impactor’s size and velocity tell the tale. What we’d like to see is a minimum impact velocity below 15 kilometers per second to ensure the survival of the interesting prebiotic molecules like HCN. The simulations show that the impact velocity around stars like the Sun is reduced for lower mass planets, the effect being enhanced if there are planets in nearby orbits. Impact velocities drop even more for planets around low mass stars in tightly-packed systems (here again, think TRAPPIST-1), for here the comets tend to be delivered on low eccentricity orbits, a significant factor because impact speeds around low-mass stars are typically high. From the paper:

…the results of our N-body simulations demonstrate that the overall velocity distribution of impactors onto habitable planets is very sensitive to both the stellar-mass and planetary architecture, with the fraction of low-velocity impacts increasing significantly for planets around Solar-mass stars, and in tightly-packed systems. It will be these populations of exoplanets where cometary delivery of prebiotic molecules is most likely to be successful, with significant implications for the resulting prebiotic inventories due to the exponential decrease in survivability with impact velocity.

We learn from all this that we have to be attuned to the mass of the host star and the nature of planetary distribution there to be able to predict whether or not comets can effectively deliver prebiotic materials to worlds in the habitable zone. If this seems purely theoretical, consider that telescope time on future missions to study exoplanet atmospheres will be a precious commodity, and these factors may emerge as an important filter for observation. But we’ll also find out whether the correlations the authors have uncovered re lower mass, tightly packed planets are demonstrated in the presence of the biosignatures we are looking for. That may tell us whether comets are a significant factor for life’s emergence on distant worlds as well as our own.

The paper is Anslow, Bonsor & Rimmer. “Can comets deliver prebiotic molecules to rocky exoplanets?” Proceedings of the Royal Society A (2023). Full text. Thanks to my friend Antonio Tavani for the pointer to this work.

Eavesdropping on the Neighbors

by Paul Gilster | Nov 15, 2023 | Astrobiology and SETI | 13 comments

Given that interstellar communications have been on my mind recently, I was delighted to receive this essay from Don Wilkins. Based in St. Louis, where he is a now-retired aerospace engineer, Don has plenty of experience in avionics and has the chops to know how to make widely-dispersed aircraft talk to each other. Here his scope is a bit wider: What are the implications of ‘lurker’ probes, the conceivably ancient (or newer) technologies from an extraterrestrial civilization that might be monitoring our planet? If such exist, their communications become a SETI target, and the question of how their network might operate is an intriguing one. I had no idea, for example, that the idea of gravitational lensing for such communications had made its way into the SETI field, but Don here acquaints us with several studies that tackle the concept, along with other insights as found below.

by Don Wilkins

If an expansionist star faring civilization exists, it is likely to construct an interstellar communications and control network, beaming information and commands across interstellar depths.[1-7] Information concerning discoveries along with status of developments within a system, to avoid duplication of effort, as an example, are transmitted at high data rates using a star’s gravitational field as a focusing element. Such concepts are familiar to readers of Centauri Dreams.

Failure to find “von Neumann probes” in the Solar System can be related to a hypothesis and a fact. The hypothesis posits aliens hide their presence from a developing civilization until the appropriate time to formally contact the younger society – or perhaps the aliens only want to observe, rather than interact with primitives.

A von Neumann probe is difficult to locate in the vast, uncharted spaces of the Solar System. The Solar System volume is approximately 5 × 10¹⁴ cubic astronomical units (AU³) when measured to the outer boundaries of the Oort cloud. Eight planets, hundreds of moons, asteroids, comets and “empty” space provide a multitude of nooks and crannies shy aliens might use to avoid curious neighbors.

An interstellar communications network is characterized by very long delay paths, frequent network partitions needed for delay tolerance and error correction.

The inverse square law mandates a reduction in data rates by three-fourths if the distance between a receiver and transmitter is doubled. If a mesh network links interstellar probes, data rates can be increased orders of magnitude over a communications system relying on direct messaging among the probes.

Consider: The home world of the aliens may be tens of thousands of parsecs distant from a hypothetical node in the Solar System. A mesh of nodes woven through the stars and, located at convenient points in interstellar space, such as nebulae, provide three advantages. Reduced distances between nodes enable faster transmission rates using the same power. A multitude of dispersed nodes provides enhanced reliability through alternate routes to the preferred destination. The mesh network is self-routing avoiding slow, risky centralized communications architectures. These advantages come at the cost of message complexity but are worth the trade-off.

As an aside, if communications relays are located to optimize bandwidth, it may be difficult, if not impossible, for a civilization such as ours to intercept tight beamed transmissions among the probes. Even if a transmission is intercepted, and if the aliens use store and forward architectures and use multiple frequencies to transmit portions of the messages, only combining transmissions at the receiver, it may be impossible for us to identify signals as of intelligent origin or translate them into a coherent message. Transmissions are burdened with metadata, routing information, further complicating understanding of interceptions, Figure 1.

