SETI’s task challenges the imagination in every conceivable way, as Don Wilkins points out in the essay below. A retired aerospace engineer with thirty-five years experience in designing, developing, testing, manufacturing and deploying avionics, Don is based in St. Louis, where he is an adjunct instructor of electronics at Washington University. He holds twelve patents and is involved with the university’s efforts at increasing participation in science, technology, engineering, and math. The SETI methodology he explores today offers one way to narrow the observational arena to targets more likely to produce a result. Can spectacular astronomical phenomena serve as a potential marker that could lead us to a technosignature?
by Don Wilkins
Finite SETI search facilities searching a vast search volume must set priorities for exploration. Dr. Jill Tarter, Chair Emeritus for SETI Research, describes the search space as a “nine-dimensional haystack” composed of three spatial, one temporal (when the signal is active), two polarization, central frequency, sensitivity, and modulation dimensions. Methods to reduce the search space and prioritize targets are urgently needed.
One method for limiting the search volume is the SETI Ellipsoid, Figure 1, which is reproduced from a recent paper in The Astronomical Journal by lead author James R. A. Davenport (University of Washington: Seattle) and colleagues. [1]
Image: This is Figure 1 from the paper. Caption: Schematic diagram of the SETI Ellipsoid framework. A civilization (black dot) could synchronize a technosignature beacon with a noteworthy source event (green dot). The arrival time of these coordinated signals is defined by the time-evolving ellipsoid, whose foci are Earth and the source event. Stars outside the Ellipsoid (blue dot) may have transmitted signals in coordination with their observation of the source event, but those signals have not reached Earth yet. For stars far inside the Ellipsoid (pink dot), we have missed the opportunity to receive such coordinated signals. Credit: Davenport et al.
In this approach, an advanced civilization (black dot) synchronizes a technosignature beacon with a significant astronomical event (green dot). The astronomical event, in the example, is SN 1987A, a type II supernova in the Large Magellanic Cloud, a dwarf satellite galaxy of the Milky Way. The explosion occurred approximately 51.4 kiloparsecs (168,000 light-years) from the Sun.
Arrival time of the coordinated signals is defined by a time-evolving ellipsoid, with foci at Earth (or an observation station within the Solar System) and the source event. The synchronized signals arrive from an advanced civilization based on the distance to the Solar System or other system with a technological system (d1), and the distance from the advanced civilization to the astronomical event (d2). Signals from civilizations (blue dot) outside the Ellipsoid coordinated with the source event have not reached the Solar System. Stars inside the Ellipsoid (pink dot) but on line between the advanced civilization and the Solar System will not receive the signals intended for the Solar System. However, the advanced civilization could beam new signals to the pink star and form a new Ellipsoid.
The source event acts as a “Schelling Point” to facilitate communication between observers who have not coordinated the time or place of message exchanges. A Schelling point is a game theory concept which proposes links can be formed between two would-be communicators simply by using common references, in this case a supernova, to coordinate the time and place of communication. In addition to supernovae, source events include gamma-ray bursts, binary neutron star mergers, and galactic novae.
In conjunction with the natural event which attracts the attention of other civilizations, the advanced civilization broadcasts a technosignature signal unambiguously advertising its existence. The technosignature might, as an example, mimic a pulsar’s output: modulation, frequency, bandwidth, periods, and duty cycle.
The limiting factor in using the SETI Elliposoid to search for targets is the unavailability of precise distance measurements to nearby stars. The Gaia project remedies that problem. The mission’s two telescopes provide parallaxes, with precision 100 times better than its predecessors, for over 1.5 billion sources. Distance uncertainties are less than 10% for stars within several kiloparsecs of Earth. This precision directly translates into lower uncertainties on the timing for signal coordination along the SETI Ellipsoid.
“I think the technique is very straightforward. It’s dealing with triangles and ellipses, things that are like high-school geometry, which is sort of my speed,” James Davenport , University of Washington astronomer and lead author in the referenced papers, joked with GeekWire. “I like simple shapes and things I can calculate easily.” [2]
An advanced civilization identifies a prominent astronomical event, as an example, a supernova. It then determines which stars could harbor civilizations which could also observe the supernova and the advanced civilization’s star. An unambiguous beacon is transmitted to stars within the Ellipsoid. The volume devoted to beacon propagation is significantly reduced, which reduces power and cost, when compared to an omnidirectional beacon.
