I’ve spent so much recent time on two SETI/METI papers by James, Gregory and Dominic Benford because they contain powerful arguments for re-thinking our current SETI strategy. By analyzing how we might construct cost-optimized interstellar beacons, the authors ask what those beacons might look like if other civilizations were turning them toward us. The results are striking: A distant beacon operating for maximum effect consistent with rational expense would offer up a pulsed signal that will be short and intermittent, recurring over periods of a month or year.
It will, in other words, be unlike the kind of persistent signal that conventional SETI is optimized to search for. Searches designed to sweep past stars quickly, hoping to find long-lasting beacons whose signature would be apparent, would rarely notice oddball signals that seem to come out of nowhere and then vanish. Tracking such signals, looking for signs of regularity and repetition, calls for a different strategy.
Image: Looking toward galactic center, a line of sight that may offer our best chance to intercept an interstellar beacon. The center of the Milky Way is behind the center of the photo. As many as thirty Messier objects are visible, including all types of nebulae and star clusters. The lines through the picture were caused by airplanes, and the dark objects in the foreground are trees. Credit: NASA/Dave Palmer.
But let’s stop for a moment. Given our inability to know who might be transmitting to us, how can we make any assumptions, given that we’re relating our own optimum cost strategy for beacons to theirs? The principle of parsimony seems reasonable to us — economic demands have a shaping effect on any project, especially long-term ones like interstellar communication. But can we assume the principle is equally valid for alien civilizations?
The Benfords’ METI paper analyzes these concerns, and their SETI paper summarizes nicely:
The optimum cost strategy leads directly to a remarkable cost insensitivity to the details of economic scaling. The ratio of costs for antenna area and system power depends on only the ratio of exponents… and not on the details of the technology. That ratio varies on Earth by only a factor of two. Both these costs may well be related principally to labor cost; if so it cancels out. This means fashions in underlying technology will matter little, and our experience may robustly represent that of other technological societies.
It’s an interesting argument to chew on and one developed at much greater length than I can summarize here — a read through the relevant sections will expose you to elegant and ingenious reasoning. And if we assume that cost-optimized beacons do exist, the interesting thing is that current SETI methods would be unlikely to find them. In current practice, search dwell times (the time when the apparatus is looking at a specific place) are only a few seconds long in survey operations, while ranging between 100 and 200 seconds for targeted searches. Couple that with long integration times on the order of 100 seconds — this means that short pulses tend to be integrated out of the results.
If we’re looking for short, pulsed signals rather than persistent ones, we’re also hampered by the fact that in most SETI searches to date, the amount of time between when the observer first looks at a target and then looks back at it in an attempt to confirm it is often a matter of days. It can be years when we’re talking about data from the SETI@Home effort. What it would take to identify beacons of the kind described here are searches of the galactic plane that operate for lengthy time frames, on the scale of years. Slow and steady does the job.
Our distant beacon will, given these assumptions, show up in our detectors for a much shorter period than conventional SETI assumes. It’s intriguing that previous searches have indeed noticed at least a few pulsed, intermittent signals that resemble this description of a beacon’s activities. A survey of galactic center reported on in 1997 by W.T. Sullivan (reference below), for example, could confirm nothing in repeat observations that would indicate a persistent beacon. But it did record “…“intriguing, non-repeatable, narrowband signals, apparently not of manmade origin and with some degree of concentration toward the galactic plane…” Again, these are one-time signals that do not repeat within the search period.
A close look at GCRT J1745-3009, an unusual transient bursting radio source some 1.25° south of galactic center, first discovered in 2002 and re-observed several times since, is a useful chance for the Benfords to do a beacon analysis using their parameters. It’s an interesting source, to be sure, with explanations from masers to flare stars, double neutron star binary pulsars, white dwarf pulsars and more all failing to fit the observations. The paper’s analysis shows that GCRT J1745-3009 is unlikely to be artificial, but if it is a cost-optimized beacon, it must be targeted given the fact that the field of stars it covers is quite small.
A rational follow-up strategy might be as follows:
1) Stare at the direction of GCRT J1745-3009 at higher frequencies as both cost optimization and higher information-carrying ability argue. Another information-bearing signal could be at the optimum high frequencies, ~10 GHz. A temporal analysis should be conducted to search for structure in the bursts, since measurements to date have not looked for any message content.
