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The exposition is that a civilization exists about 200 light years from earth and has been capable of radio communication for several thousands of years. They are not trying to hide from detection but have not been actively trying to communicate either.

I know we have several programs going on to try and detect such extraterrestrial radio signal. What plausible astronomical, technological, or human-error-related factor could I use to explain humanity not having picked up their signal?

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    $\begingroup$ I don't have numbers at hand, but the biggest problem is the fast size of the sky, so we're probably looking at the wrong direction. Or your aliens are using very very high frequency we cannot detect or a modulation that "sounds" like background noise for us humans. $\endgroup$ – Alexander von Wernherr Dec 5 '16 at 8:54
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    $\begingroup$ Hard-science is not a useful tag here. You asked for plausible concepts, not actual science into the matter. $\endgroup$ – MichaelK Dec 5 '16 at 13:38
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    $\begingroup$ One over r^2 law for one thing. Their signals could be reaching us fainter than the background noise of their neighbor star. $\endgroup$ – Mad Physicist Dec 5 '16 at 15:15
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    $\begingroup$ I wasn't going to say this before, but I will now: This question has the hard-science tag. That means that answer must satisfy the criteria given in the tag wiki. I added the post notice to a number of answers which did not satisfy the criteria. I encourage people to downvote and/or flag posts that do not satisfy the criteria and may need the post notice added. I do think the tag is appropriate, and I think that except for some edge cases, it is absolutely up to the asker as to whether or not the tag is used. $\endgroup$ – HDE 226868 Dec 5 '16 at 15:44
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    $\begingroup$ Tagged "hard-science" but look at the final sentence. You're never going to back up speculation on what human error might occur in a hypothetical situation with "equations, empirical evidence" and such. $\endgroup$ – WGroleau Dec 5 '16 at 21:35

17 Answers 17

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Radio Waves Are Hard To See, And We Didn't Think To Look At Them

There are about 134,000 stars within 200 light years of Earth. That's a lot of stars. We can't look at even a significant fraction of all of them closely enough to detect the equivalent of human-made radio waves.

A 200 ly radius is a sphere 33,493,333 ly3 in volume. The stellar density near Earth is about 0.004/ly3. Put them together and you get about 134,000 stars.

This answer on Physics.SE says we're just getting to the point where we could see radio and TV transmissions from the Earth at 200 light years away. Unless the aliens happen to point a directional transmitter right at the Earth, we're not going to pick up their omni-directional broadcasts that happen to go off into space. These diffuse as the square of the distance, so their detectability drops of very rapidly.

Spotting a radio signal 200 ly out requires a very expensive, high gain radio antenna. That sort of sensitivity requires it to be highly directional, so you can't look at the whole sky at once. SETI choses particularly likely stars and likely bands. Project Phoenix did just this in the late 90s. They picked 800 "likely" stars within a 200 ly range or about 0.6% of all the stars in that range.

Your civilization's star might not match SETI's idea of a star that's worth looking at. Or they might be transmitting in frequency ranges ranges that SETI doesn't think to look at. Or it might have simply been bad luck that their star didn't get on SETI's list. Or, as others have correctly suggested, they're not inefficiently broadcasting omnidirectional high power radio waves anymore.

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    $\begingroup$ I like this answer, because it means the writer can just have a moment where someone says "Well why didn't we find them earlier?" and the response can be "Because we weren't looking there <points at planet>." $\endgroup$ – C Bauer Dec 5 '16 at 18:24
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People tend to underestimate just how hard it is to detect radio transmissions from a distant source.

When a spacecraft communicates with the Earth using dish antennas on both ends, the received signal strength for a pure sinewave (unmodulated) carrier can be calculated as

$$P_R=\frac{P_TA_TA_R}{\lambda^2R^2},$$

where $P_T$ is the transmitter power, $A_T$ is the transmitter antenna's effective surface area, $A_R$ is the same for the receiver, $\lambda$ is the wavelength and $R$ is the distance between the transmitter and receiver.

