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SETI’s Best Chance: Find the Beacon

If we’re going to get lucky with SETI, it’s probably going to be through the reception of an interstellar beacon rather than the chance detection of an electronic emission from space. Sure, chance catches are possible, and for all we know odd receptions like the WOW! signal of 1977 might be cases in point. But we can’t confirm such signals because they’re one-shot affairs, whereas a beacon, designed to be received over interstellar distances, just might give us other options.

Understanding the Interstellar Beacon

So what can we say about beacons? In a guest editorial for the SETI League, former NASA SETI signal detection analyst Bob Krekorian takes a shot at the problem. Krekorian assumes a space-faring species will put its transmitter inside the habitable zone, designed to exist as close to the parent star as feasible to take advantage of the huge amounts of energy available there. If we were building such a beacon, we might decide to place it between the orbits of Earth and Venus, for energy reasons and efficient servicing.

Image: Looking toward galactic center in the infrared. Finding an extraterrestrial beacon may mean re-examining the assumptions that drive the search. Credit: Suzan Stolovy (SSC/Caltech) et al., JPL-Caltech, NASA.

In any case, imagine a beacon that rotates synchronously as it orbits its star. Rather than sending out a beam in all directions, it operates so that, in Krekorian’s words, “a strip of declination is illuminated at its orbital opposition and conjunction modes.” The advantages of this more targeted method seem considerable, as the author goes on to explain:

For potential contactees, the result is a gain of two orders of magnitude in received power over an omni-directional antenna and elimination of the Doppler drift accelerations from both planetary rotation and orbital motion. Assuming a continuous wave or a pulse signal, the signal types expected, detection is a much easier proposition, since the signal trajectory does not cut across detection channels in the time-frequency plane during the observation period.

All of which has further implications for how we proceed:

When the assumption is made that the signal remains stationary in a channel, coherent detection (match filtering) methods can be used to significantly improve detection sensitivity. Finally, one of the biggest challenges facing potential SETI recipients like us or anybody is the monumental real time signal processing (computer operations per second) effort that is required, if all possible signal drift paths resulting from planetary rotation are considered.

A Galactic Test for Membership?

The problems in finding such a signal are, of course, enormous, and it’s interesting to speculate, as Krekorian goes on to do, about ways to increase the information rate in a communications channel. The Shannon-Hartley theorem shows us the connection between channel capacity (bits per second), bandwidth and signal-to-noise ratio (SNR). The interplay of these factors is intriguing because it sets up a potential test (what Krekorian calls ‘an entrance fee’) for joining any organization of communicating civilizations.

You would think, for example, that increasing bandwidth is always a preferable option, for it allows you to pump more bits per second into your signal. But this added channel capacity also adds more noise into the system, which means your signal-to-noise ratio becomes problematic. Having found a signal, here’s how we could cope with this at our end:

SNR is directly proportional to telescope collecting area, so once we find the signal, larger collecting areas can be built to recover SNR and take advantage of the high data rates. This puts some of the burden of maximizing channel capacity on the receiving party, maybe the entrance fee required to join the galactic telecommunication federation.

To Find an Interstellar Roadmap

Intriguing stuff, and it’s important to note Krekorian’s point here, which is that as SETI was formerly practiced at NASA, it proceeded in two ways. A targeted search looked at a single star for 1000 seconds, and an all-sky survey looked briefly throughout the celestial sphere. Both of these strategies would fail if we assume that a stellar orbital beacon of the kind Krekorian outlines is broadcasting. Let’s assume that any civilization smart enough to build such a beacon and powerful enough to keep it in operation would know the pitfalls emerging civilizations like ours would encounter. In that case, what would they do to make it obvious?

Is there, then, an interstellar roadmap that is, in Krekorian’s words, “obvious to every emerging civilization like us based on the fundamentals of physics, astrophysics and mathematics, especially number theory, to connect one to the broadcast”? Perhaps, as Krekorian muses in an earlier SETI League editorial, the acquisition signal will be in the form of a pointer that directs us to the primary communications channel. In any case, we would expect that extraterrestrials choosing to make themselves known would make the problem of detecting their signal as straightforward as possible. In the absence of government funding, we now rely on the private sector to uncover possible strategies.

