Wikipedia:Reference desk/Archives/Science/2016 April 24

= April 24 =

SETI not successful at finding extraterrestrial intelligence
If there are so many possible earth-like planets in the Milky Way Galaxy, which makes the chances of finding alien life very high, why hasn't SETI been able to detect extraterrestrial civilizations? — Preceding unsigned comment added by 24.207.71.235 (talk) 04:56, 24 April 2016 (UTC)
 * Because you need some synchronization to recieve information over radio signaling and SETI is only listening for 26 years now which is a joke in relation to the assumed age of our universe. Ask again when we have listened for 500 Million years without finding anything :D --Kharon (talk) 05:36, 24 April 2016 (UTC)
 * P.S.: Ok, ok, 500 million may be to conservative aproach scientifically speaking and i get you can not be that pacient.. so how about asking again after listening just 500 years which roughly relates to the assumed age of the universe like 1 Minute in a 60 years Lifespan. --Kharon (talk) 06:04, 24 April 2016 (UTC)


 * Fermi paradox and rare Earth hypothesis are relevant. A large number of possibly earth-like planets doesn't make it likely that we'll detect intelligent life. We don't how how likely it is that a possibly earth-like planet has intelligent life on it (at the time we look). -- BenRG (talk) 06:05, 24 April 2016 (UTC)
 * If the OP can give numbers to use in the Drake equation that estimates the number of communicative extraterrestrial civilizations in the Milky Way, the question will be better posed than asking in qualitative terms of "so many" and "very high chances". AllBestFaith (talk) 08:46, 24 April 2016 (UTC)


 * There's also the sensitivity of our radio telescope. There may be an intelligent civilisation with radio transmitters 1 kiloparsecs away from us (not very far astronomically speaking), but our telescopes may not be sensitive enough to detect them. Suppose a human-like society exists just 3 parsecs away (we don't really expect them closer than that) and that society has FM radio stations like we have. We would have to point our most sensitive radio telescopes at their planet, staring for years before we'd be able to detect their radio station as a small bump in the radio spectrum above background noise. We have other things to do for our radio telescopes. And FM radio may be a short-lived technology. Already we are moving to broad-band digitally encoded radio, which is far harder to discriminate from random background noise.


 * There's more hope if the aliens use powerful transmissions, beamed in a narrow beam directly towards Earth. This could give a decent range at a bitrate somewhat higher than 1 bit per century. Maybe a bit per second would be possible over 100 parsecs, I didn't calculate what transmitter the aliens would need. But even then, both the transmitter and the receiver would have beams of an arcsecond width and those would have to be pointed right at one another at the right time. The odds aren't very good for that to happen. PiusImpavidus (talk) 11:16, 24 April 2016 (UTC)


 * If there are aliens in a distant galaxy, the length of their day is not likely to be the same as ours. Even if it was, the aliens are not likely to have divided it up into the same 24hr/60min/60sec divisions that we use. So, whereas we use the second as our basis for the data rate in our transmissions, who knows what basis they would use for theirs? Wouldn't a different basis make transmissions mutually incomprehensible? Maybe all the noise we hear is actually the aliens communicating amongst themselves. Akld guy (talk) 12:53, 24 April 2016 (UTC)
 * @Akid guy If you are suggesting that terrestrial data communications are all locked to a 1 Hz timebase, that is incorrect. Common digital modulation methods convey data at many different rates derived from independent clock sources and receivers are designed to synchronise themselves to incoming signals. AllBestFaith (talk) 14:49, 24 April 2016 (UTC)
 * Even if they were locked to a multiple of 1Hz - it wouldn't matter because of doppler shift. I'm quite sure that SETI aren't looking only on exact numbers of hertz - that would be a ridiculous thing to do. SteveBaker (talk) 19:28, 24 April 2016 (UTC)
 * I didn't say anything about a 1 Hz signal. We base our bit rate timings on 1 second, but if an alien culture based their "second" on some small division of the length of their day, the length of their "bit" might be a very small fraction of the length of ours. So small in fact, that it sounds like noise. Maybe we don't yet have the technology to extract anything meaningful from such short bit rates, let alone decode any encryption that they might use. Akld guy (talk) 20:12, 24 April 2016 (UTC)
 * That would be a matter of more advanced technology, not of the day length. If our day were a hundred times as long, but our technology were the same, we wouldn't make our data transfers a hundred times slower. —Tamfang (talk) 07:39, 25 April 2016 (UTC)
 * @Akid guy 1 Hz or Hertz means a frequency or rate of once per second which is exactly what you are harping on about, like the ticking of a clock. Mechanical clocks are hopelessly inadequate to measure the durations of bits in modern digital communications that now are routinely sized in milliseconds, microseconds, nanoseconds and picoseconds. High bit rate implies short bit duration and there is no such thing as a human standard bit. Of course some competence is required to draw connections (if any) between the Sexagesimal time divisions of Babylonian astronomy and the theory in SETI that any communication signal is distinguishable from thermal noise by its Autocorrelation. In this way an inquisitive alien who picks up a GPS satellite transmission and first wonders why earthlings seem to transmit "noise" to themselves, on closer inspection deduces that GPS satellites transmit periodic PRBS for time measurements. AllBestFaith (talk) 12:56, 25 April 2016 (UTC)
 * I'm not impressed with the way you stated that I'm "harping on". I responded to questions. I'm also well aware of what a Hertz is, thank you very much. See my profile. I began building radio equipment in the 1960s and have written software in both Basic and Assembler and have interfaced it via a serial port, developing my own KISS code. Your condescension, by referring to a mechanical clock, is totally out of place. Read what I said (unfortunately I spoke of bit rate when I should have said bit length). Our bit length is based on our second. If an alien culture has a different day length, their bit length is almost certainly going to be different from ours, and it may be so much shorter that we perceive valid data as simply "noise". Akld guy (talk) 16:35, 25 April 2016 (UTC)