By way of example, if I plug in the numbers for Pioneer 10 near the end of its mission (8 W transmitter, 80 AU from the Earth, a transmission frequency of roughly 2.3 GHz, a transmitter antenna diameter of 2.75 m, and 70 m dish antenna on the ground), I get a value of -165 dBm (decibel over milliwatts).

So now let me move the signal source from 80 AU to, say, 120 light years, which is a 100,000-fold increase. This reduces the signal strength by a whopping 100 dB. Further, let me assume that it is not transmitted by a dish transmitter aimed directly at the Earth (why would it be?) but rather, it is omnidirectional. At the wavelengths used by Pioneer, that's another ~30 dB decrease in the signal strength. On the other hand, let me assume that the transmitter is a lot more powerful, say, 8 MW instead of 8 W. That is a 60 dB increase in signal strength.

So a 70 m dish antenna (the largest used by NASA's Deep Space Network) would now see a signal at -245 dBm. Or rather, it would not see that signal; it is orders of magnitude below its detection threshold (maybe around -195 dBm).

OK, forget the DSN. Let's say we use the new Chinese radio telescope, FAST. Its dish is a whopping 500 meters in diameter. That is a 17 dB boost in the received signal level. Let me be generous and bump it to 20 dB; so now we are getting a -225 dBm signal.

It is still some three orders of magnitude less than what we can actually detect. And remember, this is just to detect the presence of a signal. Any modulation on that signal reduces the signal level, and detecting the modulation itself, even at a very low data rate, would be still orders of magnitude away.

So no, contrary to popular belief, civilizations that use radio are not shining beacons in the (radio) sky. Aliens won't be beaming back to us Hitler's 1936 Olympic opening speech from dozens of light years away. These "stray" radio broadcasts are immensely difficult simply to detect (never even mind decode or demodulate). And it's not a limit of technology, these are hard physical limits; an antenna of a given size only intercepts so many photons, so much radio energy, even if otherwise it is a "perfect", flawless instrument.

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    $\begingroup$ +1 Excellent answer. I think it's worth pointing out that -200dB means the power of the signal is scaled by 10^-20. That's an itty bitty signal. $\endgroup$ – Samuel Dec 6 '16 at 0:23
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    $\begingroup$ Yes, -200 dBm is $10^{-23}$ watts.Which explains why DSN receivers are cryogenically cooled to near absolute zero, to reduce thermal noise and make such tiny signals detectable. $\endgroup$ – Viktor Toth Dec 6 '16 at 1:18
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Quite simply because they've had the technology for thousands of years. Their compression and encoding has progressed past the point where we can identify it as a coherent signal.

There's a tiny window where their broadcast technology would be comprehensible to our receivers during which we could possibly pick up their signal. That would be at the point only recently after they'd invented radio, but before compression and digital encoding. Even if we'd picked up a signal from that point, the chances are we wouldn't recognise it simply due to different technologies.

However that window is out by thousands of years in this case, and it's unlikely that such a technological coincidence would occur in tandem with a grossly overpowered domestic radio broadcast signal.

It's not missing their existence you have to justify, it's detecting them. Our chances of spotting them even under the best of circumstances are minuscule.

Being the more advanced race, they'd have a window for detecting us starting around now and lasting for a few years, but if they don't pick us up at the start of the window, our signals will start to be too complex for them to recognise.

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  • $\begingroup$ Do you have any evidence on how minuscule our chances of spotting them are? For example, if another identical Earth were 200 ly what would be our chances of spotting them? I don't think this answer does enough to justify the hard-science tag. $\endgroup$ – kingledion Dec 5 '16 at 16:26
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    $\begingroup$ This seems like it might be a good answer, but I'm having trouble parsing it. In the phrase "it's out by 1800 years", what is "it", and what does it mean for "it" to be "out by 1800 years"? $\endgroup$ – David K Dec 5 '16 at 17:49
  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – HDE 226868 Dec 6 '16 at 1:36
  • $\begingroup$ It is also possible that a civilization could develop and implement efficient data compression technology before developing radio communication. $\endgroup$ – Jasper May 15 '18 at 0:58
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Directed transmissions

Transmitting radio into space is just a waste of energy, so they shape the lobes of the transmissions carefully, making sure as much as possible is sent to the receivers. This they do by either sending directly at receivers, or for broadcasting they transmit only in the ground plane.