Kepler is going to give us a better idea of how likely we are to find such a beacon. If terrestrial worlds in the habitable zones of their stars turn out to be relatively common, we’ll be spurred to ask why we have yet to receive a signal and ponder whether SETI’s methods are not, as Krekorian implies, severely behind the times. If terrestrial worlds are found to be scarce, the problem is magnified by the potential distance between us and possible senders. SETI will continue in either case, but so must the relentless effort to discover what methods would most reliably result in a signal, assuming there is one out there to be had.

Related: The Guardian reviews Paul Davies’ The Eerie Silence.


Comments on this entry are closed.

  • david lewis March 29, 2010, 10:19

    Maybe such beacons are designed so that emerging civilizations can’t detect them. Only civilizations able to harness the resources of their solar system would have the knowledge needed to tap into such a network.

    Imagine a galaxy with a huge number of emerging technological civilizations. Now imagine the vast majority of those civilizations self-destruct. They are inherently violent. What can the non-violent civilizations do?

    1. Try to help the violent civilizations? But how? If such violence is basic to the emerging civilization they can either modify the race – moral considerations there. Or aid them past the violence, which could be dangerous to all life in the galaxy if the emerging civilization doesn’t change and uses the technology it eventually gets destructively.

    2. Don’t hide your communications and allow all emerging civilizations to participate in the network. However if they are much like humans that would take an immense emotional toll. One day there is this vibrant emerging techno civilization emerging next door, and the day after …. silence. Not to mention there is the danger of a violent civilization still getting it’s hands on some technology being shared across the distances, and using it destructively.

    3. Hide your communications so that only those who have shown themselves to be stable can participate in the network. Those who have already expanded out into their solar system and from whom not much can be hidden anyway.

  • Zarpaulus March 29, 2010, 10:33

    Unless galactic civilization has some form of monopoly on faster-than-light travel (I’m thinking a wormhole network would be most realistic), it would probably be safer to not make any kind of beacon.

  • Ron S March 29, 2010, 10:55

    “Assuming a continuous wave or a pulse signal, the signal types expected, detection is a much easier proposition, since the signal trajectory does not cut across detection channels in the time-frequency plane during the observation period.”

    This doesn’t solve the frequency drift “problem” since it only addresses one side of the communications path. Earth is both rotating and revolving, adding quite a large and continuously changing doppler shift. Since the beacon is, presumably, not directly targeting us, it is up to us to make the compensation by assuming the as-yet undetected beacon’s source and transmitting pattern, and then compensating for our own relative motion.

    “Finally, one of the biggest challenges facing potential SETI recipients like us or anybody is the monumental real time signal processing (computer operations per second) effort that is required, if all possible signal drift paths resulting from planetary rotation are considered.”

    I disagree. The computation “problem” is going away, unless we suddenly hit a brick wall in the progression of Moore’s Law (not really a law, just a technology trend line), which could happen but not anytime soon.

  • James M. Essig March 29, 2010, 11:12

    Hi Paul;

    I also wonder if the noise from a high power output extraterrestrial space craft propulsion system could produce some sort of regular or unnaturally irregular signal.

    Perhaps our best chance at observing such is within the domain of electromagnnetism given the strong coupling between electrical current flow changes and electromagnetic signal production.

    Another interesting concept is the possibility of gamma ray bursts produced by the collision of any relativistic ETI spacecraft with objects such as comets, space boulders and rocks etc, by accident.

    The WOW incident is very intriging and I hope we find more such signals.

