by Paul Gilster | Jan 3, 2024 | Astrobiology and SETI |
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).
by Paul Gilster | Dec 28, 2023 | Culture and Society |
An old pal from high school mentioned in an email the other day that he had an interest in Adam Frank’s work, which we’ve looked at in these pages a number of times. Although my most recent post on Frank involves a 2022 paper on technosignatures written with Penn State’s Jason Wright, my friend was most intrigued by a fascinating 2018 paper Frank wrote for the International Journal of Astrobiology (citation below). The correspondence triggered thoughts of other, much earlier scientists, particularly of Charles Lyell’s Principles of Geology (1830-1833), which did so much to introduce the concept of ‘deep time’ to Europe and played a role in Darwin’s work. Let’s look at both authors, with a nod as well to James Hutton, who largely originated the concept of deep time in the 18th Century.
Adam Frank is an astrophysicist at the University of Rochester, and one of those indispensable figures who meshes his scientific specialization (stellar evolution) with a broader view that encompasses physics, cultural change and their interplay in scientific discourse. He fits into a niche of what I think of as ‘big picture’ thinkers,’ scientists who draw out speculation to the largest scales and ponder the implications of what we do and do not know about astrophysics for a species that may spread into the cosmos.
Now in the case of my friend’s interest, the picture is indeed big. Frank’s 2018 paper asked whether our civilization is the first to emerge on Earth. Thus the ‘Silurian’ hypothesis, explored on TV’s Doctor Who in reference to a race of intelligent reptiles by that name who are accidentally awakened. The theme pops up occasionally in science fiction, though perhaps less often that one might expect. James Hogan’s 1977 novel Inherit the Stars, for example, posits evidence for unknown technologies discovered on the Moon that apparently have their origin in an earlier geological era.
Image: Astrophysicist Adam Frank. Credit: University of Rochester.
I won’t go through this paper closely because I’ve written it up before (see Civilization before Homo Sapiens?), but this morning I want to reflect on the implications of the question. For it turns out that if, say, a species of dinosaur had evolved to the point of creating technologies and an industrial civilization, finding evidence of it would be an extremely difficult thing. So much so that I find myself reflecting on deep time in much the same way that I reflect on the physical cosmos and its seemingly endless reach.
Consider that we can trace our species back in the Quaternary (covering the last 2.6 million years or so) and find evidence of non-Homo Sapiens cultures, among which the Neanderthals are the most famous, along with the Denisovians. Bipedal hominids show up at least as far back as the Laetoli footprints in Tanzania, which date to 3.7 million years ago and were apparently produced by Australopithecus afarensis. Frank and co-author Gavin Schmidt also note that the largest ancient surface still available for study on our planet is in the Negev Desert, dating back about 1.8 million years.
These are impressive numbers until we put them into context. The Earth is some 4.5 billion years old, and complex life on land has existed for about 400 million of those years. Let’s also keep in mind that agriculture emerged perhaps 12,000 years ago in the Fertile Crescent, and in terms of industrial technologies, we’ve only been active for about 300 years (the authors date this from the beginning of mass production methods). Tiny slivers of time, in other words, amidst immense timeframes.
So as Frank and Schmidt point out, we’re talking about fractions of fractions here. There is a fraction of life that gets fossilized, which in all cases is tiny and also varies according to tissue, bone structure, shells and so forth, and also varies from an extremely low rate in tropical environments to a higher rate in dry conditions or river systems. The dinosaurs were active on Earth for an enormous period of time, from the Triassic to the end-Cretaceous extinction event, something in the range of 165 million years. Yet only a few thousand near-complete dinosaur specimens exist for this entire time period. Would homo sapiens even show up in the future fossil record?