Already today we have WiFi routers that do beam shaping to optimize transmission. And this is after having radio for just about a hundred years.

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  • $\begingroup$ Do they not use satellites for communication? If they use satellites then there will be at least some transmissions going into space. This doesn't provide any analysis of how much and advanced civilization could reduce the amount of energy going into space compared to what we emit now. $\endgroup$ – kingledion Dec 5 '16 at 16:28
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    $\begingroup$ @kingledion Satellites if anything have always had highly directional transmissions. Considering the distance and that pesky Inverse-Square Law it has more or less been an absolute necessity to not transmit isotropically when it comes to satellites. And those transmissions are — invariably — aimed right at the ground. $\endgroup$ – MichaelK Dec 5 '16 at 16:32
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    $\begingroup$ However, signals heading outbound to satellites are going right into space... $\endgroup$ – kingledion Dec 5 '16 at 16:37
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    $\begingroup$ @kingledion And they too are highly directional. Meaning that — taking into account the rotation of this alien planet — we would get a blip at the most as the lobe sweeps past us. And — again — the Inverse-Square law. Geostationary orbit is a few tenths of a light second out, whereas 200 light years is at least 10 billion times the distance. 10 billion times means an attenuation of approximately 200 dB. $\endgroup$ – MichaelK Dec 5 '16 at 16:45
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    $\begingroup$ @MichaelKarnerfors Then you should put all that information in your answer. $\endgroup$ – kingledion Dec 5 '16 at 16:48
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You don't need anything. Our technology could not pick up our own radio emissions at a 200 light-year distance, so there's no reason to think that we should be able to pick up broadcasts from an alien civilisation. The various SETI programmes are looking for directed transmissions aimed specifically at us, not broadcasts.

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  • $\begingroup$ I edited in the link you gave in a comment, and removed the post notice. If the link needs more context, you can of course edit as necessary. $\endgroup$ – HDE 226868 Dec 8 '16 at 17:53
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Essentially we cannot detect the radio communications of a civilization at 200 light years.

  • Michael Kjörling's answer to the question How far away would an alien civilization need to be for us to not notice them is very interesting and has plenty of equations. Money quotation:

    At current levels of sensitivity, targeted microwave searches could detect the equivalent power of strong TV transmitters at a distance of 1 light year (within which there are no other stars).

    The conclusion of the answer is that we won't detect them unless they point a transmitter with enormous power, i.e., comparable to the total electric power produced on Earth, straight at us, and we happen to look for it at the right time on the right frequency.

  • The common view is that civilian radio transmissions not intended to be receivable over interstellar distances are simply not detectable. On the contrary, high-power directional signals, such as those generated by radiotelescopes (when used as transmitters) or by military radar installations, are detectable over huge distances, but the probability of their detection is very low because they cover a narrow cone and are emitted for a very short time; see "The Benefits and Harm of Transmitting Into Space" by Jacob Haqq-Misra et al. (2013).

  • The big factor in the detection of alien civilizations is of course the span of time over which they use high-powered radio. See "Calculating the probability of detecting radio signals from alien civilizations" by Marko Horvat; the conclusion of that article is that unless there are very many alien civilization or somehow the aliens develop their technology very much slower than us then the probability of detecting an alien civilization in the galaxy is very small.

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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. Also, can everyone please be nice? There was a bit of escalation here that was unnecessary. $\endgroup$ – HDE 226868 Dec 8 '16 at 17:49
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Precondition

"Several thousand" means at least two thousand. Since they are only 200ly away, this means that their first radio signals (which were likely close to what we would consider "radio") are already history. What we receive now, if we receive anything, is 1700+ years ahead of our state of development.