  • NS March 29, 2010, 13:54

    Here’s a YouTube video on Cost-Optimized Interstellar Beacons that touches on some of the same issues as the above. It’s part of the SETI Institute lecture series but it hasn’t been linked from that site yet:


  • Administrator March 29, 2010, 14:15

    Excellent, NS! Long-time readers know that I’m a great admirer of the Benford brothers’ work on interstellar beacons, and I didn’t know about this video. Thanks. A quick search on ‘benford’ on this site will also turn up previous articles here that deal with how the authors analyze such a beacon.

  • NS March 29, 2010, 14:17

    One of the suggestions that James Benford makes in the above video is that continuous beacons are very inefficient and therefore unlikely. A beacon that periodically illuminates us is what we should be looking for. The first detection is likely to be as a transient, and may be ignored by SETI searches that dismiss non-repeating signals. He thinks data on transients should be re-examined for signs of artificial origin.

  • Gary Williams March 29, 2010, 16:45

    Maybe I’ve missed something but I’m assuming that the beacon in question, even with the energy of the star, would be almost a narrow field transmisson? If so, you’d need to be in the right part of the sky to receive that transmission and infer that it was an artificial signal.

    One other point – Does the beacon have to be near the star? Could it be place about 500AU out where there is a natural gravitiational focus from the Sun? I’d assume that this would work for other stars as well.

    Final point, yes, that WOW signal is interesting because of the wavelength it was transmitted on and the fact it’s not been fully solved yet!

  • bigdan201 March 29, 2010, 17:31

    When imagining aliens, I think its important not to let anthropomorphic bias get in the way too much. I think ETI will have certain things in common with us, but they will be fundamentally different. I dont see them as the noble savages of the Navi, nor do I see them as relentless invaders as portrayed in various scifi. They might be as different to us as we are from ants. Ants live in our world, they eat and reproduce etc, but theyre so different that we can only study their society from a distance – we cant relate or FEEL any way about them (unless a fire ant bites your butt, but thats another story).

    It is tricky to predict or extrapolate what alien motivations or civilization might be like. But we can be certain that they have energy needs, and that they reproduce in some way. advanced ETI will deal with the same physics/math/chemistry as us. It is also likely that they will be social and aggressive, since the development of intelligence favors social predators (humans are social and aggressive). But then again, ETI could be a hivemind so who knows?
    The Wow! signal is intriguing. ETI may not be trying to contact everyone else, and we may accidentally catch a beacon for some other purpose, or the energy output of some artificial event like folding space. But if ETI does try to make itself known, it is likely that they will rely on math/physics/chemistry as being constants that any other intelligence would know about.
    By the way, its important to realize that the Earth has only been broadcasting for a bit less than 100 years.. anyone outside of 100 light years has not heard us yet. The same applies to ETI, which may not be visible or audible to us if theyre far enough away.. or if we do see them, they might be vanished by now, like a very distant star. This is all conjecture, though.

    By the way, have you guys read Rendevouz with Rama by Arthur C Clarke? its a great book, and does very well in describing an alien craft/species while sticking with hard science.

  • Eniac March 29, 2010, 18:34

    Even a narrow, rotating beacon would waste energy shining between stars, most of the time. The most efficient way to send radio signals is pencil beams continuously directed at individual stars which are selected as most promising to contain whoever you would like to communicate with, and each with just enough energy to be detectable at the target, no more, no less.

    If orbital Doppler shift really is a problem, compensate by varying the frequency accordingly. Just as any SETI detector on Earth would obviously know how to compensate for the Earth’s orbit and rotation.

    Such pencil beams could probably be generated efficiently and scalably by an enormous orbiting cloud of individual solar powered transmitters configured as a 3d phased array. The only limit on the power of such an array would be how many you can mass produce during their average lifetime.

  • NS March 29, 2010, 19:29

    I’ll look at the Benford papers. Thanks for the references.

    I guess cost/efficiency would be considerations for any culture whose resources aren’t unlimited, so trying to analyze interstellar communication from that perspective may be as close to a cross-cultural point of view as we can get. I agree that we probably can’t guess ETI’s ultimate motivations.

