Fascinating! This works the other way, too, right? Most any alien civilization will only have a brief period of “noiseless” emissions, followed by millennia of “noisy” ones? Doesn’t this make the whole SETI effort a waste of time?
ETA: I just realized that, while SETI may be useless for detecting unintended planetary transmissions (like our once less-noisy, now more-noisy entertainment and local communications emissions), it IS good for finding INTENTIONAL interstellar communications signals, which are sure to be relatively noise-free.
Remind me – WE have been sending out intentionally noise-free signals lately as well, no? Perhaps as part of the same SETI program?
That last part is self-evident piffle.
Why? The idea is hardly new. If the entropy of the stream is maximal it is indistinguishable from noise, and yet has maximal information content. Redundancy in the information implies less than maximum entropy. Lack of redundancy implies that all information is thus useful, rather than some being redundant, and thus of no additional use beyond what has already been received.
Wait, the Heaviside Layer is a real thing? And here I thought T.S. Eliot made that up.
Powers &8^]
With the unfortunate side effect that a one-bit glitch destroys the entire signal. Information Theory is a two-edged sword.
Quite true. It is clearly a limiting case. However you don’t need to add much redundancy back to allow it recover. Further, the way in which that redundancy is added may not be obvious, and require understanding of the coding process to even detect. Clearly the absolute limit case would not be used - one will always have some amount of redundancy present. Whether there is enough for our alien listeners to realise that there is unnatural modulation in use or not is another matter. In the presence of lots of background noise we may find that it is simply never viable.
Oh, it’s real, all right. It’s up, up, up, past the Russell Hotel.
This is what they call Active SETI, or sometimes METI, “Messaging to Extra-Terrestrial Intelligence.”
One example was the 1974 Arecibo message that Frank Drake and some associates sent to celebrate the revamping of the Arecibo telescope. They sent it to the M13 globular cluster, so it’ll get there in only 25,000 years…
A part of me suspects that ordinary alien signaling will be layered (like TCP/IP or SNA) and have something very similar to HDLC at the equivalent layer. Maybe we should be on the lookout for 0x7E bytes. 
Communications actually intended for interstellar distances, of course, require a full ECC level of redundancy. Can’t rely on sequencing or mere error detection when turnaround is measured in years.
If you are designing an error correction code from scratch now one would turn to things like Reed Solomon codes, possibly with cross interleaving. The neat thing here is that you can tune the coding to be resilient to quite significant damage by spreading out the information across a wide distance in the stream, and adding enough redundancy, also spread widely, to be able to reconstruct the data in the face of long periods of interference or outage. Where long might be tens, hundreds, potentially even millions of consecutive bits lost in the base stream. You can tune it to be resilient to the expected distribution of interference or outages. The downside is that the parameters to the coding become much more difficult to deduce if you are presented with the raw stream. Although it contains redundant information actually detecting the redundancy is non-trivial.
Active SETI on the other hand is as dumb as possible. Basically it says, “Hi there, we’re made of meat. Meat, you hear that! Meat! Here is the menu. Lovely meat!” and repeat, sometimes even with pictures on the menu.
Yeah, I definitely subscribe to the “How not to be seen” side of that coin.
Fascinating, thanks! Gotta love the Teen-Age Message. “Hey, little green dudes, you dudes are, like, totally awesome, like, dude, whoa!”.
Interesting that the first of these intentional messages will reach a star in Libra in 2029.
I realize they’re pretty amateur, in terms of the coding and physics parameters, but hey, it’s a start.
I can find a description but not a transcript of the TAM.