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The Precursors are looking for recruits for their interstellar army. Obviously they don't need primitives who can only just bang rocks together, so they've set up a test - a beacon, lying above the galactic plane, using pulsed lasers to beam prime numbers (ie a group of three pulses, then a group of five pulses, then seven pulses, etc) towards thousands of stars that may evolve technological life. We have not detected it yet because it lies in the Zone of Avoidance; but that's about to change.

My question is this: Given that the idea is to contact only technological life, and that the signal is blocked by the galactic core (for story reasons), what wavelength of light would the Precursors be likely to use?

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    $\begingroup$ 666 nm, which happens to be red light. $\endgroup$ – MolbOrg Mar 1 '17 at 15:20
  • $\begingroup$ According to the Wikipedia article: "Projects to survey the Zone of Avoidance at radio wavelengths, particularly using the 21 cm spin-flip emission line of neutral atomic hydrogen (known in astronomical parlance as HI), have detected many galaxies that could not be detected in the infrared. Examples of galaxies detected from their HI emission include Dwingeloo 1 and Dwingeloo 2, discovered in 1994 and 1996 respectively." $\endgroup$ – MichaelK Mar 1 '17 at 15:22
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    $\begingroup$ Also: the center of the galaxy — that causes the Zone of Avoidance — is at least 5 000 light-years wide. This means that if your Precursors intend to contact races beyond the galactic center, then the message will take at least 5 000 years to arrive. And then there will be — in the very least — 5 000 years before the recipients join the fray. What kind of army is expected to last for 10 000 years? And if your Precursors are so primitive that they cannot go out there and recruit help in person, but the "help" can travel that distance... do your Precursors really want to get in touch? $\endgroup$ – MichaelK Mar 1 '17 at 15:29
  • $\begingroup$ @MichaelK Oh, it's much better than that - the beacon is on the very far edge of the galaxy, 70,000 light years away.The ability to reach the beacon and find out about the war is part of the test to make sure you're ready to fight in it. As for what kind of army is expected to last for 10,000 years (actually closer to 10 billion years), well...the term "Cosmic horror" exists for a reason. $\endgroup$ – Werrf Mar 1 '17 at 15:36
  • $\begingroup$ It might be helpful to know how these prime numbers are encoded, e.g. a prime number wavelength, a repeating binary pattern of off/on or amplitude modulation or frequency modulation. Each of these has its own considerations. I think I could answer though, given that information. $\endgroup$ – ocket8888 Mar 1 '17 at 22:29
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Since you’re not carrying a complex signal but just blinking, you don’t need to consider how higher frequencies can carry more information. Though I wonder how “hey, prime numbers!” can be interpreted as “come join our eternal army”.

It should be something that will carry the distance, not be blocked by gas and dust and whatnot.

It should best be unnatural: a tight frequency might itself be unnatural, but it might go unnoticed unless it was something that didn’t match a natural signal at all, without needing spectrographic analysis.

If it’s meant to be noticed by ground-based observers, it needs to get through the atmosphere of candidate planets. That may correspond with the native’s vision since that’s what sunlight gets through too. Aim for typical visible range and people will see a blinking star without instruments.

In a story I’m working on, a green beacon has this effect. But the blinking is just an attract layer, with terabytes encoded in more subtle ways in the signal.

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The best wavelength for such communication is the subject of several articles on wikipedia. Of these, The Water Hole has received a great deal of attention:

The strongest hydroxyl radical spectral line radiates at 18 centimeters, and hydrogen at 21 centimeters. These two molecules, which combined form water, are widespread in interstellar gas, and their presence radiates radio noise at these frequencies. Therefore, the spectrum between these frequencies form a "quiet" channel in the interstellar radio noise background.

It's such a promising choice that its name, The Water Hole, is actually a reference to the belief that its a natural place for intelligent life to search for each other.

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