2) look in the opposite direction, 180 degrees from the Center, to see if there’s another beam communicating toward GCRT J1745-3009.
The method could tell us whether we’ve by chance intercepted an interstellar communication link. This particular source seems an unlikely candidate, but it is interesting enough to trigger subsequent study and gives us a chance to put the cost-optimized strategy to work. Another interesting candidate is the so-called ‘WOW’ signal seen at Ohio State in 1977. A year-long campaign directed at galactic center might uncover a follow-up, but it also gives us an idea of the kind of effort the search for the fleeting signals of such beacons might involve.
Image: The WOW! Signal. Credit: The Ohio State University Radio Observatory and the North American AstroPhysical Observatory (NAAPO).
We will need to be patient and wait for recurring events that may arrive in intermittent bursts. Special attention should be paid to areas along the Galactic Disk where SETI searches have seen coherent signals that are non-recurring on their limited listening time intervals. Since most stars lie close to the galactic plane, as viewed from Earth, occasional pulses at small angles from that plane should have priority.
Thus the call for systematic scans of the entire galactic plane, emphasizing the importance of galactic center, with steady observing periods covering a span of years. For reasons discussed in the METI paper, the 10 GHz range is optimum as opposed to lower-frequency, more conventional SETI choices. Re-thinking our SETI strategy offers the chance to maximize the chances for detection, just as, the Benfords show, analyzing the demands of interstellar messaging (METI) can provide significant insights into the kind of signal we might find.
The papers are James Benford et al., “Cost Optimized Interstellar Beacons: METI,” available here, and Gregory Benford et al., “Cost Optimized Interstellar Beacons: SETI,” available here. As I’ve mentioned before, these papers work as a unit and should be read together, the one illuminating the other. The Sullivan paper I discuss above is Sullivan et al., “A Galactic Center Search For Extraterrestrial Intelligent Signals,” Astronomical and Biochemical Origins and the Search for Life in the Universe, IAU Colloquium 161, Publisher: Bologna, Italy, p. 653.
If we want to consider how to pick up transmissions from a distant civilization, it pays to consider the most effective strategies for building interstellar beacons here on Earth. This is the method James, Gregory and Dominic Benford have used in twin papers on SETI/METI issues, papers that should be read in conjunction since the METI questions play directly into our SETI reception strategies. It pays to have a microwave specialist like James Benford on the case. Our METI transmissions to date have used radio telescopes and microwaves to send messages to nearby stars. Longer distances will cost more and take much more power.
How much would a true interstellar beacon cost, one not limited to the relatively short ranges of recent METI transmissions? Count on something on the order of $10 billion. As to power, Jim is able to quantify the amount. To reach beyond roughly a thousand light years with a microwave beacon, an Effective Isotropic Radiated Power (EIRP) greater than 1017 W must be deployed. A beacon designed for communication across galactic distances goes even higher, with ranges up to 1020 W. We can compare this to the Arecibo radio telescope, famously used for sending a message to the Hercules Cluster (M13) in 1974. Arecibo can muster 1013 W, and most (short-range) METI messages have managed 1012 to 1013 W.
We learn in these pages that microwave emission powers have increased by orders of magnitude, while the introduction of new technologies has changed our methods for emitting powerful signals since the days when Project Ozma was but a gleam in Frank Drake’s eye. The paper lays out the necessary background, noting that most high power devices operate in bursts of short pulses and are not extremely narrow band. Economical beacons are likely to be pulsed, as Drake himself would note back in 1990. The Benfords liken this strategy to that of a lighthouse, with a pulsing beam that moves and calls attention to itself.
Go through these pages for a useful survey of high power microwave (HPM) technology as it developed, with implementations through the decades ranging from the relativistic klystron to modern intense relativistic electron beam technology. The paper notes that arrays of antennas are the only practical way to produce the large radiating areas necessary for an interstellar beacon, arrays like those already used in radio astronomy work. The construction of an efficient beacon is then considered against the constraints of cost optimization, considering the crucial relations between cost-optimal aperture and power.