From the paper:
The likelihood of objects surviving and being discovered is similarly unlikely. Zalasiewicz (2009) speculates about preservation of objects or their forms, but the current area of urbanization is <1% of the Earth’s surface (Schneider et al., 2009), and exposed sections and drilling sites for pre-Quaternary surfaces are orders of magnitude less as fractions of the original surface. Note that even for early human technology, complex objects are very rarely found. For instance, the Antikythera Mechanism (ca. 205 BCE) is a unique object until the Renaissance. Despite impressive recent gains in the ability to detect the wider impacts of civilization on landscapes and ecosystems (Kidwell, 2015), we conclude that for potential civilizations older than about 4 Ma, the chances of finding direct evidence of their existence via objects or fossilized examples of their population is small.
Image: The Cretaceous-aged rocks of the continental interior of the United States–from Texas to Montana–record a long geological history of this region being covered by a relatively shallow body of marine water called the Western Interior Seaway (WIS). The WIS divided North America in two during the end of the age of dinosaurs and connected the ancient Gulf of Mexico with the Arctic Ocean. Geologists have assigned the names “Laramidia” to western North America and “Appalachia” to eastern North America during this period of Earth’s history. If a species produced a civilization in this era, would we be able to find evidence of it? Credit; National Science Foundation (DBI 1645520). The Cretaceous Atlas of Ancient Life is one component of the overarching Digital Atlas of Ancient Life project. CC BY-NC-SA 4.0 DEED.
Intriguing stuff. The authors advocate exploring the persistence of industrial byproducts in ocean sediment environments, asking whether byproducts of common plastics or organic long-chain synthetics will be detectable on million-year timescales. They also propose a deeper dive into anomalies in current studies of sediments, the same sort of analysis that has been done, for example, in exploring the K-T boundary event but broadened to include the possibility of an earlier civilization. I send you to the paper, available in full text, for discussion of such testable hypotheses.
Back to deep time, though, and the analogy of looking ever deeper into the night sky. In asking how long a civilization can survive (Drake’s L term in the famous equation), we ask whether we are likely to find other civilizations given that over billion year periods, they may last only as a brief flicker in the night. We have no good idea of what the term L should be because we are the only civilization we know about. But if civilizations can emerge more than once on the same world, the numbers get a little more favorable, though still daunting. A given star may be circled by a planet which has seen several manifestations of technology, a greater chance for our detection.
A cycle of civilization growth and collapse might be mediated by fossil fuel availability and resulting climate change, which in turn could feed changes in ocean oxygen levels. Frank has speculated that such changes could trigger the conditions for creating more fossil fuels, so that the demise of one culture actually feeds the energy possibilities of the next after many a geological era. How biospheres evolve – how indeed they have evolved on our own world – is a question that exoplanet research may help to answer, for we have no shortage of available worlds to examine as our biosignature technologies develop.
Culturally, we must come to grips with these things. In an essay for The Geological Society, British paleontologist Richard Fortey discusses the seminal work of James Hutton and Charles Lyell in the 18th and 19th Centuries in developing the concept of geological time, which John McPhee would present wonderfully in his 1981 book Basin and Range (I remember reading excerpts in The New Yorker). The Scot James Hutton had literary ambitions, publishing his Theory of the Earth in 1795 and changing our conception of time forever. Hutton knew Adam Smith and spent time with David Hume; he would also have been aware of French antecedents to his ideas. But despite its importance, even Lyell would admit that he found Hutton’s book all but unreadable.
It took a friend named John Playfair to turn Hutton’s somnolent prose into the simplified but clear Illustrations of the Huttonian Theory of the Earth in 1802, making the idea of deep time available to a large audience and leading to Lyell. Which goes to show that sometimes it takes a careful popularizer to gain for a scientist the traction his or her work deserves. The emphasis there is on ‘careful.’
