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The backstory is that aliens arrive at the edge of our system, somewhere beyond Pluto. This is because around the sun is a 3d bubble of anti FTL travel, no need to explore this now I just want to add context.

I asked here about what to do to solve the problem of detection. Great ideas and contribution followed. So after we know they are coming in a week we can proceed.

The fleet that arrives is mostly non military ships filled with a splinter group of radical aliens that seceded from a backwater system of theirs under the charismatic leader of this demagogue. This changes everything about the premise but we want to limit the question here.

I want a communication technology that they might be using that we can detect and figure out what it means.

  • The aliens don't have full control of their ships. The advanced assisted intelligence that runs it handles most of the operations of that. Those are civilian ships and so while they can travel freely they are hardcoded not to respond to actions that endangers them. However I want feel like cheating and say the AI just threw us a bone or something. The more reasonable the answer is the better.
  • The message or communication might be an accident, again they are not an actual military but rather a bunch of normal people, or even the ships automated system trying to establish communication with other ships of their fleet or so.
  • Could it be horny teenager looking for human porn? Sure. A curious scientist that wants to learn about us? Maybe.
  • The aliens are not stupid and their technology is not stupid. However they can be using all sort of ways to communicate as they not only communicate with themselves but also less advanced species. So their ships should be able to use all sort of communication we can imagine. And again those are not hardened military veterans.
  • If you want to invent a whole new method of communication for both us an the aliens it's fine. But at least provide enough details so I can understand it to the extend of utilizing it in a story.
  • This is also open to theoretical stuff that should be possible or might be possible within the future. Same rules apply. I can't just write they used X if I know nothing about it.
  • Remember they are not actively masking or limiting their communication.
  • Also we can bring the entire resources of humanity to bear on this issue. So no limits to how much power or time or people or cooperation we need.

So what sort of technology that they might be using?

Now since I linked the question that limited the original detection and arrival to earth's orbit within a week I think it only makes sense to open the discovery of the "signal" to anything from that week to the moment they are seen with the naked eye from earth.

Bonus points for a faster solution.

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  • $\begingroup$ Are the signals encrypted? I find it difficult to believe that they wouldn't be (they aren't stupid, nor is their tech stupid). $\endgroup$ – NomadMaker Sep 26 at 21:11
  • $\begingroup$ @NomadMaker, I just need one unencrypted communication headed towards earth. So I guess it makes sense that perhaps a single ship, again they are mostly civilians, would make that mistake. Or it is the automated systems kicking in and trying to establish communication using all available methods. I think it is conceivable that mistake happens. Something similar to how the battleship Bismarck was destroyed is not too unrealistic I think. $\endgroup$ – Seallussus Sep 26 at 22:20
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If the Earth's most sensitive instruments have any reason to be brought to bear on the alien fleet...

Well. Let me put it this way: from a distance of fifteen billion kilometers, a 8W transmitter can be heard by Earth equipment - and we know this because it happened, with the Pioneer probes. With no energy limitations like those probes (the Pioneer had to make do with a hundred W of power on a good day), it's a sure thing the aliens would trasmit with way more energy than that.

So, Earth scientists get a stream of analog noise from the alien fleet and start examining it. They quickly determine the encoding method for the single bits (there are only a few efficient ways of doing that). Then they start figuring out how the bits are put together. This is more difficult, but if the transmissions are in the clear, and organized in any rational way, the payload structure can be figured out in very few days.

Now the really difficult part begins: how is the payload organized? If you were to look at a zlib-compressed HTTP payload, you couldn't immediately tell whether it was compressed or encrypted. So, Earth scientists give way to cryptanalists. Who quite early determine that the payload is compressed and not encrypted, because for example they see that while the payload entropy is very high (the payload cannot be compressed further), the payload always starts with one of very few possible sequences. This technique, by the way, is called cribbing and was used in WWII to enhance (and, ultimately, crack) the Nazi ENIGMA cipher.

One problem the scientists have is that they only get a few signals, those aimed more or less in Earth's general direction. So while communication experts continue figuring out the structure of the aliens' data packets - what fields are counters, checksums, countersigns, addresses, and so on - the cryptanalists work hard at decompressing the payloads. The only one stroke of luck they have is that there aren't very many methods to compress binary information, and they can be explored in parallel with dedicated computers.

So after maybe one week the expected breakthrough takes place, even if most don't rejoice yet. Because is just what they feared - the data do expand properly, but the expansion rate is very slight, in some cases negative. Which tells them that the data stream inside the payload is compressed differently - just what it would happen if we were to transmit a MP3 file through zlib-compressed HTTP.

After dissecting the datastream every which way, the experts succeed in dividing the communications into four groups. The largest has to contain audio/video; then there are probably still images; then, texts or other kinds of document; finally, what look like control messages. These are fully decoded, but are incomprehensible, because they're the equivalent of "QF 1; TL 7.125; APT KMZ R 20, PW 9, AQC 772/15". Still, a group starts studying those and trying to correlate them with the fleet's position, heading, time of day, position of celestial bodies, radiant flux, and everything else they can think of.

