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This seemed to be the closest group for this question.

Every few months there are a few articles asking "Are we alone in the universe" which focus on the Fermi Paradox, the Drake Equation, and end by talking about SETI and its lack of progress. As I understand it SETI is looking for signs of technological civilizations beyond our solar system, both in our galaxy and beyond through radio astronomy.

My question is about what would count as a "sign" of a technological civilization, and is there a maximum distance beyond which such a signal would be indistinguishable from other radio emissions from stars and galaxies?

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What signals do SETI receive? SETI uses radio telescopes to look at large portions of the sky across a range of wavelengths. They believe that we are most likely to find alien civilisation buy looking for microwaves.

How would we know if they were from aliens? There are radio signals from many different sources bouncing around the universe, many of which we don't quite understand yet. However, natural sources of energy tend to produce a range of wavelengths. SETI are hoping to find "narrow-band signals", which are strong signals at a single wavelength. These (as far as we know) only come from devices made to produce them, so only a technologically advanced species would emit these signals.

Is there a maximum distance for detecting these? Yes, these signals would decrease with distance. It is not so much that they become indistinguishable form natural sources, as they are a different type of signal, but as the waves travel they spread out, like dropping a pebble in a pond, so the signal is much weaker as you get further away. Also, space is not empty. If the waves are emitted from a planet in a galaxy, there will probably be lots of dust and gas (interstellar medium) in the galaxy which will absorb the radio waves, so it might be like looking for car headlights on a foggy day - we will see something vague and fuzzy but it will be much weaker if there is more matter between us and them.

If you want to find out more about SETI they have a website with FAQs and news. If you want to get involved there is a project called SETI@home which uses spare processing power on your home computer to analyse the data they are receiving and search for these narrow-band signals.

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Other answers covered most of it but I would like to bring a couple of points up:

  • As the distance between the radio emitter and the radio receiver increases, the signal will be received from a smaller patch of sky, requiring better telescope resolution (see: radio telescope effective area and solid angle), and also requiring more time to scan the sky, i.e. making everyone's life more difficult.
  • Natural light is unpolarised and is, basically, white noise (even though there are few natural sources of polarised light). SETI hopes to find signals in small bandwidths (distinguishing them from the broad bandwidth noise) near natural lines that are familiar to intelligent life, such as between the 21 cm band that is a characteristic of water. This is purely an assumption that aliens will be smart enough to emit a radio frequency at 21 cm and we will be there in the right place to detect it.
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  • $\begingroup$ Seems like 1.43 GHz might be a little slow for signal transmission. Even my primitive Wi-fi network is running at 5 GHz. $\endgroup$ – Wayfaring Stranger Aug 8 '15 at 0:25
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    $\begingroup$ @WayfaringStranger, 1.43 GHz would be used for deliberate SETI signaling. Any species performing radio astronomy is likely to be listening on the 21 cm hydrogen line, so by transmitting at or near that frequency, you maximize the chance of being detected. $\endgroup$ – Mark Aug 9 '15 at 6:22
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Besides radio, SETI is also searching for signs of artificial technology in data from infrared (space) telescopes. The idea is that civilizations make good use of energy, and solar energy is the most abundant and easy to access. Fusion power for free. Fissile elements are mostly inaccessibly hidden deep inside planets. If a civilization absorbs a substantial part of a star's light for its industry, the natural star light would be replaced by redshifted waste heat radiation from that industry. Something we should be able to detect with today's technology.

Jason T Wright seems to be the one man driving force behind this kind of actual search today, within the SETI and wider science community. Here are some links to a talk, slides, a paper and a blog by Jason. Lots of galaxies have been studied, so at least it isn't common that civilizations extract most of the energy from entire galaxies. The universe might be too young for that.

Personally, I fear that our focus on life and intelligence as we know it, expressed in the in my opinion naive Kardashev scale may miss that there might exist completely different kinds of phenomena out there. Life on Earth is all just one single instance. We have no diversity, no statistics, but we do know that the combinatorics of chemistry is mindboggling. But we can only do what we can do, and it would be a bit strange if there's nothing like us out there, considering how extremely ancient and successful DNA-life is on Earth, which doesn't seem to have unique circumstances. A problem might be that biology seems to use all tricks, except radio.

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  • $\begingroup$ Is your last sentence saying that radio is universal? $\endgroup$ – NuWin Aug 11 '15 at 4:26
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    $\begingroup$ @NuWin No, rather the other way around. Radio is so very useful in our civilization, but as far as I know, radio never occurs in biology, although electricity is used alot. Maybe the water is blocking it. Biology loves wires. $\endgroup$ – LocalFluff Aug 11 '15 at 7:41
  • $\begingroup$ That makes sense now. But then why did the people at SETI base their project on finding ET's through radio?? Is there any scientific benefit for their findings/approach because it seems like there isn't any ETs using radio (I think). $\endgroup$ – NuWin Aug 12 '15 at 5:56
  • $\begingroup$ @NuWin I'm afraid radio SETI started almost as soon as we invented radio telescopes. We love our new cool gadgets, and maybe sometimes overestimate what they will achieve. I still think we should listen, but this stuff might be stranger than we can imagine. I don't know of any astrophysical spin offs from SETI. Astronomers seem to be very fussy about what and how and when to observe and don't use any random data. $\endgroup$ – LocalFluff Aug 12 '15 at 6:02
  • $\begingroup$ Ahh I see. I always thought there was a reason why the SETI project chose radio.. It turns it out it was just a bunch of smart nerds trying something new. But I agree we should still listen. $\endgroup$ – NuWin Aug 12 '15 at 6:11

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