The signal

Over two millenia of "radio availability" makes it likely enough that their signals are at the very least something like a forward error-corrected and compressed digital (or trigital, who said an alien bit can't have three states?) signal. Forward error correction would mean they could send a lot more energy-efficiently, which is good for them but not so good for you trying to receive their signals from far away, and compression would mean that whatever you receive, if you receive something, looks like noise.
They might as well use a form of "radio" that we are unfamiliar with (unusual high frequencies?) or entirely unable to interprete. Their form of radio might even be some form of quantum communication, which by design is only receivable by designated subscribers and makes "eavesdropping" from the outside impossible. Quantum communication is something we are presently almost getting to work with photons (well, saying "almost" is wrong... it works, it just isn't ready for mainstream yet). An advanced alien race might as well have some form of "quantum radio" with designated recipients, why not.

The Wow! Signal

They actually tried to communicate (not with us, but maybe with a starship of theirs), and we actually did receive their message. Unluckily, we were unable to decipher or identify it as what it was, nor answer to it. Eventually, the signal was considered "just noise", or something a comet produced.

Reflection

An entirely different, more conservative, and plausible explanation would be ionospheric (or similar, whatever they have on their planet) reflection, in the same way as shortwave radio is reflected on our planet. This isn't even limited to the planet's atmosphere, their solar system could have an Oort cloud which contains some kind of particles that reflect 99% of all transmissions. Thus, they could even communicate with space ships within their solar system, or a colony on another planet, and we would still have trouble catching a glimpse.

Black matter

Black matter might be another perfectly plausible reason why we didn't spot them. We know with rather good certitude that such a thing as black matter exists (though we don't really know what it is exactly, or how exactly it complies with our known laws of physics, but we are confident something is there). There might just happen to be some black matter that, for some reason, absorbs radio signals in between us and the aliens. Most radio signals broadcast quasi-light-like, which means if something is in between sender and receiver, the signal is partially or completely obstructed. That is the main reason why TV stations (or GSM stations) have their antennas on high towers or mountains -- their range is primarily limited by obstructing objects and finally Earth's curvature, not so much by the transmitter's power. It is also the reason why orbiting the moon is a no-fun experience for astronauts (there's no such thing as telling "Houston, we have a problem" with the Moon in between).

Technical limits

Finally, even without any special conditions, detecting another civilization which isn't actively trying to communicate is a daunting task, to say the least. Or, you could say, pretty hopeless.

There's the thing about arc length. For something hundreds of light years away, you have to aim darn precisely, and you had better have a really, really directional antenna. But this is not the biggest problem.

The transmitter of a non-trivial nationwide TV station, which is among the strongest "normal" radio emitters that we have (about 2,000 times stronger than e.g. a typical GSM base) typically has EIRPs in the 100kW range. "Kilowatt" sounds like a lot, but it is really a quite small number.

The radiant flux density is EIRP/4π*d2

Note the innocent little "2", and note that d is actually 1018 meters. That's... a mindboggingly huge number, especially after you square it.

Anything "kilo" is too ridiculously close to zero for being measured if you divide it by 1036. Thus, catching any kind of "normal" signal that isn't deliberately directed at us with an extra super strong designed-for-interstellar-hello transmitter is pretty much hopeless.
Certainly, one can more or less arbitrarily scale up the size of the receiving antenna array. Naively, if you don't get enough Watts per square meter out, you just throw in more square meters, and there you go. But the problem is that you also catch more noise. Some of this noise is many orders of magnitude larger than the signal. The chance of actually catching something meaningful is rather dim.

Plus, there's quite a few stars out there. Neglecting everything else, and assuming that we are perfectly able to receive radio from any planet up to 200ly away, there's only about a 1:100,000 chance that we randomly pick the correct star system. Trying them all is possible, of course, but it takes several minutes to rearrange an antenna array, so this task would take anywhere from years to decades.
It might just happen that we only found them now because it took us so long to pick the right star.

