Centauri Dreams
Imagining and Planning Interstellar Exploration
The Inner System Viewed from Saturn
With the Cassini mission continuing through 2017, we’ll doubtless have many fine views of Saturn to come, but the images below merit special attention, enough so that I decided to close the week with them. We’re looking at an annotated, panoramic mosaic made by processing 141 wide-angle images, sweeping across 651,591 kilometers. That covers the planet, its inner ring system and all its rings out to the E ring. Moreover, the view presented here is in natural color, so we see the color as it would be seen by human eyes rather than as distorted during observations at other wavelengths.
You may remember the ‘Wave at Saturn’ campaign from last summer, when the word went out that Cassini would be snapping a view of the Earth from Saturn space. In the mosaic (click the image to zoom in) we can see the Earth as a blue dot to the lower right of Saturn, but Venus is visible too to the upper left, and Mars shows up as the faint red dot above and to the left of Venus. A close look will reveal seven of Saturn’s moons, including the intriguing Enceladus to the left. Enceladus is worth mentioning because the E ring, about 240,000 kilometers from Saturn, is made up of fine icy particles from the erupting geysers in Enceladus’ south polar terrain.
“This mosaic provides a remarkable amount of high-quality data on Saturn’s diffuse rings, revealing all sorts of intriguing structures we are currently trying to understand,” said Matt Hedman, a Cassini participating scientist at the University of Idaho in Moscow. “The E ring in particular shows patterns that likely reflect disturbances from such diverse sources as sunlight and Enceladus’ gravity.”
The second image (below) has been brightened and color-enhanced to tease out the ring structure. Note the blue color of the E ring, which is caused by the diffraction of sunlight. In both images, the Earth, Venus, Mars, Enceladus, Epimetheus and Pandora were brightened by a factor of eight and a half relative to Saturn to make them easier to see, although you’ll still need to zoom in by clicking to make them out. The outer rings (from the G to the E ring) were likewise brightened relative to the already bright main rings. Full background on these images can be found on this JPL page.
Getting a view like this is tricky because trying to see the Earth from Saturn means looking close enough to the Sun to endanger sensitive spacecraft detectors. Thus the need to find a time when the Sun is entirely behind Saturn as seen from Cassini. The spacecraft’s wide-angle and narrow-angle cameras were used to capture 323 images in a little over four hours, with the red, green and blue spectral filters combined to create the natural-color view. Although this is the second time Cassini has viewed it, the Earth has only been imaged from the outer Solar System three times, the first being the famous ‘pale blue dot’ image from Voyager. This is also the first time Earth’s inhabitants were told in advance about a photo that would include their entire world.
What a Strange Asteroid Can Tell Us
The Pan-STARRS survey telescope in Hawaii has reminded us how much we still have to learn about asteroids. We saw yesterday that the Chelyabinsk impactor could be studied through physical evidence as well as the ample photographic records made by witnesses on the ground. But P/2013 P5, discovered by Pan-STARRS and then the object of Hubble scrutiny, is in the main belt between Mars and Jupiter, and rather than appearing as a mere point source, the object shows six comet-like tails that have confounded all those who have looked at it.
“It’s hard to believe we’re looking at an asteroid,” said lead investigator David Jewitt, a professor in the UCLA Department of Earth and Space Sciences and the UCLA Department of Physics and Astronomy. “We were dumbfounded when we saw it. Amazingly, its tail structures change dramatically in just 13 days as it belches out dust.”
Image: This NASA Hubble Space Telescope set of images reveals a never-before-seen set of six comet-like tails radiating from a body in the asteroid belt, designated P/2013 P5. The asteroid was discovered as an unusually fuzzy-looking object with the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS) survey telescope in Hawaii. The multiple tails were discovered in Hubble images taken on Sept. 10, 2013. When Hubble returned to the asteroid on Sept. 23, the asteroid’s appearance had totally changed. It looked as if the entire structure had swung around. Credit: NASA, ESA, and D. Jewitt (UCLA).
What could cause an object to resemble, as this UCLA news release notes, a rotating lawn sprinkler? Jewitt’s team, which has published its findings in the Nov. 7 issue of the Astrophysical Journal Letters, rules out an impact scenario because that would have caused a sudden release of a large amount of material. The paper notes that the six dust tails have been observed for five months and show no fading over that period.