Figure 1 Message Encapsulation for Network Transmission

Given unknown compression techniques, error correction methods, message structure, and splitting messages among different frequencies and routes, intercepting and understanding traffic on the alien network may be impossible.

Frank Drake and others proposed direct communications using simple messages among the stars. “Hello” messages are sequences of bit strings easily reformatted into images depicting the Solar System and humans. If alien probes established interstellar mesh networks, it is highly probable those networks ignore Drake-type messages.

Von Neumann probes could explain the Great Silence. If robots are spreading into every nook and cranny of the Galaxy, the efficiency of radio transmissions as the contact medium is open to doubt. The physical presence of a probe in a distant star system removes any doubts about the nature of the signal. An alien spaceship provides instantaneous interchanges rather than slow century long conversations. Information density as represented by the probe is considerably greater than a radio signal could provide. Aliens may wait for contact through probes rather than relying on energy-hungry beams subject to misunderstanding launched into the unknown.

Researchers hypothesize signals intended for solar gravitational lens receivers could serve as technosignatures of advanced alien civilizations. Figure 2 diagrams a link between a transmitter in orbit about an alien star and a receiver within the Solar System.

Figure 2 Architecture of a communications network based on an Einstein Ring [8].

In addition to detecting stray communications from distant transmitters, other detection methods are possible. A possible technosignature is light reflected from a station-keeping light sail. Assuming the communications node uses a light sail to maintain the system’s position, the sail reflects the Sun’s light back into the Solar System. A “star” whose spectrum suspiciously matches that of the Sun could point to the existence of an alien communications node.

Another technosignature possibility is an area of space which is slightly warmer than expected. This could be a clue that something is trying to hide by spreading its waste heat into a low temperature, innocuous blob.

If an alien probe is located, we could message the robot. “Active ” SETI, where humans transmit to aliens, rather than listen for alien communications, has roused protests from those who worry aliens are hostile. Communicating with alien nodes within the Solar System does not provide aliens with information which they do not already possess and avoids the concerns of active SETI opponents.

Researchers avoid the issue of identifying a signal by content. An alien relay using gravitational lensing is effectively motionless in relation to the Sun. Monochromatic signals should present Doppler shifts resulting from the Earth’s orbit and rotation assuming terrestrial based sensors.

Kerby and Wright generated criteria for a long-duration node using gravitational lensing [8]:

1. Close or moderately-distanced companion stars impart huge delta-v costs on a relay spacecraft, and thus close binary or multiple-star systems should be avoided.

2. The presence of gas giant planets either limits the maximum gain total of the stellar relay (depending on the reflex semimajor axis imparted on the host star) or imparts a delta-v cost of a few m s⁻¹ per year to maintain alignment.

3. A more massive host star requires proportionally higher delta-v costs to resist its inward gravity.

4. A host star that is rapidly rotating will be deformed away from a spherical shape, resulting in a lens that does not have an exact focal point from which to transmit, resulting in significantly lowered gains.

The search for alien communications nodes orbiting nearby stars has already begun,

Table 1. Nearest network node: location priority list (top 10)

Note: In game theory, a Schelling point is a choice made without communication among the involved parties. Sagittarius A* may be chosen as a communications hub by aliens simply because it is such a visible feature located in the center of the Galaxy.

M. Gillon and other researchers examined Wolf 359 and TRAPPIST–1 for evidence of communications nodes in the vicinity of those stars.[5] No evidence of alien communications nodes was uncovered although there are any number of possible explanations for failure. The search continues and, contingent on funding, will expand to between ten and twenty stars.

For six months, a team led by G.W. Marcy searched for continuous and short period laser emissions from Proxima and Alpha Centuri. [6] Search equipment can detect power levels as low as one hundred Watts. No artificial laser light was observed in frequencies between 380 to 950 nanometers.

From the paper:

The intended recipient of the transmission is located at an unknown location near Proxima Centauri, leaving uncertain the most likely location of the local transmitter. Thus, during a year of observations, a transmitter located near the SGL focus line could appear within a region of roughly 400 arcsec from the coordinates given above. Parallax and aberration can be predicted for any instant, but the unknown location of the receiver near Proxima Centauri leaves the SGL location unknown within roughly 60 arcsec. We search within the full 400 arcsec domain for both sub-second pulses and long-lived emission of monochromatic sources. Our total field of view of 2.2 x 2.2 deg easily includes that 400 arcsec domain. Exploring the relatively large area around the anti-solar position of Proxima Cen expands the survey to include emitters located off-center that target only a fraction of the focal ring surrounding the Sun.

Two telescopes were used in the search. Candidate detections in the first were checked against exposures taken in the second telescope. No correlations were found.