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  • $\begingroup$ By "black matter", are you referring to dark matter? $\endgroup$ – HDE 226868 Dec 5 '16 at 15:42
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    $\begingroup$ @kingledion: Neglegting the small constants which multiply up to approx another 1-2 orders of magnitude (like 4*pi, or like the first digit of the 200 light years in meters), you get 10^-36 W/m^2. The best radiotelescope arrays that we have today are more like 10^-29, which is a tiny 7 orders of magnitude amiss, and it is not clear what they are able to receive at all (in order to receive something meaningful, you need information about the kind of signal, which we do not have -- think modulation or such). $\endgroup$ – Damon Dec 5 '16 at 17:31
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    $\begingroup$ Current earth technology has long surpassed digital and "trigital" - we use PSK or QAM for transmitting 256, 512 or more "signals" in the time of one digital bit. This means 8 or 9 bits gets transmitted at one time. Work is in progress for up to 4096 or 12 bits. So, if they use 12-bit/4096QAM, even directed at us, we wouldn't see it. $\endgroup$ – MikeP Dec 5 '16 at 19:15
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    $\begingroup$ this answer is so full of misguided, made up stuff ... i would'nt even know where to begin - let me just throw this one thing here : quantum communication is impossible, neither is it "in development" / "not ready for mainstream yet" nor will it be, thats entirely made up. What has been proven experimentally is quantum encryption, which has got nothing to do with quantum communication since the communcation itself is done like it always has been done before : via optical fibers and lasers, even quamtum nonlocality can be explained via the pilot wave theory $\endgroup$ – specializt Dec 6 '16 at 11:17
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    $\begingroup$ @MikeP i think you might want to re-read ... pretty much everything you have read about the topic, you seem to have mixed up most of it. There is no "trigital" in current technology, literally everything is digital. QAM is a modulation for digital information onto analog signals - this basically means that the amplitude (Y) of a sinus wave is segmented into parts, all of which are declared to be a combination of binary bits. There is no "at one time", the X-positions on a sinus wave literally are infinite but the clock cycle divides that wave into a specific amount of steps $\endgroup$ – specializt Dec 6 '16 at 11:53
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Guglielmo Marconi invented the radio in 1894 and it took us less than a decade to realize the need for encryption. At first it was wartime messages only, but it quickly spread to the general populus. Properly tuned cryptography functions are indistinguishable from static, as one would get from a plethora of astonomical bodies or even the vacuum of space. If this species' discovery of radio even remotely resembled our own, we would only have been able to detect their radio chatter for a tiny slice of their history.

This is also one of the main explanations of the Fermi Paradox as to why we haven't found intelligent life yet.

https://en.wikipedia.org/wiki/Fermi_paradox#Civilizations_broadcast_detectable_radio_signals_only_for_a_brief_period_of_time

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Your alien civilization stopped most of its powerful radio & TV broadcast transmissions as soon as it discovered lasers, fiber optics, and how to send laser pulses over those fibers.

Even when broadcast transmissions were common, the antennae were shaped to keep them hugging the ground (as @Michael Karnerfors mentioned, to do otherwise is wasteful).

We still send transmissions into space, but only ones highly directed at satellites, and then only at the minimum power needed for good reception.

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    $\begingroup$ Even the most powerful laser beam made today wouldn't be able to reach a 200 light years away star. $\endgroup$ – BЈовић Dec 5 '16 at 12:23
  • $\begingroup$ @BЈовић source? $\endgroup$ – njzk2 Dec 5 '16 at 20:43
  • $\begingroup$ That’s the same as Separatrix’s answer from several hours ago. $\endgroup$ – JDługosz Dec 6 '16 at 6:21
  • $\begingroup$ @njzk2 There was a question something very similar some months ago, but now I can not find it anywhere :( $\endgroup$ – BЈовић Dec 6 '16 at 15:09
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The radio waves are absorbed by our atmosphere. Our choice of radio wave lengths (and vision) is dictated by what can reasonably pass through our atmosphere, if that doesn't line up with what passes through their atmosphere we will not see their ground based transmitters with our ground based receivers.