 * I recommend http://www.seti.org/faq - I'll quote one of their FAQ points: "If an extraterrestrial civilization has a SETI project similar to our own, could they detect signals from Earth? In general, no. Most earthly transmissions are too weak to be found by equipment similar to ours at the distance of even the nearest star. But there are some important exceptions. High-powered radars and the Arecibo broadcast of 1974 (which lasted for only three minutes) could be detected at distances of tens to hundreds of light-years with a setup similar to our best SETI experiments."
 * ...and that's an important point. The aliens would have to be beaming a very high powered (by our standards) signal - and aiming it right at us - and timing that precisely so that it arrives when we're listening - and they'd have to be within "tens to hundreds of light-years" of us.  If you look out to 100 light years, there are maybe a dozen potentially inhabitable planets.
 * SETI also say "...while there are hundreds of billions of stars in our galaxy, only a few thousand have been scrutinized with high sensitivity and for those, only over a small fraction of the available frequency range."
 * So it's really no surprise at all that we're not finding anything.
 * SteveBaker (talk) 19:28, 24 April 2016 (UTC)


 * About 1980, i think, Science had an article about what Earth would look like in radio waves as seen at various nearby stars. (I've seen the claim elsewhere that Earth puts out as much radio as a small star; though we may be dimmer now, with conversions to narrowcast.)  Observers could infer the length of our day from periodicities, the size of Earth and its orbit from Doppler shifts, and some hints of the geographic distribution of cities (brightest on the limb, because most of the energy goes out horizontally). —Tamfang (talk) 07:39, 25 April 2016 (UTC)

One possibility is that life intelligent enough to develop radio communications tends to be "intelligent" enough that it also develops ways to snuff itself. So the window for something like SETI could be quite brief in the context of the age of the universe. Shock Brigade Harvester Boris (talk) 00:08, 26 April 2016 (UTC)
 * This is discussed in detail in our Great Filter article. Scary stuff. SteveBaker (talk) 15:37, 26 April 2016 (UTC)

What about the fact that no aliens have visited earth so far?24.207.71.235 (talk) 03:23, 26 April 2016 (UTC)
 * Well, for most aliens, they can't know were here yet. We've only been dramatically affecting the atmosphere, changing the appearance of the planet and emitting radio waves for around 100 years - and given the speed of light, that means that only aliens living within 50 lightyears stand ANY chance of knowing were here and getting here.  Even that presumes that they have near-lightspeed spacecraft and the capability of maintaining the lives of their crew for that long.  Within 50 light years, there are maybe only three or four inhabitable planets.  With realistic capabilities to travel here, no aliens could possibly know that we exist AND be able travel here for at least a thousand years.
 * The only alternative would be that aliens with so much capability are visiting other star systems routinely and continually - and that would require a truly insane degree of technology and energy. If that were the case, we'd already know about them.
 * SteveBaker (talk) 15:37, 26 April 2016 (UTC)


 * Though I'm a fan of the berserker (Saberhagen) idea, a possibility I have not really seen mentioned is something I remember reading that "a perfectly encoded signal is indistinguishable from noise." (I heard this claimed as a consequence of Shannon's theorem, though if it is I don't actually see how)  But there's a notion in my mind that the seemingly random entanglements of particles and even isolated patches of space, the happenstance of which way each ring of six hydrogens points in a drop of water, and countless other such minutiae might conceal a powerful signal.  That whole alien civilizations' worth of data and transhuman understanding exist in each drop of pee during its brief flight, as oblivious to its slow descent as we are to the gradual continental drift between here and the Andromeda Galaxy.  In this notion, civilizations would wink out from our view not by self-destruction, but by some sort of realization of higher levels of consciousness until they become, somehow, integrated into the whole of the cosmos.  The appearance of this process, however, seems like it might be disturbingly similar to various death-cults on Earth... Wnt (talk) 18:59, 27 April 2016 (UTC)


 * It's true that a perfectly encoded signal is indistinguishable from noise - but nobody in their right mind would start off a conversation with a perfectly encoded signal. To the contrary - you want a signal that's very inefficiently encoded so that it stands out from the noise and will be instantly recognisable.  That's why we have the trope that the first alien contact will be a string of prime numbers.  But if their civilisation encodes numbers and performs arithmetic using interpretive dance choreography - it may take a while for us to figure it out. SteveBaker (talk) 22:43, 27 April 2016 (UTC)
 * This is an important difference though. Now we're only talking about people trying to contact us.  If their signals to each other resemble noise, then we won't simply overhear them as they go about their normal business.  And the odds of them trying to contact us depend on matters of ethics - unless they feel like it's a good, worthwhile thing to mess with the heads of emerging cultures, they won't, because they don't have any selfish reason to do it, and indeed, you're talking about using up a tremendous amount of potential bandwidth to emulate a crude low-tech signal to someone who probably won't hear you.  Our current attitude toward uncontacted tribes gives us some hints, though it's by no means definite as that changes century to century. Wnt (talk) 17:24, 28 April 2016 (UTC)