At the receiving end, the listeners would determine which stars could see the supernova and which would have time to send a signal to the listeners. The listening astronomers would benefit by limiting their search volume to stars which meet both criteria.
For example, astronomers on Earth only observed SN 1987A in 1987, thirty six years ago. If the advanced civilization beamed a signal at the Solar System a century ago, our astronomers would not have the necessary clue, the observation of SN 1987A, to select the advanced civilization’s star as the focus of a search. Assuming both civilizations are using SN 1987A as a coordination beacon, human astronomers should listen to targets within a hemisphere defined by a radius of thirty-six light-years.
The following is written with apologies to Albert Einstein. The advanced civilization could observe the motion of stars and predict when a star will come within the geometry defined by the Ellipsoid. In the case of the Earth and SN1987A, the advanced civilization could have begun transmissions thirty-six years ago.
The recently discovered SN 2023ixf in the spiral galaxy M101 could serve as one of the foci of an Ellipsoid. 108 stars within 0.1 light-year of the SN 2023ixf – Earth SETI Ellipsoid. [3]
Researchers propose to use the Allen Telescope Array (ATA), designed specifically for radio technosignature searches, to search this Ellipsoidal. The authors point out the utility of the approach and caution about its inherent anthropocentric biases:
“…there are numerous other conspicuous astronomical phenomena that have been suggested for use in developing the SETI Ellipsoid, including gamma-ray bursts (Corbet 1999), binary neutron star mergers (Seto 2019), and historical supernovae (Seto 2021). We cannot know what timescales or astrophysical processes would seem “conspicuous” to an extraterrestrial agent with likely a much longer baseline for scientific and technological discovery (e.g., Kipping et al. 2020; Balbi & Ćirković 2021). Therefore we acknowledge the potential for anthropogenic bias inherent in this choice, and instead focus on which phenomena may be well suited to our current observing capabilities.”
1. James R. A. Davenport , Bárbara Cabrales, Sofia Sheikh , Steve Croft , Andrew P. V. Siemion, Daniel Giles, and Ann Marie Cody, Searching the SETI Ellipsoid with Gaia, The Astronomical Journal, 164:117 (6pp), September 2022, https://doi.org/10.3847/1538-3881/ac82ea
2. Alan Boyle, How ‘Big Data’ could help SETI researchers intensify the search for alien civilizations, 22 June 2022, https://www.geekwire.com/2022/how-big-data-could-help-seti-researchers-intensify-the-search-for-alien-civilizations/
3. James R. A. Davenport, Sofia Z. Sheikh, Wael Farah, Andy Nilipour, B´arbara Cabrales, Steve Croft, Alexander W. Pollak, and Andrew P. V. Siemion, Real-Time Technosignature Strategies with SN2023ixf, Draft version June 7, 2023.
Reminds me of Clarke’s “The Star,” where a capricious Creator destroys a comparable civilization in order to herald the birth of Christ with the Star of Bethlehem.
Would be ironic indeed if the only other otherwise potential target civilization within the ellipsoid was destroyed by the source event used to draw attention to the signal.
It’s a large universe, with sometimes perhaps arbitrary results as viewed by those inhabiting one space in time.
I remember that story from my mis-spent youth. After all this time, it still makes me sad. What a powerful, wonderful writer Clarke was.
Thank you for the memory nudge.
I am afraid I just do not understand the rationale for synchronizing events and directed beacons. The beacon arrives at some time (known) after the event is detected. The sender knows when it is on the ellipse for each target and sends te signal. For the receiver, the event must be known (e.g. the supernova example) and to search the ellipse for stars that could send the signal that will be received around time T after the event.
1. Why would ET send a targeted beam to a target without knowing something about the target. IOW, checking all stars on the ellipse at time T may be futile as there is no evidence that the star could be inhabited, nor ET knowing the Earth is inhabited.