Lyell’s Principles of Geology, published in three volumes between 1830 and 1833, famously traveled with Darwin on the Beagle and, as Fortey says, “donated the time frame in which evolution could operate.” He goes on:
“…once the time barrier had been breached, it was only a question of how much time. The stratigraphical divisions of the geological column, the periods such as Devonian or Cambrian, with which we are now so familiar, were themselves being refined and put into the right sequence through the same historical period. Just to have a sequence of labels helped geologists grapple with time, and, in a strange way, labels domesticate time.
But domestication co-exists with wonder. I imagine the most hardened geologist of our day occasionally quakes at the realization of what all those sedimentary layers point to, a chronological architecture — time’s edifice — in which our entire history as a species is but a glinting mote on a rockface of the future. Our brief window today is reminiscent of Hutton and Lyell’s. Like them, we are compelled to adjust to a cosmos that seems to somehow enlarge every time we probe it, inspired by new technologies that give birth to entire schools of philosophy.
John Playfair would write upon visiting Siccar Point, the promontory in Berwickshire that inspired Hutton’s ideas, that “The mind seemed to grow giddy looking so far into the abyss of time.” We are similarly dwarfed by the vistas of the Hubble Ultra Deep Field and the exquisite imagery from JWST. Who knows what we have yet to discover in Earth’s deep past?
The paper is Schmidt and Frank, “The Silurian Hypothesis: Would it be possible to detect an industrial civilization in the geological record?” published online by the International Journal of Astrobiology 16 April 2018 (full text). Gregory Benford’s Deep Time: How Humanity Communicates Across Millennia (Bard, 2001) is a valuable addition to this discourse. For a deeper dive, Fortey mentions Martin Rudwick’s Bursting the Limits of Time: The Reconstruction of Geohistory in the Age of Revolution ( University of Chicago Press, 2007). Fortey’s own Life: A Natural History of the First Four Billion Years of Life on Earth (Knopf, Doubleday 1999) is brilliant and seductively readable.
by Paul Gilster | Dec 6, 2023 | Astrobiology and SETI |
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.
by Paul Gilster | Oct 25, 2023 | Communications and Navigation |
I sometimes imagine Claudio Maccone having a particularly vivid dream, a bright star surrounded by a ring of fire that all but grazes its surface. And from this ring an image begins to form behind him, kilometers wide, dwarfing him and carrying in its pixels the view of a world no one has ever seen. The dream is half visual, half diagrammatic, but it’s all about curving Einsteinian spacetime, so that light flows along the gravity well to be bent into a focus that extends into linear infinity.
My slightly poetic vision of what happens beyond 550 AU or so doesn’t do justice to the intrinsic beauty of the mathematics, which Maccone learned to unlock decades ago as he explored the concept of an ‘Einstein ring’ as fine-tuned by Von Eshleman at Stanford. When I met him (at one of Ed Belbruno’s astrodynamics conferences at Princeton in 2006), we and Greg Matloff and wife C talked about lensing at breakfast one morning. Even then he was afire with the concept. He’d been probing it since the late 1980s, and had submitted a mission proposal to the European Space Agency. He had written a short text that would later be expanded into the seminal Deep Space Flight and Communications (Springer, 2009).
Maccone said in his presentation at the Interstellar Research Group’s Montreal symposium that he was delighted to see the Sun’s gravitational focus moving into the hands of the next generation, citing the 2020 NASA grant to Slava Turyshev’s team at JPL, where a Solar Gravitational Lens mission is being worked out at the highest level of detail as an entrant into the sweepstakes known as the Heliophysics 2024 Decadal Survey. To see how far the concept has gone, have a look at, for example, Self-Assembly: Reshaping Mission Design, or A Mission Architecture for the Solar Gravity Lens, among numerous entries I’ve written on the JPL work.
