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METI: Learning from Efficient Beacons

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.

Comments on this entry are closed.

  • tacitus October 30, 2008, 14:47

    10 billion dollars? Wow, that’s cheap — dirt cheap to any technological society that has overcome the sorts of problems we’re still dealing with in our daily existence.

  • ljk October 30, 2008, 17:22

    Ten billion dollars may sound cheap to anyone familiar with
    budgets involving space science, but you that to the average
    person – especially in this current economy – spending that
    kind of money to signal aliens will not go over well, to put it
    mildly. Even telling them that their tax dollars no longer go
    to funding SETI won’t help here with that kind of a number.

    Any rich billionnaires out there willing to fund a project that
    might bring humanity into Galactic Enlightenment, to say
    nothing of immortalizing him or her forever? S. R. Hadden
    thought SETI was a good investment.

    When we do build a METI beacon some day (and I know it
    will happen, be it by a government or some private group),
    I hope they start planning in advance for a reply.

  • Adam October 30, 2008, 19:08

    A day’s flutter on the DAX isn’t it?

  • ljk October 30, 2008, 23:58

    Wait a minute about that globular cluster comment in the image
    credits: Aren’t most globular clusters very old, which means
    they have had billions of years to develop planets and life –
    assuming you always need a planet to make life.

    Even if GCs have too many stars too close to each other to
    make and keep stable planetary systems, who is to say that
    advanced ETI aren’t there utilizing the resources of many close
    stars. Or that they moved them together there themselves?

    Besides, in just 50,000 years, we may know if anyone is living
    (or lived) in Messier 13.

    See this article on the subject:


  • Paul Titze October 31, 2008, 0:50

    Spending $10 billion dollars transmitting to ETIs which might not be in our neighborhood (hence wait possibly at least 1000 years for a reply) or even if there are ETIs in our neighborhood they might not be listening on our beacon microwave frequencies is a big ask… much better continue with SETI and spend the rest of the money on breakthrough propulsion physics research and go have a look yourself if there are any ETIs in our neighbohood. If you meet a neighbor at least you can have a decent conversation over coffee on their planet ;-) (instead of sending messages back and forth every 1000 years).

    Cheers, Paul.

  • Ronald October 31, 2008, 3:53

    I think it would make a lot more sense then to spend that sort of money on large space-based interferometers to directly image and spectro-analyze earthlike planets and their atmospheres. One step at a time and first things first.
    As David Lewis mentioned yesterday: a living planet with a biosphere is one big beacon.

    Anyway, in comparison with the 800 billion US $ that the US spends on defense per year (or the current financial bail-out), almost any space science project is dirt cheap.

  • Administrator October 31, 2008, 8:23

    Larry writes:

    Wait a minute about that globular cluster comment in the image
    credits: Aren’t most globular clusters very old, which means
    they have had billions of years to develop planets and life –
    assuming you always need a planet to make life.

    Larry, the cluster in the photo is young, embedded in a star-forming nebula called NGC 3603. That’s why I made the comment, comparing areas where most stars are younger with those we would target farther in toward the galactic core.

  • andy October 31, 2008, 10:38

    Wait a minute about that globular cluster comment in the image

    It says “cluster”, not “globular cluster”… there are other types of star clusters, e.g. open clusters.

    As for globular clusters being good places to look, there are various issues including extremely low metallicites (not much material to build terrestrial planets out of), and the very high stellar densities would tend to disrupt planetary systems. Not a very good place to put a Matrioshka brain, even if you believe anyone would build such a structure: there’s a high risk of a stellar encounter causing a star to come crashing through the Dyson shell. I don’t really have any idea what the views of such a Matrioshka brain entity would be, but at a guess, I would think this would be a somewhat undesirable eventuality.

    (I can’t get at the aeiveos link, it seems to be nonexistent)

  • ljk October 31, 2008, 15:38

    Here, this link just worked for me.

    Globular Clusters and Astroengineering:


    Maybe globular clusters are too busy and too metal-poor to be
    of much use for an interstellar civilization. But now those open
    clusters you mention….

  • Ronald November 1, 2008, 11:55

    What andy mentioned about the unsuitability of globular clusters is probably also true for most galactic halo stars, dwarf galaxies and elliptic galaxies: old metal-poor stars.

    But no need to despair: most (about 2/3 of all) galaxies are spiral, of which about half are bar-spiral subtype, like our own. We are actually quite common.