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Schwerin says "radio waves are hard to see," which gave me an idea. These folk have no sense of hearing, and so their writing was pictographic as is their "speech" (sign language). And long before we discovered radio, they had developed to the point that video was preferred over "text."

Although SETI has picked up their transmissions, video of living beings signing inherently doesn't have much pattern, especially when several thousand unrelated signals are mixed together.

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  • $\begingroup$ Actually, would even audio have enough pattern for SETI to detect? Even if all the signals of a populated planet were mixed? $\endgroup$ – WGroleau Dec 5 '16 at 13:49
  • $\begingroup$ um ... sonic waves cannot exist in outer space, there is no "audio" in interstellar space. At all. Well a human MIGHT be able to "hear" random noises while floating inside a dense gas cloud but thats about it, once he leaves the cloud ..... utter silence. What is "have enough pattern" even supposed to mean? $\endgroup$ – specializt Dec 5 '16 at 14:39
  • $\begingroup$ SETI is picking up radio waves and looking for patterns, which may include AUDIO modulation. $\endgroup$ – WGroleau Dec 5 '16 at 15:06
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    $\begingroup$ This is a proposal, not a hard science answer. $\endgroup$ – kingledion Dec 5 '16 at 16:30
  • $\begingroup$ actually thinking that a alien civilization use the same modulation as humans is ... childish and dumb, if not statistically nigh-impossible. Even IF aliens use analog audio modulations it will almost assuredly not be compatible to anything on earth, let alone even be recognizable - which is even more true for digital signals since trinary, quaternary quantisations are very possible - in fact the possibilities for modulation are literally infinite. I guess whoever came up with that idea (looking for analog audio) doesnt have much knowledge about technology ... probably a hardcore mathematician $\endgroup$ – specializt Dec 6 '16 at 9:56
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Place your planet near a strong astronomical radio source, like a pulsar or other neutron star. The brightness of the pulsar would drown out the relatively week transmissions of a transmitting civilisation unless we were actively looking for their transmissions and knew where to look.

For extra fun, you could combine your pulsar with an active star-forming region, where the hydrogen and other gases that are collapsing into stars are irradiated by the light of earlier stars and release their own radio signals.

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    $\begingroup$ Do pulsars emit at the same frequency as radio communications? The spectrum of radio is pretty broad. If so, how would a civilization be able to use radio in their own atmosphere with a pulsar nearby filling the airwaves with static? $\endgroup$ – kingledion Dec 5 '16 at 16:30
  • $\begingroup$ The pulsar doesn't need to be particularly near their system, just on a straightline path from earth to the other system. I looked but couldn't find much information readily available on what frequencies pulsars emit on, unfortunately. $\endgroup$ – Werrf Dec 5 '16 at 16:33
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Even were you looking in the right direction and for the correct signal, the inverse square law says that the emissions would be detected at extremely weak levels.

A 1W transmitter for example would at 200 light years be received at an energy density of 2 microwatt per square light year. That's not a whole lot for your typical radio telescope that has a size of maybe 400 square meters (for a 20 meter diameter dish, not exactly a tiny one).

And that's the order of magnitude of most radio transmitters in use, even a kilowatt transmitter would only change that signal you receive to 2 milliwatt per square light year.

Remember that a light year is 9.5e15 meters, squared that becomes 9e17 square meters.

Of course that goes out the window if the transmission is highly directional, but the idea holds (as the signal will always spread out, even a laser, parallel by design, will spread out over long distances to where a beam shot from say the moon will have a spot size of several centimeters in diameter when arriving in low earth orbit).

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This is an excellent question, and most answers are cool, too. But I'd like to add something.