Finally, a team used the Green Bank Telescope (GBT) and Breakthrough Listen (BL) backend to listen in the L and S bands for nodes using the Sun as for gravitational focusing [7]. The search covered possible nodes for Alpha Centauri AB system and HD 13908. This search was also unsuccessful although the work was regarded as proof-of-concept.

Searching for nodes in an interstellar network with a terminus within the Solar System has just begun. Earth based sensors can make relatively low-cost searches for Lurkers. Even if the probability of success is low, the enormous rewards of success merit the investment.

References

1. M. Gillon, A Novel SETI Strategy Targeting the Solar Focal Regions of the Most Nearby Stars, Acta Astronautica 94, 629 (2014)

2. M. Hippke, Interstellar Communications I Network, Overview and Assumptions, arXiv 1912.02616v2 (2019)

3. M. Hippke, Interstellar Communications II Deep Space Nodes with Gravitational Lensing, arXiv 2009.01866v1 (2020)

4. M. Hippke, Interstellar Communications III Locating Deep Space Nodes, arXiv 2104.09564v1 (2020)

5. M. Gillon, A. Burdanov, and J.T. Wright, Search for an Alien Communication from the Solar System to a Neighbor Star, arXiv 2111.05334 (2021)

6. G.W. Marcy, S.K.Tellis, and E.H. Wishnow, Laser Communications with Proxima Centauri using the Solar Gravitational Lens, Monthly Notices of the Royal Astronomical Society, 509-3, 3798-3814, https://arxiv.org/ftp/arxiv/papers/2110/2110.10247.pdf, (2022)

7. Nick Tusay, et al, A Search for Radio Technosignatures at the Solar Gravitational Lens Targeting Alpha Centauri, arXiv 2206.14807v1 (2022)

8. Stephen Kerby and Jason T. Wright, Stellar Gravitational Lens Engineering for Interstellar Communication and Artifact SETI, The American Astronomical Society, 2021 November 19, https://iopscience.iop.org/article/10.3847/1538-3881/ac2820

The Order of Interstellar Arrival

by Paul Gilster | Oct 30, 2023 | Astrobiology and SETI | 49 comments

Writers have modeled the arrival of an extraterrestrial probe in our Solar System in a number of interesting science fiction texts, from Clarke’s Rendezvous with Rama (1973) to the enigmatic visitors of Ted Chiang’s “Story of Your Life,” which Hollywood translated into the film Arrival (2016). In between I might add the classic ‘saucer landing on the White House lawn’ trope of The Day the Earth Stood Still (1951), based on a Harry Bates short story. All these and many other stories raise the question: What if before we make a radio or optical SETI detection, an extraterrestrial scout actually shows up?

Graeme Smith (UC: Santa Cruz) goes to work on the idea in a recent paper in the International Journal of Astrobiology, where he focuses on the mechanism of interstellar dispersion. The model has obvious ramifications for ourselves. We are beings who have begun probing nearby space with vehicles like Pioneer and Voyager, and in our early stages of exploration we could conceivably be reached by an extraterrestrial civilization (ETC) before we can make such journeys ourselves. Smith is asking what form such contact would take. His paper cautiously tries to quantify how interstellar exploration likely proceeds based on velocity and distance in a steadily advancing technological culture.

This takes us back to the so-called ‘Wait Equation’ explored by Andrew Kennedy in 2006, where he dug into what he called ‘the incentive trap of progress.’ Kennedy made the natural assumption that as an interstellar program of exploration proceeded, it would continue to produce faster travel speeds, so that one probe might be overtaken by another (thus A. E. van Vogt’s ‘Far Centaurus’ scenario, where a starship crew comes out of hibernation at Alpha Centauri to find a thriving civilization of humans, all of whom came by much speedier means while the original exploration team slumbered enroute).

The question, then, becomes whether we should postpone an interstellar launch until a certain amount of further progress can be made. And exactly how long should we wait? But Smith looks at this from a different angle: What kind of probes would be first to arrive in a planetary system where a civilization like ours can receive them? Would they be the ‘lurkers’ Jim Benford has written about, left by beings who expected them to report home on what evolved in our Solar System? Or might they be more overt, making themselves known in some way, and advanced well beyond our understanding?

Image: Is this really what we might expect if an ETC arrived on Earth? From the 1951 version of The Day the Earth Stood Still. Frank Lloyd Wright is said to have been involved in the design of the craft for this movie, though some believe this to be no more than a Hollywood legend. It’s an interesting one if so. And about that spacecraft: Is it too low-tech to be realistic? Read on.

In Smith’s parlance, a civilization like ours is ‘passive,’ a specific usage meaning that it is able to probe its own system with spacecraft but does not yet have interstellar capabilities. He imagines two ETCs, one in this passive state and one capable of interstellar flight. Smith’s calculations then consider probes launched by an extraterrestrial civilization that are followed by increasingly advanced probes over time. You can see from this that the farther away the sending civilization is, the more likely that what will arrive at the passive ETC will be one of its more advanced probes, the earlier ones being still in transit.