Image: A meter-class telescope with a coronagraph to block solar light, placed in the strong interference region of the solar gravitational lens (SGL), is capable of imaging an exoplanet at a distance of up to 30 parsecs with a few 10 km-scale resolution on its surface. The picture shows results of a simulation of the effects of the SGL on an Earth-like exoplanet image. Left: original RGB color image with (1024×1024) pixels; center: image blurred by the SGL, sampled at an SNR of ~103 per color channel, or overall SNR of 3×103; right: the result of image deconvolution. Credit: Turyshev et al., “Direct Multipixel Imaging and Spectroscopy of an Exoplanet with a Solar Gravity Lens Mission,” Final Report NASA Innovative Advanced Concepts Phase II.
The astounding magnification we could achieve by using bent starlight was what drew me instantly to the concept when I first learned about it – how else to actually see not just pixels from an exoplanet around its star, but actual continents, weather patterns, oceans and, who knows, even vegetation on the surface? But at Montreal, after his praise for the JPL effort that could become our first attempt to exploit the gravitational lens if adopted by the Decadal survey, Maccone took a much more futuristic look at what humans might do with lensing, delving into the realm of communications. What about building a radio ‘bridge’?
The concept is even more audacious that reaching 650 AU with the payloads we’ll need to deconvolve imagery from another star. In fact, it’s downright science fictional. Suppose we achieve the technologies needed to send humans to Alpha Centauri. We have there in the form of Centauri A a G-class star much like the Sun (although we could also use the K-class star Centauri B). Both of these stars have their own distance from which gravitational lensing occurs, Align your spacecraft properly to look back towards the Earth from Centauri A and you can now connect to the ‘relay’ at the lensing distance from the Sun. You’ve drastically changed the communications picture by using lensing in both directions.
The consequences for contact and data transfer are enormous. Consider: If we want to talk to our crew now orbiting Centauri A and try to do so with one of the Deep Space Network’s 70-meter dish antennae using today’s standards for spacecraft communications, we’d have no usable signal to work with. Assume a transmitting power of 40 W and communications over the Ka band (32 GHz) at a rate of 32 kbps (these are the figures for the highest frequency used by the Cassini mission). The distances are too great; the power too weak. But if we factor in a receiver at the lensing point of Centauri A directly opposite to the Sun, we get the extraordinary gain shown in the diagram below.
This raises the eyebrows. Bit Error Rate expresses the quality of the signal, being the number of erroneous bits received divided by the total number of bits transmitted. Using a spacecraft at the solar gravitational lens distance from the Sun talking to one on the other side of Centauri A (alignment, of course, is critical here), we have a signal so strong that we have to go over 9 light years out before it begins to degrade. A radio bridge like this would allow communications with a colony at Alpha Centauri using power levels and infrastructure we have in place today.
Obviously, this is a multi-generational idea given travel times to and from Alpha Centauri. But it’s a step we may well need to take if we can solve all the problems involved in getting human crews to another star. Maccone told the audience at Montreal that in terms of channel capacity (as defined by Shannon information theory), the Sun used as a gravitational lens allows 190 gigabits per second in a radio bridge to Centauri A as opposed to the paltry 15.3 kilobits per second available without lensing.
Realizing that any star creates this possibility, Maccone has lately been working on the question of how a starfaring society of the future might use radio bridges to plot out expansion into nearby stars. He is in fact thinking about the best ‘trail of expansion’ humans might use to keep links being built and used between colonies at these stars. This turns out to be no easy task: The first goal must be to convert the list of nearby stars being studied (the number is arbitrary) into Cartesian coordinates centered on each star (their coordinates are currently given in terms of Right Ascension and Declination with respect to the Sun). Maccone calls this an exercise in spherical trigonometry, and it’s a thorny one.
A network of radio bridges between stars could evolve into a kind of ‘galactic internet,’ a term Maccone uses with an ironic smile as it plays to the journalist’s need to write dramatic copy. Be that as it may, the SETI component is intriguing, given that older civilizations may even now be exploiting gravitational lensing. It would be an interesting thing indeed if we were to discover a bridge relay somewhere at our Sun’s gravitational lensing distance, for its placement would allow us to calculate where the receiving civilization must be located. Using a gravitational lens for communications is, after all, extraordinarily directional. Might we one day discover at the lensing distance from the Sun an artifact that can open access to a networked conversation on the interstellar scale?
