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Is Our Civilization Detectable?

I haven’t even finished the first line of this post and I’m already in a digressive mood. The mental sidetrack comes from yesterday’s talk about the Square Kilometer Array, whose primary installations are now to be built in both South Africa and Australia. By observing an object through many instruments simultaneously, astronomers can use the technique called interferometry to combine incoming data and emulate a much larger instrument. The SKA’s sensitivity promises to be high enough to allow the detection of possible leakage radiation from another civilization, which prompted me to recall a quote I had buried in my archives:

“I know perfectly well that at this moment the whole universe is listening to us — and that every word we say echoes to the remotest star.”

The words are those of Jean Giraudoux, a French writer and diplomat whose plays, written between the two world wars, gained him an international audience (Christopher Fry was among the admirers who adapted Giraudoux’s work into English). Here the play is The Madwoman of Chaillot, a dizzying fantasy about corrupt business people who are trying to dig up Paris to retrieve its supposed oil reserves, and Countess Aurelia, the eccentric who with the help of an oddball ensemble of misfits puts these ‘wreckers of the world’s joy’ on trial. Giraudoux wrote the play in 1943 during the German occupation and died before he could see it performed.

Among the many plays, novels and essays the prolific Giraudoux wrote, this one eventually caught the eye of Carl Sagan, who used it to introduce a chapter about radio SETI in Intelligent Life in the Universe, the 1966 title he produced in collaboration with the Russian astronomer I.S. Shklovskii. I always enjoy paging through this book to see how our ideas of astrobiology and SETI have changed over the years. The SKA story invariably reminded me of Sagan’s concern over our own leakage radiation to the stars — he imagined the radio voice of Enrico Caruso traveling outward forever at the speed of light:

By now, the signal has propagated some 40 light years into space. If there is an advanced technical civilization within 20 light years of the Sun, they may have received that signal 20 years ago, correctly interpreted it as the result of another technical civilization, and immediately beamed their response to us. We should receive that signal any day now. But if the nearest technical civilization is many hundreds of light years away, we will have to wait a little longer.

And I have to add this entertaining riff, evidence that Sagan occasionally needed to lighten up:

…the characteristic signs of life on Earth which may be detectable over interstellar distances include the baleful contents of many American television programs and the mindless outpourings of rock-and-roll stations. It is a sobering thought indeed that the Beverly Hillbillies may be our only interstellar emissaries.

I’m not sure who it was who added Chuck Berry to the Voyager Golden Record, but whoever it was, good for him/her.

Whispers in the Night

People sometimes assume that stray signals would be easily snared at interstellar distances, but we’re learning that it would take a mammoth installation to make such a catch. The film Contact, made from Sagan’s novel of the same name, uses the wonderful device of a broadcast returned to us, a transmission from the 1936 Olympics in Berlin. Receiving such a signal parroted back to us would surely flag the detection of an extraterrestrial civilization and cause researchers to begin the necessary work to look for embedded information inside it.

The people behind the Square Kilometer Array talk about the ability of this instrument, once its vast telescopic resources are in place and connected to powerful computing facilities, to pick up something as weak as the extraterrestrial equivalent of an airport radar around another star. It’s a fantastic prospect, implying our ability to add a new layer to our existing SETI investigations. Is it possible that instead of scanning the skies for beacons, we might simply begin to pick up the extraneous signals of a civilization going about its daily life? The goal is energizing, but hearing claims about extraterrestrial detections always makes me uneasy.

Image: Signals from our civilization are gradually working their way into the galaxy. But would a conventional radio telescope be able to detect them at our present level of technology? Would the SKA?

Back in late 2010, James Benford discussed leakage radiation at a meeting of the Royal Society in Britain, asking whether the kind of installations we currently have on Earth could detect signals this weak if sent from a nearby star. It turns out a typical radio telescope like the Parkes instrument in Australia, if located near Alpha Centauri, would not be able to detect our TV transmissions at all. Benford pointed out that signal information is transmitted in bands on each side of the central frequency and that broadcast antennae aim their transmitted power mostly toward the surface. Signals that get into space are not coherent and are unlikely to be noted.

Sizing up the SKA

We’ll learn much more about the Square Kilometer Array as its various components come online in the late years of this decade and beyond, but the paper presenting Benford’s analysis, written with John Billingham (SETI Institute), indicates that talk about picking up airport radars and other leakage radiation may be overly optimistic. From the Benford/Billingham paper:

The assertion of Loeb and Zaldarriaga (2006) that SKA can see leakage radiation at 100 pc (316 ly) is based on the assumption that the sources are continuous, so long integration times make the leakage detectable. However, this is not true of Earth leakage. Integrating over days to months doesn’t work when the TV station you’re observing is transmitting in your direction for a time typically ~hour, before it disappears around the limb of the Earth, as stated by Sullivan. Forgan and Nichol (2010) show that, even if Loeb and Zaldarriaga were right, the probability of detection is very low.