What’s left? Sublimation of near-surface ice and electrostatic levitation of dust are among the possibilities the paper examines. The most likely solution, though, is an eruptive dust ejection scenario that could have resulted from a high spin rate. The spin could be accounted for by the pressure of sunlight exerting a torque on the 215-meter object. From the preprint:
The surviving hypothesis is that P5 is a body showing rotational mass-shedding, presumably from torques imposed by solar radiation… Rotational re-shaping and breakup under radiation torques are two of the most interesting subjects in asteroid science… Unfortunately, the expected observational signature of a rotationally disrupting body has yet to be quantitatively modeled, making a comparison with P5 difficult. This is, in part, because the appearance is likely to be dominated by small particles that carry most of the cross-section of ejected material while most of the mass resides in large particles which precipitate the instability.
The weak gravity of the asteroid, in other words, could be simply insufficient to hold the rapidly rotating object together, its dust drifting into space to form the tail-like structures we see. Interestingly, the orbit of P5 near the inner edge of the asteroid belt associates it with the Flora family of asteroids, evident collision fragments that follow similar orbits. One recent paper has noted that the spins of other members of this group show evidence for the same kind of radiation torque that P5 exhibits. Ahead for astronomers is follow-up work to learn whether dust leaves the asteroid in its equatorial plane, which would be strong evidence for rotational breakup.
And maybe, as David Jewitt suggests, we’re looking at the main way that small asteroids die. “In astronomy, where you find one, you eventually find a whole bunch more. This is just an amazing object to us, and almost certainly the first of many more to come.”
The paper is Jewitt et al., “The Extraordinary Multi-Tailed Main-Belt Comet P/2013 P5,” Astrophysical Journal Letters Vol. 778, No. 1 (2013), L21 (abstract)
Piecing Together the Chelyabinsk Event
We’re still trying to learn how frequently asteroid events like the spectacular fireball over Chelyabinsk occur. The Chelyabinsk object was the largest to fall to Earth since the Tunguska explosion in 1908, which leveled thousands of acres of forest in Siberia. This BBC story discusses Peter Brown (University of Western Ontario) and colleagues’ recent paper in Nature and goes on to quote Brown as saying that a few days’ to a week’s warning would have been valuable so that we would have been prepared for what happened near the Siberian city. True enough, but what’s significant here is that the Brown team studied 20 years of data from sensors positioned around the world to estimate the frequency of such events.
The upshot: About sixty asteroids up to 20 meters in size entered Earth’s atmosphere during this period, a significantly higher number than was previously assumed. Brown’s team reports we’ve been underestimating the strike rate of asteroids between 10 and 20 meters in size by between two and ten times. That would make a Tunguska-class impact likely every few hundred years rather than every few thousand. As for the most recent event, Brown says this:
“Something like Chelyabinsk, you would only expect every 150 years on the basis of the telescopic information. But when you look at our data and extrapolate from that, we see that these things seem to be happening every 30 years or so.”
Image: The flash above Chelyabinsk, Russia, from the fireball streaking through the sky on Feb. 15, 2013. The picture was taken by a local, M. Ahmetvaleev. Credit: Copyright M. Ahmetvaleev.
Meanwhile, a new paper in Science gives us more information about the Chelyabinsk object itself. In sharp distinction to what happened in Tunguska in 1908, the Chelyabinsk incident was well reported and photographed, allowing teams under Peter Jenniskens (NASA Ames) and Olga Popova (Russian Academy of Sciences, Moscow) to follow up the story with first-hand accounts from witnesses and to study numerous videos that captured the event, helping them to reconstruct the asteroid’s properties and trajectory.
We learn the following: The impact speed of the meteor, as determined by calibrating video images, was roughly 19 kilometers per second. The object’s passage into the atmosphere caused it to fragment into pieces about 30 kilometers above the surface, at which point its light appeared brighter than the Sun even for people as far as 100 kilometers away. Between 4000 and 6000 kilograms of material fell to the ground, with much of the debris being vaporized. One 650 kilogram fragment was recovered from Lake Chebarkul on October 16, 2013. A 3.4 kilogram rock fell near the town of Timiryazevskiy and another chunk hit a house in Deputatskiy.
Image: A photograph of the meteor streaking through the sky above Chelyabinsk. Credit: Copyright M. Ahmetvaleev.