2. The shape of the ellipse is not fixed but determining by the semi-major and semi-minor axes. This would mean searching for beacon stars for a wide range of ellipses for any given event for a range of time Ts.
3. Which stellar events to pick? There are so many, that the number of ellipses rapidly increases.
At some point, are we not monitoring the whole sky?
The search, if strictly maintained, might ignore an ETI signal sent to us that was not synced to an astronomical event, and we would miss it. (Rather like Nelson putting a telescope up to his blind eye.)
Despite the claim this approach limits the search space, are we really not better off looking for systems with habitable worlds and searching for biosignatures and technosignatures? Biosignatures indicate that a planet in the star’s HZ is not just inhabitable, but inhabited. If they ascribe to the “Dark Forest” concept, they will not be sending signals. However, we may still detect technosignatures such as stray radio or laser pulses, glints from flat surfaces that provide spectral data, etc.
If we had an all-sky observing program, and if there are a number of civilizations sending out directed em beams based on this idea, could we determine a pattern from events, received signals, and possible ellipses, to determine if apparent random signals were in fact signals synchronized to various events?”
If we were to send synchronized signals to target stars defining ellipses with an event, sending the data when we touched each ellipse, assuming a maximum range, what would our directed beam energy look like over time to service the target stars? We could certainly filter the target stars by having inhabitable/inhabited planets, and if so, how would it further constrain the needed energy?
It does seem that the “ellipsoid” is a variable, expanding feature – really, just the physical length of time between whatever you think is a significant event, and when the hypothetical responding signal is detected on Earth. It narrows the distance to the signal only if we imagine we know which significant event was synchronized with the signal. Couldn’t aliens do something more practical to let us know exactly where they are, like use a tunable monochromatic laser to slowly beam their way through an entire simulated spectrum of their particular star a few times?
I find that all of these sorts of ideas to limit energy use are constrained by what we think is “reasonable” energy use. We generate ideas that we then suggest are logical for ETI to do, and perhaps adjust our search methodology. This is almost the opposite of Dysonian thinking.
If advanced ETI is out there, but relatively rare in space and time, then if they want to indicate their presence, they will need to create omnidirectional beacons using the technologies they can bring to bear. Even million-year civilization lifetimes would just be isolated blinks in cosmic time. No Stapledonian universe of a multitude of civilizations present at the same time (if time means anything given the distances). Such a long-lived civilization could have sent probes out to monitor every star in the galaxy. If any interesting “intelligence” emerged, the homeworld will know about it in 100,000 years at worst. If “intelligence” is rare and worth encouraging (c.f. Clarke’s reasoning behind 2001: ASO) then civilizations will appear more frequently over time if the “farmer” is successful. We would have to hope that we are one of the successful “crops” if we hope to locate others. If we are the first and most isolated in space and time in our galaxy, SETI will prove disappointing.
Our current ideas seem to follow 2 approaches. Better telescopes for remote observation, and sending out interstellar probes to make local observations, and possibly more. If we survive long enough to become a K2 civilization, we could send a vast number of probes out into the galaxy. Why would we not expect any advanced ETI to do at least that much?
I want to emphasize that the shape of the ellipsoid is variable. If the semi-major axis (A) is fixed between the event and the target star. then the semi-minor axis (B) varies as 0 < B < A. So it starts close to 0 and ends as a near circle. One can pick any value of B to restrict the search, but that tells one nothing about where ETI is, should it exist. Then given the possible number of events that are observable with ever more powerful telescopes, the number of ellipses is magnified. And let us not forget these are 3-D ellipsoids, so the surface has to be searched, even if filters are applied to limit the possible targets.
This is a layman’s question. There must have been quite a few major events such as supernovae etc. in the Milky Way in the last 36 years. Would it be as much as 1 per year on average? How many events have occurred in that time frame that might have been used to point in a rough direction? Would there be events pointing in almost every direction?
Imagine a receiving civilization that has as its foundational culture or religion the careful cataloging of data from observations of these significant astronomical events. During one of their observations the precious data is buried beneath the strong beacon signal of the sending civilization and lost forever. This causing great outrage.
