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SETI and Detectability

by James Benford

We recently looked at a paper by Duncan Forgan and Robert Nichol on the question of detecting extraneous emissions from an extraterrestrial civilization using technology like the Square Kilometer Array. James Benford (Microwave Sciences) has some thoughts on the issue growing out of his own work with brother Gregory on interstellar beacons and SETI reception in general. No one has put the question of interstellar beacons to tighter scrutiny than the Benfords, with particular regard to bringing the SETI discussion, as Jim puts it, “onto a quantitative basis, as opposed to rampant speculation, as is typical of the playing-tennis-without-a-net approach taken previously.” The Benfords’ work on interstellar beacons appears this month in Astrobiology. I give full citations at the end of this post.

The Forgan & Nichol paper on detection of leakage radiation does neglect our continuing use of microwave beams not only for radar, but also for likely future beaming of power for space purposes, such as power transfer. This has been noted by several others in their comments to this website.

Understanding Leakage Radiation

I feel that leakage radiation should include both what we have generated, such as TV and radar, but should also include future likely radiation for power beaming.

Examples of applications that have been studied are transferring energy from Earth-to-space, space-to-Earth, and space-to-space using high power microwave beams. Such beams have been quantified for spacecraft launch to orbit, orbit raising to GEO, launch from orbit into interplanetary and interstellar space, and deployment of large space structures (see “Space Applications of High Power Microwaves”, James Benford, IEEE Trans. on Plasma Sci., 36, pg. 569, 2008). For example, even a single microwave beam powered launcher of Kevin Parkin would produce an increase of the effective isotropic radiated power (EIRP) of Earth by five orders of magnitude beyond that of Arecibo, our present highest EIRP radar. I feel that in the future we will be radiating in the microwave in these ways, not just as leakage from communication channels.

Strategies for Detection

Should we observe such activities by ETI, the signals would appear to us, Thomas Hair points out, as transient events. But such quick, powerful bursts would be verifiable by a ‘staring’ strategy, with smaller dishes looking continuously at the skies, most profitably at the galactic plane. Once such a burst appears, watching time can be focused on such possible sites, perhaps with some dishes linked so their effect could be coherent, raising the detection capacity of the network.

Project Argus, led by the SETI League, is the right direction to go. They’ll need short integration times to spot such transients. I would expect the transients would recur as ETI’s launches, transfers and such repeat. But even a frequent launcher aims at different parts of the sky, as planets rotate. So, we must be patient.

Distance, Receptivity and Cost

I also found that the distances Forgan & Nichol give, which are taken from the Leob & Zaldarriaga paper, although portrayed as discouraging for detecting leakage from ETI, are in fact overly optimistic for conventional TV and radar. Their work assumes that receivers are going to be integrating over hours-days-months. That doesn’t work when the TV station you are observing is transmitting in your direction only for a time typically ~hour, before it disappears around the limb of the Earth, as shown by Sullivan in his seminal paper. There is also a confusion in both papers over the distinction between transmitter power and EIRP. Forgan & Nichol misunderstand the EIRP quantity, saying on their page 1 that radars produce ‘isotropic radiation with billons of watts’. Although EIRP, the product of radiated power and the antenna gain, is that high, the radiated power is of order megawatts, since the gain is ~1000. The problem of observing leakage due to TV stations is identical to that of detecting a single strong station, as shown by Sullivan, so their estimates of detectability of leakage in these papers are systematically high.

One important implication of this is that the case made by advocates of sending messages to the stars (METI), who argue the ‘we’ve already announced ourselves’ is wrong. We have not been noticed at interstellar range by societies of our order of magnitude of technical and economic capability. One might argue that aliens will be far richer, so will have vastly larger antennas. But if so, why are they not radiating at oxygen-obvious planets like ours?

Michael Simmons, who suggests a phased array radiating rapidly cycling, short, high-powered transmissions directed at millions of nearby stars, might look at my cost equations to estimate what the expense will be (see “Messaging with Cost Optimized Interstellar Beacons,” referenced below). Hint: they won’t be cheap.