Evidently shock fractures in the rock, produced by an impact that may have occurred over four billion years ago, helped cause its breakup in the upper atmosphere. This NASA news release quotes Mike Zolensky (NASA JSC) on the small iron grains found just inside the veins as a contributing factor. “There are cases where impact melt increases a meteorite’s mechanical strength, but Chelyabinsk was weakened by it,” said Zolensky. In further Chelyabinsk investigations, Jiri Borovi?ka (Academy of Sciences of the Czech Republic) and colleagues report in a second paper in Nature that the Chelyabinsk object may well be a fragment from asteroid 86039, a two-kilometer object whose orbit is strikingly similar.
The papers are Brown, et al., “A 500-kiloton airburst over Chelyabinsk and an enhanced hazard from small impactors,” published online in Nature 6 November 2013 (abstract) and Borovi?ka et al., “The trajectory, structure and origin of the Chelyabinsk asteroidal impactor,” published online in Nature 6 November 2013 (abstract). See also Popova et al., “Chelyabinsk Airburst, Damage Assessment, Meteorite Recovery, and Characterization,” published online in Science 7 November 2013 (abstract).
SETI, METI… and Assessing Risk like Adults
David Brin is a familiar name to science fiction readers worldwide, the award-winning author of the highly regarded ‘uplift’ novels that include Startide Rising (1983), The Uplift War (1987) and Brightness Reef (1995). Among his numerous other titles are The Postman (1985), Kiln People (2002) and Existence (2012). But Brin is also known as a futurist whose scientific work ranges over topics in astronautics and astronomy to forms of dispute resolution and the role of neoteny in human evolution. His Ph.D in Physics from the University of California at San Diego followed a masters in optics and an undergraduate degree in astrophysics from Caltech. He was a postdoctoral fellow at the California Space Institute and the Jet Propulsion Laboratory. Brin has served on advisory committees dealing with subjects as diverse as national defense, space exploration, SETI and nanotechnology, future/prediction and philanthropy. His essay The Great Silence (available here) is but one of his many investigations into SETI and METI that weigh the consequences of contact with other civilizations, a subject that occupies him in the post below.
by David Brin
In a previous Centauri Dreams posting (SETI, METI and Existential Risk), J.N. Nielsen made an earnest effort to appraise both sides of the current debate over “messaging” to the cosmos, or METI. He started with an apt comparison to the way that passive-listening sonar differs from the brashly-seeking active kind — the “pinging-and-echoes” we’ve all heard in submarine movies.
Alas, he then poses the current dispute over METI in oversimplifying terms: “Is the universe such a dangerous place that it behooves us to maintain radio silence?”
This posits a glaring misapprehension — that the dissidents who mistrust METI are acting out fear of the unknown, dreading possible consequences of shouting for attention into the sky. In fact, however, Dr. Jim Benford, the late Dr. John Billingham, former senior US diplomat Michael Michaud and I have all resigned from SETI-related international commissions — and taken the matter public — in protest over behavior that has taken on an increasingly cult-like quality.
This troubling trend became apparent a couple of years ago, the one and only time that a substantial and open discussion of the matter took place in a neutral and respected venue, before a special gathering organized by the Royal Society. The proceedings of that gathering will soon be published by the Journal of the British Interplanetary Society, and I urge all interested readers to keep their eyes open for that event, when the whole matter will be laid out before you all, in far greater detail than I can do here.
(For those who are impatient to learn more (a trait I find most-becoming!) here is a link to articles and speculations by David Brin about the Search for Extraterrestrial Intelligence (SETI).
Keeping my response to Mr. Nielsen succinct in this place, let me say that he gets the “dissidents’ motives entirely wrong. We freely admit that the odds of any one bad-outcome scenario from METI broadcasts or beams might be small. We are, however, capable of admitting one thing that METI zealots obstinately refuse to avow — that we don’t know what those odds are, not even within many orders of magnitude. In the new and important realm of Risk Analysis, that kind of uncertainty ought to prompt adults to at least talk about ways to reduce the uncertainty.
But talk and discussion and appraisal of risk are exactly what the zealots — aided and abetted by certain folks at the SETI Institute — bend all their efforts to avoid. The Royal Society meeting was a fluke and efforts to expand the discussion are met with stony silence or open disdain.