The SKA may be too small for the task of picking up leakage radiation after all. Back in 2010, Seth Shostak (SETI Institute) wrote an essay about the matter in the Huffington Post:

Evidence of our existence has already washed over about 15,000 star systems, as the FM, television, and radar signals that were first transmitted during the late 1930s wick into space.

That isn’t news to many, of course, but maybe this is: These signals are not hard to find. If there are any aliens within a few hundred light-years, these clues to our existence could be found with an antenna the size of Chicago. For any society able to threaten us across such distances, that’s a pretty easy construction project.

Shostak was writing in the context of Stephen Hawking’s concerns about the dangers of extraterrestrial contact, and he’s doubtless right that a society advanced enough to cross the interstellar gulf to threaten the Earth would find building a Chicago-sized antenna feasible. But in our own terms (and what I’m getting at in this post is what we can detect), such an antenna would blow the budget for the entire century, being far larger than what the Square Kilometer Array can provide. Benford and Billingham went to work on the cost issues involved, assuming that Shostak was right that it would take an antenna this large:

Certainly, with ever-larger antenna area, at ever greater cost, advanced ETI can detect us. From the above analysis, we calculate that at 50-ly range, the antenna area must be ~1 km2. To assess Shostak’s claim, note that Chicago’s area is 24,800 km2 = 2.48 1010 m2. At the present value of SKA antennas, 2.4 k$/m2, the cost is 60 T$, comparable to Earth’s GNP of 70 T$. So if comparable to us, ETI would have to devote their entire science budget for a time perhaps of order a century to build Shostak’s antenna, a sobering prospect.

Sobering indeed. Obviously, a sufficiently advanced civilization would be capable of technologies and budgets that defy our analysis, but Benford and Billingham help us answer the question of whether or not our present technology — or that of the near future — would be capable of detecting our own transmissions from a nearby star. The answer is almost certainly no. Leakage radiation is going to be tough to detect even with country-spanning installations like the SKA. And we might answer Giraudoux’s Madwoman of Chaillot by saying that perhaps every word we say really is echoing to the remotest star, but the civilizations capable of hearing those words are going to be so far beyond our powers as to defy the imagination.

The Benford and Billingham paper is “Costs and Difficulties of Large-Scale ‘Messaging’, and the Need for International Debate on Potential Risks” (abstract) / full text.

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Comments on this entry are closed.

  • David H May 31, 2012, 10:27

    I’m not familiar with Seth Shostak’s claim about the capability of a ‘Chicago-sized’ antenna, however, the important question is whether Shostak was referring to collecting area or array extent. Building a telescope with a collecting area of 25 thousand square kilometres is out of the question, however, one that extends to the diameter of Chicago and greater but doesn’t fill the whole region is entirely anticipated. The former requirement relates to sensitivity, the greater the collecting area the greater the sensitivity. The latter relates to the ability of the telescope to resolve objects in the field of view that have been detected. The wider spread the array, the finer the resolution available. Also of interest is that the low-frequency SKA will employ antennas that will approach a cost two orders of magnitude less than the cost quoted here. Of course, cost per square metre should account for the infrastructure and running costs also. I do not know whether the quoted cost of k$2.4 (for dishes) does this.

  • James D. Stilwell May 31, 2012, 10:48

    How about an antenna built inside a 30 mile wide moon crater….
    next century….
    just wondering…
    JDS

  • Mike May 31, 2012, 13:08

    Yeah, there are very good reasons to build the SKA without having to over sell its SETI capabilities. Anyway having vastly improved sensitivities can’t be anything but a good thing regardless of purpose.
    But regarding SETI, consider the Wow! signal, still unclear what exactly it was. Leakage from a ET civilization won’t likely be “airport radars” but perhaps some totally unknown industrial process or massive engineering project and quite likely to be transitory or intermittent.
    So the wider the bandwidth, the higher the sensitivity and the quicker the signal proccessing the better. That’s good for astronomy and if we happen to stumble on an artificial radio source, oh Wow!

  • Paul May 31, 2012, 13:27

    I would be curious to know just how far away the author would place the closest intelligent aliens? Just as an informed guess.
    Obviously, no one knows. But based on notions like the Fermi Paradox, or lack of SETI results (which could be misleading), what does the writer privately imagine?
    20 light years? 50? Hundreds or thousands?
    I believe I read that Hawking (who is no specialist) doubts that anyone is within 600 light years. I can’t remember what Sagan thought.

  • Paul Gilster May 31, 2012, 14:48

    Paul writes:

    I would be curious to know just how far away the author would place the closest intelligent aliens? Just as an informed guess.

    I can’t place them anywhere, I’m afraid — I simply have no idea. If you’re asking purely for a hunch, then I’d go with my belief that intelligent life in the galaxy is out there but quite rare, and would guess the nearest functioning technical civilization to be halfway to galactic center. But again, like anyone I’m just guessing at this point.