  • david payton March 29, 2010, 22:36

    It occurs to me that ETI doesn’t have to be different from us to keep them from telling us how to enter into the galactic network… Has it occured to us to send information telling a budding civilization how to communicate with us? Have we sent information out that tells another civilization how to find us? Say a picture of the galaxy from our perspective? I don’t know but I think we have only deliberately sent messages two times… One of those times was mostly nonsensical memorabilia. This is a problem… If we have decided to send messages (which apparently we haven’t) then we need to start now with whatever tech is handy… And as the tech gets better so does the signal we produce… I figure we probably need to start sending a signal so that our ETI friends will know that we want to talk.

  • kaonashi March 30, 2010, 1:31

    So many people are still trying to find ETI in the radio spectrum. I feel that if ETI is supposed to be more advanced than we are, then perhaps ETI has advanced beyond using radio, and is communicating via some other method. That could be the “entrance fee” also, figuring out more advanced communication techniques.
    Think of it this way: People in George Washington’s time would have had no way of accessing radio-based communications, just as we currently have no way of accessing ETI communications.

  • Eniac March 30, 2010, 11:46

    Maybe we are like native Americans in the 1400’s standing at the beach, looking out across the Atlantic, wondering if there is someone out there, despite the persistent absence of smoke signals.

  • Eniac March 30, 2010, 12:38

    Of course, those pencil beams do not have to be continuously on. For reasons given by Krekorian as cited in the article, periodic blips are more efficient than a continuous signal.

    Still, a rotating beacon strikes me as primitive and inefficient.

    It seems to me, if you are looking for the very most efficient way to transmit, you would have that powerful 3-d phased array programmed to send a regular series of blips optimally focused in the direction of each of a target list of millions or billions of the closest stars.

    Blips towards a given star could simply be periodic. However, it would probably be better to chose a more complex predictable sequence, in order to eliminate the possibility of a natural phenomenon. Periodic, perhaps, to aid initial detection, but with additional blips thrown in at precise times in between. Those precise times could correspond to numerical values that would require knowledge of math or physics to generate. Golden ratio, prime numbers, pi, fine structure constant, etc. etc. Once the artificial nature of the signal has been proven this way, there would be sufficient motivation for the listener to build a bigger receiver, reduce the SNR, and decipher higher band-with information modulated onto the blips themselves.

  • andy March 30, 2010, 16:28

    If you want a beacon to have a high chance of being seen, it’s probably a good idea to make it appear similar to some class of astrophysically-interesting object (but sufficiently distinct that it can be recognised as a beacon). This means that civilisations doing astronomical surveys are more likely to spot it. If you then want to have some kind of message with high bandwidth, you then have to mimic some object that requires rapid measurement to detect.

    I’m guessing the ideal beacon will somewhat resemble a millisecond pulsar.

  • JohnHunt April 1, 2010, 1:40

    Eniac essentially states my idea. Around the sun, orbit solar powered beacons which target the star systems in the galaxy and not the space in between. With self-reproducing factories on asteroids, the number of beacons you could produce and hence the frequency at which you could signal could be in the billions. If you could produce 200 billion beacons then you could signal each star in the Milky Way galaxy however fast the beacons could charge before signaling.

    An alternate approach would be to beam very small nanocraft at some fraction of the speed of light towards a type of star which is noteworthy and can be seen across the galaxy. Decelerate the craft, have them land on a large asteroid and produce small sunshades in the billions. Have those shades tack themselves into orbit around their stars and then have them synchronously blink the star in an obviously artificial manner.

    My point is this. With advanced intelligence, it is entirely possible to make one’s self known to any civilization in our galaxy who is at a technical level as ours. In my mind, this eliminates the explanations that they are out there but are, for some reason, unable to signal us. So either they aren’t out there or they are but they are choosing to not contact us.

