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Sending a signal at interstellar distance requires enormous power - something of order not too far from energies of the stars. Even if we harness the power of stars, modulating that signal would take a lot of energy - though the actual power required could be reduced - at cost of bandwidth. Imagine a swarm of statites near Oort Cloud, designed for such task: crafts with solar sails large enough to eclipse the Sun as seen from other star to a visible degree, able to turn the sails to reveal it or obscure it - modulate Sun brightness as seen from the distant destination - but the process of turning the statite takes a week. So our bandwidth is of order of 1 bit per week.

Does SETI search signals modulated as slowly? Transmissions of order of bit per week, month, year?

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  • $\begingroup$ The advanced race from the distant Venit system have already found a way to modulate the opacity of the sail without having to move it. It's of course called the Venitian Blind. $\endgroup$ – uhoh Aug 22 '17 at 7:53
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    $\begingroup$ @uhoh: The large ones have an awful tendency to tangle and jam... and here we're talking LARGE. $\endgroup$ – SF. Aug 22 '17 at 8:01
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SETI analyses signals in the 1000MHz to 9000MHz range. So you would expect communications modulated on these frequencies to utilise the high frequency to include more data rather than less.

That said, the data on, say, the hydrogen line at 1420 MHz and the methanol line at 6667 MHz are stored, and analyses are run on these over short, medium and long timeframes and yes, cycles or variations of this length are detected, and in fact have been used to identify interesting astronomical objects (variations in signal can imply occultation by something like a moon or other orbiting body)

This does present a problem - if low frequency variation could be occultation or other natural process, why would an alien civilisation use it to communicate? Rather than a large, slow solar sail, why not thousands of small sails acting in concert, able to change position rapidly and give you changes sub 1-second?

TL;dr - yes, SETI does consider low frequency variations, but it is not an ideal channel for communication.

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  • $\begingroup$ " Rather than a large, slow solar sail, why not thousands of small sails acting in concert" - for these scales, thousands of large slow solar sails - or billions of small ones. $\endgroup$ – SF. Aug 22 '17 at 8:54
  • $\begingroup$ The more the merrier :-) $\endgroup$ – Rory Alsop Aug 22 '17 at 8:56
  • $\begingroup$ So SETI listens mainly on the 1-9 GHz? That's pretty busy for our current commercial applications. I suppose that's why they do it, the hardware is cheap thanks to mass production. The ETI might however communicate by sending out engraved golden record discs with the word "hello" (a word deliberately constructed by Graham Bell, btw, to make his first phone audible) repeated in different ways. Sounds extremely unlikely, but it has actually happened! So there's really no reasonable restraint on what we should expect. $\endgroup$ – LocalFluff Aug 22 '17 at 10:27

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