  • FrankH May 31, 2012, 15:01

    I think another issue with detecting TV signals like ours, in addition to the one mentioned in the Benford & Billingham paper, is that not only is the Earth rotating and sweeping the signal across the sky, other stations are also sending out their signals, and they won’t be the same program. TV signal formats also vary widely across the Earth.

    From far enough away (much less than a LY) the Earth is effectively a point source of very weak radio noise. The Earth’s TV “signal”, if it can be extracted from this noise, will be a random mix of video data, interspersed with a random mix of sync pulses (which will not be synchronized with each other). In other words, slightly spiky noise. It may look artificial, but no real information (like a TV program or a single frame) is going to be available.

  • Kamal May 31, 2012, 15:04

    I am sure others have done much more subtle calculations of mean distance
    to radio-transmitting civilization, but turning the question on its
    head: ASSUME there are 10K civilizations out there, and they are
    “evenly” spread out across the galaxy: what would be the rough
    separation between them?
    The result is sobering.

    From a plan-view the area of the MW galaxy is 10^5^2 square LY.
    Dropping 100^2 civilizations on a grid puts each civilization
    at a distance of sqrt(2) * 1000 LY =~ 1400 LY.
    That’s w/o taking 3rd dimension into account.

    I think we just cant grok how **BIG** the galaxy is.

  • Greg May 31, 2012, 17:03

    Very thought provoking article, as usual. It goes with what I was saying yesterday.
    Sullivan’s statement ” Integrating over days to months doesn’t work when the TV station you’re observing is transmitting in your direction for a time typically ~hour, before it disappears around the limb of the Earth”
    made me wonder about something else, in reversing the problem of detecting civilizations by their leakage radiation, If a technically advanced civilization basically has come to the same conclusion as we have, that detecting a leakage signal is technically too difficult except for the most advanced civilizations, how do you intentionally let the Galaxy know you are there? Most would say a directional focused signal, yet if you don’t know where to point your signal and for length of time to be observed the probability of being detected is very low. What would be needed is an omnidirectional signal that would be strong enough and lasted long enough, ie. years, most likely located in orbit around the sun, allowing it to be detectable as an artificially produced signal. This would allow for a much simpler receiver antennae by the detecting civilization. The question is what technological scale is needed to be able to do this?

  • Alex Tolley May 31, 2012, 17:08

    What is the likelihood that the strongest artificial signals are ours, bouncing back from nearby interstellar objects, like rogue planets?

  • Christopher Phoenix May 31, 2012, 17:08

    No one really knows how common extraterrestrial life is, let alone intelligent extraterrestrial life. I imagine that lower life forms like bacteria are relatively common, higher forms of life like the alien equivalent of shellfish and dinosaurs are somewhat rarer, and intelligent technological civilizations separated by at least several hundred light years- but that is just a wild guess.

    We don’t yet know how common life bearing terrestrial planets are, or what other kinds of planets could support life. Furthermore, we don’t know how common intelligence arises as a “solution” to the challenge of survival. We only know of one such planet and one such civilization- us, and we can’t extrapolate from only one data point. All our guesses rely on assumptions that we can’t yet know are valid. For all we know, ETI could be buzzing around in our galactic neighborhood, or the nearest alien home planet may be a quarter of the way to the galactic center.

    If the aliens have star travel, they may migrate great distances from their plane of origin and take of residence in new places. I think this may drastically change the distribution of intelligent life in the universe. The Drake equation seems to assume that aliens will be limited to their plane of origin, but if aliens can evolve on one planet and later settle on many hundreds more, we may be much more likely to encounter them someday.

  • Astronist May 31, 2012, 17:38

    The nearest intelligent aliens: I would suggest there’s no need to guess. We have one firm fact to go on. Of all the possible civilisations detectable with our current instruments and observing procedures up to a maximum distance of L light-years, the nearest is greater than L away from us. As for how far greater than L: it’s okay to admit our ignorance! (Something which science sometimes finds hard to do.) So long as we make the leap to space colonisation, our descendants will surely learn much more. But the critical step is to expand our civilisation into space (and the successful completion of SpaceX’s Dragon flight to the ISS raises hopes considerably that we will continue to grow).

    Stephen
    Oxford, UK

  • ljk May 31, 2012, 17:56

    In his 1980 PBS Television series Cosmos and undoubtedly much earlier, Carl Sagan said that if there were 1 million technological civilizations in the Milky Way galaxy, the nearest one to Earth on average would be about 200 light years away.

    http://www.youtube.com/watch?v=0Ztl8CG3Sys

    Frank Drake had a much more pessimistic number, about ten thousand.

    After fifty years of mostly sporadic SETI and a few really random METI, about all we can say since Project Ozma kicked things off in 1960 is that the galactic neighborhood is much quieter on the cosmic radio band than we initially thought and hoped.

    There have been optical and infrared searches as well, but they are a more recent development and are far fewer in number than the radio searches.

    And despite 65 years of modern era UFO sightings (yes, the anniversary date is coming up soon), we have yet to capture even one of those slippery alien spacecraft or any of their crew. At least officially….