Human expansion to nearby stars would likely be a matter of millennia, but given the age of the galaxy, it would represent just a sliver of time. Whether humanity can survive for far shorter timeframes is an immediate question, but I think it’s refreshing indeed to look beyond the current work on reaching the solar gravitational lens to the implications that would follow from exploiting it. The radio bridge is great science fiction material – we might even call it the stuff of dreams – but solidly rooted in physics if we can find the tools to make it happen.
by Paul Gilster | Sep 13, 2023 | Missions |
If we’re going to get to the stars, the path along the way has to go through an effort like Breakthrough Starshot. This is not to say that Breakthrough will achieve an interstellar mission, though its aspirational goal of reaching a nearby star like Proxima Centauri with a flight time of 20 years is one that takes the breath away. But aspirations are just that, and the point is, we need them no matter how far-fetched they seem to drive our ambition, sharpen our perspective and widen our analysis. Whether we achieve them in their initial formulation cannot be known until we try.
So let’s talk for a minute about what Starshot is and isn’t. It is not an attempt to use existing technologies to begin building a starship today. Yes, metal is being bent, but in laboratory experiments and simulated environments. No, rather than a construction project, Starshot is about clarifying where we are now, and projecting where we can expect to be within a reasonable time frame. In its early stages, it is about identifying the science issues that would enable us to use laser beaming to light up a sail and push it toward another star with prospects of a solid data return. Starshot’s Harry Atwater (Caltech) told the Interstellar Research Group in Montreal that it is about development and definition. Develop the physics, define and grow the design concepts, and nurture a scientific community. These are the necessary and current preliminaries.
Image: The cover image of a Starshot paper illustrating Harry Atwater’s “Materials Challenges for the Starshot Lightsail,” Nature Materials 17 (2018), 861-867.
We’re talking about what could be a decades-long effort here, one that has already achieved a singular advance in interstellar studies. I don’t have the current count on how many papers have been spawned by this effort, but we can contrast the ongoing work of Starshot’s technical teams with where interstellar studies was just 25 years ago, when few scientific conferences dealt with interstellar ideas and exoplanets were still a field in their infancy. In terms of bringing focus to the issue, Starshot is sui generis.
It is also an organic effort. Starshot will assess its development as it goes, and the more feasible its answers, the more it will grow. I think that learning more about sail possibilities will spawn renewed effort in other areas, and I see the recent growth of fusion rocketry concepts as a demonstration that our field is attaining critical mass not only in the research labs and academy but in commercial space ventures as well.
So let’s add to Atwater’s statement that Starshot is also a cultural phenomenon. Although its technical meetings are anything but media fodder, their quiet work keeps the idea of an interstellar crossing in the public mind as a kind of background musical riff. Yes, we’re thinking about this. We’ve got ideas and lab experiments that point to new directions. We’re learning things about lightsails and beaming we didn’t know before. And yes, it’s a big universe, with approximately one planet per star on average, and we’ve got one outstanding example of a habitable zone planet right next door.
So might Starshot’s proponents say to themselves, although I have no idea how many of those participating in the effort back out sometimes to see that broader picture (I suspect quite a few, based on those I know, but I can’t speak for everyone). But because Starshot has not sought the kind of publicity that our media-crazed age demands, I want to send you to Atwater’s video presentation at Montreal to get caught up on where things stand. I doubt we’re ever going to fly the mission Starshot originally conceived because of cost and sheer scale, but I’m only an outsider looking in. I do think that when the first interstellar mission flies, it will draw heavily on Starshot’s work. And this will be true no matter what final choices emerge as to propulsion.