I leave you to the relevant equations and turn toward the implications in terms of transmission strategy. Mindful of the concept of a galactic habitable zone, the Benfords focus on stars that lie inward toward the galactic center at distances of greater than 1000 light years, a strategy they hope will target the highest number of possible civilizations. So if you are broadcasting in an attempt to reach likely ETIs, here is a method: Broadcast in a limited way, targeting the 90 percent of the galaxy’s stars that lie within nine degrees of the sky’s area, in the plane and hub of the galaxy, with special attention to areas along the galactic disk.
Whatever races might dwell further in from us toward the center, they must know the basic symmetry of the spiral. This suggests the natural corridor to communicate with them is along the spiral’s radius. (A radius is better than aiming along a spiral arm, since the arm curves away from any straight-line view of view. On the other hand, along our nearby Orion arm the stars are roughly the same age as ours.) This avenue maximizes the number of stars within a Beacon’s view, especially if we broadcast at the galactic hub. Thus, a Beacon should at least broadcast radially in both directions. Radiating into the full disk takes far more time and power; such Beacons will rarely visit any sector of the plane. Of course such distances imply rather larger Beacons.
Image: Beautiful, but this young cluster imaged by Hubble is an unlikely home to alien civilizations. Better to look in the direction of the galactic plane, and think in terms of older stars with higher prospects. Credit: NASA/ESA/STScI/AURA).
Thus we scan the plane of the galaxy often, broadcasting toward and away from galactic center frequently, with occasional bursts toward the densest collection of nearby stars along the Orion arm. And this is interesting: Broadcast toward the locations of transient but powerful bursts that have occasionally shown up in past SETI surveys. The paper concludes that cost-efficient beacons will be pulsed, narrowly directed and broadband in the 1-10 GHz region, with a preference for higher frequencies. If we apply these thoughts to other civilizations, assuming their own tendency to minimize costs and obtain the most efficient result, we develop the necessary counterbalancing strategy for SETI.
Given current SETI methods, would we detect such ‘searchlight beam’ beacons with short ‘dwell’ times as they sweep past? More on these issues next time. The paper is James, Gregory and Dominic Benford, “Cost Optimized Interstellar Beacons: METI,” available online.
Several interesting papers on SETI have appeared in recent days, among them Gregory, James and Dominic Benford’s attempt to place SETI in the context of economics. Equally useful is Duncan Forgan’s new look at the Drake Equation, presenting a way to estimate the distribution of the crucial parameters. I’ll bypass the Forgan paper temporarily because I’ve asked Marc Millis to tackle it as soon as he gets back from the Jet Propulsion Laboratory, where he’s gone to attend a workshop. Forgan’s study has direct bearing on a Tau Zero initiative we hope to have in place by the end of the year and thus is a natural for Marc to write up.
But back to the Benfords, who have offered up twin papers (as seems reasonable for the brothers), one on SETI (with Gregory as principal author) and the other on its METI offshoot (transmitting messages rather than listening for them). James Benford is lead author on the latter. This work is so rich that I won’t try to encapsulate it in a single post, but rather draw on the ideas here over the course of a few (perhaps non-consecutive) days. In any case, I don’t want to be rushed in these discussions, where the issues are as large as the distances involved, and our philosophical approach may be as significant as our technology. And the two papers should be read in connection with each other in any case, for the analysis in the METI paper is directly relevant to developing the argument of the SETI discussion.
We’re close to the 50th anniversary of the Project Ozma attempts, and in SETI’s first half-century we have found no clear detections in our search of nearby stars. But over the course of the decades, our initial SETI assumptions have been challenged. We’ve gone from a Sagan-esque optimism that the galaxy is aswarm with civilizations to a far more measured (and surely more realistic) view that takes into account factors like the galactic habitable zone, and the sheer difficulty facing living things on the long evolutionary road toward intelligence.
Image: The spiral galaxy M74 (NGC 628), perhaps the home of technological civilizations that, like us, are trying to figure out whether they are alone. Credit: GMOS/Gemini North Observatory.