    Regarding that quote from Carl Sagan:

    “…the characteristic signs of life on Earth which may be detectable over interstellar distances include the baleful contents of many American television programs and the mindless outpourings of rock-and-roll stations. It is a sobering thought indeed that the Beverly Hillbillies may be our only interstellar emissaries.”

    Any good anthropologist will tell you that you really get a more accurate assessment about a society and culture by rummaging through their refuse piles rather than staring at their official monuments and similar declarations.

    Besides, as everyone knows, all advanced ETI only listen to Western classical music. Just look at Star Trek: With a few detours for jazz, everyone else listens to classical music and only watches Shakespeare plays – even the Klingons!

    However, if ETI are fans of Green Acres, that will prove they are a truly advanced species.

    Then Paul said:

    “I’m not sure who it was who added Chuck Berry to the Voyager Golden Record, but whoever it was, good for him/her.”

    I will have to look it up in my copy of Murmurs from Earth to be certain, but Timothy Ferris was in charge of collecting the music for the Voyager Interstellar Record. Though everyone had a hand and input into what kind of music they thought should represent humanity.

    In 2000, Ann Druyan was part of a lecture series on SETI at Harvard where she talked about the Voyager Record. Apparently their selection of Johnny B. Goode helped get Chuck Berry through a rough point in his life at the time.

    Though some Western classical music scholars sniffed that the Voyager Record team selections reminded them of the kind of stuff they play on NPR classical stations during pledge drives, I find it more than likely that whoever does find these records will have never heard this kind of music before – assuming they recognize it as such. For the record (heh), over half the Voyager Record is music.

    http://digitalmusics.dartmouth.edu/~larry/published_articles/voyager_.pdf

    http://voyager.jpl.nasa.gov/spacecraft/music.html

    http://azer.com/aiweb/categories/magazine/22_folder/22_articles/22_extraterrestrial.html

    http://davidszondy.com/future/timecapsule/voyager.htm

    http://home.comcast.net/~WJClancey/ClanceyVoyager.pdf

    http://goldenrecord.org/

  • ljk May 31, 2012, 18:12

    This needs to be shown again: The Planetary Society had a recent blog post about how far humanity’s electromagnetic signals have spread into the Milky Way galaxy after roughly just over one century.

    The representative artwork is sobering:

    http://www.planetary.org/blogs/emily-lakdawalla/2012/3390.html

    Those advanced ETI are only going to be able to hear us if they have some really big ears in space.

  • Tim Whitworth May 31, 2012, 19:58

    Of course, the ideal astronomical instrument would have amplifiers much nearer to the source. I’d also bet that technology “a million years more advanced than ours” could do a lot towards finding the signal with a much smaller instrument, and would build a far larger instrument anyway because it’s cheap for them to do so.

  • Ron S May 31, 2012, 20:29

    Sagan: “It is a sobering thought indeed that the Beverly Hillbillies may be our only interstellar emissaries.”

    I think it’s no bad thing that ETI sees that we have a sense of humor.

  • Astronist May 31, 2012, 22:12

    Tim Whitworth: “the ideal astronomical instrument would have amplifiers much nearer to the source”. Yes, exactly!! We will detect ETI in our own Solar System, or not at all. And the answer to the question “Is our civilisation detectable?” is then: yes, absolutely, providing the aliens have a probe of some sort in our Solar System. But would the society that despatched that probe be outside our range of detection, the distance L I referred to above? Would it be possible for a civilisation to have sufficient large-scale space activity to send a probe to us while at the same time not having enough activity (Dyson swarms of space colonies, starship exhaust trails, Solar System colonies) to be detectable by us at the present time? I would suggest that the answer is that this would be not impossible, but quite unlikely, given the vastness of cosmological time. The window of time in which we might receive a probe but before we were fully integrated into their expanding society would be of relatively short duration.

  • amphiox June 1, 2012, 0:34

    I want to raise an intriguing possibility:

    Technological intelligent life may be MORE common in the galaxy than non-intelligent complex life, or even simple life.

    Why? Because technological intelligent life might not need habitable worlds. Though they need a habitable world to get started, once (if) they achieve starfaring technological status, they can spread to any star system they want to, and live in artificial habitats, whereas non-intelligent complex life is restricted to very habitable worlds, and simple life is restricted to at least minimally habitable worlds.

    (Of course, I am excluded the simple/non-intelligent complex life that is associated with a technological intelligence as commensals/parasites/domesticates and so forth in this analysis).

  • Andrew W June 1, 2012, 2:56

    Are we sure we’ve got the Chicago that Shostak was referring to?

    Area
    • City 234.0 sq mi (606.1 km2)
    • Land 227.2 sq mi (588 km2)
    • Water 6.9 sq mi (18 km2) 3.0%
    • Urban 2,122.8 sq mi (5,498 km2)
    • Metro 10,874 sq mi (28,160 km2)

  • Andrew W June 1, 2012, 3:09

    Arrays of thousands of square km are more likely to be built in space, and given the completely different circumstances compared to the SKA, extrapolating from the construction costs of that array are absurd.