To make things perfectly clear, what we METI dissidents have asked — and ALL that we have asked — is for a series of open, eclectic, vigorous and thorough discussions to be held, probing this matter of risk from all sides, before individuals and small groups jump the gun, peremptorily blaring yoohoo ‘messages,’ arrogating the right to speak to aliens on behalf of all humanity. So far, these stunts are being pulled by a small subset of radio astronomers and fans — or commercial owners of tracking dishes that had been built by taxpayer funds — who deliberately avoid collegiality with other sciences, fleeing input from experts in pertinent fields like history, biology, anthropology, planetary science or, indeed, any other discipline that might shed light onto a vast range of conceivable outcomes from First Contact.
Some of which (by the way) do not depend upon any ability to travel physically between the stars.
At risk of tedious repetition, because it has proved relentlessly necessary — all that we dissidents have asked for has been the kind of earnest and eclectic pre-discussion that worked so well, decades ago, when the genetic biology community engaged in the extremely successful and positive sum “Asilomar Process” to explore their then-nascent field and thereupon decide on procedures to get both safety and rapid discovery.
That has been our request and position. Anyone who claims that we have sought anything else — out of “fear of slathering Cardassian invaders,” snarked one deceiver — is a damned liar.
(By the way, just envision those open and public symposia I talked about, drawing on all realms of pertinent human excellence, revolving around every topic that might shed light upon either aliens or ourselves. Wouldn’t such symposia make for terrific television? Might they not capture the imagination of — and edify — millions or even billions? Why would anyone oppose such exposure to the full range of ideas and analyses or our place (and possible others’) in the universe? The pro-METI opposition to this speaks a lot about their honesty and fealty to science.)
Getting down to specifics
Having explained that key point, let me go on to say that Mr. Nielsen shows a calm logic that would be welcome in any discussion. Alas, he also displays the blithe assurance we see all-too often in this field — and that was rife among the commenters underneath his posting — that one does not have to read-up any of the background material, in order to be an instant expert. Just arm-wave a pat few sentences of “logic” and you’ve solved the most perplexing puzzle in the cosmos!
Nielsen’s dismissal of the Benford paper on interstellar communications economics – for example – is toe-shallow. Indeed, does Nielsen contend that advanced ETCs — no matter how rich they are — would not still very likely prefer one means of making contact that is six to ten orders of magnitude cheaper than another?
A factor of a million or a billion may affect your choice of method, even if you are a super-Kardashev society of near-gods.
Thus, the Benfords clearly proved that “pinging” potential new civilization sites – at intervals – would be far more efficient and likely than gaudy, omni-directional beacons. Indeed, that strategy is consistent with the famous “Wow” signal… and with the fact that no gaudy, omni-blaring tutorial beacons have ever been found. The one null-result of SETI that is already definite, sufficient and proved.
Nielsen is at his best – though still very superficial – in discussing possible ways and reasons for intentional silence. Certainly I have listed in my Great Silence catalogue many variations on that theme, ranging from cowering dread all the way to beneficently allowing Earth culture to grow without cultural interference. Indeed, some of these are drawn out dramatically in my novel Existence. Still, I have trouble when Nielsen says the following:
“…just as these peer civilizations would likely incorporate our curiosity about the universe, they would also likely incorporate our willingness to take risks.”
This is yet another blithe assumption, a dilettante arm-waving that begs, rather than asks key questions. Nielsen posits that the descendants of the alien equivalents of bears or lions or geese or herbivores would share a similar psychology with we humans, who happen to come from exogamously-exploring, gregariously-interdependent yet individualistic apes, a combination that is – in truth – rather rare in nature. Oh, sure, there may indeed be others like us! But then his broad generalization is broken.
Add to that the social effects we’ve seen, in which 99% of human societies strove hard to repress curiosity and inventiveness, and you have reason to suggest that Mr. Nielsen talk things out and expose his assumptions to critical scrutiny. Preferably before publication.
Like the following:
“If our civilization determines that METI is too great an existential risk to bear, then existential risk perception begets risk aversion and possibly culminates in permanent stagnation.”
Um… where on Earth did THAT come from? Yes, I just said that most human societies squelched curiosity. But that was a personality trait arising organically from feudalism, not an outgrowth of any single act of risk analysis.
The crux
Our exceptional enlightenment civilization is outgoing and daring — traits that other human societies would have deemed foolish but that we consider noble. That spirit is under siege right now, as forces try to re-assert feudalism. But one thing is for sure. A little pre-discussion about the highly outward-looking topic of extraterrestrial civilization is not going to be the thing to bring down a curtain of stagnation! Our outwardness will not change if we act with a little prudence and pause to analyze METI risk factors, like adults. The way the biologists did, at Asilomar. Mr. Nielsen is presenting us with a strawman, and a deeply pernicious one, at that.