The Benfords — Jim at Microwave Sciences, Gregory at the University of California’s Irvine campus, and Dominic (Jim’s son) at NASA GSFC — believe that advanced societies, if they are to be found, ought most likely to exist toward the galactic center, and probably at distances of over a thousand light years. We’re thus talking, in all likelihood, about interstellar beacons rather than targeted transmissions when it comes to SETI. And if beacons are indeed at play, what can we say about their costs, and do our own standards of terrestrial cost have any application in an ETI context?
The message here is that any SETI search has to make assumptions about the beacon builders, and if we can determine something about the economics of the situation, we may learn how to target our searches more effectively. Here’s the essence of the argument about ETI:
We assume that if they are social beings interested in a SETI conversation or passing on their heritage, they will know about tradeoffs between social goods, and thus, in whatever guise it takes, cost. But what if we suppose, for example, that aliens have very low cost labor, i. e., slaves? With a finite number of slaves, you can use them to do a finite number of tasks. And so you pick and choose by assigning value to the tasks, balancing the equivalent value of the labor used to prosecute those tasks. So choices are still made on the basis of available labor. The only case where labor has no value is where labor has no limit. That might be if aliens may live forever or have limitless armies of self-replicating automata, but such labor costs something, because resources, materials and energy, are not free.
Our point is that all SETI search strategies must assume something about the beacon builder, and that cost may drive some alien attempts at interstellar communication.
SETI always seems to come with a built-in willingness to think the best of extraterrestrial cultures. If an alien civilization is sending out a message, it must be doing so out of altruism. The Benfords, though, are interested in exploring motivations from a different angle. They’d like to relate them to the only case of a technological civilization we know of, ourselves, and speculate based on human history. From that perspective, there are two reasons for sending out messages across vast time scales.
Think about what people do. You can go to the Tower of London and explore the chambers where famous prisoners like Thomas More were kept. Invariably, on the walls, you’ll find graffiti, names written into the stone. People have an apparently robust need to engage in one-way communication, putting a note in a bottle and throwing it. Indeed, the Pioneer and Voyager spacecraft are examples of the impulse. Is it likely that any of these tiny vessels will ever be intercepted? Yet putting our names, our stories, our music and our pictures on board outgoing vehicles is a method that resonates. We have a need to encapsulate who we are.
A second reason is the drive to communicate the optimum things about our culture, what Matthew Arnold called “…the best that has been thought and said in the world.” Here the Benfords cite time capsules and monuments as examples of our need to propagate our culture. The contemplation of a legacy is involved here, especially in a scenario where human lifetimes are rising. Here again the communication can be one-way. The statue of King Alfred my wife and I admired in Winchester some years back was not built to impress people within a tight time frame, but to stand as a monument that would reach future generations.
So imagine scenarios like this: A civilization with an ability to plan over millennial time scales foresees problems that are beyond its capabilities. A SETI beacon might encapsulate a call for information and help — send us everything you have on stellar warming…
Here’s another: A civilization in its death throes decides to send out an announcement of its existence. We were here and are no longer, but as long as this message endures, so in a sense do we. And let’s not discount sheer pride of the sort that could keep a beacon in operation long after the beings that built it were gone. Robotically maintained, it might boast of achievements set against the backdrop of the ruin that may eventually attend all technological cultures. Or perhaps we’ll run into interstellar proselytes, out to convert the galaxy to a particular set of beliefs by placing their highest values into their outgoing signal.
High powered signals may even be unintentional, such as the signatures of planetary radars that scan for asteroids, or perhaps the industrial activities of a space-faring civilization that boosts payloads deep into its system using beamed power. Beam-driven sail expeditions might leave such a signature (recall Niven and Pournelle’s The Mote in God’s Eye). These activities seem more likely to be detected than the far fainter signatures of isotropic radio and television broadcasts, and would carry the unmistakable message of a working technology.
Image: Does the best SETI strategy involve listening for directed messages from nearby stars? Or do we concentrate on short burst transmissions from deep in the galactic plane? Credit: VLA/NRAO.
