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If we imagine an extra-terrestrial civilization which is on an exact technological par with Earth, and has been leaking radio signals into space exactly the way we have been doing, how far away can it be from Earth and still be detectable as a technological world? The focus of my question is on the nature of incidental radio emissions into space - both how detectable they will be, and how recognizably artificial they would be to us here and now.

Consider an extra-terrestrial civilization on an Earth-like world develops technologically just as we have. At their equivalent of our 20th century, they start broadcasting radio signals around their world in the same bands and at the same power levels as has been used here. And, just as has happened here, some of those radio waves leak out into space. SETI just happens to have a dish (say one of the VLA dishes) pointed right at their world. How far away could they be before all those signals become indistinguishable from noise? Let's assume a similar ebb and flow of signal strength and clutter just as we have produced over the decades. How would degree of clutter from numerous transmission sources affect the overall signature as a technological world? Would a technological world like Earth have to be relatively nearby for cluttered artificial emissions not to be missed or dismissed as natural? Can we detect (and recognize as such) the incidental emissions leaked from a late-20th-century-equivalent world, if such existed, hundreds or thousands of light years away?

I could express the same question in the reverse case: A hypothetical technological world equipped with radio telescopes exactly equivalent to what exist on Earth today, with one pointed at us. At 20 light-years distance, would our circa 1996 emissions be both detectable to such equipment and recognizably artificial? What about our c.1986 emissions arriving on such a world 30 ly away? What happens as we go further out (and further back into Earth's radio history)? Is a less cluttered spectrum more easily recognizable as artificial? As we have gotten noisier, have we become more easily detectable? Have we become more or less easily recognizable as a technological world through our incidental emissions? Have we passed through a peak, and if so, how many Sun-like stars are now inside that radio "shell"?

Anthony X
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  • I answered a very similar question on [worldbuilding.se] where it was posed as How far away would an alien civilization need to be for us to not notice them? You may be interested in that one. – user Aug 14 '16 at 14:14
  • @MichaelKjörling so, bottom line, if a SETI dish were trained on Alpha Centauri, and there was a civilization just like ours there with a comparable receiver trained on us, neither of us could detect the other's incidental emissions? Or, if a civilization were listening to us from, say, a few hundred light years away, by the time our present-day emissions arrived there, they'd need something vastly larger and more sensitive than anything currently existing on Earth? – Anthony X Aug 14 '16 at 14:52
  • Pretty much, yes. If the assumptions I made in my answer over on Worldbuilding are reasonably correct, then it would be tolerably easy to detect a signal sent by an extrasolar intelligent civilization for the express purpose of being detected, assuming that we know where (frequency and direction) and when to listen; but really, really difficult to detect the incidental RF emissions of a current-Earth-like civilization in another star system. Incidental RF emissions is something we strive to minimize because that energy is essentially wasted as well as contributes to radio interference. – user Aug 14 '16 at 15:30

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