  • andy November 2, 2008, 11:43

    I wouldn’t be so quick to write off elliptical galaxies. The “old and metal-poor” thing does not seem quite so clear-cut, particularly for the giant ellipticals. From what I can tell, the metallicity should not be a showstopper in many giant ellipticals. Dwarf galaxies are a different matter, these do seem to be very metal-poor.

  • James Benford November 10, 2008, 11:54

    tacitus: A galactic-scale Beacon for 10 $B is a stretch for our level of civilization, but perhaps not for advanced ETI, hence would be for them ‘plausible luxuries’.

    Paul Titze: We’re not arguing for METI Beacons, just quantifying them to keep the discussion realistic. You mention that they, like we, might ‘not be listening on our beacon microwave frequencies’. That was true of us in the early SETI days, but is now ending. The advance of electronics (as in the Allen Array) and the observation of transients (see the Lorimer & Hyman references in our SETI paper) are causing searches for transients across much wider frequencies in the microwave. Both the Allen Array in California and Parkes in Australia Down Under can now see transients, but only Allen has the larger bandwidth. (I’m preparing to propose to get time on Parkes to look for transients at certain locations in the southern sky.) // As for probes, the speed of light is the fastest way to contact ETI that we know of, so probes are the slow boat. And if advanced ETI exists at levels higher than ours, they could come calling in your ‘breakthrough propulsion physics’ ships. But they haven’t, as far as we can tell.

    Ronald; Looking for biospheres is going to happen with the next couple of generations of space-based telescopes. A biosphere is noticeable, but that’s not civilizations, which we’re talking about, but life, which is a start.

  • ljk May 17, 2009, 11:43


    Scientists to Gather Messages to the Cosmos for ‘Earth Speaks’ Project


    Date Released: Thursday, May 14, 2009

    Source: SETI Institute

    For nearly fifty years, the Search for Extraterrestrial Intelligence (SETI) has used radio telescopes to scan the heavens for signs of alien technology. But scientists still do not agree about whether we should reply to an extraterrestrial signal, and if we do, what we should say.

    To help answer these questions, on May 15, 2009, Dr. Douglas Vakoch, Director of Interstellar Message Composition at the SETI Institute, will launch Earth Speaks, a research project to collect messages online from people around the world.

    “Earth Speaks invites people to ponder the question, ‘What would you say to an extraterrestrial civilization?'” said Thomas Pierson, Chief Executive Officer of the SETI Institute in Mountain View, California. “By submitting text messages, pictures, and sounds from across the globe, people from all walks of life will contribute to a dialogue about what humanity might say to intelligent beings on other worlds.” he explained.

    The occasion for the launch is the visit to the SETI Institute by Kamau Hamilton, a sixth-grade student from the Central Harlem Montessori School, whose winning idea for the Kids Science Challenge (kidsciencechallenge.com) inspired the SETI Institute to invite other children to submit their own “Sounds of Earth.” (Hamilton’s sounds include the jangling of bracelets and the squeak of sneakers over a hardwood floor). Kamau will be a special guest at the Institute’s annual open house, Celebrating Science, on May 16th, 2009 from 2-4 pm. The public is welcome.

    Now all people can submit their messages to Earth Speaks at http://messages.seti.org, where they also enter labels or “tags” to help researchers categorize the messages. “By studying the tags used by many different people, we can capture the major themes that run through thousands of individual messages,” said Vakoch “That sets the stage for creating interstellar messages that begin to portray the breadth and depth of the human experience.”

    “Earth Speaks uses technology of the twenty-first century to understand human aspirations around the globe in ways not possible before,” said Dr. Jill Tarter, Director of SETI Research at the SETI Institute. “This is a potentially a huge resource that can be explored to look for cultural universals.” Questions about message content take on increased importance as the SETI Institute’s Allen Telescope Array (ATA) begins a new phase of research with its galactic plane survey for radio signals from other civilizations. As search technology becomes more powerful, the chances of detecting distant civilizations beyond Earth increase.

    The SETI Institute currently has no plans to transmit messages into space. “The question of whether we should send intentional messages to other civilizations is too important to be answered hastily,” said Pierson. “Through Earth Speaks, the SETI Institute also hopes to foster an open and thoughtful debate about the pros and cons of sending messages to other worlds.”

    Although radio and televisions signals are leaking from Earth into space continually, such signals are weak in comparison to focused transmissions aimed at specific stars. Thus, any future intentional messages could provide extraterrestrial civilizations with their first direct evidence of life on Earth.

    “First impressions matter,” said Vakoch. “The initial messages we send to an extraterrestrial civilization could set the tone for a conversation lasting hundreds or thousands of years.”