Given all these motivations — and we don’t know how to choose between them — microwaves seem to be the best carriers of a civilization’s message, if only because interstellar absorption is minimal at those wavelengths. Can we make any guesses based upon cost and perceived value about how such a culture would proceed with a microwave beacon? If so, our conclusions would have a material impact on how we conduct the SETI search. From the paper again:
…even altruistic Beacon builders will have to contend with other competing altruistic causes, just as humans do. They will confront arguments that the response time for SETI is millennia, and that anyway, advanced societies leak plenty of microwaves etc. into deep space already. We take up these issues below. It seems clear that for a Beacon builder, only by minimizing cost/benefit will their effort succeed. This is parsimony, meaning ‘less is better’ a concept of frugality, economy. Philosophers use this term for Occam’s Razor, but here we mean the press of economic demands in any society that contemplates long term projects like SETI. On Earth, advocates of METI will also face economic constraints.
The Benfords believe that evolution will always seek for economy of effort. Their analysis is bracing stuff and it calls into question a SETI strategy that is not optimized for short bursts of pulsed microwaves and the observation times necessary to see repeat events. Another question is inevitable — have we inadvertently picked up such signals already and dismissed them? More on this in coming days. The paper, “Cost Optimized Interstellar Beacons: SETI,” is available online. We’ll look at “Cost Optimized Interstellar Beacons: METI,” available here, tomorrow.
Two asteroid belts around Epsilon Eridani? So we learned yesterday, a fascinating find and one I want to discuss today, but only after celebrating Epsilon Eridani itself. Can any star have a more interesting pedigree? This is one of the Project Ozma stars, the other being Tau Ceti, that Frank Drake targeted in the first attempt to listen in on extraterrestrial civilizations. The Centauri stars seemed less likely then, in an era when multiple systems were thought to be hostile to planetary formation. But Epsilon Eridani and Tau Ceti were both single, Sun-like stars, surely possible homes to planets not much different from ours.
Or so we thought. We’ve since learned that Tau Ceti’s chances as a home to flourishing civilizations are diminished by the likelihood of intense cometary bombardment, while Epsilon Eridani itself is young enough (850 million years) that any parallel with our own Solar System, where life has had billions of years to attain technology, breaks down. But these stars are close enough to us to make them realistic targets for interstellar probes, assuming we develop the needed technologies and have the will to launch them. And if exploration in our own system has taught us anything, it’s that we should be prepared to be surprised, no matter how well established are our preconceptions.
So we move 10.5 light years in the direction of the constellation Eridanus to a star that looks a lot like ours once did, a point that Massimo Marengo (Harvard-Smithsonian Center for Astrophysics) is quick to note. “Studying Epsilon Eridani is like having a time machine to look at our solar system when it was young,” says Marengo, co-author of the discovery paper. That calls up the possibility that this system may evolve as ours has, perhaps one day yielding civilizations of its own, even if none were available for Frank Drake to overhear in 1960.
The new findings show that Epsilon Eridani has an asteroid belt at a distance of some 3 AU, more or less in the same place as the asteroid belt around our Sun. A second belt, found using the Spitzer Space Telescope, exists at about 20 AU, in the area where Uranus orbits in our system. Still further out, extending some 35 to 100 AU from the central star, is a ring of icy materials reminiscent of our own Kuiper Belt, but holding 100 times more material. Current thinking is that when the Sun was Epsilon Eridani’s age, the Kuiper Belt looked more or less the same, with much of its material swept away in the Late Heavy Bombardment.
Image: This artist’s conception shows the closest known planetary system to our own, Epsilon Eridani. Observations from NASA’s Spitzer Space Telescope show that the system hosts two asteroid belts, in addition to previously identified candidate planets and an outer comet ring. The system’s inner asteroid belt appears as the yellowish ring around the star, while the outer asteroid belt is in the foreground. The outermost comet ring is too far out to be seen, but comets originating from it are shown in the upper right corner. Credit: NASA/JPL-Caltech.
The intriguing gaps between the rings of material around this star are laden with possibility. We already have some radial velocity evidence for the existence of a Jupiter-mass planet in an elliptical orbit with a semi-major axis of 3.4 AU and an orbital period of 6.9 years. That would place this world near the location of the innermost belt identified in this work. But it would also create a problem, according to the paper: Ranging from 1 AU to 5.8 AU as it moves through its orbit, such a planet “…would quickly clear that region not only of dust particles but also the parent planetesimal belt needed to resupply them, inconsistent with the dust distribution implied by the Spitzer observations.” The problem can be eased if the degree of this planet’s orbital eccentricity is diminished, which would allow an orbit entirely within the innermost belt.