But they are a peaceable people so they set their anger aside and wait for the beacon to cease so that their observations can continue.
After the third time this happens they’ve had enough. They declare a forever war against the responsible civilizations and launch their fleets of FTL warships.
An SF story seed right there.
Perhaps, though only for a short story of the kind Asimov and others of his generation wrote a long time ago.
Other than providing a bit of humor, my only point is that we have a peculiar tendency to promote an unusual astronomical circumstance to a SETI/METI strategy, and then invent ETI that are compatible with that strategy. I am not convinced.
In Clarke’s 3001: The Final Odyssey (1997), Earth discovers that a Nova was likely caused by the monoliths as a judgment on the system’s ETI that did not meet approval. The suggestion was that Earth would be the next to meet this fate. This is the scenario for the plot for the last 50-odd pages of the novel.
2 years later, in a [very] short story for Nature, “Improving the Neighbourhood” (1999), Clarke suggests that a star that went “double nova” (strongly hinted that it was our sun and Earth) was due to an accident trying to liberate infinite energy.
If you do write a story on this topic, I would love to read it.
Or the signal is interpreted as fulfilling some great prophecy. And they launch a pilgrimage fleet.
I.e the sending civilization is about to get swamped by alien Jehovah’s Witnesses. Gotta be careful what you wish for ????
The search will be better circumscribed and directed but the enormity from which the selection has to start will leave out many if not most possibilities.
It seems the longer we wait without finding something the more elaborate our reasons are why we haven’t seen anything yet, and the more baroque our schemes to speed up the process.
Its been less than a century since Karl Jansky detected the first radio signal of cosmic origin. Just 90 years. In astronomical terms, that is NOTHING. We’ve only just started looking. Its like a whale emerging from the water, taking a quick look at an overcast sky, and seeing nothing, saying; “There’s nothing out there”.
Even if advanced technical civilization are fairly common in the universe, we may search carefully for thousands of years and never find any evidence of them. The search has just started, The universe is very big and the speed of light is very slow.
“Piano. Piano. Slowly, slowly catchee monkey.”
This assumes that any actively listening or transmitting civilization will last long enough for any signal to be worthwhile. 2-way communication is unlikely unless both civs are extant over the 2-way time delay. Maybe transmissions just have no value. Transmissions are also ephemeral unless they can be repeated for very long periods, with a relatively short period. One could also interpret this scenario to imply that only very long-lived civs transmit, and that the period between transmissions is long so that only long-lived civs can receive the transmission and reply.
Speculation upon speculation.
Ah, Alex, you’re absolutely right. But the question is important. It matters. Perhaps it matters more than any thing else. That’s why we keep on asking it.
Even if we never know the answer, we must ask it and we must keep asking it.
They built cathedrals in the Middle Ages, we build radio telescopes. Because it matters. Let’s just hope that IF they’re out there, they feel the same way.
“Observatories are holy places. They are as impressive and beautiful as a medieval cathedral and by necessity are usually located in lonely and desolate landscapes. Like cathedrals, they are temples to the ineffable, to the incredibly remote, and to our faith in being able to connect with it–places of worship, in a way, sacred places. I know it’s sentimental and impractical of me, but if this site is to be abandoned, let it not be replaced with a farm or village or reservoir or some other practical symbol of the economy. Let it naturally decay into ruins, as a monument to our boldness, and to our stupidity. Centuries from now, men will stand in that place and say ‘we once explored the stars from here’.”
from “Thoughts Upon Hearing the Arecibo Radio Observatory was About to be Closed”
–Centauri Dreams, Nov 23, 2010
For either METI or SETI, the generalized case for the ellipsoid strategy assumes a higher rate for ETI than a galaxy wide strategy. Assume that not every ETI will respond or message and that some aren’t friendly, and the rate for ETI must increase. It seems to me that the general case for the ellipsoid strategy assumes many valuable targets outside the ellipsoid. Imo, the argument that the ellipsoid strategy provides better return on investment than a galaxy wide strategy is slim. At a low enough rate for ETI, but still above 2 per galaxy, the ellipsoid strategy for METI and SETI can’t provide a return on invest.