  • Tobias Holbrook April 1, 2010, 6:58

    //Unless galactic civilization has some form of monopoly on faster-than-light travel (I’m thinking a wormhole network would be most realistic), it would probably be safer to not make any kind of beacon.//
    Why? Scared about alien invaders? Remember, there ain’t no stealth in space – we’d detect their starship coming from quite a few light years off. Enough time to build our defences… relatvistic missles, unless they accelerate up to speed in system, aren’t much of a threat; targeting issues.

    Unless they have FtL. At which point they’re already here, because they’d be advanced enough to send out probes to every promising star without listening for radio beacons.

  • Duncan Ivry April 2, 2010, 18:16

    About “Hide your communications”

    If there are already several communicating civilizations out there (with, say, millions or billions of people each), then it would be difficult for them to hide the information of how to communicate.

    In a former period of my life I have been a professional information security person, and one thing those persons learn is: You can’t keep an information secret, if too much people know it, or at least know about the existence of the information. It is an open secret (pun intended) in the information security business, that the greatest risk originates from the insiders — but not because they are eval, but because they are a little bit stupid or because they are blessed with good intentions (e.g. “we are all interstellar brothers and sisters”).

    We should not assume, that members of advanced alien civilizations are not able to make “advanced” mistakes.

  • Daniel April 3, 2010, 2:01

    “… then it would be difficult for them to hide the information of how to communicate … In a former period of my life I have been a professional information security person, and one thing those persons learn is: You can’t keep an information secret, if too much people know it, or at least know about the existence of the information. “

    I don’t agree with this.

    While I wasn’t involved in professional information security, I was involved in electronic warfare in the military. The easiest way to hide yourself is and still go about your business is to maintain radio silence, and communicate via other non-electromagnetic means (whether it be telegrams by foot/vehicle, or a commander’s ‘mission intent’).

    I think applying concepts from information security to SETI would be shortsighted. Sure, there would be useful concepts, but in my mind the world of Signals Intelligence and Electronic Warfare is a much better analogue as we don’t know if other civilizations out there (if there are any out there) are hostile, involved in intergalactic war, or maintaining radio silence due to the aforementioned hostilities/wars (and given evolution is probably universal, competition between species is probably going to produce at least one highly predatory intelligent alien civilization).

    Perhaps they don’t want to be found (and why that is, is a much more interesting question).

  • Duncan Ivry April 3, 2010, 14:33

    Daniel: “The easiest way to hide yourself is and still go about your business is to maintain radio silence …”

    You skipped one important part: “if too much people know it, or at least know about the existence of the information”, then keeping something secret by hiding is not efficient. This is professional knowledge supported by a lot of experience.

    (With military people you will basically have the same problem — okay, not to the same amount, because these people are usually more disciplined. But in the military there are stupid people — oh, yeah, there are — and people with so called good intensions as well. Take enough of them knowing some secret, and that’s it.)

    But we are talking about the case of alien civilizations, which do just *not* maintain radio (or what else) silence, but keep on talking inside a group of privileged civilizations, trying to keep the know-how secret.

    I insist: Take several alien civilizations with millions or billions of individuals and with high-tech all around, then you will find some groups inside those civilizations who will give secrets away to others.

    Even a military dictatorship, which tries to embrace the whole planet, will not be able to keep the relevant information secret for hundreds or thousands of years. Why do I say “even”? A military dictatorship usually keeps just the most incompetent people inside their ranks.

    By the way, something like interstellar or even intergalactic war is … er … a little bit over the top, I think.

  • Eniac April 5, 2010, 22:41

    By the way, something like interstellar or even intergalactic war is … er … a little bit over the top, I think.

    I agree. Furthermore, it is even questionable whether there can be such a thing as “interstellar civilization”, given the immense time lags involved. What decisions are there that can wait for a 10 year round-trip for approval? Which emissary would go on a 30 year round trip and remain loyal to an authority they have no contact with? And that would only be the most local level of interstellar administration.

    Interstellar distances and the light speed limit pretty much preclude any sort of meaningful organization, and with no organization, all you get is individual civilizations, each sitting on their own star system. They might have common origins, but no interactions to speak of, let alone wars.