The paper goes on to offer the evidence for three possible planets in this system:
Planet A: the long-suspected but still unconfirmed Jovian-mass planet with aorb = 3.4 AU that may be associated with the innermost warm debris belt detected by Spitzer. Planet B: also perhaps Jovian-mass, associated with the second warm debris belt at r ? 20 AU inferred in this paper, also possibly helping define the sub-mm ring inner edge via 2:1 resonance, keeping the zone between 20 and 35 AU clear… Planet C: located at r ? 35 AU, with mass less than a few × 0.1 MJup according to dynamical models, preventing small grains from drifting inward past the inner edge of the sub-mm ring.
It’s intriguing to note that while Spitzer would not have been able to detect it, at least one of these worlds may be within range of the James Webb Space Telescope, scheduled for launch in the next decade. The image below shows a comparison between our Solar System and what we’ve thus far discovered around Epsilon Eridani. Note the size of that outer belt of icy materials compared to the Kuiper Belt!
Image: This artist’s diagram compares the Epsilon Eridani system to our own solar system. The two systems are structured similarly, and both host asteroids (brown), comets (blue) and planets (white dots). Credit: NASA/JPL-Caltech.
Both of the inner planetary possibilities orbit near the rim of one of the asteroid belts, and the third would be located near the rim of the outer ring of comets. Future work should tell us much more, including the ever lively question of whether terrestrial worlds may lurk inside the innermost asteroid belt. Whatever the case, a star close enough to be an early target for any interstellar technology we develop will always hold our interest. The paper is Backman et al., “Epsilon Eridani’s Planetary Debris Disk: Structure and Dynamics based on Spitzer and CSO Observations,” slated for publication in The Astrophysical Journal and available online.
Ponder the image below, which scientists at Tel Aviv University are interpreting in terms of the structure of the universe itself. The work draws on the well established notion that large galaxies are found on bubble-like structures — the soap bubble analogy is inevitable — with smaller dwarf galaxies scattered along the bubble surface. The Tel Aviv team thinks it has discovered visible traces of a filament of dark matter around which galaxies form. Filaments would be found at the juncture of two bubbles where the membrane is presumably thickest.
Thus the image, which shows fourteen galaxies studied at the university’s Wise Observatory. Here the galaxies are thought to stretch along a line extending from the lower right to the upper left corner. In its paper, the team calls the grouping “…a single kinematically well-behaved ensemble.” The area studied is intriguing not only because the galaxies found here seem to be forming in a line, but also because thirteen of them show new star formation. That latter fact drives the possibility that the galaxies are indeed forming on a dark matter filament, one that would attract new baryonic matter that then forms into stars. (Image credit: AFTAU).
The paper presents extensive observational data to support this dark matter (DM) conclusion:
We propose that the observational evidence argues in favor of interpreting the galaxies as located on a DM ?lament that is itself located in a low-galaxy-density region, and is accreting intergalactic cold gas focused by the ?lament. We are therefore witnessing in the classical NGC 672 and NGC 784 groups of relatively bright and nearby late-type galaxies the basic phenomenon of hierarchical clustering, the direct formation and growth of small galaxies out of intergalactic gas accreted on a dark matter “backbone”. This nearby galaxy collection offers, therefore, an ideal opportunity to study the phenomenon of hierarchical clustering in signi?cant detail.
If the Tel Aviv team is correct, then what we are seeing here is the first stage of the clustering of matter that will eventually form into a major galaxy. Noah Brosch, who is director of the university’s Wise Observatory, emphasizes that the phenomenon is occurring at relatively close hand. “Our studies show that you don’t need to go to the edge of the universe to find dark matter,” says Brosch. “It may be only 15 million light years away, more or less in our backyard.” Close enough for an advanced civilization to get a mission to one day? Brosch is surprisingly sanguine about not just interstellar but intergalactic travel: “Our technology is abysmally limited right now, but it could definitely happen.”
The paper is Zitrin and Brosch, “The NGC 672 and NGC 784 Galaxy Groups: Evidence for Galaxy Formation and Growth Along a Nearby Dark Matter Filament,” in press at Monthly Notices of the Royal Astronomical Society and available online.