This is a highly technical talk compressed into an all too short 40 minutes, but let’s just go deep on one aspect of it, the discussion of the lightsail that would be accelerated to 20 percent of lightspeed for the interstellar crossing. Atwater’s charts are worth seeing, especially the background on what the sail team’s meetings have produced in terms of their work on sail materials and, especially, sail shape and stability. The sail is a structure approximately 4 meters in diameter, with a communications aperture 1 meter in size, as seen in the center of the image (2 on the figure). Surrounding it on the circular surface are image sensors (6) and thin-film radioisotope power cells (5).
Maneuvering LEDs (4) provide attitude control, and thin-film magnetometers (7) are in the central disk, with power and data buses (8) also illustrated. A key component: A laser reflector layer positioned between the instruments that are located on the lightsail and the lightsail itself, which is formed as a silicon nitride metagrating. As Atwater covers early in his presentation, the metagrating is crucial for attitude control and beam-riding, keeping the sail from slipping off the beam even though it is flat. The layering is crucial in protecting the sailcraft instrumentation during the acceleration stage, when it is fully illuminated by the laser from the ground.
How to design lensless transmitters and imaging apertures? Atwater said that lensless color camera and steerable phased array communication apertures are being prototyped in the laboratory now using phased arrays with electrooptic materials. Working one-dimensional devices have emerged in this early work for beam steering and electronic focusing of beams. The laser reflector layer offers the requisite high reflectivity at the laser wavelength being considered, using a hybrid design with silicon nitride and molybdenum disulfide to minimize absorption that would heat the sail.
I won’t walk us through all of the Starshot design concepts at this kind of detail, but rather send you to Atwater’s presentation, which shows the beam-riding lightsail structure and its current laboratory iterations. The discussion of power sources is particularly interesting given the thin-film lightweight structures involved, and as shown in the image below, it involves radioisotope thermoelectric generators actually integrated into the sail surface. Thin film batteries and fuel cells were considered by Breakthrough’s power working group but rejected in favor of this RTG design.
So much is going on here in terms of the selection of sail materials and the analysis of its shape, but I’ll also send you to Atwater’s presentation with a recommendation to linger over his discussion of the photon engine, that vast installation needed to produce the beam that would make the interstellar mission happen. The concept in its entirety is breathtaking. The photon engine is currently envisioned as an array of 1,767,146 panels consisting of 706,858,400 individual tiles (Atwater dryly described this as “a large number of tiles”), producing the 200 gW output and covering 3 kilometers on the ground. The communications problem for data return is managed by scalable large-area ground receiver arrays, another area where Breakthrough is examining cost trends that within the decades contemplated for the project will drive component expenses sharply down. The project depends upon these economic outcomes.
Image: What we would see if we had a Starshot-class sailcraft approaching the Earth, from the image at two hours away to within five minutes of its approach. Credit for this and the two earlier images: Harry Atwater/Breakthrough Starshot.
Using a laser-beamed sail technology to reach the nearest stars may be the fastest way to get images like those above. The prospect of studying a planet like Proxima b at this level of detail is enticing, but how far can we count on economic projections to bring costs down to the even remotely foreseeable range? We also have to factor in the possibility of getting still better images from a mission to the solar gravitational lens (much closer) of the kind currently being developed at the Jet Propulsion Laboratory.
Economic feasibility is inescapably part of the Starshot project, and is clearly one of the fundamental issues it was designed to address. I return to my initial point. Identifying the principles involved and defining the best concepts to drive design both now and in the future is the work of a growing scientific community, which the Starshot effort continues to energize. That in itself is no small achievement.
It is, in fact, a key building block in the scientific edifice that will define the best options for achieving the interstellar dream. And while this is not the place to go into the complexities of scientific funding, suffice it to say that putting out the cash to enable these continuing studies is a catalytic gift to a field that has always struggled for traction both financial and philosophical. The Starshot initiative has a foundational role in defining the best technologies for interstellar flight that will lead one day to its realization.