Indeed, no one on this planet has pursued Otherness and exploration of the topic of the alien more relentlessly than I have, in both fiction and in science. I do not view the cosmos with trepidation. I am proud to be a member of the only civilization on Earth (and perhaps anywhere) that ever stared at the stars with eagerness, salivating with intense hunger.
But overlaid upon both curiosity and romance is something else called maturity. I have children. Care for their posterity is a duty I intend to fulfill. If that means spending a decade listening and learning and discussing, instead of yammering like idiots into a cosmos that is unexpectedly and perhaps dauntingly quiet? Well then call me the adult in the room.
To impatient adolescents that is a crime. Well then…. mea culpa.
On the other hand, maybe folks could stop yelling “gotcha!” in a topic that is actually rather complex. Indeed, those complexities are the spice, the juice, the excitement in this field. If your contribution is a quick, snarky “of course” about… aliens? Well… take my word for it. You have no idea what you are talking about.
James Benford: Comments on METI
Pardon this extended introduction to Jim Benford’s response to Nick Nielsen’s Friday essay, but it comes at a serendipitous time. Jim’s recent online work has reminded me that we in the interstellar community need to work to see that as many resources as possible are made available online. In the absence of specialized bibliographies, useful information can be hard to find in more general indices. And it’s always dismaying to read an intriguing abstract only to realize that the paper itself is behind a pricey firewall. Access to academic libraries certainly helps, but online databases still vary in what they make available, which is why I always check the home pages of the authors of a given paper to see if they have posted a copy of their work themselves. Scientists can do much to get the word out, as Jim’s new site attests.
You’ll find it at http://jamesbenford.com/. Over the weekend, after Nick had discussed METI (Messaging to Extraterrestrial Intelligence) on Friday, I resorted to Jim’s site to pull the Benfords’ key papers on cost-optimized interstellar beacons back up on screen. Preprints of these are available on the arXiv site, but how much more useful to have a single site with a researcher’s papers not just in preprint but in final form. Jim and brother Greg’s work on how beacons might be built, what their broadcasting strategies would likely be, and how we might go about finding them is moving SETI in interesting directions and it’s all here in one place.
So are references to Jim extensive work on microwave beaming to sailcraft, on fusion and pulsed power and plasma physics at large. Here we’re still limited to bibliographic entries because of publishers’ policies, but the bibliography is itself a valuable tool, and we can hope, as publishers gain more experience with networked resources, that open access to scientific work (particularly that funded by taxpayers!) will increasingly become the norm. Interstellar studies needs good bibliographers and researchers can help by cataloguing their own work.
But enough of this extended introduction to the response Jim sent to Nick. What to do about sending messages to other stars? For that matter, just how far are our own electromagnetic signals traveling? We have much to learn as the investigation continues.
by James Benford
I’m very pleased to see Nick Nielsen’s essay. Fits my attempts to widen the discussion of METI. METI is an issue that really should be debated further; as it is increasingly possible that someone will announce us. These are not easy questions; they are ultimately social questions, so should be widely discussed.
I would suggest another paper of mine which treats METI directly. It’s in the Special Issue of JBIS on the METI Debate that I am editing:
“Costs and Difficulties of large-scale METI, and the Need for International Debate on Potential Risks”, John Billingham and James Benford, in press, JBIS (2014).
A draft version is on my new website, at
http://jamesbenford.com/papers-articles/seti/
A few comments:
1) Opponents of METI do not ‘maintain that the “leakage” of the ordinary (unintentional) EM radiation of a technological civilization cannot be detected at interstellar distances”. No, they say just that past signals are undetectable using our present and projected technology. Future leakage may well be larger – see the following point.
2) Several of the responses to my quantitative arguments showing that neither Earth-scale radio telescopes or even the Square Kilometer Array (SKA, that radio astronomers hope to build, but have yet to find the funding for) are able to detect leakage radiation or messages from our radio telescopes only at very limited range of at most a few light years. Their responses simply say that ETI will have far larger radio telescopes as big as Chicago or New Hampshire.
It’s not that easy. Such assumptions are not without implications. The proper scientific thing is to consider the implications of such assumptions. Larger-scale civilizations will leave larger footprints.