As others have pointed out, there is a lot of overlap in the motivations for radio and optical METI, general astronomical study that narrow the number of likely targets for METI and filling a galaxy with probes. As well, general study and probes solve some of radio and optical METIs weaknesses: recipients don’t need to respond, a generalized message is weaker than a bespoke, and not every recipient has to be friendly.
Imho, using a METI as a general search strategy has disadvantages and gets replaced with techniques that allow for bespoke communication strategies that are very difficult to generalize.
There is still a case for SETI. We are bound by the anthropic principle, we can only see the local rate for ETI and the ET people who showed up and there are more than one motivation for METI that don’t include friendly communication. The Vogons had a point about human complacency.
I think an artificial directed beacon makes far more sense. There are many things to decide with this approach of course. What are the target stars? What type of beacon? What is the on/off rate and for how long can it be sustained? It seems worthwhile but strictly for reasons of curiosity and determination to at least attempt to find a receiving civilization. It’s an extremely long shot and I think listening and watching is far more cost effective such as the eight potential SETI signals discovered recently by reviewing data from 800 or so nearby stars using a deep learning search algorithm. Adding a beacon of our own would be interesting and exciting though.
“Observatories are holy places.” Indeed. As are cathedrals. To the extent that they are perceived as in some way transcending infinities of time, space, or both.
Yet there are world-views (Indic, and also Kabbalah) in whose ascriptions of finitude to time and space with a homogeneous isotropic Divine where such an entity is acknowledged, excludes the possibility of attributing holiness/sacredness to anything less than the entirety of the multiverse.
Hello, the idea is attractive but how would E.T think? if it is able to synchronize itself on a novae or a pulsar, it surely expects an answer through the same channel: would we then be technologically developed enough to show it that we have “seen” it and even respond to it? Can a type 0 or 1 civilization like ours communicate with a type 3…not sure* :) and the reverse: would a type 3 WANT to communicate with us [because it would have probably already spotted because of its technological development]…not sure! Technologically, we would surely have little to teach it & biologically, either it would not “understand” the Living as we conceive it, or it would have knowledge of the assemblies of amino acids that we are and in this case, I think it has there is a good chance that we will end up in their lab test tubes… not a very pleasant story :) Joking aside: communicating is seeking or even exchanging Information. So the question is “why communicate” and what are we looking for / what is E.T looking for? I want to put forward here the idea that the objective of the message can determine the technical means and therefore the very nature of the message: if I make a cloud of smoke in the desert, I will report myself but the precision of the message will be poor in information: the receiver will not know if I am alone or with others, large, small etc. On the contrary if I use a very targeted powerful laser with terabits of data, or a graviational wave as a new “morse” code, E.T in will learn a lot about me! In other words, shouldn’t the search for technosignatures start from what we observe to deduce the message and therefore find its author?
Paul, thank you for making us dream!
Fred in France. F5CEY
* I have often asked myself the question with pulsars: would we be able to tell the difference between a “natural” pulsar and an E.T signal?
Thank you, Fred. A pleasure to have you here. The pulsar question you close with has indeed caused its share of controversy, especially early on as we were learning what pulsars are.
The technosignature might, as an example, mimic a pulsar’s output: modulation, frequency, bandwidth, periods, and duty cycle.
Why would they do that? It would be mistaken for a natural pulsar?
Unless it’s the dark forest way to communicate with someone specific Thomas, while avoiding tipping off someone else? It all seems very unlikely as the transmitter would have had to establish prior communications with someone. We need that good old tachyon radio for FTL communications. Wasn’t that the old SF trope?
Sorry but can someone help me with example numbers and dates ?
SN1987A is 168,000 LY away, ie. the horizontal straight line.
How can the distances d2+d1 in the diagram also equates 168,000 LY to ensure synchronization ?
Surely the ETI (black dot) cannot send a message to us before he knows that SN1987A exploded. How can we then receive simultaneously his message along the flash from SN1987A ?