I am often seeing articles based on the Fermi Paradox appearing from various news outlets, journals, and Facebook pages. They usually tend to agitate me due to factors that I believe the writer has disregarded. A while ago I had a related question arise in my mind which I sort of answered myself without doing too much research. I thought I would ask the question here, as I could well have been too dismissive, and there might be a chance I can enjoy some of these F.P. articles again.

Could alien electromagnetic (radio and optic) signals be detected against the backdrop of an electromagnetic source billions of times stronger (their host star)? If yes, could it be detected with today's technology? Any estimate as to have far it could travel whilst staying coherent (or at least detectable)?

You could look at the question the other way round: How powerful would the radio source producing the message have to be and do we have technology which could produce such a signal today?

  • $\begingroup$ This feels like it might be appropriate for the Skeptics SE, since I believe this is one of the main methods used today to find intelligent life elsewhere in the universe. $\endgroup$
    – Frostfyre
    Aug 24, 2016 at 19:22
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    $\begingroup$ Anyone involved in SETI? This question is for you! $\endgroup$
    – PipperChip
    Aug 24, 2016 at 19:33
  • $\begingroup$ If Aliens are anything like us...we've tried with the Arecibo message. 1974 sent to Messier 13 en.wikipedia.org/wiki/Arecibo_message $\endgroup$
    – Twelfth
    Aug 24, 2016 at 21:46

2 Answers 2



See the breakthrough listen project, and in particular this presentation for details on what could be detected at what distance with what’s coming available now. That is exactly what you are asking.

For more general information on suppressing background of imaging a planet near a star, see various other seminars in the series. The same technology that will let you image a planet or take a spectrum of ots atmosphere would apply to radio wavelenths too.

Second, radio signals produced by a civilization might include those that are extrodinarily atypical for the host star. Peaks at particular narrow frequencies and polarization would show up, for example.

You might not detect FM audio channels, but rather think about ground-to-aircraft radar systems and industrial activity.

  • $\begingroup$ The sensitivity of that technology is staggering! Ok, I will stop being a skeptic. Thanks :) $\endgroup$
    – Varrick
    Aug 24, 2016 at 22:53
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    $\begingroup$ Be a skeptic (using the correct definition of the term). That’s what science is all about. If popular articles seem wrong or leave you lacking, seek out better sources (like the SETI seminars I linked to). $\endgroup$
    – JDługosz
    Aug 24, 2016 at 23:16
  • $\begingroup$ I meant about this particular question :) I should have researched it myself yes, but it's nice to get a broader insight from you guys from time to time! $\endgroup$
    – Varrick
    Aug 24, 2016 at 23:18

I don't think that's entirely the point of the Fermi Paradox;
Kurzgesagt has an awesome youtube video on this link here and the follow-up video here

Either way; to answer your question is a tricky thing to do (see the Wow! signal)

What is an Electromagnetic (EM) signal? Radio waves such as those we've sent out to Search for Extra-Terrestrial Intelligence (SETI) are simply light waves traveling at a specified frequency, but so much can imitate or manipulate this frequency; here are just a few:

We first have to take into account Cosmic Microwave Background Radiation (CMB Radiation) that could distort what is heard.

Next, we have Black Holes. These super-dense remnants of very large stars (>5 solar masses) will warp Space and Time to such an extent that Scientists had to develop a new theory about them in the movie Interstellar.

Thirdly, we have Pulsars, although they won't affect any preexisting radio communication, they will add their own as they spin, emitting light in pulses (hence their names)

Next, as you originally posted, the host star also emits its own radiation that could (possibly) interfere with the signal, though it should be mentioned that when we humans search for EM signals, we search for a specific frequency that, we think, would be used to avoid this kind of interference.

As any EM signal is simply a light wave, it has the same (or similar) properties in terms of decay, that is to say, if uninhibited, it will continue to propagate outwards in the direction it is fired in until such a point that the light wave becomes so long as to become in-discernible against the CMB. (Currently the most distant object seen so far with telescopes is UDFj-39546284, an early forming galaxy at over 13.2 billion light years away)

The point of the Fermi Paradox is to ask "If there's this many stars in the known universe, where are all the aliens?" it's NOT asking "Are we alone in the universe?" because that's not a paradox.

(TBH, the Fermi Paradox isn't really a true paradox anyway and wasn't even made by anyone named Fermi)

Considering all this information, it's not out of the realms of impossibility that a sufficiently advanced civilization (one that can use telescopes to see other worlds and has radio communication) could potentially send a signal that would reach us here on Earth.

The only problem is, Space is BIG, very big. Even in our fastest ships (Voyager 1 and 2), it would still take 18,000 years to get to our NEAREST stellar companion, Proxima Centauri, (At approximately 4.2 LIGHT YEARS away)

  • $\begingroup$ Re « wasn't even made by anyone named Fermi»: The basic points were raised by Enrico Firmi… $\endgroup$
    – JDługosz
    Aug 24, 2016 at 22:47
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    $\begingroup$ I don't think your digression ever got back to the question being asked! Radiation is… here are many atural sources… space is big… so? could it be detected, at what distance? How powerful of a source would be need to be detected by a similar program? $\endgroup$
    – JDługosz
    Aug 24, 2016 at 22:51
  • $\begingroup$ @JDługosz - Although Fermi's name is most commonly associated with the paradox, he was not the first to ask the question. An earlier implicit mention was by Konstantin Tsiolkovsky in an unpublished manuscript from 1933.[23] (The same source as the one you provided.) $\endgroup$
    – Raisus
    Aug 25, 2016 at 9:07
  • $\begingroup$ Noting that Fermi wasn’t the first to ponder it is not the same as saying that nobody named Fermi had made that point. Fermi certainly didn't get the idea from someone who didn't publish. $\endgroup$
    – JDługosz
    Aug 25, 2016 at 10:38

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