If we use Claudio Maccone’s statistical argument that results in the nearest ET civilization being roughly 1000 light years away, no message we can transmit from Earth within the limits of our technology would be detected unless the receiving civilization were substantially wealthier than us, by a factor of 1,000. The details: If we use the ability to build larger area radio telescopes as an indicator of the scale of civilization, that means the energies consumed, and so the wealth, should be larger by a similar amount.
Are there observable features of such civilizations? I think so, especially if they are nearby. If there were a civilization at Alpha Centauri with 1,000 times the energy consumption of earth, they would be able to do extraordinary things. They might modify their climate, beam power around their solar system, construct and launch starships, such as the ‘sailships’, beam-powered sails that Project Forward is conceptualizing. They would be observable.
So just imagining that ET is far more powerful than we are is not a simple escape clause from the arguments I made.
3) My work with John Billingham shows what METI would cost us: ~ 10 B$ for 1000 light year range. Countering METI would cost more than METI, but of same order. To suppress METI, radiate the same signal exactly 180° out of phase so that it cancels, suppressing the message.
4) “Energy and material limitations will cease to be relevant for all practical purposes.” That’s doubtful, as economic history shows. I remember when nuclear power was going to make electricity ‘free’. Didn’t happen. What does happen is prices for some things fall. For example Al, rare before 1800, now used for Coke cans.
5) Spectroscopy of exoplanet atmospheres could reveal markers of life like oxygen, but markers of civilization are very hard to see. Can anyone give an example of something detectable? CFCs aren’t, they’re too small.
SETI, METI, and Existential Risk
To broadcast or not to broadcast? The debate over sending intentional signals to other stars continues to simmer even as various messages are sent, with no international policy in place to govern them. Writer Nick Nielsen looks at METI afresh today, placing it in the context of existential risk and pondering the implications of what David Brin has dubbed the ‘Great Silence.’ If risk aversion is our primary goal, do we open ourselves to a future of permanent stagnation? Or is announcing ourselves to the universe something we have any real control over, given the ability of an advanced civilization to detect our presence whether we send messages or not? Mr. Nielsen, a contributing analyst with online strategic consulting firm Wikistrat, wonders whether our counterparts around other stars aren’t wrestling with the same issues.
by J. N. Nielsen
At the Icarus Interstellar Starship Congress in Dallas last August I had the good fortune to be present for James Benford’s talk about METI, “Shouting to the Galaxy: The METI Debate.” METI is an acronym for messaging extraterrestrial intelligence, which is to say the active propagation of EM (electro-magnetic) spectrum communications from Earth to extraterrestrial targets, in contradistinction to SETI, which is the search for extraterrestrial intelligence, which does not seek to send messages from Earth but instead only listens for EM spectrum signals from the stars.
METI may be contrasted to SETI as active sonar to passive listening: active sonar pings the depths of the ocean and waits for the sound to be bounced back by objects below; passive listening sends out no pings, but simply waits for whatever sounds happen to come one’s way. The analogy is useful because it points to one of the controversies in the METI/SETI debate: submarines, which are typically the targets of active sonar, generally prefer not to be found, so they wait quietly in the depths and rely mostly on passive listening. It is those who hunt the submarines that employ active sonar.
Are technological civilizations floating in the cosmos like submarines floating in the sea? Is the lesson of what Paul Davies calls the “eerie silence” and David Brin has called the “Great Silence” that there is a reason to maintain a low profile and to listen only? Is the universe such a dangerous place that it behooves us to maintain radio silence? This was discussed in Benford’s presentation. Benford outlined the arguments made by both those for and against METI. Primarily these arguments came down to the advocates of unregulated METI maintaining that interstellar travel is impossible (therefore making METI safe), while opponents of METI maintain that the “leakage” of the ordinary (unintentional) EM radiation of a technological civilization cannot be detected at interstellar distances, though intentional METI signals could well be heard (meaning we are safe for the time being, but METI would raise our risk profile).
This debate leaves aside the possibility that one might disagree with both premises, as I do, and maintain that, given a certain technological threshold, interstellar travel is possible, and that, also given a certain technological threshold, it is likely that unintended EM spectrum radiation leakage is detectable. These two technological thresholds—those of interstellar flight and detection of the EM leakage of technological civilizations—are not likely to be one and the same, so that a gap is opened up between the possibility of detecting ETI at interstellar distances and actually attempting an interstellar journey to meet ETI face to face. This gap could be one explanation of the Fermi paradox, but I will not explore this possibility further at the present time.