    My favorite idea is to build them in free space, perhaps tethered to a small asteroid, using the asteroids gravity to maintain tension on a reflector made from a thin reflective film.

  • Michael Spencer June 1, 2012, 7:47

    On the issue of ‘civilization density’–that is, making any sort of estimate of the number of Others out there–remember that apparently life arose here on earth only once.

    Yes, it’s possible that our life killed off other life. We don’t know. But we do know that there is no other obvious genesis, recent reports of arsenic-based organisms not withstanding (and probably disproven).

    I wonder about this. I wonder why Earth isn’t continually throwing up new forms. And the answers to the question, depending upon your own predilections are sobering. One cannot escape the simple observation that even on our own fecund, pregnant and horny planet, life happened exactly once. Once.

  • ljk June 1, 2012, 8:56

    Ron S May said on 31, 2012 at 20:29:

    Sagan: “It is a sobering thought indeed that the Beverly Hillbillies may be our only interstellar emissaries.”

    “I think it’s no bad thing that ETI sees that we have a sense of humor.”

    Assuming they have one. And humor is so much more subjective than music – assuming they get that concept, too.

  • jkittle June 1, 2012, 9:49

    Most likely we are leaving other signatures ( beyond radio waves) that are more obvious to an advanced civilization- or at least soon will be.
    -For example – the presence of certain gasses in the atmosphere ( like fluorocarbons) or perhaps neutrinos of unusual energy as leakage from powerplants or accelerators. The truth is we do not know how they will be looking for us and the detectors they might use. If i were a planner in an interstellar civilization , I would want to see probes sent as far and as wide as possible to scout out my galactic neighborhood. A probe even on the outskirts of our solar system could detect a lot of interesting “biomarkers” from earth ( and in a few centuries from humans on mars perhaps) to phone home about

  • Ronald June 1, 2012, 11:22

    Although I am usually known as very optimistic, when it comes to alien intelligences and civilizations I tend to be pessimistic, even much more than Drake, or maybe I should say realistic.

    There have been a few posts with comments on this superb website with regard to the estimated (guesstimated) number of habitable terrestrial planets in our MW galaxy, for example https://centauri-dreams.org/?p=11625&cpage=1#comments.

    Remarkably several different researchers and I myself as an informed amateur came to similar conclusions: a few hundred million.
    That is based on the reasonable assumptions that we have to look for habitable planets primarily around solartype stars (roughly F9-K2) in our galactic disk, or rather the habitable part of it (i.e. not too close to the galactic core). See the referred post and comments for more details.

    A few hundred million may sound like a lot but it is actually not, as a starting point for platforms of life, with all obstacles to overcome on the way to higher intelligence and civilization, each subsequent obstacle reducing the number of candidates left for the next level.

    My own guess is that life is rather common, but higher (multi-celled, specialized organs) life rather uncommon, and advanced (self-aware)intelligence and civilization exceedingly rare, probably not more than between 1 (us) and a handful in the entire MW galaxy.

  • Paul June 1, 2012, 11:40

    Paul Gilster made the reasonably construed guess that intelligent civilizations might not be any closer to us than halfway to the galactic center. Given that it is so quiet out there that not even the interstellar crickets can be heard chirping, it seems he may not be too pessimistic.
    Of course, no one really knows!
    The galactic center is 27000 light years distant, so that would put our green friends over 13000 light years away with their large antennas. Ouch.
    However, despite Kepler’s difficulties with spotting earth sized planets in their host star’s habitable zone, my hunch is that there are some Earth Too’s well closer than that.
    Those babies are ours, and we need to find them, get to them, and cherish them. Sometimes I wish the Mother Ship would come rescue me, but I guess it’s up to us after all.

  • Greg June 1, 2012, 13:47

    What I can derive from the facts in the article is a civilization would actually need to be transmitting intentionally to the galaxy to be noticeable. We may have several civilizations within a couple of hundred light years but the “quite” we are hear is simply the fact that no one is intentionally broadcasting.

  • jkittle June 1, 2012, 14:01

    just a thought- as we are out there searching for ( and finding) planets that may be habitable, it is reasonable to suppose that other civilizations might be doing the same. A system by system apprach greatly reduces the degree of difficulty for the problem, both in terms of the number of systems to monitor and the ability to literally focus our instruments on specific worlds. and we accumulate interesting data even as we wait. As more data is accumulated some marginal worlds can be placed at a lower priority for the search( even excluded) and the search of more distant, more optimal worlds added. Eventually we can rule out the vast majority of star systems and end up monitoring just a few million worlds, focusing on the best and closest candidates. This this is a process of honing our skills and narrowing our searches to be more productive, while expanding our search across the galaxy. To be a proper scientist- we can verify the null hypothysis over and over again until we 1) find a world or 2) exclude worlds withing in certain distance from earth.
    Now, given that other putative civilizations will be doing the same it is Fermi’s proposition turned on its head- if they are out there – we will find them even as they seek to find us. If not then we can be sure that we are either alone or not advanced enough to prove otherwise. right now we are firmly in the ” not ripe” catagory.