To follow the latest Benford thoughts on SETI matters, see James Benford, Gregory Benford and Dominic Benford, “Messaging with Cost-Optimized Interstellar Beacons,” Astrobiology Vol. 10, No. 5, pp. 475-490 (abstract / preprint), and the same authors’ “Searching for Cost-Optimized Interstellar Beacons,” Astrobiology Vol. 10, No. 5, pp. 491-498 (abstract / preprint).


Comments on this entry are closed.

  • Thomas Hair July 13, 2010, 14:00

    The Benfords’ articles on galactic scale beacons are spot on and while ten billion is alot of money, they show that it is nonetheless do-able. As for the Forgan and Nichol article it should be troubling for everyone in the field that the International Journal of Astrobiology plans on publishing something based upon the demonstrably false assumption that we are going or will go radio quiet in the near future…By the way we won’t even be going radio quiet in the far future unless we plan on running telephone lines all the way to Alpha Centauri.

  • NS July 13, 2010, 18:11

    Gerry Harp of the SETI Institute believes that to verify a signal is a result of ETI would require days or weeks of continuous reception:


    It’s not clear at least to me that any signal other than a beacon would meet that requirement. My guess is that as our detection methods improve (and if indeed there are any ETIs) we will start observing more and more events that meet all the criteria for a genuine ETI signal — except repeatability. Maybe we already have (e.g. the ‘Wow’ signal). I certainly hope that Thomas Hair and James Benford are right that some routine ETI activity will give us that repeatability, or that an ETI will find it worthwhile to build a beacon.

  • Michael Simmons July 13, 2010, 20:57

    The thing to know is that at present their maybe several high energy transients occurring monthly and nobody is likely to see them.

    You might find this interesting.

    Read the report “Argus: An L-Band All-Sky Astronomical Surveillance System” in the technical section and the show and tell page.

    The report states “For a point of reference,
    the Green Bank Telescope (GBT) has a mechanically steered
    7854 aperture and in L-Band [23].
    Argus would require about eight million elements to achieve the
    same sensitivity on a gain-over-temperature basis.”

    A space based array (see ESA SMOS mission) doesn’t have to compete with thermal noise from the ground around it and hence requires much less elements.

    I’m imagining a large sphere in space which has its surface covered with millions of little wide band spiral antennas, monitoring the whole sky and being able to capture in wide bandwidth sub millisecond transients.

  • Thomas Hair July 13, 2010, 21:36

    I am not sure what “verify” means in this context. It could mean very different things to different people just like the legal concept of reasonable doubt. So, once again I am skeptical, this time of Gerry Harp’s claim…a signal may be obvious upon receipt or it may take considerably longer, but to state numbers with such certitude in an endeavor that is so reliant on statistical analysis seems questionable at best.

  • Adam July 14, 2010, 6:13

    Hi All

    Power-beaming, for domestic and propulsive purposes, does seem like a potentially huge “signal” but wouldn’t such look an awful lot like a high energy natural transient source? How would we ever verify its artificiality?

    James B. reiterates a good point about radio & TV leakage – unless ETs have reason to be using immense antenna to stare at tiny patches of the sky, then they’re not going to detect, let alone pick up anything intelligible, from our terrestrial communications. Bandwidth is too wide and the power is insufficiently focussed for a typical signal to escape the Solar System, let alone reach other stars, before it’s indistinguishable from noise. Deliberately sending off high power messages is unlikely to find a listener at the other end, but such a signal would be the first sign we existed.

  • Eniac July 14, 2010, 14:26


    A space based array (see ESA SMOS mission) doesn’t have to compete with thermal noise from the ground around it and hence requires much less elements.

    I’m imagining a large sphere in space which has its surface covered with millions of little wide band spiral antennas, monitoring the whole sky and being able to capture in wide bandwidth sub millisecond transients.

