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So this is a variant of this other question.

  1. I know stars are big, they radiate a lotta energy, they have spectral lines. But how dense are these spectral lines and is there a noise floor at frequencies other than the spectral lines where it would be most logical for intelligent beings to squeeze in an EM signal? Are these holes in the spectrum quiet enough? I just can't imagine some mortal species competing with a canopy of stars to get a message through unless there are some really quiet spectral holes.
  2. Suppose such holes exist (and I imagine astronomers know about them, it's just that I don't), given inverse-square attenuation, how far can we expect to hear a 100 Megawatt transmitter with a decent directional antenna that is pointed to where ET will expect our star (and planet) to be in the hundreds of years in their future? I am assuming that ET picks a star that they think might have a habitable exoplanet, they know the distance to that star, and project where in their space that star will be when their beacon would arrive at that star.

I mean, can we expect to hear anything from further than 100 lightyears? 1000 lightyears? Then, within that sphere, given the Drake equation and whatever are the going parameters for it, what would be the likelihood of a civilization to appear in that window of space and time? It seems to me to be low. (100 LY)³ isn't all that big of a chunk of our galaxy.

Essentially, my question is: Does SETI even have a fool's hope?

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    $\begingroup$ A civilization that wants to communicate over interstellar distances has to be very long lived. It will not mind investing in communication systems with very high angular resolution (in the nanorad range). Separating the emissions of a star from those of a coherent transmitter located e.g. 10-100 AU from the star are not a problem over distances of 1000s of parsec. At that point it simply becomes a matter of generating the necessary transmitter power for a desired bandwidth and a tradeoff between baseband frequency and antenna size. $\endgroup$
    – FlatterMann
    May 28 at 6:46
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    $\begingroup$ So you're saying that SETI does have (at least) a fool's hope. $\endgroup$
    – robert bristow-johnson
    May 28 at 15:30
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    $\begingroup$ My personal opinion is that SETI is a technology looking desperately for an application. If another intelligence wants to communicate with us, they are not going to waste all that energy to flood the entire inner solar system with an unfocused radio signal. A rational communication strategy would be to pick a "silent" region in the outer system and to direct most of the beam energy there. They know that there will be at most a couple centuries between the discovery of optical and radio astronomy and the capability to deploy a receiver in the outer system. They can wait. $\endgroup$
    – FlatterMann
    May 28 at 18:28
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    $\begingroup$ I don't expect ET to be able to differentiate much between the locations of Earth and Neptune. They're pretty far away. I'm expecting, just from probabilities, further away than 100 ly. They're gonna squirt energy at our solar system and I doubt they're gonna be making antenna arrays as big as the entire orbit of an outer planet. $\endgroup$
    – robert bristow-johnson
    May 28 at 18:46
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    $\begingroup$ You are greatly underestimating both ET and the capabilities of our own astronomers. Look up the spatial resolution of the event horizon telescope at the distance of the black holes they are imaging. And this is just a sub-optimal technology step. Large optical space arrays can probably resolve planetary surfaces at the mile-scale over distances of 1000 light years. $\endgroup$
    – FlatterMann
    May 28 at 18:53

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Most of the spectrum is similar to that of a blackbody.

The quietest portion is up in energy, in the soft x-rays and up. And the low energies (radio waves) are comparably quiet compared to conceivable modulated artificial emitters.

For communication one would use something that can be created and modulated well, too.

That is why radio waves are considered, too, for SETI. Their modulation will stand out from the steady background of the star.

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