  1. The thesis that their technology may be far more evolved and thus difficult for us to understand yet is awesome but narrow. I'd say that we do not know how their technology would develop, meaning that they might use different radio transmission than us. As you know, currently all radio is based on Amplitude Modulation, Frequency Modulation and Phase Modulation, meaning that a carrier signal is used to reliably transmit ONLY the amplitude, the frequency or the phase of the transmitted signal as a function of time. That's great for encoding audio in our math system, but their math could look at the universe from a completely different point of view. For all I know, they might not communicate through audio at all (like a giant octopus race mentioned in the Midday World cycle by brothers Strugatsky), thus meaning that they'd transmit only text or other information type. Or they might transmit amplitude and phase simultaneously, for example. The history of our science implies that there is only one way for the science to develop, but (alert: Mass Effect spoiler)

    in Mass Effect, for example, Reapers say that their influence on us caused us develop "along the paths they desire" which implies there are ways of development we might be unable to comprehend. So, there's that.

  2. There may be some obstacles for the radiowaves on the border of the Sol system of which we do not know. Imagine that other stars emit radionoise, for example. I am not an expert, but I find it's quite possible. Correct me if I'm wrong :) Or, there could be a black hole that would catch all transmissions into itself.

  3. At last, they might be so far from Earth that it would take thousands of years for a radiowave to travel between us. The difference in planetary movement speed or in distance could cause Doppler's effect and modify the wave.
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  • $\begingroup$ #1 I'm certainly not an expert, but I don't think the contents being audio matters, or even the modulation (granted our equipment was tuned to pick it up). Wouldn't it just have to be distinguishable from randomness or cosmic noise? And I certainly don't know this for sure, but wouldn't the Doppler effect be expected to preserve the non-randomness? $\endgroup$ – xdhmoore Dec 7 '16 at 0:51
  • $\begingroup$ @xdhmoore The concepr of noise is dependant on the equipment used. What is noise for us is not necessarily noise for those who transmit in another way $\endgroup$ – feakuru Dec 7 '16 at 7:45
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  1. Given the distance, even 0.01 degree can means a whole different planet altogether.

  2. Plantes rotates around their sun. They are not stationary. Thus you can make it that everytime Earthlings pointed their detection machine in the right direction, there is an object inbetween. For example, another star.

  3. Like others has suggested, since they have the technology years earlier, their encoding could be much more advanced than our own, their radio could use a very high range of frequency that the human's equipments can't capture.

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    $\begingroup$ Point 2 is mostly incorrect. There's no need to be "in a direct line with Earth", not sure what that means. The only requirement is our line of sight to the planet is not blocked by their star nor ours, that's a small fraction of our orbits. The closest black hole we know of is V616 Monocerotis about 3000 ly away. Also they don't have any greater gravitational effect on radio signals than a star of equal mass. OTOH a star is pouring out all sorts of EM radiation of its own acting like trying to spot a candle next to a spotlight. $\endgroup$ – Schwern Dec 5 '16 at 9:54
  • $\begingroup$ What is the "0.01 degree" relative to? What angle does it describe? $\endgroup$ – HDE 226868 Dec 5 '16 at 16:37
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Any number of these make detecting a signal very difficulty, unless one knows the exact code used (or code algorithm), frequency, bandwidth, modulation, and has a high-enough signal to noise ratio. Then there is the actual analog/digital encoding, compression, encryption, symbol set/language, binary/trinary/etc, and more.

  • Spread-Spectrum
  • PSK (Phase shift Keying)
  • QAM (Quadrature amplitude modulation)
  • PM (Phase Modulation
  • SM (Space Modulation)
  • Code Division
  • Orthogonal frequency-division multiple access

SETI says that it is unlikely that a civ would detect us: http://www.seti.org/faq#obs12

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Yes - chosen modulation schemes make it difficult to decode the data within the spectrum. But every wireless transmission scheme has a sync method - a way to find the system prior to establishing a connection - which may enable detection - and indicate the PRESENCE of another civilization even if we cannot decode any of the content.

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  • $\begingroup$ Hi. Welcome to Worldbuilding. Your answer starts with "Yes", but this isn't a yes/no question. I think you may have been intending to add a comment instead of posting an answer. If you did intend to answer, please edit your answer to be in a form that matches the phrasing of the question. $\endgroup$ – SRM - Reinstate Monica Dec 6 '16 at 2:09

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