Although we do not now possess the science or technology to detect EM signatures of ETI at interstellar distances, we must account for the possibility not in terms of our present technology but rather in terms of technology that would be available to a technological civilization somewhat in advance of our own. Any peer civilization (i.e., any technological civilization like us) is going to be looking for peer civilizations because an intrinsic curiosity, at least in part, defines our civilization, and is likely to be similarly present in any civilization capable of science and therefore capable of developing an industrial technology. In looking for peer civilizations, any advanced ETI will show at least as much ingenuity as we have shown in the search for ETI, since ingenuity of this kind is another quality that, at least in part, defines our civilization.
Why would an ETI be looking in our direction in the vastness of the cosmos? In so far as Earth occupies the very interesting position in the cosmos of being a small, rocky planet in the habitable zone of a main sequence star, geologically active, with a large moon, and with a large Jovian planet in the outer solar system to clear away the orbit of the inner solar system of potentially damaging debris, the earth does in fact occupy a privileged position in the universe—though, interestingly, not privileged in virtue of the structure of the universe (such as occupying the center of the universe), but rather in virtue of a contingent confluence of circumstances conducive to life.
We are now, at the present level of our technology, probably less than twenty years from the spectroscopy of exoplanet atmospheres, which could reveal markers of life and civilization. Any advanced peer civilization would have already worked on the spectroscopy of exoplanet atmospheres, and in so doing they would have performed this spectroscopic analysis for the kinds of planets that would likely host peer civilizations—small, rocky planets in the habitable zones of main sequence stars. In other words, ETI would have already by now done the spectroscopy of Earth’s atmosphere, and in so doing they would have focused in on the Earth as a place of great interest, in the exact same way that we would focus on an “Earth twin.” This would mean that they would focus all their best radio antennas on us, just as we could focus intensively on a planet that would likely host life and civilization.
Benford has elsewhere detailed the considerable expenses that would be entailed by any large SETI or METI effort with a realistic ability to communicate over interstellar distances. (Benford has written a series of papers on the cost of interstellar messaging beacons, which are referenced below.) I am not convinced by arguments that the cost of interstellar beacons would be too high to be practicable. Estimating the costs of mega-engineering projects incorporates a range of assumptions intrinsic to economies of scarcity; once any technology-capable species transcends its homeworld and becomes a spacefaring civilization (i.e., once it has extraterrestrialized its civilization) energy and material limitations will cease to be relevant for all practical purposes.
With all the materials of a solar system, the continuous energy output of a star, and robots to do the building, a radio telescope capable of detecting unintentional EM spectrum leakage from a technological civilization light years distant might be no burden at all. Such a project is not far beyond our present technological capacity. For this reason I believe it would be relatively easy for an advanced ETI of a peer civilization to build a custom antenna for nothing other than the possibility of detecting our EM leakage, since they would have already identified Earth as a promising target for SETI and perhaps also METI.
In the question and answer session following Benford’s talk a new wrinkle in all this appeared. My co-presenter from Day 2 of the Starship Congress, Heath Rezabek, suggested that anyone opposed to unregulated METI could broadcast a counter-signal to a METI signal and essentially silence that signal. Subsequent to the Congress, this idea came to be called the “Rezabek maneuver,” and was further elaborated by Heath Rezabek, Pat Galea, and James Benford, who suggested several strategies for the masking of a METI transmission. Harold “Sonny” White, known for his research into the physics of superluminary interstellar travel suggested, “Sounds like we need to add another factor to the Drake equation: Rezabek Ratio — defined as the ratio of civilizations that mask their presence using the Rezabek maneuver to the open civilizations that leave the light on…”
The possibility of a METI counter-signal is an idea that can be scaled up beyond the scope of a single planet, so that it is possible that the Great Silence is not something natural, but could be imposed or generated. One metaphor that has been used to explain the eerie or great silence is that no one shouts in a jungle. This is plausible. If the universe is a dangerous place filled with predators, you don’t want to call attention to yourself. But it is just as plausible that everyone is “shushed” in a library as that everyone keeps quiet in a jungle.