If an active ETC is evolving rapidly in technology, or is exceedingly distant, then a vehicle of relatively advanced state may be more likely to first reach a passive collecting civilization. In this case, there could be a considerable mismatch in the technology level of the first-arrival probe and that of the passive ETC that it encounters. This would presumably have ramifications for what might eventuate if an artefact from an ETC were to arrive within the Solar System and enable first-contact with terrestrials. Hypothetical reverse engineering, for example, might be difficult given the technology gap.

Assuming the probe speed scales linearly with launch date, Smith uses as an example the Voyager probes and spins out increasingly fast generations of probes, noting how many such generations will be required to reach first the closest stars and then stars farther out, and calculating the time that separates the first encounter spacecraft with the initial, zero-generation probes. The situation accelerates if we assume probe speeds that scale exponentially with launch date. I send you to the paper for his equations, but the upshot is that this scenario heightens the likelihood that a first encounter probe will display a major disparity in technology from what it finds at the receiving end.

And depending on the distance of the sending civilization, the disparity between the ETC technology and our own could be such that we would have difficulty understanding, even comprehending, what we were looking at. Smith again (italics mine):

The key implication of this paper can be summarized as followed: if an actively space-faring ETC embarks on a program to send probes to interstellar destinations, and if the technology of this ETC advances with time, then the first probe to arrive at the destination of a less-advanced ETC is less likely to be one of the earliest probes launched, but one of more advanced capability. There may thus be a substantial disparity between the level of technology comprising the first-arrival probe and that developed by the receiving ETC, if it has no interstellar capability itself. The greater the initial separation of the two ETCs, or the greater the rate of probe development by the active ETC, the greater is the potential for a technological mismatch at first encounter.

Image: A language that can alter our perception of time, under study in the film Arrival, where the probes in question represent a technology that is baffling to Earth scientists.

The situation would change, of course, if the receiving civilization is also one possessing interstellar capabilities, in which case contact might not even occur on the home world or system of the receiving culture. As we are a passive civilization in Smith’s terms, we are likely to encounter a markedly advanced civilization if an artifact ever does show up in our system. David Kipping calls this result ‘contact inequality,’ and remember, “…increasing the distance D_max of the first-contact horizon increases the likely generation number of a probe of first encounter, thereby enhancing a contact inequality with a passive ETC.”

Something entering our Solar System from another civilization should be highly sophisticated, well beyond our technological levels, and perhaps utterly opaque to our scrutiny. The scenario of, for example, the Strugatsky brothers’ Roadside Picnic seems more likely than that of The Day the Earth Stood Still. The 1972 novel depicts the ‘stalker’ Red Schuhart as he enters a ‘zone of visitation’ where an alien civilization has come to Earth and left behind bizarre and inexplicable traces. Now they’ve moved on. What is human culture to make of their detritus? From the novel:

He had never experienced anything like this before outside the Zone. And it had happened in the Zone only two or three times. It was as though he were in a different world. A million odors cascaded in on him at once—sharp, sweet, metallic, gentle, dangerous ones, as crude as cobblestones, as delicate and complex as watch mechanisms, as huge as a house and as tiny as a dust particle. The air became hard, it developed edges, surfaces, and corners, like space was filled with huge, stiff balloons, slippery pyramids, gigantic prickly crystals, and he had to push his way through it all, making his way in a dream through a junk store stuffed with ancient ugly furniture … It lasted a second. He opened his eyes, and everything was gone. It hadn’t been a different world—it was this world turning a new, unknown side to him. This side was revealed to him for a second and then disappeared, before he had time to figure it out.

Image: From the 1979 film Stalker, based loosely on the Strugatsky novel. No spacecraft, no aliens here, just the mystery of what they left and what it means.

It should hardly surprise us that an arriving interstellar probe would be well beyond our technology; otherwise, it couldn’t have gotten here. But if we factor in what Smith is saying, it appears that depending on how far away the sending ETC is, the technology gap between us and them becomes greater and greater. We’re talking about baffling and perplexing morphing into the all but unknowable. Indistinguishable from magic?

No grand arrivals, no opportunities for trade, no galactic encyclopedias. This is first contact as enigma, and if I had to put money on it, I suspect this is closer to what would happen if contact is achieved by a visitation to our planet. I return to Rendezvous with Rama, where odd geometric structures and a ‘cylindrical sea’ are found within the probe slingshotting around the Sun, and the vehicle departs as mysteriously as it came, leaving behind only one overwhelming fact: We are not alone.

The paper is Smith, “On the first probe to transit between two interstellar civilizations,”
International Journal of Astrobiology 22 (2023), 185-196 (abstract).