  • Paul Gilster June 1, 2012, 16:00

    Jim Benford writes with this comment:

    “… the comment by David H seems to be saying you can get cheaper receivers or transmitters merely by thinning the ‘Chicago-size’ array. If that is what he means, he’s not aware of the ‘sparse-array curse’. I’ve attached a short piece that explains it. Basicly, there’s no free lunch here.

    “He also mentions that the low-frequency array antennas are much cheaper. Yes, but they also have corresponding lower gain at such low frequencies.”

    And here’s Jim’s piece on the ‘sparse array curse’:

    Antennas can focus on a small spot by the technique of phasing many small antennas, and synthesize an “effective” area equal to the diameter spanned by the antennas. So why can’t this technique be used in reverse to send a beam with a beam-spread characteristic of a large antenna by using a phased array of small antennas? The reason is a relatively obscure theorem in the physics of radio transmitters, known in the field as the “thinned array curse” (sometimes the “sparse array” curse.)

    The thinned-array curse is very simple. If a transmitting antenna is filled by an area-fraction F (where F is less than 1), then the power that is lost by emission into side lobes of the beam (and hence is NOT directed into the main beam) is proportional to 1-F. The receiving antenna has the same property; a large receiving array will simply not get the photons for greater signal detection.

    This theorem can be derived from the brightness conservation principle, which is directly derived from the third law of thermodynamics: an antenna that is not fully filled cannot make a spot on the ground that is brighter than a filled array of the same diameter, and hence any array phasing that makes a smaller spot on the ground must also direct less power into the main beam.

    The problem is fundamental, and cannot be solved by any amount of clever innovation: there are no technological solutions to a fundamental law of physics.

  • bigdan201 June 1, 2012, 17:28

    I wonder about this. I wonder why Earth isn’t continually throwing up new forms. And the answers to the question, depending upon your own predilections are sobering. One cannot escape the simple observation that even on our own fecund, pregnant and horny planet, life happened exactly once. Once.

    Biogenesis is a major question. However, I’m not pessimistic. First of all, even if life happened here only once, it happened very early in the earths history. And since then, it has diversified to the extreme and survived and flourished through all manner of calamities and extinctions. And that’s assuming that there was only one biogenesis, and that our form of life didn’t just out-compete another form(s).

    Exobiology is also a major question mark. However, when you consider extremophiles, and the fact that life is found in every conceivable niche on earth, simple extrapolation becomes optimistic. Life is made of and requires certain elements that are found in abundance in the universe. Solar systems are also in abundance. I don’t see anything radically special about the earth, so I predict that we will find plenty of life out there.

    What we’ll find is still anyone’s guess. My guess is that simple life will be very common, with an inverse relationship between abundance and complexity. Intelligent/sentient species like our own do exist, but they’re not very common (<100 per galaxy at a given time?)

    Also, keep in mind that alien civilizations, like human civilizations, are not only separated by space, but also by time. Our first contact with aliens may well be with awe-inspiring ruins and artifacts from the distant past.

  • Greg June 1, 2012, 17:44

    Paul, my statment should have been like this,

    “What I can derive from the facts in the article is a civilization would actually need to be transmitting intentionally to the galaxy to be noticeable. We may have several civilizations within a couple of hundred light years but the “quiet” we hear is simply the fact that no one is intentionally broadcasting.”

    Poor spelling due to being at work and no time to spell check :)

  • Wojciech June 1, 2012, 18:30

    Personally I think that searching for radio signals is a waste of time. Too much risk of too big difference in age of civilizations and too many assumptions about them sending radio signals to us.
    With current advances in telescopic observations it seems that a better solution would be to build gigantic hyper telescope constellations that would be able to not only detect but image other planets with life, and signs of civilizations(road systems, night lights, or in case of very advanced civilizations possible mega-structures)

  • mike June 1, 2012, 20:12

    i read that a probe sent to jupiter was sent into that planet to avoid collision with it’s moons to avoid contamination. the probe had an atomic generator and some (r.c. hoaglang et al at the enterprisemission.con)) speculated it may ignite jupiter to become a second sun.
    i propose that we do just that, send concentrated atomic waste from earth to jupiter to get fusion ignition. then long after humans are gone alien observers can debate if the twin suns are natural or intentional.
    since it is a thought experiment i propose that we do a survey to look for such parings.

  • Ronald June 2, 2012, 4:41

    @mike: do not trust anything that Hoaxland is claiming, it is sheer unfounded nonsense. Jupiter cannot be ignited to become a ‘second sun’, i.e. ignite its nuclear fusion, because its mass is simply way too small for that. BTW this idea of ignition of Jupiter to become a 2nd sun comes from A.C. Clarke’s 2001 series.