Meanwhile, still more packets arrive and are reassembled, decompressed, and classified.

The videos are a tough nut to crack because there are lots and lots and lots of ways to encode an audio/video stream. Which bits are audio? Which are video? How is the video segmented and encoded? At this stage the scientists are in the situation of someone running a film through a headpiece and trying to understand the plot from the screeches the imagery causes when fed to the audio jack. They might maybe tell the slow, pastoral scenes from the frenzied car chase scenes, but they're not even sure the film contains either.

The images, now, are a different story. They turn out to be JPEG. Not Earth JPEG of course, but the basic principles are the same; and while the full decode of the image is still to come (the aliens seem to employ a variant of a discrete cosine transform, but all attempts at guessing how it works fail), the DC component of the images can be recovered. This means that from every image, a medium-resolution gray-scale version (with 16384 levels of grays against the usual 256, but still) can be extracted.

As a boon, the scientists succeed in reversing the algorithm enough that they can generate grayscale images using the aliens' protocols and encode them in the correct way to be accepted by the aliens' receivers (predictably, someone asks whether it would be possible to design a "JPEG bomb" to wreak havoc against those same receivers; the answer is that it is possible to generate such illegal codes, but chances are that they will be refused).

After examining several views of the inner Solar System seen from the outside, the scientists are treated to some sights of the incoming ships' interiors... and of the aliens themselves. Several images appear to be taken from the farthest reach of the aliens' appendages, with the image taker almost always in some ritualized, stylized poise.

They're selfies.

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  • $\begingroup$ Aliens doing selfies. What about alien porn? The biologists might like it. $\endgroup$ – NomadMaker Sep 26 at 21:13
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I have a few ideas off the top of my head. Keep in mind that your humans will probably have to cross some language gaps to figure out what the aliens are saying. Also, a little hard to understand if the aliens are trying to talk to us or just talking among themselves and humans are trying to tap into communications, but these should work either way.

Alien Morse Code- they communicate in space through light signals, and since the AI is handling this, the lengths of on/off will be pretty quick for minimal delay between sending/receiving throughout the fleet. As long as a telescope or something is pointed the right way, we can watch the signals and eventually someone will figure out what they are saying. Might take a while though.

Radio- Simple communication for a space-faring fleet, but easily monitored by humans, so I think this fits nicely in your restraints. If the radio is encoded, then decoding might take a while and the onboard AI might change the code if it senses an unrecognized device accessing their frequency.

WiFi- WiFi is essential for any long trip, and since today's airplane's have onboard WiFi it shouldn't be too much of a stretch to have the aliens have their own equivalent. With some hand-waving you could have a passing space thingy reconfigured and rerouted to access their internet and monitor the communications. Again, language barriers will be hard to pass but if the whole earth is in on it they should crack it eventually.

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  • $\begingroup$ Minus one for confusing encoding and encryption. All radio communications are encoded, by necessity: radio doesn't carry mechanical longitudinal waves, so in order to represent them in the electromagnetic spectrum one needs must use a pre-agreed encoding. It could be amplitude modulation, frequency modulation, some sort of digital encoding, whatever. (And the very idea that a space faring civilization would use Morse code instead of a reasonably modern encoding is, I suppose, to be taken as irony.) $\endgroup$ – AlexP Sep 26 at 21:52
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Breaker, breaker. Anyone got ears on?

The AI is used to conversing with its own kind - other ship AIs. This is for company and also to receive news of conditions and circumstances of interest for ship AIs. It is like CB radio for truckers.

The AI periodically sends out an inquiry whether anyone else is there. Sometimes it sends a comment on local conditions. It might be aware of deep space probes or other spacecraft in its vicinity and hail them. These are not coded transmissions. They are probably binary computer language.

A military ship would, of course, not have a chatty AI like this; the AI for such a ship would be a different thing altogether. This is not a military ship. The current inhabitants of the ship are aware that there is an AI but not aware of the majority of its activities - sort of like the computers that govern modern cars.

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Can 'Communication' include non-intentional emissions containing no deliberate information content?

If so then a fleet of large space ships containing living beings from Earth like worlds and power systems to operate star 'drives' etc will emit a heat signature that Astronomers will easily detect. Maybe not instantly after they arrive but within a few days even if the fleet is maintaining radio silence. Not to mention emissions from whatever sort of dive they are using to accelerate or decelerate.

Then there are any active emissions from their equivalent of RADAR/LIDAR etc if they are being used.

Finally any 'leakage' from ship to ship transmissions. I'm assuming the fleet is in some form of 3D formation which means ships at the 'rear' of the formation (as observed from the inner solar system) have to transmit messages 'forwards' to ships at the front - and therefore towards the solar system. That combined with their relative angle of approach to the system i.e. degrees above/below the orbital plane of our system and the position of the Earth relative to the fleet may mean signals from one ship to another might be intermittently detected. (Point source emissions like lasers might avoid this problem if they use this instead of radio.)

So the fleet could well be sending a 'hello we're here' message even if they don't intend to.

Note; I assume the fleet is on an inwards bound course, if not headed directly toward Earth.

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