    Exactly. Even better, a large “cloud” of receivers flying in formation. The individual receivers would be solar powered, mass-produced and cheap, and there would be no structure or central energy source to add to the cost.

    These could just as easily be transmitters, and when not busy beaming power to places, such a transmitter array can be used to “cycle” through a large number of promising targets, the most efficient way to operate a beacon. If the cycling is such that the intervals between pulses to each target form a mathematical pattern, the signal will be unmistakably artificial.

    If the purpose of the array is normally to beam power, using it for METI during times of low demand would be practically free.

  • Michael Simmons July 14, 2010, 20:23

    I just feel its important to consider that beacons might send a single 200ms (say) spread spectrum pulse with prime number spaced frequency components sent once every several years. Only a all sky monitoring system would be able to detect it.
    Upon reception of the same signal it would transmit a much stronger and longer directed signal with content.

    I think once a single seti signal was found beyond reasonable doubt the money ( billions, look at how much the LHC cost) would suddenly appear.
    Any other races out their would probably react the same.
    Lets say the chance of detecting a signal without extra special effort is 1 in a million per year. In couple of million years we/they should found one and built this huge antenna array.

  • Pat Galea July 15, 2010, 3:00

    Thomas: “unless we plan on running telephone lines all the way to Alpha Centauri.”

    Now there’s an idea I hadn’t considered for the Icarus comms system! :-)

  • James Benford July 18, 2010, 21:13

    Thanks to Michael Simmons for pointing out the Argus All-sky radio telescope, a really interesting instrument. However, that’s not what I was meaning when I wrote ‘Project Argus is the right direction to go’. I meant the project to build amateur telescopes of 3 to 5 m diameter to observe larger patches of the sky. The concept is for as many as 5000 such systems, of which 121 now exist. It would take ~1000 antennas to have the same effective area as Green Bank. I don’t know why the names are almost alike.

    Adam asks if a natural transient can be distinguished from a beacon. That’s a good question. The basic answer is that it depends on what it looks like. The most likely object to cause one to wonder is a pulsar; I address that in the paper How can we distinguish transient pulsars from SETI beacons? (http://arxiv.org/abs/1003.5938). I think the easiest to make a beacon distinguishable in an unmistakable way is to modulate its amplitude, frequency or phase in a non-natural way. For example, Eniac suggests modulating the revisit time, Michael Simmons a prime number frequency modulation in spread spectrum.

    As several remarks make clear, there is a synergism between power beaming, such as space solar power, METI beacons and radio telescopes. A technology with several applications becomes robust, as there are economies of scale. So I expect the future will have considerable activity in high power microwave technology.

  • ljk July 21, 2010, 0:01

    Finding frugal aliens

    ‘Benford beacons’ concept could refocus search for signs of intelligent extraterrestrial life

    Steve Zylius / University Communications

    Astrophysicist Gregory Benford — standing before the UCI Observatory — believes an alien civilization would transmit “cost-optimized” signals rather than the kind sought for decades by the SETI Institute.

    For 50 years, humans have scanned the skies with radio telescopes for distant electronic signals indicating the existence of intelligent alien life. The search — centered at the SETI Institute in Mountain View, Calif. — has tapped into our collective fascination with the concept that we may not be alone in the universe.

    But the effort has so far proved fruitless, and the scientific community driving the SETI project has begun questioning its methodology, which entails listening to specific nearby stars for unusual blips or bleeps. Is there a better approach?

    UC Irvine astrophysicist Gregory Benford and his twin, James — a fellow physicist specializing in high-powered microwave technology – believe there is, and their ideas are garnering attention.

    In two studies appearing in the June issue of the journal Astrobiology, the Benford brothers, along with James’ son Dominic, a NASA scientist, examine the perspective of a civilization sending signals into space – or, as Gregory Benford puts it, “the point of view of the guys paying the bill.”