  • Tim Whitworth June 2, 2012, 9:03

    We cannot assume that the earth is not special. The earth has intelligent, technological life on it, which may actually require certain conditions.

  • ljk June 2, 2012, 15:00

    Paul said on June 1, 2012 at 11:40:

    “However, despite Kepler’s difficulties with spotting earth sized planets in their host star’s habitable zone, my hunch is that there are some Earth Too’s well closer than that. Those babies are ours, and we need to find them, get to them, and cherish them.”

    Maybe we should check out these Earth Toos (?) first to make sure they aren’t already someone’s baby. Just be on the safe side before starting humanity’s galactic empire building.

  • ljk June 2, 2012, 15:12

    Wojciech said on June 1, 2012 at 18:30:

    “Personally I think that searching for radio signals is a waste of time. Too much risk of too big difference in age of civilizations and too many assumptions about them sending radio signals to us.

    “With current advances in telescopic observations it seems that a better solution would be to build gigantic hyper telescope constellations that would be able to not only detect but image other planets with life, and signs of civilizations(road systems, night lights, or in case of very advanced civilizations possible mega-structures).”

    The SETI Institute can barely keep up the $2 million funding it needs annually just to stay afloat and you want constellations of hypertelescopes in space? Well, then, humanity can pretty much write off finding alien life for a good long time.

    http://www.zdnet.com/blog/foremski/setis-search-for-alien-life-is-in-trouble/2292

    If humanity no longer has the knowledge or interest in supporting SETI, then it deserves to remain cut off from the rest of the galaxy and left to its own devices. We will then see how long this self-centered species’ technological society with the rampant consumption of its home planet can last as it is now without expanding and searching into space.

  • Daniel June 2, 2012, 17:26

    Do these analyses take into account the 2.7K blackbody radiation that permeates the universe? Perhaps my skimming of the text missed that, but it seems like an important and neglected point.

    Given the 1/r^2 decay of the intensity of the electromagnetic radiation being emitted by earth, it is relatively straightforward to calculate how far one would have to be before the intensity of our artificial radio transmissions fall below the level of the cosmic blackbody. Beyond that distance, no antenna, regardless of how large, will be able to distinguish our artificial emissions from the background noise of the universe. A larger antenna will collect more of our emissions, but also more noise. I seem to recall that the relevant distance is only a few light-years. In which case, building a big antenna to look for artificial radio emissions is provably futile, unless you’re aiming at one of the few closest stars.

    Can somebody corroborate or correct me?

  • Kris June 2, 2012, 17:46

    @jkittle above:

    I completely agree that we need to think about the problem of detectability NOT in terms of radio leakage or METI, but in terms of Earth’s spectroscopic signature. That is why Hawking is completely out to lunch with his recent blabbering about suppressing an intentional radio signal.

    Our ‘pale blue dot’ has been broadcasting its habitability loud and clear to the Galaxy for more than the 75,000 years it would take to each every other star system in the milky way.

    As NASA have recently shown and will demonstrate further within the next 15-20 years, it is possible to spectroscopically image a distant planet with Kardashev Type-1 level technology. Any other advanced civilization in the galaxy should be able to do the same.

    If they are out there in the Milky Way, they definitely know that there is abundant life on Earth.

    In my view, the success of the recent Kepler mission gives the most force to Fermi’s paradox. They’ve had lots of time to observe the Earth, so where are they?

  • Wojciech June 3, 2012, 5:50

    “In my view, the success of the recent Kepler mission gives the most force to Fermi’s paradox. They’ve had lots of time to observe the Earth, so where are they?”
    The first civilization able to catalog and observe life bearing worlds would be at considerable advantage to all of its successors.
    We are on the start of developments that(at least in theory) would allow us to create devices allowing to scan the whole galaxy for life.
    Once you have that it changes the whole Fermi Paradox-you could send probes(if you could or wanted) to selected worlds instead of whole galaxy.

    To cut it short:
    -we were undetected per fluke
    -there are none besides us, either never were or died out
    -we were detected by the first civilization or the only civilization is conservationist in nature and prefers others to develop on their own before they reach certain technological level(interstellar travel? colonization of certain sphere of space?), as contact with somebody advanced 100 millions ahead would shatter our civilization
    -they never reached interstellar travel

    As to hypertelescopes, the recent idea is to create small telescopes costing 1-10 million dollars each. I can imagine with automation we could combine them into swarms arranging themselves into constellations. A hypetelescope could still cost a couple of billion of dollars in the future, but it would feasible project, perhaps of international nature.

    In any case detection of radio signals-while noble-seems to be fading song of the past, compared to tools we will have at our disposal.

  • Tim Whitworth June 3, 2012, 7:05

    It’s possible to pick up repeating signals that are weaker than the background interference by using statistical models.

  • Ron S June 3, 2012, 21:58

    Tim: “It’s possible to pick up repeating signals that are weaker than the background interference by using statistical models.”

