[richk449]

> But even if we assume radio waves, Shannon says that the you can send the most information when using a random code. As you approach the Shannon limit the signal becomes more noise-like as the encoding tends towards random.

The waveforms we currently use for high data rate communications are already pretty close to the Shannon limit. I'm willing to bet the aliens will tolerate a 30% reduction in data rate if it means using an identifiable signal.

Does Shannon's theorem really imply that the closer an information stream is to the limit, the more it resembles noise?

[zh3]

There's a simple proof of this; an optimally compressed signal will have all symbols occuring with equal frequency (otherwise you'd be able to compress it more). So yes, an optimally compressed signal is indistinguishable from noise.

However that's the theory; in practice the range of frequencies the signal occurs over and how strong it is at each frequency tells you a lot about whether it's likely to be an information carrier or not.

[jhoechtl]

They send sognals to be heard and discovered. Why would they try hard to disguise if it's meant to be heard by another civilisation?

[anonsivalley652]

Let's apply common sense.

0) First, would it be a good idea to announce your world's existence? Robotic explorers who might exist in your neighborhood might decide your world is worth harvesting or colonizing.

1) If yes, then wouldn't sending a high-power announcement signal need to be obviously unnatural, i.e., containing very little data? Short trains of pulses with pauses that count up in binary perhaps?

1.1) "Visible light" seems like an anthropocentric assumption because there's not necessarily a reason other entities couldn't sense microwaves, IR, UV or even possibly fissile radiation. Furthermore, cybernetic "evolution" seems like there would be many possibilities for different senses. Maybe there is something particular efficient about the visible spectrum, such as polarized laser light remaining distinguishable extreme distances?

2) If said entity wanted to send a little more data as well, wouldn't they use a slightly different carrier or modulation but it wouldn't necessarily reach as far? Or, send it, as mentioned, more slowly?

[sliken]

Seems unlikely. Assuming a civilization could send a force to take over earth from many light years away, what could they possibly find of use?

Any element is easier to get elsewhere (without such a big gravity well) and no monkey descendants grumpily lobbing nukes at ya.

Sure, maybe they would harvest the asteroids and small moons while ignoring our complaints over sent over radio waves. No direct impact on earth. Maybe they would zorch a few suspicious looking sats with high energy signatures while they sat in high orbit and observed.

If we particularly amused them, or they thought we were uniquely tasty they could just grab a few of us and figure out how to replicate us.

Maybe our internet and fascination with cat videos would amuse them, some advanced crawlers running on quantum computers (for those pesky prime number based encryption) might cause some serious, but temporary disruption.

Maybe they would install a world leader with orange hair to sabotage any world wide scientific progress to help ensure we don't make significant progress any time soon.

But the sci-fi troupes of stealing humans to eat or vacuuming up the atmosphere or oceans seems exceedingly unlikely.

If anything they might drop a blackhole into the sun to harvest 10% of the mass that falls in and is converted into energy. That way then can use the energy for something more useful. But if that was common we'd be seeing lots of other stars disappear strangely.

All in all seems rather egocentric to think any really advanced civilization would particularly care about us.