This conclusion is quite similar in some respects to the conclusion in the last paragraph of David Brin’s classic paper (“The ‘Great Silence’: the Controversy Concerning Extraterrestrial Intelligent Life,” Quarterly Journal of the Royal Astronomical Society, Vol. 24, NO.3, P.283-309, 1983):
“It might turn out that the Great Silence is like that of a child’s nursery, wherein adults speak softly, lest they disturb the infant’s extravagant and colourful time of dreaming.”
There is a relationship between the idea of the universe as a jungle and the universe as an imposed quiet zone: that safety is preferable to risk. In other words, these two approaches to the Great Silence imply risk aversion. I don’t think that we can safely assume risk aversion, especially in light of the fact that in SETI and METI we are contemplating peer civilizations, and just as these peer civilizations would likely incorporate our curiosity about the universe, they would also likely incorporate our willingness to take risks.
Who is likely to hear any signals broadcast in a METI effort? Non-peer civilizations without any equivalent of our science, technology, curiosity, and risk-taking habits could likely only be detected by an actual mission to the homeworld of such a civilization. They won’t be listening for us. Who might be listening for METI signals? Predatory peer civilizations may be listening, or may even broadcast an interstellar beacon, just as surface warships send out sonar pings into the ocean, listening for a response. If a successful SETI effort leads to a METI effort on our behalf, that may all be part of a plan – and not our plan.
We would do well to be cautious, but it would be folly to suppose that risk can be eliminated. To exist is to be subject to existential risk. There is no risk-free norm to which we can return by following some program of risk aversion or to which we will naturally be returned by abstaining from particular actions. But this should not be taken as cause for despair: METI is a classic risk/opportunity tradeoff (cf. Existential Risk and Existential Opportunity). Risk and opportunity cannot be separated.
If we contact ETI it could be the greatest thing to ever happen to our civilization, or it could be the worst thing. When we think about existential risk (if indeed we do think about existential risk at all), it is usually only as an existential threat that must be mitigated, whereas existential risk may be understood both in terms of its possibilities as well as its limitations. One dimension of the risk/opportunity tradeoff is that the same action (or inaction) that we take to mitigate existential risk may unintentionally magnify risk, just as one and the same action (or inaction) we pursue in order to seize an opportunity may come to unexpectedly foreclose on that opportunity. (Either scenario might describe a METI initiative.) No outcome is inevitable; that is why we call it risk. If the outcome were inevitable, it would lie outside the scope of risk. (Cf. Existential Risk and Existential Uncertainty)
If our civilization determines that METI is too great an existential risk to bear, then existential risk perception begets risk aversion and possibly culminates in permanent stagnation—which means that we must not only think about existential risks to Earth-originating life, intelligence, and civilization, but also existential risks to all life, intelligence and civilization, anywhere, since other sources of life in the universe may come to the same conclusion and an entire galaxy (or more) might be plunged into permanent stagnation, flawed realization, or subsequent ruination as a consequence of this perceived existential risk.
Many of those who contemplate intelligent life in the universe suggest the possibility of civilizations a million or more years old. I myself don’t think this is likely, but in this event an early, pre-emptive silencing of the universe by an advanced civilization (an imposed Great Silence) might well prevent later civilizations from broadcasting their presence to the rest of the universe—existential threat and existential risk thus gives way to existential isolation and existential despair on a cosmic order of magnitude. In this scenario, the Great Silence is a fitting tribute to a mournful universe in which timid civilizations choose silence, or have silence imposed upon them.
This strikes me as an unlikely scenario. If we are true to our history, and we rush in like fools where angels fear to tread, we will announce our presence to the universe both intentionally and unintentionally. Our peer civilizations in the universe (if there are any) are likely to do the same.
References:
James Benford’s papers on interstellar beacons:
1 “Messaging With Cost Optimized Interstellar Beacons”, James Benford, Dominic Benford and Gregory Benford, Astrobiology, 10, pg. 475, (2010).
2 “Searching for Cost Optimized Interstellar Beacons”, Gregory Benford, James Benford and Dominic Benford, Astrobiology, 10, pg. 491, (2010).
3 “Building And Searching For Cost-Optimized Interstellar Beacons”, James Benford, Dominic Benford and Gregory Benford, in Communication with Extraterrestrial Intelligence, Ch. 18, ed. Douglas A. Vakoch, SUNY Univ. Press, NY, pg. 279-306, (2011).
4 “Smart SETI”, Gregory and James Benford, Analog, vol. CXXXI, no.4, April (2011)
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