    Indeed, and it is routinely done but not straight-forward path to success. These statistical methods require knowing the signal’s signature. For SETI the presence of a signal and its signature are unknown, so you have to test for all (or at least a lot) of them. That is pretty much what SETI@Home is all about: taking data and trying out every conceivable signature on it.

  • Ron S June 3, 2012, 22:05

    Daniel: “Do these analyses take into account the 2.7K blackbody radiation that permeates the universe?…”

    You’re asking a lot of fundamental questions in your post that are a bit much for me to type answers to since it’s adds up to an advanced course on radio-astronomy, electromagnetism, antenna theory and so forth.

    But first, the sky temperature is generally higher than 2.7K in all directions since our own galaxy is noisy with synchrotron radiation from gas clouds and the like, and of course lots of much hotter point sources. The techniques used to dig out the CMBR, and to do so with high accuracy are quite advanced.

    As for much of your other questions, much can be done to detect artificial signals since their time and frequency signatures are statistically quite different from most astronomical sources. Filtering, high-directivity antennas and additional (routine) methods, including statistical analysis, can accomplish a great deal.

  • Rob Henry June 4, 2012, 0:05

    To calculate our chance of detection we must model the detecting or signal sending ETI. Given the Fermi paradox, I put it to you that we already know that if ETI’s exist in abundance, they must all without exception have an incredible aversion to spending money on any outward expansion. Advanced such ETI’s may still by populated with peculiar individuals who contemplate communication with other ETI’s. Their technology would be so advanced that they would know when gravitational lensing would allow a signal to be cheaply sent or received and this would make the task possible for small groups – so long as they were not obsessed with any one target in particular.

    Offhand, I can’t help noticing how well this fits with the characteristics of the WOW! signal that we would then expect to be incredibly strong, and not repeated for a very long time.

  • bigdan201 June 4, 2012, 8:10

    Our ‘pale blue dot’ has been broadcasting its habitability loud and clear to the Galaxy for more than the 75,000 years it would take to each every other star system in the milky way.

    There are many other variables at work. There may be many other pale blue dots in the galaxy with similar signatures as the earth. Spacefaring aliens might have been still too far away to find it worth their while to cross the light-years. Or they may have decided not to for their own reasons. Maybe aliens did visit and catalogue the planet in the distant past, and we’re none the wiser. They might be cut off from us due to being on the other side of the galactic core. They might prefer planets that are different from the earth.

    I won’t keep going, I’ve made my point. There is too much speculation when it comes to ETI. The Fermi Paradox rules out vast, contemporary interstellar civilizations within our sector of the galaxy. Other than that, who’s to say what’s out there?

  • David H June 4, 2012, 10:32

    Paul Gilster – hi. You wrote:

    ‘Jim Benford writes with this comment:

    “… the comment by David H seems to be saying you can get cheaper receivers or transmitters merely by thinning the ‘Chicago-size’ array. If that is what he means, he’s not aware of the ‘sparse-array curse’. I’ve attached a short piece that explains it. Basicly, there’s no free lunch here.

    “He also mentions that the low-frequency array antennas are much cheaper. Yes, but they also have corresponding lower gain at such low frequencies.”’

    I am familiar with issues inherent in sparse-array imaging, however, this is a technique that is both employed (MWA, LOFAR) and will be employed (SKA-Low). My question was simply one seeking clarification of Seth Shostak’s comment about a ‘Chicago-sized’ antenna, that is, whether he was referring to collecting area or array extent. My further comment about cost was simply a reflection on the relative cost of dishes and antennas, not one of merit.

    All the best, David.

  • amphiox June 4, 2012, 14:55

    To your list, Wojciech, you have to add the following:

    – they detected us, but decided to study us by building bigger telescopes, not by sending probes to us
    – they detected us, and sent a probe, but the probe hasn’t reached us yet
    – they detected us, and sent a probe, and the probe arrived 10 million years ago, did all it wanted to do, and left (or is out there floating dead in space somewhere in the Kuiper belt)
    – they detected us 15 million years ago, noted no sign of technological civilization, and decided not to do anything more
    – they detected us 15 million years ago, noted no sign of technological civilization, decided to keep watching, and did keep watching for 10 000 years, and then stopped because their own civilization crashed
    – they detected us, sent probes, and are watching us with them as we speak

  • Mr. Econotarian June 4, 2012, 15:13

    Optical SETI is the way to go.

    With a 10-m Keck-type telescope and a 2.5-MW CW laser pointed at the ETI for 15 minutes, a Kepler-type satellite could detect the signal out to to 250 pc.

    The interesting part is that ETIs (and ourselves) are both likely to have Kepler-type satellites for searching for planets, looking for when the signal from a star gets slightly darker. We just have to look for when it gets slightly brighter!

  • Greg June 4, 2012, 15:23

    @amphiox, if they are over 200 light years away they probably are totally unaware we exist since 200 years ago we had no radio technology, now imagine thousands of light years distant?

  • Paul K June 4, 2012, 16:47