    “Our grandfather used to say, ‘Talk is cheap, but whiskey costs money,’” the physics professor says. “Whatever the life form, evolution selects for economy of resources. Broadcasting is expensive, and transmitting signals across light-years would require considerable

    Full article here:


  • ljk October 10, 2010, 10:11

    Q&A: Refining the search for extraterrestrial life

    A UC Irvine astrophysicist discusses what the SETI Institute has done wrong and how we might enhance our chances of receiving a signal from ‘out there.’

    Full interview here:


  • ljk December 4, 2011, 2:07

    Let’s build a beacon to tell aliens who we were

    12:55 02 December 2011

    by Chris Wilson

    Somewhere in the cosmos, 36 light years away from us in the direction of the Hercules constellation, a series of electromagnetic waves stretching across 30 million miles of space carries a message from Earth. Each of the 1,679 signals it contains falls in one of two frequencies – an FM signal that translates to a bunch of ones and zeroes.

    This message originated in 1974, when it was broadcast from the Arecibo radio telescope in Puerto Rico to commemorate the facility’s renovation. The authors of the message, Carl Sagan and SETI founder Frank Drake, hoped that any aliens who happened to receive it might notice that 1,679 is the product of two prime numbers, 23 and 73, and if you arrange all the zeroes and ones in a grid of 23 columns and 73 rows, you get a series of simple, ASCII-like pictures, including a double helix and a crude image of a person. Whether or not an alien civilization could crack the code, they would at least notice something funny about these FM signals. They’re 10 million times stronger than the background noise from our sun.

    But no extraterrestrials will even get the chance. The Arecibo transmission was aimed at a system 25,000 light years away that will have long since orbited out of the signal’s path by the time it arrives in the vicinity. The odds of any two civilizations ever overlapping in time are extremely small. Even if we left the Arecibo telescope squealing out its signal until its power ran out and its hardware rusted, there’s virtually no chance that the emanations would get anywhere in particular, and hang around long enough to be seen or heard. The only way we’ll make contact is if we can make a beacon that keeps going for millions or billions of years after we’re gone.

    The practice of sending signals to prospective alien neighbors is called active SETI or METI (Messaging Extraterrestrial Intelligence). In 1999, and again in 2003, a Russian scientist named Alexander Zaitsev took it upon himself to send powerful signals to nearby star systems using a high-power radio transmitter in Ukraine. This upset a lot of his colleagues, who thought it was a bad idea to announce ourselves to the cosmos – more than we already do with leaked radio communications – when we have no idea what an extraterrestrial civilization’s intentions might be. Stephen Hawking has warned that we could face a Columbus-coming-to-America situation, with Earth playing the part of the Native Americans. Michio Kaku pointed out that Cortes conquered the entire Aztec empire in less than two years, with technology that was only perhaps 1,000 years more advanced. If a bunch of aliens showed up here, the tech disparity might be many times greater in magnitude.

    I’ll accept full responsibility if the Earth is wiped out by aliens, but I don’t think Hawking or Kaku have much to fear from the Russian. His broadcasts are sporadic, and the odds that anyone would be in exactly the right place to detect them are functionally zero. Not until someone develops a signal that can be repeated for eons will there be any feasible threat from a slime-thirsty race of space goblins.

    Full article here:


    To quote:

    “The most fruitful work takes place on the blog Centauri Dreams, which is affiliated with a foundation that promotes interstellar travel.

    “The charge is led by a pair of twin brothers, physicists James and Gregory Benford (the latter is also a science fiction author), who approach the field of alien-directed beacon-design first from an economic approach. In a paper in the journal Astrobiology, they lay out how one might build such a device without spending the whole GDP. No technology available in the near-term will allow us to deliver powerful signals every minute of the day over a span of multiple epochs, they argue. But we might be able to make a beacon that works more efficiently, by targeting only those star systems where life seems most likely, and then pinging them each in turn, repeating the cycle every few months or so. Presumably, if a curious civilization caught one transmission, it would train its telescopes on that exact spot until the next part of the beacon’s message arrived. This more sensible approach – a sort of Energy Star specification for SETI – would save enough power to keep the beacon running for millions of years.”